Here are some more shots taken with the Optomax 200mm F/3.5 that I reviewed here
Because I was impressed by the overall performance of it's smaller brother I decided to try out the Tokina SZ-X SD 28-105mm F/4-5.3 to see if it could deliver a similar performance over a more versatile focal range (albeit with slightly slower maximum apertures)
Build quality and finish:
A similar story to the 28-70mm, this lens has a lot of metal and an overall solid feel. This lens is a push/pull design as opposed to the traditional 2 touch design of the smaller 28-70mm but otherwise has a very similar finish.
And once again a similar story, very similar!. Wide open we have decent central resolution at the wide end and decent resolution across the frame at the long end. Stopped down to F8 produces razor sharp results ideal for landscapes. Because of the relatively slow maximum apertures bokeh is not a consideration all too often but when coaxed into producing it the lens shows a relatively smooth performance for an older zoom lens, one benefit of the slow apertures is that axial chromatic aberrations (sometimes referred to as bokeh CA) is extremely well controlled. That just leaves colour and contrast, both excellent across the aperture range!
By clicking on these images you can go to flickr where a larger version is available:
It has to be another recommendation, apart from low light work where the slow maximum apertures would be a disadvantage this is a great all rounder and puts in a decent optical performance, best of all people are practically giving them away, go and buy one!
Build quality and finish:
There is nothing lacking here, the lens is mostly metal and has smooth focus and zoom rings. Overall construction is very good considering this was not a top of the line lens, the only possible criticism is that there is no internal focus mechanism so using a polarizer filter can be tricky as the front element rotates during focusing but this is not unusual for an older zoom, especially one that was modestly priced.
In short; great! The lens has superb colours, almost too saturated sometimes! In terms of sharpness the centre of the frame is pretty good at 28mm wide open and stopped down to F/8 is sharp across the frame, just as sharp as most of my 28mm primes! Wide open at 70mm sharpness is already very decent across the frame and is superb at F/5.6. Contrast never seems to be lacking and even the bokeh is smooth in most situations although at 28mm with close backgrounds it can be understandably busy. Overall I was impressed by the results from this lens, it's best stopped down at the wide end which I tend to do with most lenses as I usually do landscape work at wide angles and at 70mm it's really very good wide open so can be useful for portraits. The lens has a macro mode and while it doesn't allow for a huge magnification it's worth noting that sharpness remains decent even at or near the minimum focusing distance.
By clicking on these images you can go to flickr where a larger version is available:
It's well made, light, has great image quality and a decent macro mode, the only thing I found lacking was a non rotating front element, this is another bargain lens that you can buy for a ridiculously small price and get great results from, I recommend it!
An update about cheap polarizer filters!
A while ago I wrote about some Sakar CPL filters that were bought cheaply from ebay and how well they performed however after some research I have discovered an issue that is worth mentioning.
It turns out these filters work perfectly well on lenses up till about 70mm in focal length, after that they will degrade picture quality because they are not uniformly flat enough and when placed in front of a lens will create a prism effect and distort light rays. The effect worsens with longer focal lengths and larger physical apertures (ie the actual size of the lens opening and not the aperture setting)
At first I thought it was just an issue related to the cheapness of the lenses and that by buying a more expensive filter I would be safe to use it on lenses with a longer focal length but after some research I discovered that even expensive filters can have very negative effects on long lenses, that is why a lot of super tele lenses have dedicated drop in filters at the rear of the lens, this smaller filter is a lot easier to make totally flat so it does not impact on image quality.
The problem is discussed in greater depth here:
There is also a discussion here on the red movie making forum where a user has used VERY expensive Schneider filters and still experiences severe image degradation which he likens to smearing Vaseline over the lens:
So to conclude this short entry, the cheapy ebay filters still work incredibly well for their price but I would advise not using them on lenses over 50mm to be safe, I have tried a Tian Ya CPL that was much better and worked till about 150mm so it really depends on which filter you get but I would say 50mm to be safe and anything more is a bonus, we are talking about £3 filters here! If you want to use a CPL on a longer lens then be prepared to dig deep into your pockets because it's not just the cheapy ebay ones that spoil the party on Tele lenses, no wonder there is such split opinion over filters and their effect on image quality!
On a side note, I have also used Green.L brand UV filters and have noticed no problem with these even on a 500mm lens so in my experience it's the polarizing filters that can be tricky!
My 350d gave up the ghost recently so I decided to buy another body.
This seller had a tidy one listed, "perfect working order with no issues" and so on so I bought it.
The camera turned up without a dust cap on the body, the lens mount stuffed full of bubble wrap, the viewfinder was caked in dust and dead bugs etc.
Worst of all the focus confirm lights were all misaligned so far that they didn't even hit the boxes in the viewfinder, they were all too far to the right. As a consequence the lights do not hit the little reflectors/etched areas and the lights are so dim you could hardly see them unless you turned the lights off!
I emailed the seller and he claimed this was normal for ANY used camera.
He said it was a regular issue with this particular model, even though he said "no issues" in the description.
When I demanded a refund he told me he had found another body that "works correctly" at which point I said if this is the case then you were lying by claiming the first body isn't faulty.
I sent the camera back but he did not refund my original postage, on top of the postage to send it back I was £18 down.
I left him negative feedback and he sent me a ridiculous threat about taking me to court (he must have heard that silly story in the news a few years back) he then sent me a request to change my feedback which I denied!
Then he sent a request to cancel the transaction, that option is supposed to be for when an item gets lost or damaged and cannot be sent, he stated to ebay that I had "changed my mind" he was obviously trying to get his selling fees back but seeing as he ripped me off I immediately cancelled his request. He then accused me of being a scam artist, I pointed out that to scam someone you need to make money from them, not buy faulty goods and not get all your money back.
He has now sold the camera to someone else even though he still possesses money I paid towards it. The new owner left positive feedback but I emailed them and told them the situation, they said they had not fully checked the camera over but now it seems they have realised it is faulty too!
In the meantime, someone else has bought a camera from this guy that was faulty and he has accused the buyer of damaging it himself and is refusing to refund him!
Basically this guy is dishonest, arrogant, hostile, unprofessional and very poorly informed in terms of legal matters and distance selling rules etc.
He tries to pass off physical damage and malfunction as "normal wear and tear"
AVOID THIS SELLER!
Named and shamed!
In my last update I mentioned I had bought a Kodak Gear 78-215mm F/4.5-5.6 in Canon EOS mount for peanuts and that I was quite optimistic about it's capabilities considering it's price and I'm pleased to say I was right. This lens is essentially a re-badged Tamron that was also available under the Tiffen name. The true focal length is 80-210mm and the Kodak version can be had with both 80-210 and 78-215 written on the side but they both report as a 80-210 in EXIF information.
Build quality and finish:
The lens has a lot of plastic in it's build but is certainly well put together and even the manual focus ring, while small compared to days of old is still superior to most of Canon's own efforts in this price range and benefits from a rubber grip. The lens is very compact and light which makes it ideal for when you don't want to be weighed down by too much equipment. The lens even comes with a custom Kodak bag that has room for the lens plus one more and a few filters, a really nice touch and along with the lens hood exhibits how stingy Canon et all are with their lower end products, often making you pay extra for such accessories. The focus motor is a bit noisy but not too bad, you can't expect a silent motor in this price range. The focus action is acceptably swift but accuracy appears to become worse the further towards the long end of the zoom you go.
This is where the lens shines for me, the colours it produces are very vivid and eye catching and contrast was strong regardless of aperture of focal length. The sharpness was good wide open up till about 135-150mm where it began to tail off slightly but by stopping down to F/8 bought the sharpness right back. You can actually get fairly sharp images wide open at full tele but the autofocus seems to be inaccurate enough to miss slightly most of the time, this may well be partly thanks to my 350D too but I found it safer to just stop down when at the long end. With a camera that has a better AF system the lens may perform better in terms of focusing at 210mm and if you had live view you could obviously tweak the focus manually.
By clicking on these images you can go to flickr where a larger version is available:
Overall this is another one of those bargain autofocus lenses that can be picked up for next to nothing and delivers surprisingly good results, I have better lenses but they cost many times as much and are also much heavier. These lenses crop up on eBay a few times a month and if you need a telephoto zoom on a budget or like me you just like a bargain I recommend giving it a try!
I was thinking the other day how I don't get around to making posts as often as I would like so in an effort to combat that I will begin making entries to document all the smaller photography goings on in my life that do not warrant an entire entry (at least not yet).
Recently I have purchased lots of bits and bobs from ebay (as usual) as well as two cameras! A new camera is quite a significant event to me so to have two new arrivals inside a month is an exciting thing indeed!
Some recent purchases have been:
Vivitar 200mm F/3.5: This lens is massive for what it is and is in a style I do not recognise it is mostly black with a large silver band towards the base, my initial tests show it is quite good but unfortunately the aperture mechanism has become disengaged somehow, typical! it survives 30 years then crumbles in my hands after 5 minutes! It should be easy enough to fix :)
A Kodak "gear" 78-215mm F/4.5-5.6 zoom lens in Canon EF mount: This is a re badge Tamron lens and is really an 80-210mm and is also sold under the Tiffen brand. I got this lens purely because it was dirt cheap and I was curious. The presentation of this lens is excellent, it comes complete with a well made a storage pouch/bag and actually has a better grip on the zoom ring than the Tamron version. Optically it seems quite good, I haven't tested it extensively but it seems to be a pretty decent performer considering how much it's worth.
50 bags of silica gel: These were just a bag of gel pouches like you get in packaging and new shoes, I bought them just to put into lens pouches and my camera bag and other places I store lenses just to keep moisture (and hopefully fungus and mould) at bay.
A knock off version of the Canon ES-62 lens hood for the 50mm F/1.8: This is really great quality and easily up to the standard of the official canon one at a fraction of the price, it even has the bayonet ring which you screw into the filter thread and then just twist the hood on and off, some cheaper ones just screw straight in and then you can't put the lens cap on again. I have bought a few copied lens hoods recently and they have all been excellent.
A Yashica ML zoom 42-75mm F/3.5-4.5: I got this for two reason, firstly it was cheap and secondly it is a less common focal length and I was curious. I haven't tried it yet.
A 10 piece Tian Ya graduated filter set: This is a cheap set of filters that are compatible with the Cokin P series and appear to offer substantial value for money. It was after reading this encouraging post that I decided to try them out and although I haven't used them a great deal yet they seem to be great for the money, you can't really used them for too many situations because they will make things look too unnatural but for intensifying sunsets and sun rises etc they should prove an interesting addition to the kit bag.
LOTS of lens caps: Just a bulk buy of front and rear caps for various lenses, they were a bargain and the front caps were all centre pinch which of course makes putting a lens cap on with a hood fitted much less infuriating!
A 3 axis hotshoe mounted spirit level: I am nearly as bad as holding a camera straight as I am holding it steady so this is a handy little accessory. I already had one but it was inferior and had a machining mark right across where the bubble sat so it was hard to read properly.
A Ricoh Rikenon 50mm F/2: I have bought and sold a few of these in the past and it's a great lens, very sharp and nice bokeh too, I got it because it was too cheap to pass up!
A Casio Exilim EX-Z77: Yes ok I got 3 cameras recently but this one doesn't really count as it was 99p and described as faulty. It turned out that just the battery was dud so I got another one for £2! Unfortunately it's not as good a performer as I had hoped, edge sharpness is poor through most of the focal range as is purple fringing control and the entire frame becomes noticeably soft through diffraction if the lens stops down and with no way of controlling it manually half of the shots comes out muddy looking. I wasn't expecting too much but I was hoping the lens might have been a bit more consistent, only the very best point and shoots are genuinely sharp at the pixel level but I was at least hoping I could get some decent A4 prints from it's 7 mega pixels (my 4 mega pixel Canon G3 manages that just fine) I'll probably put it back on ebay but I'll try cleaning the lens first just in case it makes a difference.
Miranda 28mm M42 lens: Dirt cheap, haven't tried it yet.
Vivitar 75-205mm F/3.8 2 touch: The one that's made by Kiron, been through a few of these and they are excellent for an older zoom. Unfortunately this lens has some oil stains inside which I may or may not be able to clean as zoom lenses are impossibly complex to disassemble. The seller is not interested because they claim to have been unaware of the problem at the time of sale, I tried to explain that a shop would not be able to shrug off responsibility that way but they were still unwilling to help so I left them a negative feedback, bollocks to em'!
A Samsung GX-1S DSLR: CHEAP! The first DSLR Samsung ever made, this was listed as faulty because the mode dial wasn't activating all the settings, I'm pretty sure I will be able to fix this as it's normally just dirt and/or corrosion on the contacts underneath the dial. The camera is in perfect shape otherwise and came complete with both the 18-50mm and 55-200mm kit lenses. I was impressed by the quality of the kit lenses, non rotating front elements and hoods with removable cut outs to make the use of rotating filters possible with the hood still attached, much better than Canon and Nikon kit lenses for quality of finish! The only gripe I have so far is that I can't open the raw files in capture one, it's annoying because the files are all but identical to the equivalent Pentax model so it must just be the headers that capture one is getting upset about, I read about a piece of software that can fix this but have been unable to track it down. I don't know if I'm going to keep this camera or fix it and sell it on for a profit but it's already growing on me, I'll post a review of it soon either way.
A Tamron 103a 80-210mm Adaptall lens: This was 99p, I already have one but I couldn't let it go at that price, it's a great lens, I'll review it at some point.
A Panasonic DMC-G1!!!! An EVIL (Electronic viewfinder interchangeable lens) camera! I have bought this to replace my Olympus E-410, using magnified live view is by far my preferred method of focusing manual lenses but trying to make out what's on the LCD in daylight is next to impossible. The Panasonic G1 has an electronic viewfinder and it's probably the best one ever fitted to a camera so now I can use live view even with the sun behind me! Other advantages over the (admittedly already versatile) E-410 are the ability to use pretty much any lens with the right adapter (Canon FD, Leica M39, C-mount, Olympus Pen, Pentax 110, Minolta MD, Deckel and so on and so on) and the excellent rotating rear LCD, how I have dreamt of an interchangeable lens camera with the same excellent feature of my trusty old Canon G3! Hopefully the camera has better noise control too, when you turn the detail killing noise filter off on the E-410 even ISO 100 shots have noise visible. Expect more about this camera soon!
An Olympus 4/3 to Micro 4/3 adapter: This will allow me to use all the adapters I have already invested in for the E-410
A C-mount to Micro 4/3 adapter: So I can try out all sorts of weird and wonderful Tv and Cine lenses!
A Canon FD to Micro 4/3: Because I already have a few FD lenses laying around.
A sun 28-80mm Macro zoom: Dirt cheap and I have a soft spot for Sun lenses, Haven't tried it yet.
A Canon conversion lens adapter LA-DC58C: This is for my Canon PowerShot Pro1 and allows filters and wide angle converters etc to be used and also has a tripod mount. I got it mainly because it only cost 99p!
A Canon Wide Converter WC-DC58N: This is for my Canon PowerShot G3 and takes the wide angle from 35mm to 24mm in 35mm equivalent terms. This was also bought mainly because it was very cheap and I always lusted after one when my G3 was new but could never afford it!
And finally a Kamasa 30 piece precision screwdriver set so I can stop mashing the screws of lenses up by using those cheapy pound shop "jewellers screwdrivers" hopefully I will be able to repair more lenses now instead of ruining the screws and rendering the lens un repairable.
Whew! that's about it I think! I'm currently waiting on my lens adapters for my new G1 and in the meantime will be selling my E-410 and a few lenses most probably. I shall no doubt post some more lens reviews etc soon!
Some lenses are just destined to be unpopular and this is a classic example. The problem starts not so much because of the actual deficiencies of the lens but because of snobbery. Time and time again we hear about people who have "all the gear but no idea" and how many times have we frequented forums where members have spent thousands on the kind of equipment and software we can only dream of only to post the most mediocre photos that could have been taken with a compact? These people are the worst for spreading false information, often commenting on equipment they have never owned purely out of snobbery.
Disclaimer of sorts:
Before I proceed I want to make it clear that this IS a less than stellar lens, it does have a low build quality, it does have distortion and chromatic aberrations but none of these factors account for why most people hate this lens, people hate this lens because it's CHEAP and people who have limited photographic skill but lots of money need to feel they are justified in spending thousands on lenses.
Any lens can capture a good photo, I honestly believe that. If you have a stunning sunset you can take a picture of it with your phone and it will still be a good photo, the subject is already there and will be the same no matter what medium is used to record the scene. The difference the equipment makes to me is in the actual image quality, you can watch a great movie on an old black and white TV set or on a brand new 1080p system and it will still be a great movie, the expensive equipment does however increase the viewing quality and in turn raises the overall enjoyment. So while the gear doesn't make the photo I do believe that there is a level of quality to which we must adhere to at least do a scene justice, the more expensive the equipment gets the more the law of diminishing returns comes into play but at a more affordable level there is a great leap between let's say a camera phone or cheap compact and an entry level DSLR with even a mediocre lens, in some cases you could pick up the DSLR for less than the phone/compact but even brand new an entry level DSLR and kit lens will not cost more than a few hundred pounds or dollars more than one of those ultra high pixel count cameras or camera phones. The DSLR will have much, much greater per pixel sharpness, better colour, better noise control by a ridiculous margin, almost incomparable features and functions and even with the cheapest of kit lenses will have a better lens with less optical flaws and improved sharpness and contrast. Now to make the equivalent upgrade over the entry level DSLR you would not have to spend a few hundred or even a few thousand, there is simply no DSLR/lens combo available that will improve upon an entry level DSLR as much as that DSLR improves upon a camera phone or a cheap compact.
The point I'm trying to make is that once you reach a certain level of image quality the returns of spending more money quickly become harder and harder to discern. There is a level of image quality that is nowadays affordable to reach by most people, any entry level DSLR and lens will will take photos that are almost immeasurably superior to a cheap compact or camera phone. There are a lot of people who own very expensive cameras and lenses yet don't have the technical skill or sheer photographic eye to make the most of entry level equipment let alone top of the line professional gear. There are people out there that would not be able to use a Canon 35-80mm F/3.5-5.6 III to it's full ability yet will passionately slate it and condemn it as "The worst lens canon ever made" having never touched one and having never taken a decent photo in their lives.
The purpose of this entry then is not to say that this lens is anything special because it's not but to say that it is vastly better than popular opinion would suggest, it's not a top performer and you WILL see a difference between this lens and let's say, a Canon 17-55mm F/2.8 but just try to remember that I bought the 35-80mm for about the same price as the hood for the 17-55mm costs! Let's say you picked up a second hand 350D body for £150 then you picked up a 35-80mm for £15, that £165 combo will shame any camera phone or cheap compact for image quality and functionality. But if you were to buy the aforementioned 17-55mm along with an EOS 7D to put it on would this £2200 combo shame the £165 350D/35-80mm in the same way? Of course not, it would be a considerably better but it would not be the same giant leap, the same monumental step forwards and it certainly would not represent the same value for money. It's hard to break the image quality down into measurable units but certainly the 350D/35-80mm would represent a better quality/pound ratio than the 7D/17-55mm.
So what is this lens if it is not great? It's simply good enough. It represents that first jump in image quality over basic imaging devices and a price/performance ratio that more expensive lenses will never reach. It will take decent shots, it will do most scenes justice, it will allow you to make decent A3 prints and it will deliver enough sharpness once stopped down to mid apertures to keep up with most digital sensors. It is a world away from a camera phone but only a city away from a Canon L lens. Obviously the aperture is slow and there is less scope for using the depth of field creatively but it is still immensely more useful in this respect than any compact will ever be purely because a compact camera is limited by it's tiny sensor.
Build Quality and finish:
This lens is actually kind of solid feeling in a strange way, it's very light and very basic in it's construction but it seems to at least be made out of hard plastic that wouldn't break easily, it's more substantial looking than the Canon 50mm F/1.8 which isn't saying much but might at least give you some idea. The finish is very basic and as with many entry level Canon lenses the focus ring is ridiculously small, it's almost like a vestigial structure such as the human appendix, it's a throwback that through disuse has shrunken to the point of being barely functional. The autofocus motor is quite noisy and the lens cannot always be trusted to get a focus lock first time every time, this may be just the lens or a combined effort between the lens and my 350D but either way I found it wise to take a copies of each shot where possible, making sure to half press the shutter in between to allow the lens to have another try at getting it right. The failure rate wasn't high but I would say maybe 2 out of every 10 shots could be mis focused so it was worth taking several shots to assure at least one was focused correctly, this issue was more prevalent at the tele photo end of the zoom. The lens features only 5 aperture blades so out of focus highlights will not be circular once the lens is stopped down and some people will hate this but personally I appreciate most types of bokeh be it silky smooth or bold and quirky so I didn't feel hard done by in this area. Overall the lens displays all the hallmarks of an object that was built to a tight budget because it was!
Even wide open the central sharpness is perfectly decent, corners inevitably become soft, more so at the telephoto end but even so we are talking about a lens that is worth about £15. Colour and contrast are totally fine in my eye, faithful to real life with maybe a slight bias towards magenta which gives a nice warmth to images. As for flare and colour fringing, this only occurred in challenging situations such as back lit tree branches and although unpleasant were relatively mild and easy to correct in software. Overall the lens is capable of taking punchy photos with usable sharpness wide open and once stopped down to F/8 sharpness that appeared to exceed the resolution of 8mp sensor inside my 350D. Considering the used value of this lens the quality is quite remarkable.
I'm going to provide a brief comparison between a Casio EX-Z77 (typical entry level compact) a Sigma 18-50mm F/2.8 EX DC (supremely sharp mid to upper range zoom) and the 35-80mm (dirt cheap piece of junk!) It was hard to get the same field of view with the Casio due to the different crop factor and lens focal length involved but I got it as close as I could, The Sigma and Canon lenses were both set to 35mm and F/8.
First of all some web sized shots:
Even at low resolution it's possible to see a lack of detail in the Casio photo compared to the others but in terms of colour and overall appearance the images are quite close, the Casio and Canon lenses have more of a magenta hint and the sigma is slightly more neutral, Note also that the sigma exhibits less slightly distortion, particularly visible at the bottom right of the frame
Now lets do some pixel peeping, first up some 100% crops from the Casio:
Now the Canon:
And finally the Sigma:
It's plain to see that the Casio image is very soft at the pixel level with noise reduction killing detail even at the base ISO setting, the sensor is 7mp but I would personally say there's only 4mp of information being recorded at most. The 350D/35-80mm combo is immediately more impressive with improved sharpness and detail and better contrast. The 350D/Sigma combo does provide a tiny amount more detail but not by much and by looking at the green fence in particular it appears that both the Canon and Sigma optics out resolve the 8mp sensor inside the camera. Overall then the the Canon lens completely outclasses the Casio compact and runs the Sigma very close, the difference being the Canon lens cost £15 and the Sigma lens cost me £370 when it was new, now I'm not saying the Canon lens is as good because it simply isn't, the Sigma is sharper, has less distortion and is less prone to colour fringing and flaring (compare the cladding around the chimney), not to mention that fact that the Canon lens cannot physically compete with the Sigma's faster aperture and superior build quality. All I'm trying to point out is that at F/8 they are very close and with a scene like this in particular I would always be shooting at F/8 even with the Sigma so whilst the Canon is somewhat inferior overall both lenses are capable of getting a decently sharp and colourful shot in at F/8, both lenses will do the job and produce a file that will print nicely at A3.
The following pictures are all clickable and will take you to my Flickr page where you will be able to view the full size images:
80mm F/5.6 (wide open):
35mm F/5 (half a stop from wide open, focus is on the TV aerial):
80mm F/5.6 (wide open):
None of them are prize winners obviously but they give you an idea of how the lens performs.
Put basically the 35-80mm is a decent enough lens and an amazing one when it's value is taken into consideration, it receives universal scorn for being a bad lens but it seems very few people are willing to give it a try. I suppose to some people it would be pointless to even bother but to me it is interesting to use different lenses just for the sake of variety because all lenses have unique characteristics and never render a scene in quite the same way. I bought this lens to see just how bad it was and it turned out I was surprised by just how OK it was! If you see one for cheap then I recommended trying one out, take it somewhere you would normally use a more expensive lens and see how it does, make an effort to squeeze the most out of the equipment and challenge yourself and remember, if you don't like it you can always take out the front element and use it as a super macro lens capable of greater than 1:1 reproduction, but that's for another entry!
The other day I was pondering the peculiarities of my AF Sigma 10-20mm,
the lens exhibits a significant "curvature of field" effect and despite the
massive potential depth of field offered by the focal length it is possible to
take shots where there is not uniform sharpness across the entire image, at
first I thought this was a lens defect as there were many reports of these
lenses being sold with elements that were de centred but after some testing I
realised that I could take a succession of pictures with the lens wide open and
re composing the focus each time and that each picture would have a different
plane of focus despite common sense telling me everything from a few meters to
infinity should be in focus no matter what.
A typical set of results would include a picture where only the extreme background or foreground is sharp, a picture where edges are soft and a picture where the entire scene is in sharp focus.
Obviously with an ideal lens if you focus on an object that is 3m away in the centre of the frame then any other object 3m away anywhere in the frame should be sharp too but some lenses seem to exhibit a distinct shift towards the edges of the frame either inward or outward.
I have noticed on a few lenses now (mostly wider focal lengths) that if I focus on the centre using the magnification mode of live view and then move the focus point to the edge of the frame that I have to readjust focus to get the picture sharp. It seems some lenses have a flatter field of focus than others and if you just take crops from the edge of a shot that was focused in the middle it will not always accurately display the potential edge sharpness of some lenses.
It is entirely possible to get uniform sharpness for most subjects even if a lens has field curvature but you have to account for this when focusing in order to avoid soft areas. There are 3 basic types of field when dealing with camera lenses, a flat field, a field that curves outwards at the edges and a field that curves inwards at the edges.
Out in the real world this issue is not as obvious or even as relevant because the circumstances in which it arises are not as commonly found but certainly under some conditions it can be noticeable. The kind of shot that most often shows up the problem is when people take "wall shots" or use test charts as part of a lens review to demonstrate both centre and edge sharpness. Some lenses that have a curved field can end up being sold short in terms of edge sharpness because people tend to always use the centre of the image to focus such pictures, now all that needs to happen is for the tester to focus from in front of the subject if a lens has an inward curving field or focus from behind the subject if the lens has an outward curving field and the corners could instantly be rendered soft giving the false impression that a lens has soft edges when it doesn't.
Note: When I say focusing from behind I mean starting off with the lens focused beyond the subject when you begin adjusting the focus ring and likewise when I say in front I mean you start off with the lens focused somewhere in from of the subject before you begin turning the focus ring. The following illustrations will show you that where the focus comes from has an effect on the sharpness across the image when using a lens that suffers from field curvature.
In these examples we are imagining our lens has a plane of focus that curves outwards at the edges away from the photographer. The black bar represents the subject and the violet bar represents the area of focus with an outwards curve.
In this first image the photographer has focused on the edge from in front of the subject, due to the field curvature the centre of the subject falls outside of the area of focus:
If he had instead pulled the focus in from behind the subject then the entire subject would have been inside the area of focus:
So with a lens that has an outward curving field of focus it is best to either focus from in front of the subject if you are using the centre as a focusing point or focus from behind if you are using the edges. Obviously with a lens that has an inward curving field of focus the opposite is true.
It's important to remember that you are working with a narrow margin of error so you have to stop turning the focus ring just as soon as focus is achieved or you will go right through that margin or error and end up back where you started. An easier way to make sure is to take several shots with slight adjustments to the focus made between shots so that one of the shots has captured the entire subject inside that small margin, people call this focus bracketing and it is especially useful for this problem and also for AF lenses that have a tendency to be indecisive about if they are sharply focused or not.
Another way which I recommended is to use live view if you have such a luxury, using live view you can quickly confirm that you have achieved uniform sharpness before snapping away.
Obviously the more pronounced the curve is the harder it is to make sure the entire subject falls into the area of focus but even when using a wide aperture the depth of focus is usually not paper thin, all you need is for the focus depth to be enough to contain the subject even with the curvature going on. Even if a lens cannot focus on the centre and edge of a flat object simultaneously there is usually a good compromise between the two. When it comes to lens tests and reviews it still seems unfair and technically inaccurate to base edge sharpness on an image where the lens is not correctly focused at the edges, perhaps re composing focus between the centre and edge whilst making a note of the field curvature would be a fairer method. Under "real world" conditions we seldom take photos of flat surfaces with the lens wide open so it's less of a problem but it's better to be aware of such issues and how to get around them and it's certainly better to be aware of such issues when you are conducting a lens review!
This is just a brief explanation and doesn't account for the fact that some lenses have a varying field of focus depending on how far away the subject is, some lenses have a flat field with close subjects but a curved one with distant subjects and vice versa. Another variable is with zoom lenses where different focal length settings can have an effect on the field of focus. It is also worth considering that film is to an extent three dimensional and more forgiving where as digital sensors are completely flat and less forgiving but all of this is certainly both beyond the scope of this post and beyond my level of intelligence!