Most of us who have attempted night shots will no doubt have encountered that point where the camera refuses to meter for exposures longer than 30 seconds or so, you can easily shoot longer than this in bulb mode but the camera will no longer help in terms of metering. You can get around this with an external meter but this can be expensive. You can also use an exposure table but this will never be 100% in tune with the scene you are looking at so it can be unreliable but do not despair!
There is a very very simple workaround to calculate exposure times using the cameras built in meter!
With the camera set at ISO 100 the required exposure time quickly exceeds the maximum 30 seconds the camera is willing to offer but WHAT IF we set the camera to ISO 1600 or even 3200 or basically the highest standard ISO your camera will go to before you have to enable software expansion? Whatever shutter speed your camera gives at ISO 3200 for example you can just multiply by 32 and that's the exposure you need when you turn the ISO back down to 100!
So let's say you set your aperture to F/8 and the camera is set to ISO 3200 and the camera meter gives you an 18 second exposure, all you do now is turn the ISO down to 100 and multiply 18 by 32 and you will get 576 seconds.
Next you divide 576 by 60 and you get 9.6 which you round up to 10 minutes.
So to recap:
Set your camera to a high ISO
Get a meter reading
Multiply the reading by the ISO (reading x 32 for ISO 3200, reading x 16 for ISO 1600 etc)
Divide the number by 60 and then round up.
Obviously this only works if you intend to shoot at ISO 100, if you shoot at 200 then halve the exposure time and so on....
This also assumes that you have taken your meter reading already stopped down to your desired shooting aperture.
Slightly more advanced options:
The above method should be just fine for most people, if your camera only goes to ISO 1600 then you should still be able to calculate exposures of 7-8 minutes with the lens set to F/8 which should be enough to get a correct exposure with just the full moon as your light source but there are occasions when much longer exposures are desired or lighting is darker and you can push things further by taking the meter reading with the aperture of the lens opened up and then factoring that into the maths.
I'm not going to go into half stops etc but let's just say you should double the exposure time for each stop you will be closing the aperture so if you take a reading at F/4 and the camera gives 8 seconds and you want to shoot at F/8 then use 32 seconds as the start point for calculating the exposure!
There's also the matter of fine tuning, light meters are by their very nature intended to measure light so in environments where there isn't much of it they can struggle. My suggestion here is to take some test shots with the ISO turned up high, that way you can get a quick preview without having to do a full lengh low ISO exposure and you can then tweak the exposure as necessary, if you're camera suggested 8 seconds and you find 10 seconds gave a better result then use 10 seconds as the basis for your low ISO calculation!
Now grab your remote and go and try it out! All you need is a remote that can activate the bulb mode of your camera and a stop watch, you may well even have one built in to your phone!
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