How long an exposure can my camera meter?
A friend and I were talking about long exposure photography today, which brought up the issue of determining how long an exposure you needed. Like most DSLRs, my Nikon D90 will only automatically expose out to 30 seconds; if you need a longer exposure than that, you have to time it by hand in Bulb mode.
(In my opinion this is a pointless limitation, probably reflexively carried forward from the days when there was only so much space on a physical shutter speed dial. Since cameras are computers these days, you could even limit automatic exposures to no more than 30 seconds while still letting the camera count the time for longer manual exposures.)
But that's just how long an automatic exposure the camera will do, not the limit of what it will meter for you. If the exposure is at the 30 second limit and the viewfinder is still reading 1 EV of underexposure, you can fix that with an exposure that has 1 EV more time, ie a 60 second bulb exposure. So how long an exposure can I actually meter with my camera?
- in my normal settings, the viewfinder will show up to 2 EV of
underexposure. If I change the camera to use 1/2 EV steps for
exposure compensation instead of 1/3 EV steps, this increases to 3 EV
- I can extend the range of the viewfinder meter by adding negative
exposure compensation (a 1 EV underexposure with -1 EV of exposure
compensation will read as a correct 0 EV exposure). My camera allows
up to +/-5 EV of exposure compensation, pushing metering to an
effective 8 EV of underexposure.
(The allowed range of exposure compensation doesn't changed based on the step size used.)
Since every EV is a doubling of the exposure time, 8 EV of exposure past 30 seconds is 128 minutes; call it a two hour exposure. If this isn't enough, I can start pulling the viewfinder meter back by metering with a high ISO but actually photographing with a low ISO. My camera has a base ISO of 200 and is easily raised to ISO 3200, giving me another 4 EV of metering range (a total of 12 EV); that takes me to an exposure duration of over 30 hours.
So the real answer here is that my camera battery is going to die long before I get to an exposure that I can't meter. (Probably the sensor would overheat from continuous use, too.)
(In theory the D90's meter is only specified to work down to -1 LV. In practice this isn't even moonlight and I've metered without problems in much darker conditions.)
PS: since I'm unlikely to sit still for a one hour exposure, much less a two hour one, I don't even need to change the exposure compensation step size. My normal settings still give me 7 EV of exposure past 30 seconds, which is already just over an hour. Mind you, using 1/2 EV steps makes the eventual time calculations somewhat easier.