Roving Thoughts archives

Using automatic exposure locking

Back in 2008 when I set up my D90 and wrote down my settings, I said this about the AE-L button:

I don't know enough to use exposure holding, but if I do 'tap to hold' seems to be the least obnoxious way of doing it.

Boy, I was kind of innocent back then. Nowadays I have learned what autoexposure locking is for the hard way and I use it reasonably frequently. The simple way to put it is that locking the exposure is the quick way to deal with the importance of watching your exposure from shot to shot.

If you're not in full manual mode, the camera can change the metering during a sequence of photographs; it can do this even if all you're doing is changing the exposure compensation. Locking the exposure with AE-L counteracts this, giving you a stable exposure that you can make consistent adjustments to. Otherwise your attempts to adjust the exposure to get the picture right can be happening on top of quicksand, so that you dial in some negative exposure compensation to correct things but then the camera decides to expose more so in the end your exposure winds up just the same. This is both pointless and frustrating when it happens (and very puzzling if you don't notice the exposure shifting on you; here you are adding exposure compensation yet nothing is happening, or the wrong thing is happening).

I've repeatedly stubbed my toe on this so these days I've learned that if I'm taking a sequence of pictures of the same thing the first thing I should do is hit the AE-L button, especially if I'm just working to get the exposure right. Otherwise, even if I'm just lowering the camera to look at the histogram after I've taken a picture the composition can be just different enough when I bring it back up to my eye that the Nikon matrix metering changes the base exposure.

I maintain that my choice of 'tap to hold' is absolutely the right option for this on Nikon cameras, at least for what I want to use AE-L for. It would be very hard to have to keep one finger on the AE-L button all the time, even when I'm doing things like checking the histogram for specific areas of the picture.

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 of underexposure.

• 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.

The importance of watching your exposure

One of the things I've learned over the time that I've been doing photography is the importance of watching my exposure. Not in the usual sense of noticing when the camera is putting you at a too low or absurdly high shutter speed (or ISO, or aperture), although that's important too. What I mean is watching your exposure from shot to shot.

Modern cameras in matrix or evaluative metering can and do change how they decide to expose a scene depending on just how it looks to them, regardless of whether or not the actual light has changed. Or in other words, if you change your composition the camera can change to a bad exposure. If your first photograph of a mixed scene with the clear sky visible has the sky correctly exposed at 1/200th at f/11 at ISO 200, you then recompose to show more of the shadowed ground, and the camera suddenly wants to be at 1/100th at f/8 at ISO 200, you are probably going to completely blow out the sky if you just automatically click the shutter.

(As they say, I've been there and done that.)

The same thing is true in more extended circumstances. If you're strolling along a path in a park taking pictures of different things, your exposure often should be fairly constant regardless of how much of the sky or sunlit ground is visible in each photograph. And if the camera's metering seems very off, you probably have a choice you need to make; you can let the camera more or less expose for the darker areas you're looking at (and accept that the bright areas may well blow out), expose for the sky or other bright area and live with the darker areas (possibly doing tricks in postprocessing), or add fill flash or the like. Or decide that there's too much contrast in the picture and you can't get a decent version of what your eyes are seeing.

This is why I say you want to watch your shot to shot exposure; you want to realize that the 1/100th at f/8 at ISO 200 exposure has to be wrong before you click the shutter. Even if you routinely check your histograms after taking a photograph, getting a good exposure to start with will save you the aggravation of immediately re-taking a shot (and trying to narrow in on the right exposure).

(Some people will say that the answer is clearly to use manual mode and maybe center-weighted or spot metering. Both are too much work for me and tend to blow up in my face in their own ways; in practice it's easier for me to watch shot to shot exposure and correct the metering with exposure compensation when necessary.)