Last time I let my inner geek get carried away it was about the DSLR camera modes. It struck me, after another hide based camera chat (well, you have to do something whilst waiting for something to turn up!) that perhaps I should talk a bit more about exposure. Same caveats as before about this being in Canon DSLR-speak, but it applies to all.
I’m sure everyone will agree that getting the correct exposure is always desirable, but why? You can open up a picture in Photoshop and adjust the exposure, so why worry too much? There are a number of reasons, #1 of which is probably laziness. Why make life hard for yourself when you don’t need to? More to the point, there are limits to how much you can tweak a picture and there are often penalties in doing that as well. If a picture is too dark (under-exposed), you can lift it a bit, but if it is TOO under-exposed, there’ll be no detail to lift, and it will be useless. You’ll also find you end up with the parts of the picture that were in shadow can be rather noisy & unpleasant when you brighten them.
Similarly, if you over-expose too much, you’ll end up blowing the highlights (in other words the bright white bits). In this situation, you’ve burned the detail out of the image and won’t be able to recover it. It can be useful to over-expose slightly – you’ll hear it referred to as “expose to the right”, but don’t get too carried away. Why expose to the right? If you lift an under-exposed shot, you bring up noise in the shadows. If you drop an over-exposed shot, you lessen any noise and that’s always desirable.
Left, Right, eh what?
That’s all talking about the histogram. That’s the little graph you see on the back of the camera, or in software on the PC, like these three screengrabs. One under, one over, one correctly exposed.
See how the histogram shows more bulk at the lower/left hand end on the under-exposed shot, more up the right with the over-exposed one, nicely spread when it's correctly exposed? If you get the histogram rammed up the right hand end, like this one, you know you’ve got blown highlights and there’s not much you can do….
Here’s a top tip though. Look through your camera menu for something called “Highlight Alert” or similar. Enable that (if you have it) and when you look at the image on the camera’s rear screen, blown highlights will flash. It’s a handy quick check. Don’t worry about the odd glint on a chrome bumper, or on a bit of the image you know you’ll crop anyway, but if you have large blinking blown highlights, you might want to fiddle with the exposure compensation a bit.
Why does the camera get exposure wrong in the first place? Cameras work on the assumption that every picture is just a little bit average. 18% grey most of the time (think grey as in brightness on a black & white image, not grey on a colour picture). If your subject is far off that, the camera has trouble working out what it should do, but you can help by choosing the exposure metering mode wisely. There are often a number of these, but they typically include something quite simple (average across the whole picture), an evaluative one that takes note of the whole frame but places greater emphasis on the middle bit and spot metering, which only worries about the brightness of the centre spot. Whilst spot metering is often recommended for bird photography, I find it only really works well when the bird fills the frame nicely and is NOT white, black or a mixture (swans, blackbirds, magpies as examples). If you spot meter on white, you tend to get under-exposed shots. On black it tends to over-expose. On a Magpie, exposure ends up all over the place as you move between black and white feathers (incidentally, it’s why good wedding photography is hard – all those white dresses and dark suits in the same pictures). What you may find is the centre weighted evaluative metering mode is a better general purpose mode, then learn to recognise situations where a stop of over or under exposure compensation will help. Dark bird against light sky? Bird on reflective water? Over-expose by a stop. You may blow some highlights, but the bird will be closer to correct exposure and that’s (probably) what you’re after. Otherwise you’ll get a lovely silhouette against a perfect sky, but no detail. Too many blinking highlights on your rear display? Try a stop of under-exposure and see if that’s better.
I just used another techie term. Stops. Hmm, ok, let me take a breather and expand on that in a bit….
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If all your shots are over-exposed Clare, you probably have exposure compensation set to + something. Positive exposure compensation is useful for dark birds against light sky (eg Marsh Harriers) but for gulls, which are light anyway, leave exposure compensation set to the middle, possibly even under-expose slightly if the highlights are still blown. If you haven't already done so, go in to the settings on your 7D and enable Highlight Alert (it's the second blue menu, at the top). When you then view your images on the back screen, the blown highlights will flash.
The exposure compensation can be checked through the viewfinder (graduated scale along the bottom, runs from -3 to +3, you're after the mark to be in the middle). It will show up when you half press the shutter button. Alternatively, press the Q button and look at the rear screen. The same -3 to +3 scale will show up on there. Change it by scrolling the rear wheel whilst looking through the viewfinder (remember to half press the shutter button for a moment to start metering)