Tag Archives: center-weighted metering

Blowing out the sky…or not

[This was originally going to be a quick response to a question posed in a Flickr discussion thread (“Over exposed skies on D60“), but I found myself typing more and more, so I figured it would make a good blog post.]

The meters on the entry level Nikon DSLRs (and the D80, if I understand correctly) are tuned to ensure that darker areas aren’t underexposed, but this often leads to a problem: The sky, or other bright area, ends up “blown out.” There are various ways to alleviate this, none of them ideal.

Ideally, one could simply use a graduated neutral density filter, but such a filter has a built-in, natural limitation, namely that it works best when you can ensure that the sky is in the top half and the landscape is in the bottom. If the division between the two is uneven, as it is in many photos, then it’s not going to be as helpful. There is also a problem when the front element of the lens rotates, as it does on Nikon’s 18-55mm kit lenses.

Luckily, there are other methods:

1) Using matrix metering with the exposure compensation feature. Take a shot, check the histogram or highlight display to see how badly the sky is blown out, adjust exposure compensation accordingly, then try another shot. Be sure to center the frame in the same spot as the first time, so the meter receives the same input both times. In extreme cases, I’ve had to adjust exposure compensation multiple times to get it “right.” A single value is not going to work for every situation. I’ve used compensations varying from -0.3 to -2.7, although -0.7 to -1.7 seem to be the most common. The greater the compensation, the more contrast there’ll be in the final picture. At an extreme value like -2.7, the result is going to be questionable at best. With experience you can take a guess at your compensation value on that first shot, and save yourself some time.

2) Another thing to try is putting the lens on manual focus and setting “exposure lock” to “ON”. (Ironically, it’s possible this method never would have occurred to me if autofocus hadn’t been broken on my kit lens.) Expose an area right on the edge of the sky, hold the shutter button half down to lock the exposure, recompose, and shoot. With the lens on manual focus, you can focus at whatever point in that sequence is most convenient. Then check your exposure just like in (1). Thanks to metering the edge of the sky, the chances are much greater that your exposure will be correct on the first shot, but even if it’s not, you’ll need much less adjustment.

This exposure lock method is easier with autofocus off. With autofocus on, you’ll soon find yourself in a situation where your focus point, your metering point, and the center of your composition are three different spots, and it gets pretty hairy trying to handle all of that (especially if you prefer the left eye like I do, meaning the AE/AF-lock button is mashed right up against your face!). In fact, wanting to avoid this situation was the main reason I decided to learn the next method:

3) Just use manual exposure mode. If you want autofocus on, this will often work better than (2). Adjust the shutter speed and aperture explicitly, then check your results as in the other two methods. I find that manual exposure works best in the winter. In the summer, the presence of green vegetation increases the possible variability of the lighting, and adding a polarizer adds yet another variable, so eventually it gets to be too much to keep track of. That’s why I’ve never entirely given up on methods (1) or (2). However, in situations where the lighting is pretty consistent, this is actually the easiest method.

Wondering what values to start with? That’s not really all that hard. There’s an old heuristic called the “Sunny f/16 Rule” and it goes like this: On a sunny day, you can set your aperture to f/16, your shutter speed to the reciprocal of your ISO value, and the chances are good that this exposure will be right. Whether it turns out exactly right depends on various things, but it’s always a good starting point. You probably don’t want to use f/16, so when you move the aperture down, count how many steps you go, then count that many steps faster on your shutter speed. On a Nikon D40 at base ISO 200, then, the starting point would be f/16 at 1/200 second. But I’d rather shoot at f/8, which is two stops down (or six clicks on the wheel), so then I’d bump up the shutter speed two stops (six clicks) to 1/400 second. Easy! :)

4) I’ve tried abandoning matrix metering in favor of center-weighted, but gave that up after a couple of weeks because I found it was leading to even more tedious adjustment and fiddling than matrix metering. Instead of just needing negative exposure compensations, I now found many of them ending up on the positive side, making the whole process into a complete bother. How I managed to survive for years shooting film with a center-weighted camera is beyond me. :)

5) There’s always HDR, which I haven’t tried, mainly because it’s a computer intensive method (the less time I can spend in front of the computer, the better!). I will hazard a guess that this is the most complicated method of all. Or perhaps the method requiring the most art. :) I’ve seen plenty of cheesy, tasteless HDR compositions. A few of them look really superb, though (whether done with a realist aesthetic or not), and in extreme situations, I am sure that this is the only way to really get the shot.

6) I have found that a circular polarizer can sometimes help. When it does, it’s because the polarizer is actually darkening the sky, thus reducing the overall contrast. But that means it works less well when that darkening effect is minimized, such as in the late afternoon or early morning, or when the air is hazy. Unfortunately, these are precisely the times when the contrast problem is likely to be greater.

7) Whichever of the above methods you use, be sure you are shooting RAW. It has better dynamic range, and overexposed areas can often be “recovered” in post processing. That doesn’t work with JPG images.

So that’s basically it, there’s no “easy” way out, strictly speaking, unless you want to get a camera body that has some type of in-camera dynamic range expansion. I’ve seen some samples of this from the Pentax K-7, and they look somewhat fakey to me, not as good as a high-quality, manually done HDR image can. (Perhaps this is a matter of setting parameters in the camera, though.)

One final thing—sometimes you actually want to blow out the sky:

Evening Glow

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