Tag Archives: exposure

Variable ISO – a useful exposure tool

Thom Hogan posted some interesting ideas on his site yesterday, pertaining to the question of how we, as photographers, deal with setting exposure. Basically, in the film days, we were stuck with one ISO rating for an entire roll of film (and some of us habitually used only one type of film), so at that time, ISO was essentially a fixed value. To adjust exposure, we then had to adjust shutter speed and aperture.

Today, ISO can be varied from frame to frame, and on some cameras, there is quite a wide degree of choices available. Hogan’s idea, then, is that having a selection of finely adjustable aperture settings may not really be necessary. We pick one of our two or three preferred apertures (wide open, optimal sharpness, or maximum depth-of-field prior to diffraction), then vary the shutter speed and ISO (if needed) to get the exposure we want. The point then is that we don’t really need a whole slew of apertures to choose from.

This talk of varying the ISO brought to mind something similar that I’d been considering writing about. I got a longer lens late last year, a 300mm, which on my reduced size sensor equates to a 450mm field of view. That’s long enough to make it a significantly different animal compared to other lenses I’ve used. Furthermore, it does not have vibration reduction. On the other hand, while it’s a somewhat hefty lens, it’s weight is well within reason for hand-held use. The problem then becomes ensuring that the shutter speed doesn’t go down far enough to result in problems. In practice, I discovered that I was most comfortable keeping it around 1/800 of a second or higher. That speed allows me some leeway, meaning I can get away with sloppier shot discipline, basically. If I went down to 1/500, I would have to be very careful to hold it steady as I released the shutter, and the overall percentage of blurred shots would be higher regardless. Another thing I ran into relates to a peculiarity of my camera (a Nikon D40): higher shutter speeds sometimes seem to result in weirdness. This is sort of hard to describe, and I’ve never really been sure what the hell is going on when it happens. I suspect it has something to do with the electronic shutter that this camera uses, in that it can sometimes create some weird artifacting effects at higher shutter speeds. The anti-aliasing filter may also be a factor, or possibly the lossy compression, even though it makes no sense at all that shutter speed would interact with either of those things. I don’t honestly know what it is. However, the practical implication of it was that I realized it would work very well if the shutter speed was at 1/800 all the time, at least for hand-held shooting with this lens. Well, there’s an obvious solution to that, isn’t there? Shoot in shutter-priority mode.

The one question was what would happen when the lens opened all the way up. It’s only an f/4 lens, which is not fast enough for shady conditions or heavy cloud cover at 1/800. So I turned on AutoISO, a feature I hadn’t used on my camera in a couple of years, and which I had never ever tried in shutter priority mode. I actually wasn’t even sure it would do what I needed it to, which was to keep the camera at the base ISO of 200 until the lens was wide open, then raise the ISO upward if there wasn’t enough light. Turns out, this is exactly what it does, and it works very well. I totally recommend it, at least for Nikon bodies with this type of AutoISO, and similar non-VR long lenses. It would work with VR lenses, too, if I had some particular reason for not wanting to use the VR, such as bokeh optimization.

On other camera bodies, such as the D7000 which I will have in the not too distant future, it wouldn’t be necessary to do it this way. The D7000 has more flexible AutoISO settings. On the D40, when using AutoISO in the more typical aperture priority mode, the highest setting allowed for “minimum shutter speed” is 1/250th of a second, which is not fast enough for a lens like this. On a D7000, I am not sure what the “maximum minimum” is, but I know it’s at least 1/1000th of a second. So on a D7000, I could just leave it in aperture priority mode, set up AutoISO to allow a minimum shutter speed of 1/1000th of a second, and viola!, I’d be set to go. That camera also does not have an electronic shutter, so that particular weirdness would presumably be gone, meaning I’d have no reason to avoid very high shutter speeds.

In the mean time, though, I have accumulated quite a collection of 1/800 second shots, many of them at f/4 and weird ISO values like 520 that result from the AutoISO setting. I’ve so far only run into one instance where the camera maxed out at ISO1600. The resulting shots were about one stop underexposed, but actually looked pretty good anyway.

That, of course, leads to the other big consideration with this, which is how high can you push the ISO? There was a time when I would not have been willing to use ISO1600 on a D40. However, more recently, I’ve come to realize that the noise inherent in that setting is really not all that bad. The amount of detail lost is pretty minimal, and the noise reduction in Lightroom 3 does a pretty good job at keeping the noise under control. As long as the photo isn’t extremely underexposed, the result is a somewhat grainy looking picture, sort of like Ektachrome 400, back in the day. (Although, frankly, a D40 RAW image at ISO1600 has better color than Ektachrome 400 ever did.)

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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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