Monthly Archives: March 2011

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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