Know Your Gear: ISO Limitations
When we speak about creating the perfect exposure, we often allude to the Exposure Triangle. The Exposure Triangle is comprised of three components: ISO, Aperture and Shutter Speed. We often talk about Apertures and Shutter Speeds, because they are the most variable of the lot. But ISO is really the cornerstone of the Exposure Triangle, and it's one of the reasons why I often list it first when speaking of the three. Today we're going to speak about ISO, how to use it and how it affects your photographs.
What You Need To Know
I could write a dissertation about ISO; the physics behind it, the mechanics and design constraints and of course how it has improved over the years. But I don't need to do that as I think it would really confuse the issue (and most of you would probably fall asleep, not
something I'd like to have a reputation for). Besides, that's not the purpose of this article. In simple terms, ISO is a measurement of the sensor's sensitivity to light. The higher the ISO, the more sensitive to light it is. In other words, you'll need a higher ISO to take photographs under lower light conditions. With film, you would have to physically change the roll of film to change your ISO. But in what many see as an advantage over film, digital cameras allow you to change the ISO on the fly.
Not all cameras are created equal when it comes to ISO. And one cannot assume that every camera will perform the same under different lighting Some cameras handle it better than others. There are many factors that affect ISO performance, but the main contributors are the type of sensor (size, resolution and type) and the camera's on-board processor (which often provides a preliminary noise reduction). Even closely related cameras from the same manufacturer may have different ISO performance. You must experiment with different ISOs – much in the same way you experiment with different apertures or shutter speeds – to find the limitations of your camera.
How ISO Affects Your Work
Being able to change the sensitivity to light may seem like a great thing, but there is a major trade-off: The higher the ISO, the more
noise gets introduced. With film, the noise is simply in the form of a larger grain, which is not always undesirable, especially with Black & White photography. But on digital, the noise can come in the form of a higher grain with color shifts and artifacts. With some digital
cameras, high ISO is barely usable; just because a camera can support high ISO doesn't mean you'd want to use it. The good news is that many of the latest cameras offer significantly improved high ISO performance. I've seen some very impressive results from some of the new full-framed cameras on the market where even at ISO 2400 and above, the noise is rather insignificant. There are even some cropped sensors on the market that have impressive results as well. So maybe one day in the future, ISO won't even be a major concern.
Even if you had the camera with the best ISO performance on the market, you still couldn't completely ignore ISO. The noise may be minimal, but it will still be there, impacting the overall quality of your images. For quality purposes, you would still want to go with the lowest ISO feasible for the shot. The needs will differ between shots. If you're shooting stationary subjects with the camera locked down on a tripod, there would be little reason to use a higher ISO. A low ISO would mean a longer shutter speed, but with such a shot, that might be of little concern. The same would be true in a studio environment where you have absolute control over all of the lighting. The higher quality of the low ISO would give you a much cleaner image to start with when you needed to do some touching up. However, on the sidelines of an evening football game in the fall, it may look like the stadium is pretty bright, but may still not be very bright from your camera's perspective. You need fast shutter speeds to capture action under those conditions, so pushing up the ISO is going to get you the edge. But the purpose of that type of photograph is vastly different. It's about capturing the scene, maybe for the purpose of the newspaper, so you have a little bit more flexibility in the finished quality. The inverse is true as well. Suppose you're shooting a stream or waterfall in broad daylight and you want to slow the shutter as much as possible. Lens filters are the ultimate solution, but a great place to start would be to drive the ISO as low as possible. Lower ISOs will result in longer shutter speeds. So a combination of a low ISO and a Neutral Density Filter (or more) would help to make that water silky smooth.
As with all technical aspects of photography, you can't let the ISO aid in your laziness. Just because you can capture photographs in
relatively low doesn't mean you always should. It's one thing when you're at a party and you're just documenting some good times. But if you're setting up for a fine art photo, bumping up the ISO is a last resort. Instead, you should consider using longer exposures (on a
tripod, of course), off-camera flash or other ways to introduce light into the shot. Light is your paintbrush, and there is no reason to opt for higher ISO instead of painting with light. Bottom line, ISO is simply one more tool, one more setting to tweak, and it should be
treated as such. So again I repeat: Keep the ISO as low as you can so as to guarantee the best quality image you can create.