Back To Basics - Exposure: The Great Balancing Act - 4 of 5

Sensitivity

Sensitivity, often referred to as ISO, is the third and final fundamental tool that the camera provides the photographer for manipulating the volume of light captured in a photograph. Unfortunately the reason I've saved it for last is the fact that, for most people, it is the hardest to grasp. However, the power it provides warrants taking the time to understand ISO inside and out.

If you've read the previous two articles about shutter speed and aperture, than you know what's coming, another analogy relating to water pipes and buckets. However, I'll admit up front that this one is going to require a little bit of a stretch to really understand what's going on, and it's certainly not as straight forward as the other two. Looking back, we said that shutter speed was analogous to opening a valve on a pipe and adjusting the amount of time it was left open to regulate how much water was collected in a bucket. Aperture was equivalent to fixing the amount of time the valve was open, but varying the diameter of the pipe to regulate the total amount of water collected. Of course, in our cameras, we're collecting light instead of water, but the analogies are fairly easy to understand. For sensitivity however, we'll leave the pipes and the valves alone and this time, vary the size of the buckets.

I'm going to talk solely about digital sensors now, because the analogy I'm going to use fits them slightly better in my opinion. Film cameras are very similar, but their process involves chemistry, a class in which I received a "mercy" C+ in college - you don't want me trying to explain anything related to chemistry to you.

The analogy: Imagine each pixel on the cameras's sensor as a tiny bucket, but instead of collecting the water, we use it for collecting light. For simplicity, we'll say that when the bucket is empty the camera records that pixel as being true black, and when the bucket is full, the camera records that pixel as being true white. All the amounts in between full and empty represent all of the other tones and colors the camera can capture. (I said, "for simplicity", because this description leaves out the filters and other engineering magic that make this possible; however, this simplified version gets the idea across, and is easier to wrap one's head around.) So now we have tiny buckets capturing the light - if we make the buckets smaller, they require less light to fill them up, the larger we make them the more light required to fill them up. What this means is that instead of varying the amount of light that comes into the camera (i.e. with shutter speed or aperture), we're varying the cameras definition of a full "light bucket."

You'll remember, the more light we collect, the "brighter" the photo. We now added a third tool to the toolbox which allows us to essentially slide around the amount of light required for the camera to register a certain brightness. That means, we can keep our aperture AND shutter speed constant, but still change the overall exposure of a photograph. If you think about the implications of that last statement, you'll realize that sensitivity seems like a magic addition to the balancing act, but as you might expect, it too comes at a price.

Trade-Off: Noise (Film Grain)

So by now, you've probably realized that if you make the "buckets" really small, you should be able to take pictures in near darkness, only requiring a very small amount of light to expose the picture. What you may not have considered, is that by doing so you've made the camera much more sensitive to a phenomenon known as noise. Noise shows up as a random colored grain throughout the image. As it gets more and more severe, it can actually begin to distort the image (as you'll see in the examples below.) Noise for digital cameras is the result of random electronic effects that exist in all electronic circuits. For film cameras, noise is known as film grain, and is the result of tiny crystalline structures that form during the development process. (That's as close as you get to chemistry from me folks) In both cases, it can either be looked at as a nuisance or in some cases, as an artistic addition to your photographs. As was the case with the other two trade-offs, I'm going to provide some examples below so you can see what noise looks like, and how it affects your photos.

The photo subjects for this illustration are two of the creepy, 3-foot-dolls my wife has down in her doll collection. I know it's not the case, but I swear it feels like these things move around on their own sometimes. Anyway... I've included the scene I shot below, as well as zooms (click on each for the large version) which show the noise added by going to higher ISOs. These shots were intentionally taken on my trusty old D200, as its high ISO noise performance, while decent, is far behind Nikon's newest cameras. Keep in mind, noise won't be apparent on the thumbnails, you really need to zoom in to see what's going on.

Full Scene

ISO: 100

ISO: 200

ISO: 400

ISO: 800

ISO: 1600

ISO: 3200, if you zoom in on this photo, you see that the noise is starting to dominate some of the edges, completely distorting some of the finer details

At this point, you've lasted through four days of my rambling, and hopefully have started to form some ideas about how the three fundamentals: aperture, shutter speed, and sensitivity, might interact with each other. We see that each has its advantages and disadvantages, and now we know what some examples of variations of each might look like in actual photographs.

With that in mind, make sure to join us tomorrow for the exciting conclusion of Exposure: The Great Balancing Act, cause it's time to start juggling...

Tomorrow - Conclusion