So we’ve expounded on exposure, analyzed aperture & studied shutterspeed… Now it’s time to discuss the final leg of the Photologic Triangle: ISO.
Simply put, ISO is a rating of how much light-gathering ability your sensor is programmed for. Back in the day, this would be akin to “film speed.” For some reason, ISO tends to be the most difficult to understand component of exposure.
Meet Luz. His sole raison d’être is to gather light. Now, pretend for a second that light can be gathered in buckets and you need, say, 1000 buckets of light to create a “technically correct” exposure. Now let’s pretend that Luz can clone himself. What would gather light quicker? Luz times 100 or 1 Luz plus 399 of his chromosomally identical friends? Let me break it down visually

Of course, 400 Luz’s (re: ISO 400) will gather the same amount of light much faster than just one-hundred Luz’s (RE: ISO 100). You’ve now unwittingly internalized the concept of ISO.
It’s true that higher ISO values will let you gather light faster (meaning you can use smaller apertures and faster shutterspeeds), but there’s a trade-off. If you’ve got 400 light-bearing construction workers traipsing through your 12 megapixel exposure, they’re going to track in some dirt, if you get my meaning. What I’m talking about is noise. Take a look at these two samples:

Both "technically correct" exposures according to my camera's meter, but click for full size to see the noise difference.
As you can see above (click for full size), there is virtually no noise present at ISO 100. At ISO 1600, however, the noise can be quite brutal. Whenever I shoot anything, I try to use the lowest ISO rating possible to avoid noise like this. Every camera handles noise differently (some better than others), so it’s important for photographers like me to use the best equipment available. FWIW, I find Nikon cameras to be the best in this (and just about every other) department.
I tend to view ISO as a variable of last resort, since it has the most potential negative impact on image quality. I’ll only increase ISO if I can’t:
- Create more ambient light (i.e., open up a window)
- Open up my aperture
- Use flash
My view here is solely my opinion, and one formed mostly from having an older generation camera (which will soon be replaced with a Nikon D4; the evolution of noise handling technology & high ISO capability has induced a frenzy within my soul). I use flash for every commercial client, event & portrait, so pretty much all of my work you’ve ever seen has been shot at ISO 100, or worst case 400. But noise is generally your enemy, unless you’re going for that retro black & white super-grainy look.
My final disclaimer with ISO is that there is no substitute for having a correctly exposed image, even if that means picking up a little bit of noise. The attitude that is far too prevalent among photographers is that we can just “fix it in post.” That’s just an excuse to be lazy and not nail the exposure in the camera. As the sagacious Joe McNally said in The Hot Shoe Diaries, “No pixels have to die!”
So use this knowledge skillfully: ISO is the nitroglycerin of the Photologic Triangle. It’s dang useful when used properly, but it can be disastrous if used carelessly.
In the fifth & final part of this Photography 101 series, we’ll bring the whole Photologic Triangle together, summary style.