What I Know Of: ISO

If you recall from my first What I Know Of post, from the point of view of the camera, exposure is a function of three parameters -- aperture, shutter speed, and ISO sensitivity.  Yet we've talked about all the automatic and semi-automatic modes for controlling exposure, and ISO speed has been almost entirely absent from the discussion.  The exposure modes only seem to be concerned with aperture and shutter speed.  Why?

In The Beginning...

There's actually an historical reason for that. All of the exposure modes were conceived in the days of film photography.  Back in the day, ISO (or ASA or some other acronym, depending on how far back you go) sensitivity was a property of the roll of film.  In fact it was one of the key parameters you used to select your film.  You loaded the roll into your camera, then set a dial on the camera to the ISO speed of the film.  And for all 24 or 36 shots off that roll, you were locked into that ISO.¹  So ISO was sort of a "set and forget" parameter on the camera.  There was no "exposure triangle" at that time (or at least, I never heard the term.)  Exposure was thought of as a function of two variables -- aperture and shutter speed.  So the various exposure modes, which were all conceived during this time, assume that ISO is a constant and set exposure by varying the aperture or shutter speed only.

With digital cameras, ISO is now a configurable parameter of the light sensor.  You can change it from shot to shot.  Heck, people change it in the midst of composing a shot!  And I suppose rather than changing how the exposure modes worked (and confusing their entire existing customer base), manufacturers wisely chose to implement a separate mechanism for governing the ISO sensitivity of a digital sensor.  Hence, ISO behavior is configured separately from the exposure modes, but its setting has just as much effect on the exposure as aperture and shutter speed.

No, It Doesn't Mean "In Search Of..."

ISO sensitivity controls the sensor's sensitivity to light.  You can read Wikipedia's in-depth discussion of ISO, what the acronym stands for, what it actually means, and it's relationship to sensor gain and all that stuff, but it's really not necessary.  ISO is set using a numbering system in which bigger values mean more light and smaller value mean less light.  The numbering system is such that each successive value is double the one before it.

100  200  400  800  1600  3200  6400  12800

And similar to aperture and shutter speed, each successive value represents twice the sensitivity to light as the one before it.  ISO is also subject to the principle of reciprocity, so a change in the ISO setting can be counteracted by the inverse change in either shutter speed or aperture, and the resulting exposure will remain unchanged.  Now, on modern digital cameras ISO can be adjusted in finer granularity so you'll typically see ISO settings in between the ones above when you're scrolling through the values, but I've shown it this way to simply things. 

ISO sensitivity allows your camera to increase or decrease exposure without any effect on depth of field or motion blur/freeze.  So if you have exactly the right DoF and motion blur/freeze that you want, but the photo is under- or over-exposed, then you can use ISO to adjust the exposure without screwing up the other image properties.  It's truly a wonderful thing.  Except in photography there's usually a catch and in this case it's noise.  In my exposure post, I likened ISO noise to the background hiss you get when you crank your stereo.  It really is exactly the same principle.  As you can see below (click on the photo for the full-size view), high ISO noise can be downright unsightly at high levels.

Noise at ISO 1600, 100% crop

Not only is the noise ugly, but it also robs the photo of sharpness.  While it's true that there's software that can remove noise (an essential tool in my opinion), it's also true that you will still lose some sharpness even after the noise removal.

That's The Way I Roll

My general strategy for setting ISO can be summarized as:  As low as possible, while still maintaining the aperture and shutter speed I need for the photo.  In other words, I keep the ISO at the minimum setting and raise it when my preferred aperture/shutter speed settings are going to underexpose the image.  So in good, plentiful light, I'll set it as low as it will go.   Some conditions in which I'll raise the ISO are:

• When I need to freeze action, but the faster shutter speeds I require are going to underexpose the image.  This could happen at, say, an indoor basketball game.  Or when I'm taking indoor photos of my beautiful, smart, wonderful, and sugar-amped children.
• When I need wide depth of field, but the smaller aperture values I require are going to underexpose the image.  This could happen with a indoor group photo.  Or a hand-help landscape shot in low light.
• When I need an ultra-fast shutter speed.  Perhaps I'm photographing hummingbirds and proper exposure is at 1/500, but I really want 1/2000 to freeze his wings.

Auto ISO

At it's most basic, you set ISO sensitivity on a digital camera pretty much the same way as on a film camera, except instead of turning a dial, you hold down a button and spin a wheel.  Or you menu dive.  But Nikon and Canon DSLRs have automated ISO systems that can save camera-futzing time in a lot of shooting situations.

On Nikon cameras, you have 4 parameters:

• ISO Sensitvity - The ISO setting.
• ISO Sensitvity Auto Control - Allows the camera to override the ISO Sensitivity in order to properly expose the photo.
• Maximum Sensitivity - If Auto Control is on, this value is the highest ISO setting you'll allow the camera to set.  The camera will go no higher than this, even if it means underexposing the photo.
• Minimum Shutter Speed - If Auto Control is on, this is the shutter speed below which the camera will start raising the ISO to maintain proper exposure.  If the camera reaches the Maximum Sensitivity and the resulting image would still be underexposed, only then will it lower the shutter speed below the Minimum Shutter Speed.  In other words, maintaining Max Sensitivity takes precedence over maintaining Minimum Shutter Speed.

The idea is that you set ISO Sensitivity the lowest setting and the Minimum Shutter Speed to be the lowest feasible shutter speed in order to freeze action or prevent image softness from camera shake, and then let the camera take it from there.  It will automatically raise the ISO if necessary in order to the maintain proper exposure and the minimum shutter speed.  When I use Auto ISO, I usually set the Minimum Shutter Speed to be 1/<lens focal length> to counteract camera shake.
 
The Nikon system works really well.  Until you add flash.  Then things get weird on later camera models, including mine.  In fact, the system is so screwy with flash that I turn off Auto ISO when using flash and just set it manually.  Details on this are at http://francoismalan.com/2011/04/auto-iso-on-the-nikon-d7000/ but trust me, just set the ISO manually if you want to use flash.  Life's too short.

The Canon system is simpler, smarter, slightly less flexible, and has no wonkiness with flash.  (As I understand it anyway.  Please correct me if I got this wrong!)

• ISO Speed - The ISO setting
• Auto ISO - Allows the camera to override the ISO Speed in order to properly expose the photo.
• Maximum ISO - If Auto ISO is on, this value is the highest ISO setting you'll allow the camera to set.  The camera will go no higher than this, even if it means underexposing the photo.

There is no "minimum shutter speed" to set with the Canon system because the camera knows the focal length of the lens and will set the minimum shutter speed automatically.  Smart!  But I guess one disadvantage would be that you might want to set the minimum shutter speed manually to a faster speed than the camera would pick (say, for an action shot).  As I understand it, the Canon system also starts ramping up the ISO before the shutter speed actually reaches its internally-determined minimum shutter speed, which strikes me as impressive for its sophistication, but not necessarily a desirable thing from an ISO optimization point of view.  I think I'd want to stay on the lowest ISO as long as my chosen aperture and shutter speed would let me.  (Once again, other view points on this are welcome and solicited, as long as they're civil.  Maybe I'm not seeing all the pluses and minuses of this.)

Reducto Sonitus!

One final note about noise reduction.  While lower is better for ISO, you can ruin a photo by going overboard with that principle.  If your picture is a turd because it's blurry or the depth of field is all wrong, then the fact that it is noiseless is not going to improve its turdliness.  So there are times when you're simply going to have to deal with noise to get the shot.  In those instances, noise reduction software can really clean things up.  In fact I would go so far as to say that the results of a good noise reduction tool can be downright amazing.  But I'd be remiss not to mention as I did before that there is a sharpness penalty even with the best noise reduction.  In addition, it can also interpret fine-grained texture in the image as noise and zap that, so you have to back off on the controls which can then cause it to overlook real noise.

Same photo as above, but with noise reduction using AfterShot Pro.  Cool, no?

With HDR, noise reduction is a necessity because the process itself can leave a lot of noise artifacts.  I use Noise Ninja which is built into Corel AfterShot Pro and it works very well.  But the software I like best (and I haven't used them all) is Imagenomic Noiseware.  Noiseware is amazing at preserving image quality while removing noise, and also discriminating between noise and fine texture.  I'm just not willing (yet) to spend the dough on it when AfterShot performs very well and I already own it.  I'd love to hear from anybody who has something they like even better...



¹ Well, not strictly speaking.  Sophisticated film photographers will sometimes "push" film:  They deliberately set the camera to a higher ISO than the loaded film, which underexposes the film.  Then they increase the development time to compensate and raise the exposure.  The resulting print has greater contrast, but more noise, which has a very distinctive look.