Thread: ISO explained
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Old 11-22-2012, 12:57 PM   #1
Mike McCarty Mike McCarty is offline
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Joined: Dec 2001
Location: Tulsa, Oklahoma
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ISO explained

A friend recently asked me to explain the ISO setting on his camera so I thought I’d share my thoughts here. As usual if anyone would like to add, or correct, please feel free to do so.

You can think of ISO as a contract with the devil.

With any camera the lowest ISO setting is where it will produce the best quality image. The problem begins when the conditions of light become less than bright. If quality matters, and when would it not, the lowest setting is where you want to be. The trouble is - it's not always possible.

This is when you begin to compromise your principles.

The camera says: Excuse me; it's getting a little dark here.

And you say: What can I do? I really need to take this photo.

The devil, I mean the camera, says: That's fine, but I'm going to have to make a sacrifice to quality if you’re going to take this photo without blurring.

And you say: OK, just keep me informed, and forget flash, I won’t go there.

So, the camera begins to bump up the ISO. Each notch up the ISO ladder re jiggers the calculus of shutter speed and aperture (it will do this automatically if you set it to do so) such that you are able to shoot the shot in this lower light. As the light goes down further so goes up the ISO to compensate. The rub is that each notch up of ISO reduces the quality of the image by some perceptible factor.

The important number in play here is shutter-speed. When the light diminishes the shutter has to stay open for a longer period of time to let that same "calculated" amount of light to enter. The longer the shutter is open the more time is given for camera, and subject movement (read blur). When this shutter speed falls to 1/60th of a second and below your ability to hand-hold the camera without shaking is reduced. The result is that the picture will be blurred. Even if you are very steady any movement of your subject will create the same blurred outcome. The remedy is to increase the shutter speed.

From the comfort of your couch you can demonstrate this principle. Set your camera to the lowest ISO setting, we’ll call it 200, and point your camera to a fixed point and half press the shutter-release. Notice in the view-finder the shutter speed will be, for example: 1/30th of a second. Now, adjust the ISO setting up one notch and re shoot the same point by half pressing the shutter-release. You will notice in the view-finder that the shutter speed has increased to 1/60th of a second (these shutter speed numbers may be different, but the direction upward is the point). This would be a very good thing because it allows you to take the picture either hand-held, or, better freeze a 4 year old boy with the jitters. You can only make a shot "hand held" at about 1/30th of a second, and that's really being keen not to move. A 60th is more realistic and still it’s tough. Keep bumping up the ISO and you will see the shutter speed correspondingly creep up.

Here lies the bargain. You are gaining the ability to take the picture without a blurred, out of focus result; however, the quality of the image is being degraded with each click up the ISO ladder. There comes a point when the compromise no longer works because the quality is too far gone.

You make your bargain as you will. The thing about your generation of camera, the Nikon D50, and my old D70, is that they had a top limit of 1600 ISO, and when you reached that number the quality was usually so bad you would opt not to have the results. With the newer generation of digital SLR’s the ISO goes way beyond 1600. For example, with my Nikon 5100, and its 16MP sensor, I can take a pretty respectable picture at 3200 ISO. Of course you might not want to blow this up to 16x20, but the difference between this camera and my old D70 is astounding.

In the old days of film cameras there was no such thing as ISO setting the way we now have it. Back then we bought film that was either: 100, 200, 400, or 1600 ASA. These numbers indicated the respective "speeds" of the film and can be compared to what we now know as ISO - the bigger this number the faster the film. This meant that the higher the number the less light you needed. However, still, with each jump up the quality was degraded. And, you had to make your decision about this “film speed” on a per roll basis, and every picture on that roll was fixed to that speed. This meant you couldn't take a roll of 100 into a low light situation, so you had to know what you were going to encounter before you loaded the film, or like most photographers you had a good supply of all speeds on hand.

With digital every photo is programmable in regards to film speed (read - ISO). And so the principles are: the lower the number the better the quality, the lower the light the more compromise to quality is necessary, the better the camera the more quality in low light situations.

Those of us that prefer natural light photography live on the edge-of-darkness (where the devil resides). When you operate on this cusp of light and dark the shutter speed must be managed. ISO is an important management tool.

Try the couch test, it will painlessly demonstrate the process and after a while you will naturally incorporate this into the mix of your knowing.

This photo was taken with my Nikon D5100 at 3200 ISO, 1/25th second - truly the edge-of-darkness, on multiple levels.

A Happy Thanksgiving to all.
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Mike McCarty
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