The Magic Of ISO

What is ISO and should I set my camera on auto-ISO or set it manually?

Back in the days of film it came in different speeds.Kodachromes Demise

The slower the speed, the finer the grain and the “better” the print would look.

Problem was film originally had film speeds called ASA of 10 for the original Kodachrome. Such a low ASA meant the photographer was limited to shooting almost exclusively in bright sunshine if they didn’t want to use a tripod. That BTW is the reason photographers of 100 years ago shot exclusively using tripods. Until manufacturers figured out how to produce film with a sufficiently high ASA rating every shot was taken with the camera on a tripod.

ISO is the digital equivalent of film’s ASA. In a digital camera ISO is a measurement of the sensitivity of the camera’s sensor. And here’s where the magic comes in. You can adjust the sensitivity of the camera sensor via the camera’s software controls.

New full-frame DSLR cameras are capable of producing excellent, virtually noiseless (what we called grain in the film days) images at ISOs way above ISO 1600 (which was the practical limit of film’s ASA range). Smaller sensors in point-and-shoot cameras can’t hit these astronomical heights but even their tiny sensors can use ISO settings as high as 800 ISO and still produce perfectly useable images.

Now what about auto-ISO?images-1

This setting in the camera’s menus allows the camera to vary the ISO according to how much light is hitting the sensor. The ISO can be set to rise to compensate for lower light. So a shot that would be taken at ISO 100 (which in theory should produce a noiseless shot) could see the camera raising the ISO automatically if the light changed and the scene was darker. In most cameras you can set the ISO-auto controls to change to a predetermined limits.

But the big issue with auto-ISO is you have no idea what the camera is actually doing if you’re shooting in a hurry. Now that can be an advantage if you’re shooting sports and high-speed shooting is your first concern. But, when it comes to low-light photography you may not want the auto-ISO raising the sensitivity of the sensor as this will introduce more noise into the image. Again, some editing software such as Lightroom have excellent noise controls but it’s still important to shoot the best shot possible in camera as software has its limits too.

I almost never use auto-ISO for all the reasons above. When I’m shooting outdoors I set my ISO to 200. Works great. If it’s an overcast day I’ll set it to ISO 400.

Going up from ISO 200 to 400 allows me to shoot at one shutter speed higher or to close the aperture by one stop. The term stop comes from the past when lenses on manual box cameras had manual apertures that clicked from one setting (or stop) to the next.

The ability to click from one stop to the next allowed the photographer to set the aperture without actually looking at it. And where was the photographer looking if not at the controls? Back in the old days he or she was looking at a frosted glass plate in the back of the camera with a black sheet over their heads.


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