What is ISO?
I was asked to write a bit about using ISO by a member of the Flickr Alpha group, they said “my camera has only ever been set to AUTO ISO. How and when should I change it? You’ve probably seen an ISO or ASA number associated with a roll of film. This tells you how sensitive the film is to light, a higher number indicating more sensitivity to light. In digital photography ISO indicates how sensitive the image sensor is to light. If you control the ISO you are taking more control of the final image, and it will give you more possibilities in creating the final Image you want.
Native ISO VS the rest
Your camera has what we call a “Native ISO” that is what the actual ISO value of your camera chip set. Your camera will tend produce the sharpest most vibrant images at this ISO if correctly exposed. Unfortunately this value wont be suitable in every situation. So camera manufacturers have given you the ability to change your cameras ISO through software and hardware built into the camera. If I can find a list of which Alpha has which native ISO I will post a copy here in the blog.
ISO Button on an Alpha 850
The most common ISOs are:
100, 200, 400, 800, 1600, 3200, 6400. You may have to change some custom fields in your camera menu to be able to use all the available ISO values in your camera. Some cameras may start ISO 200 and not have an ISO 100 don’t worry too much about, it is because of the “native ISO” of the camera chip. Your camera may have other ISO values like 125, 320 or 640 if so great if not worry they are only minor steeps between the common ISO values the give you greater control over your camera. Some of the newer cameras are coming out with ISO 12800, the advances in technology are sometimes hidden but amazing when you understand what it means.
Basic things to think about when choosing an ISO.
The higher the ISO number = the higher the sensitivity to light
Moving from one ISO to the next value changes the exposure by half or double.
The ability to shoot in low light has a trade-off in the form of grain or noise. In digital cameras it’s not the grain of the film that becomes visible at higher sensitivities, but digital noise – the visual equivalent to the hiss you hear when you turn a hi-fi amp up to full volume when nothing is playing.
The lowest ISO will generally give you sharp images.
The higher the ISO number = the more noise/grain.
The rule of thumb is to select the lowest ISO you can get away with that will allow you to take a picture at a fast enough shutter speed and/or larger aperture.
Quick tips for different situations.
A football match outdoors on a bright day: at ISO 200 you should be able to shoot at 1/250 or faster to freeze action.
On a race track you may want to go down to your lowest ISO so you can “pan” the camera with the movement of the vehicles, thus keeping the subject sharp but the background blurred through the camera movement.
Concerts and gigs: always a tough one especially at smaller venues where there’s little lighting and the subject is moving. Best bet is to start at 3200, do a few test shots and then lower the ISO to the lowest you can take successful shots.
Churches and galleries: 1600 might be the minimum ISO you can get away with to shoot at 1/60.
When you are working in a studio environment, or with any form of controlled strobe lighting your camera will not be able to correctly calculate an exposure value. Due to the “unknown” lighting variables that the camera doesn’t control or know about you will have to use a manual ISO value. In this situation you will generally chose the lowest possible ISO or the native ISO of your chip set.
Rule of thumb 1:
Faced with the choice of introducing more noise and a fast enough shutter speed, go with the shutter speed. You can live with noise and make attempts to reduce it with software but you can’t do much with a blurred shot other than bin it.
Rule of thumb 2:
Purposely use a high ISO for a grainy effect and give mood to a scene. This might be harder to do than you think as camera manufacturers are continuously reducing (“improving”) the noise from ISO settings to the point where you might need to add it in post-production.