First things first! Wanted to apologize for slightly lengthy posts for the first few blog entries. They should be shorter for future posts, but I first need to lay the groundwork for the basics of photography which require a bit more explanation, and thus longer posts. Thanks for your patience!
I'm going to start by just copying and pasting my prior written definition from my second blog post...
"ISO - or International Organization for Standardization, refers to the sensors sensitivity to a given amount of light. This relates to film speed in non-digital cameras. The higher the number the faster the film. 100 being low speed, and 800 or more considered high speed film. By increasing the sensitivity of the image sensor, you are making it more sensitive to a given amount of light. So for example lets say you need to take a photograph in a dark room. You may start by trying to make the aperture it's largest possible size to let the most light reach the sensor. You may then try to make the shutter speed longer to get the light you need. Well if you have the shutter speed to slow (or open longer) you may capture the image at a proper exposure but your image will likely be blurred if you were hand holding the camera (as opposed to mounting it on a tripod). So what can you do about that? Increase the sensitivity of the sensor. By bumping up the ISO to a higher number, one can shoot at a shutter speed that wont cause any motion blur. Without adjusting the ISO, the image will look dark and underexposed. By increasing the ISO, the light that is available in the room will seem more apparent in your photos because of the increased sensitivity to light."
So we now know that if a situation occurs where it is dark and we need to handhold our camera for a shot, we can bump up the ISO to allow us to shoot at a faster shutter speed as to not blur the photo. Increasing ISO may also prove useful if you have a situation in lower light conditions where you would otherwise already have a fast enough shutter speed for handholding, but your subject is actually moving which may require an even faster shutter speed. Below is an example of this type of situation:
I was photographing a bridge opening on the York River where a boat was passing through the bridge. In the first photo I wanted to show the blur of the boat. At an shutter speed of 30 seconds, all you see is the blur of the lights from the boat. For the second photo I needed to freeze the movement of the boat so that you could still tell what it was. I quickly bumped up my ISO so that I could hopefully freeze the action. The photo didn't turn out all that well, but I think it helps demonstrate the concept (oh, and I had to use a tripod for the first photo in order to keep the bridge sharp because it's humanly impossible to handhold an exposure of 30 seconds)-
ISO-100, 30 seconds, F/9
ISO-3200, 1/4 second, F/2.8 - In this case I should have shot at an even faster shutter speed and a higher ISO to compensate. However, I was trying to avoid image noise as much as possible. We'll get into that in just a minute.
Similar to yesterdays post, I want to give you a stop chart to show what amounts of ISO represent one full stop of light.
- Low ISO, Less image noise, Lower sensitivity to light
- ISO-100
- ISO-200
- ISO-400
- ISO-800
- ISO-1600
- ISO-3200
- ISO-6400
- ISO-12800
- High ISO, More image noise, Greater sensitivity to light
it should be noted that your camera may have ISO settings between the ones listed above. again, many cameras let you increase or decrease your ISO in 1/3 stops of light where those listed above are full stops. if your camera allows 1/3 increments, 3 clicks of the ISO setting wheel will increase or decrease your ISO 1 full stop.
So now we've added a third and final way to adjust our exposure. Let's say we have our settings in such a way that your photo is properly exposed by only changing aperture and shutter speed. You can then increase or decrease your ISO one full stop and would in return have to increase one of the other two settings one full stop to maintain the same original exposure.
Example 1: ISO-100, F/5.6, 1/125
You can increase the ISO one full stop to ISO-200 (see chart above) and then change either the aperture or shutter speed one full stop. So let's increase the shutter speed for this example.
New settings: ISO-200, F/5.6, 1/250th second
Example 2: ISO-100, F/8, 1/250th
You can increase the ISO let's say 3 full stops and then decrease the aperture (or make the hole smaller to compensate for the increased sensitivity to light) 3 full stops.
New settings: ISO-800, F/22, 1/250th
In both examples you were able to change settings and end up with the same exposure.
So I'm hoping at this point you're starting to get the concept of ISO and how it effects your photograph. In that last photo, and in the chart above, you may have noticed the comments regarding image noise. What is image noise? Image noise is a negative side effect of increasing the ISO. The more you increase the ISO, the more image noise or graininess you'll notice in your photos (especially in the shadow areas).
Examples of noise at different full stop ISO settings (missing ISO-6400, accidently deleted it before uploading..again, I think you still get the idea though). In these examples I changed the shutter speed accordingly to maintain the same exposure when I adjusted the ISO. I left aperture the same for each photo.
ISO-100
ISO-200
ISO-400
ISO-800
ISO-1600
ISO-3200
ISO-12800
It should be noted that the greater the ISO and noise level, the less sharp your image will be. You can also reduce the noise in post processing with noise reduction software, but that's another days topic. If you want to get a jumpstart though, one of the best programs for this is called Noise Ninja.
So that's ISO in a nutshell. It can be a bit confusing, but the best way to learn it is use this guide and go out and play with the settings. Just remember if you've got your slowest possible shutter speed that you can hand hold and your widest aperture for the most light and you still can't get enough light....increase the ISO until you get a properly exposed photo.
There's your ISO "From The Ground Up!"
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