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Old Dec 27, 2006, 5:50 AM   #1
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I'm new to these settings on cameras, having only really used point and shoots before. What is ISO? I have a sony DCS-T10 and it seems to have a great effect on the image. Can someone give me an overview of what it does, and what situations different settings are used for? My main use is family photography, not scenery or portraits. Just chasing the kids around, inside and out.


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Old Dec 27, 2006, 6:33 AM   #2
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For all practical purposes, the ISO level represents how sensitive the sensor in the camera is to the light hitting it. As the ISO number goes up, the camera is more sensitive too light - so it is able to capture an image much faster than with a low ISO setting. The trade-off is the digital noise - as you bump up the ISO you'll see more noise in the image.

As for what practical purpose it serves, here's a brief write-up I just sent to a friend (since it's a cut and paste, I apologize for references that seem out of place, but I didn't feel like typing all this info over again):

Here's a brief side discussion on EXPOSURE. Exposure is controlled by 3 components: aperture, ISO andd shutter speed.
Aperture refers to how big the 'iris' of the lens opens up and thus how much light gets in. Think of it like an eye. The wider it gets the more light it lets in.
Aperture is usually measured in 'stops' - the stop values are denoted by f-stop numbers and have these values:
1.0, 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, 22
You can see the mathematical progression here. For the purposes of our discussion the important fact is: The LOWER the f-stop, the WIDER the aperture and thus the MORE LIGHT that gets in. And each stop effectively DOUBLES the amount of light. So, f4 let's in twice the amount of light that f5.6 lets in. F2.8 lets in twice the amount of light f4 lets in and 4 times the amount of light f5.6 lets in. Now, you might say - great, I just use a lower f-stop and all my low light problems are solved. Not so fast. There's physics involved. The lower the f-stop the bigger the lens needs to be. Now, here's the other kicker - f-stop is really a RATIO of aperture to focal length. What the heck does that mean? It means that to keep the f4 value, a 100mm lens needs to be larger than a 50mm lens at f4. This is why if you look at the specs for your camera it is likely variable aperture - which means it has one maximum value when you are zoomed in (say 3.5) but a worse f-stop when you are zoomed all the way out (5.6 may be the max zoomed out). Now, some DSLR lenses have what's called a contant aperture - which means you can zoom out and still maintain the best possible value for that lens. But they are larger and more costly - I'll show you how big the lens is that I use for field sports. There is also ANOTHER drawback: Apertures also control depth of field (DOF) which is how much of your image is in focus. When you focus on a subject and tak a picture, the entire photo is not always in-focus. sometimes the background is blurred Sometimes it's a slight blur and some times it's very distinctive. DOF refers to how much distance in front of and behind your focus point is actually in-focus. I can't explain the physics (not that you wouldn't understand, just that Idon't know how the physics works myslef) but basically the wider the aperture (lower the f-stop) the LESS DOF you have. So, use a wider aperture to let in more light and you end up with backgrounds more out of focus. Here's an example of a bird on our back deck where I wanted a very shallow depth of field (notice how blurred it is to the right of the bird house):

Or here's one of of a kid:

Here's one where you can see it works both in-front of and behind the subject (both the center and the background are blurred):


This is why landscape photographers use tripods so much - they are using very narrow apertures - f22 so that as much of the image as possible appears in focus. Using those narrow apertures requires more shutter speeds (remember I said the 3 components work together).

Now, for the work I do - I very much want that shallow DOF so I get the nice blurred background and the subject stands out. But, sometimes you DONT want such shallow DOF.

Now, DOF is controlled not just by aperture but also by focal length of lens and distance to your subject. But for the purposes of this discussion - if the focal length is the same and distance to subject is the same, a wider aperture gives shallower DOF. So that's the OTHER downside to using wider apertures.

Shutter speed refers to how long the shutter stays open exposing the sensor to the light
It's pretty self explanatory - 1/15 is one fifteenth of a second - that's how long the shutter stays open and the sensor is exposed to the light. It is also measured in full stops - typically:
1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000, 1/8000

Although you don't you aren't constrained to using only those values - typically cameras allow adjustments at 1/3 stop - so you have2 increments between 1/250 and 1/500 (1/320, 1/400) for example.

ISO refers to how sensitive the sensor is to light. The higher the ISO the more sensitive the sensor is. However this also means more noise.
Now these 3 work together to achieve proper exposure. As such, if you adjust 1 or 2 of them, the others need to adjust to compensate.
The progression of ISO works like this: 100, 200, 400, 800, 1600, 3200. Some cameras go down to 50 and some allow ISO adjustments in increments in 1/3 stops as well.

So, how do they all work together? Here's an example.. Let's say the camera says the proper exposure for a picture is:
ISO 200, f5.6 and 1/250

Now, every time you change one variable you have to change one of the other two (or both of them). So, each doubling of shutter speed is a 1-stop change. One or both of the other two variables have to INCREASE by a total of 1 stop to maintain proper exposure (i.e you're keeping shutter open for less time so you either need a wider opening or more sensitivity).

So, if shutter speed changes to 1/500, you can keep ISO at 200 and set aperture to f4 (letting in less light) and the exposure will be the same.
Or, you can have 1/500 and leave aperture at f5.6 and drop ISO up to 400 and get the same exposure.

Want to change the shutter speed from 1/250 in the original example to 1/1000 (that's a 2 stop difference: 1/500 is one stop and 1/1000 is a 2nd stop):
ISO 800 (each doubling of ISO = 1 stop), f5.6 and 1/1000 is the same exposure as the original example
ISO 200, f2.8 (look at the chart above - f2.8 = 2 stops from f5.6 and 1/1000is the same exposure.
Or you could set ISO 400 (1 stop) and F4.0 (1 stop) and 1/1000 to get the same exposure.

So, in low light situations, you need to keep a shutter speed that is usable - which means you may need to adjust ISO or aperture or both to get your results. Hope all this made some sense.
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Old Dec 27, 2006, 11:27 PM   #3
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Thanks JohnG,

I am a new member and this helped me also. I just bought a great new camera that came with a very bad user manual.
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Old Jan 30, 2007, 10:54 PM   #4
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Thats a very informative response JohnG, thank you.
I nearly didnt read this post as I thought I had a pretty good handle on understanding ISO but I am glad i did as I learned a lot.
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