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Old Nov 27, 2006, 1:02 PM   #1
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I been using a camera for about 2 years now and I read many books on photography and how to take pictures. I have a basic understanding of what film speed(ISO) and the aperture. From my understanding ISO is the sensitivity of the sensor. The higher the ISO, the more sensitive the sensor, the more light can be captures.

My question is how is the ISO rated? Is there a set standard or is it dependent on the camera? Lets say you have two camera and lets call them X and Y. X is a dSLR and why is a PnS digital camera. You set both camera to 200 ISO, and the aperture/shutter to the same setting. Should you get the same exposer from both camera?

My 2nd question is about aperture. I understand that the lower the number, the wider the opening which allows more light, and less DOF. My question is how is the aperture rated? I notice that on my Pentax SMC-M 50mm @ f/2.8 it is bigger then my Tokina 28mm @ f/2.8. It seems like there is a relationship with the focal distants.

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Old Nov 27, 2006, 2:41 PM   #2
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ISO - your understanding is correct. In the example you provided, if you set aperture and shutter speed the same on two different cameras and set the ISO to the same value you should get CLOSE TO the same exposure. It's probably not going to be exactly the same. You may find that Camera X in your example is slightly MORE exposed at ISO 200 than Camera Y. Or it might not be. In any event it should be VERY close - within 1/3 stop say.

As for how aperture is rated, aperture is typically rated according to f-stop. The f-stop is the ratio of focal length to aperture. The longer the focal length, the bigger the opening (and thus the bigger diameter the lens ) required to keep that same ratio. It is also why so many consumer grade zoom lenses change max aperture as you zoom out.

Here is some info regarding f-stops:


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Old Nov 27, 2006, 9:40 PM   #3
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Thanks John that really helped me out. I am that type of person that always wants to know why and how things work.
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