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Photography/these mean the same?


What is the difference between iris adjustment and exposure? Aren't they the same? This is for a digital camera.

Getting the proper exposure means allowing the proper amount of light to fall on the film or sensors, according to the sensitivity of the film or sensor array.  Film sensitivity is according to ASA number, with ASA 200 being the most common.  ASA 400, for example, would require less light.  Sensor arrays in digital cameras also have a similar selection for sensitivity.

In order to get that required amount of light to fall on the film or sensor array, there are two adjustments to the lens:  aperture and shutter speed.  The aperture opening (that you call iris) is calibrated in f-stops.  A smaller diameter aperture opening is indicated by a higher number, such as f-22.  A larger diameter aperture opening (allowing in more light) is indicated by a smaller number, such as f-2.8. When the aperture is smaller, the depth-of-field is greater.  That is the range of distance over which images in the photo have a clear focus.  If you want your main subject to be in focus, and the background to be blurry, use a wide aperture.  If you want everything to be in focus, use a small aperture.  Using a wide aperture means that a faster shutter speed is needed, such as 1/500 of a second.  Using a small aperture means that a slower shutter speed, such as 1/16 of a second, is needed in order to still get the right amount of light.  But if you want a fast moving subject to be clear, then you need a fast shutter speed, so that would mean you need a wide aperture.

So, the aperture and shutter speed can be adjusted to select between depth-of-field and stop-motion, while still allowing the right amount of light to fall onto the film or sensor array.

When using a flash, the shutter is usually fixed at 1/60 of a second, but the flash serves to stop the motion.  Then the aperture can be adjusted for depth-of-field as long as the amount of light needed remains within the adjustment range of the flash control.

Hope this helps!



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Dave Nyce


I have used SLR cameras for 30 years and have taught classes on photography at work for my co-workers. I had a darkroom for 20 years, but now am using mostly digital cameras. I can answer questions on equipment and techniques for photography, but am not an expert on specific model numbers of cameras or accessories.

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