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Old Jan 20, 2012, 3:24 PM   #1
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Default Question about Distortion

Hi everyone. It's my first post. First accept my apology because of my bad English language.
I have interested about cameras for awhile and i'm searching about them. Recently i learned about f-stop. I thought when i want to take a photo and f-stop is the low number, my photo becomes convex or concave that i don't like it. When i see a camera, first i look on the lens and numbers to understand how much distortion it has!! But i saw an exception. It was Olympus XZ-1. The range of f-stop is 1.8-2.5. Numbers are low, So i expected very high distortion. But when i saw the sample photos i confused. Those were fine and acceptable.
My questions are: I was wrong and other properties could influence on distortion?
Is there any way to understand which cameras have low distortion by properties? Or not, i have to see sample photos?
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Old Jan 20, 2012, 5:01 PM   #2
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The f-stop is a measure of how much light the iris of the camera lets through (in a very simplified way of explaining). It has very little affect on distortion.
The design of the lens is the main influence on distortion. It is difficult to create a zoom lens without some on one end or the other.
There are some camera and lens review sites that measure distortion for their reviews, as well as posting sample pictures. That would be the best way to check.

brian

P.S. Your English is fine and at least as understandable as many native speakers here in the USA.
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Old Jan 20, 2012, 5:12 PM   #3
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The f-stop is a measure of how much light can pass through the lens to the image sensor, not a measure of distortion. Lenses with large apertures (numerically small f-numbers) let in more light than lenses with small apertures (numerically large f-numbers.)

Using a larger f-stop means you can use a faster shutter speed to obtain a properly exposed photograph. With a smaller f-stop, you may need to use a slower shutter speed, possibly getting motion blur due to subject movement.

The f-number is the ratio of the diameter of the lens opening to its focal length. For instance, a lens with a 100mm focal length and a aperture of 25mm will have an f-number of f/4 (100mm / 25mm = 4.0.) Some lenses have adjustable focal lengths, but have the same maximum aperture size, but because the focal length changes, so does the f-number. For instance, if that same lens can also have a focal length of 50mm, since the aperture size remains the same, at 50mm it would have an f-number of f/2 (50mm / 25mm = 2). So that lens would be described as a 50-100mm f/2.0-4.0 lens.

Lenses also have adjustable diaphragms, or irises, that allow you to adjust the size of the aperture, and thus, the amount of light that passes through the lens. In that way, you can use smaller apertures (larger f-numbers) in brightly lit situations.

Distortion is a seperate matter, and can only be determined by use. There are many websites that make objective tests of lenses and publish the results. Some of the more popular ones are SLRGear.com and PhotoZone.de.
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Old Jan 21, 2012, 6:15 AM   #4
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Thank you Brian and TCav for input and your guidances. So i was wrong completely. And only way to understand is testing the lens.
Instances that you mentioned TCav don't work on my camera.
I took a photo with max optical zoom. Details of photo are these: f/5.6 , Focal lenght 25mm and Max apercure 4.96875
25 / 4.96875 = 5.03
Another photo without zoom: f/2.7, Focal lenght 6 mm , Max aperture 2.875 mm
6 / 2.875 = 2.08
What is wrong?
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Old Jan 21, 2012, 7:01 AM   #5
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All lens designs are different. Manufacturers can put the diaphragm anywhere in the optical path that they want. Note that a 70-300 f4-5.6 would have an effective aperture diameter of 17.5mm when the foca l length is 70mm, and 53.5mm at 300mm. The diaphragm doesn't move, but the effect of the diaphragm changes as the optical path is altered to change the focal length. The forumla is correct. (See Wikipedia's page on f-number in your own language.) Your lens is just one of many that has a more complex design than the simple example I used to describe the calculation of the f-number.
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Old Jan 21, 2012, 7:52 AM   #6
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Got it. Thank you.
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