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Old Sep 9, 2010, 12:14 PM   #1
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Default F Stop Question

Hope this is the correct place to post this. I was taking pictures yesterday with my Nikor 55-200 f4-5.6. I was experimenting with changing the f stop and I could adjust to around f10. What is the purpose of going to say f10 or f7? I notice most talk about a small f number to have a brighter lens. Whats the purpose of the other end?

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Old Sep 9, 2010, 12:55 PM   #2
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That lens, like many lenses, is on the soft side wide open. It isn't quite razor sharp until you stop down to about f/8. In addition, you may be bothered by vignetting with that lens if it's wide open (this is easily auto-corrected either in-camera with newer cameras or in PP with, e.g., Capture NX2 -- so you may not care about vignetting at all, depending on your set-up). Of course, smaller apertures also increase depth of field, if that is what you want (m[a/i]cro photography tends to try for the smallest aperture it can get away with for this reason). At some point, you hit a wall and the small size of the aperture starts introducing blurring due to diffraction from the tiny opening, but that is normally somewhere south of f/22.
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Old Sep 9, 2010, 12:57 PM   #3
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It is all about aperture. Aperture defines with F numbers.

The aperture stop of a photographic lens can be adjusted to control the amount of light reaching the film or image sensor. The aperture size will regulate the film's or image sensor's degree of exposure to light
If you lower to F number (f4) aperture would be bigger (big hole), if you set bigger F number like f5.6, aperture would be smaller and the light would be less.
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Old Sep 9, 2010, 3:04 PM   #4
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Also reducing the F-stop (numerically increasing the F-number) also results in greater depth of field.

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Old Sep 9, 2010, 4:25 PM   #5
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reducing the F-stop (numerically increasing the F-number)
reducing the F-stop = increasing the F-number = smaller aperture = low light on sensor

this will solve a lot of newbie problems i guess
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Old Sep 9, 2010, 5:16 PM   #6
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Aperture refers to the opening through which light passes. A diaphramis is used to adjust the size of the aperture.

F-Number is the mathematical expression for the ratio of the effective diameter of the diaphram to the focal length of a lens. A 100mm lens with a diaphram that has an effective diameter of 25mm will have an F-number of f/4.0. A 200mm lens with a diameter of 50mm will also have an F-number of f/4.0. Even though the shorter lens has a smaller aperture, the longer lens has a narrower angle of view, so they both let in the same amount of light. So the F-number is a measure of the light gathering ability of a lens.

Large apertures (larger F-numbers) allow light to pass through a lot of glass. Any imperfections in the optics, become more obvious at larger apertures, so the image that's projected onto the image sensor is less likely to be sharp. Using a smaller aperture reduces the size of the areas of the polished surfaces through which light passes, so the image will be sharper.

But when the aperture gets too small, light starts to bend as it passes through the aperture, in a phenomenon known as diffraction. This will cause the image that's projected onto an image sensor to become less sharp.

So a large aperture lets in a lot of light, but with less that perfect optics, the light isn't as likely to project a sharp image. Closing the aperture ("stopping down") a little narrows the available pathways for the light, so it is likely to get sharper since the light passes through fewer imperfactions. But if the aperture gets too smal,the image starts to get less sharp because the waves of light are diffracted.

When you're photographing a black bear in a cave at night, you want to capture as much light as possible, so you use a large aperture, so you can use a shorter shutter speed, so you can get out of the cave before you wake the bear.

When you're photographing a polar bear on the polar icecap in the summer, there's too much light, so you need to stop the lens down because your camera's fastest shutter speed is 1/4000, and at ISO 100 and an aperture of f/16, the shot will still be overexposed.
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Old Sep 9, 2010, 10:13 PM   #7
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Originally Posted by wolverines View Post
... I was experimenting ...
Keep doing that and you will figure things out real quick. There were some good comments in response to your question, but you should use those to guide further experiments - not as the final answer.
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