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I am using a Canon 50D with the 18-200mm lens it came with. Some of my pictures turn out un-sharp, even taking metering and Auto Focus points into consideration. I wonder if this is caused by my aperture selection.

What is the best aperture to use and how do I decide? Does the range (focal length) I am shooting make a difference?

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Welcome to I recommend you do a little bit of basic learning around the basic photo relationships of aperture,speed and focal length. This question can be perceived as 'odd' by most photographers and is not terribly useful. – cmason Jul 13 '11 at 15:11
Huh. This strikes me as a perfectly fine "newbie" question, y'all. – mattdm Dec 6 '11 at 11:23
If you can find a copy of Bryan Peterson's "Understanding Exposure" at a library or book store, I think you'll find it answers a lot of your questions! (I'm sure there are other books out there that are just as good, but this is one I've read.) – khedron Dec 6 '11 at 17:19
up vote 5 down vote accepted

It depends on the available light and how much DOF you want. But as an overall recommendation you can use f/8.0 to f/11.0. An aperture faster than f/8.0 produces soft corners and an aperture beyond f/11.0 starts to produce soft images due to diffraction.

I guess you're shooting in Aperture Priority (Av) mode, try getting used to other modes like Shutter Priority (Tv) and Manual (M) as well.

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The reason is that some of the picture taken turn out un-sharp; even taken metering and AF point into consideration. Yes absolutely correct I am shooting in AV mode while Manual once a while or mostly in landscape. I don't see any differences from the picture compare with Av and M mode so I wonder in actual what is the difference. – user5936 Jul 13 '11 at 3:19
There's no difference in pictures taken in different modes, they just help you get the correct settings in a either full manual or partial auto manner. If you want full control over your pictures, you should be shooting in Manual (M) mode. It also helps you to understand all the factors (Aperture, Shutter speed, ISO) better. It'd help us to identify the reason if you can post a sample picture of the "un-sharp" photo you're talking about. There are several reasons for a photo being un-sharp, such as: Motion blur, Handshake, Front-focus or back-focus issues, Wrong focus point selection etc. – fahad.hasan Jul 13 '11 at 5:28

It's true: "depends on available light and how much DOF you want" as ShutterBug said. So actually there isn't a "best" aperture for this lens (nor any other one). Because of the optics it has, sharpness changes a lot in this lens. You should take a look at this chart done by DPreview in their review. They tested the lens sharpness according to the aperture and focal length, which will help you understand the characteristics of your lens.

I have the same and what I do is try to think about the focus point and the focal distance I need for the photo, then decide the aperture according to that chart. If you shoot in Av you won't have to worry about the speed but I'd suggest you to try your own combinations and not leave everything to the processor.

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