Getting a sharp focus on a 50mm when the subject is at 5m - 10m [duplicate]

Question

I'm a novice photographer with a 450D/XSi. I'm using a 50mm/1.8 prime lens to start out and I noticed that a lot of times my pictures end up not being sharp for stationary scenes. I was mostly using manual focus and switched to using auto focus mainly for this reason. The problem is even with auto focus this happens.

I noticed this mostly happens when a subject is from around 5 - 10 m, and in low light situations. I try to keep the aperture at around f/2.8 to get better sharpness without opening it all the way. I increase the ISO to get better exposure (even though my camera's high ISOs are not that good). But, I do decrease my shutter to around 1/25 s. The image shown is an example of this. The settings are f/2.8, 400 ISO, 1/25s.

I think the lower shutter speed may contribute to this focus problem, but I'm not sure whether it's the only reason. Is the focal length of the lens also a contributing factor here? But, most online material doesn't say anything about focal length when giving recommendations for landscapes (I assume that if it's good enough for objects at infinity, then it should be good enough for objects at 10m). So, I assume this is not the case. Is that right? Is it possible to get some recommendations as to what sort of settings I should aim for to get the best focus given I have a good exposure given I have an objects at 5m - 10m?

Answer 1

There are three competing, inescapable, factors causing blur:

1. Depth of field: a lens produces a sharp image of objects only at a specific distance, and blurs those closer or farther. The narrower the lens aperture or opening (the higher f-number), the more the lens behaves like a pinhole and the greater the depth of field. However, to allow narrow aperture, more light, longer exposure to collect more light, or higher sensitivity (higher ISO setting) is needed. Longer exposure increases the problem of subject movement an camera shake, while higher ISO settings introduces "noise".

2. Motion blur: any camera shake or subject motion causes the image to be smeared out. Most people can hold a camera with 50-mm equivalent lens steady for 1/50 second, and with image stabilization and practice, for perhaps 1/15 second. A stable tripod is far better than hand-holding, though, for reducing motion blur.

3. Diffraction limit: No matter how good the lens, there are fundamental limits to the sharpness of an image due to diffraction effects, which make the image worse at narrow apertures.

4. In addition to the above fundamental issues, cameras may degrade sharpness when processing a JPEG image, a lossy compression scheme. In fact, I found my newly purchased camera was set to blur photos noticeably to save storage space. Set sharpness higher, if you want.

The photographer juggles these factors for each exposure. You might want to blur the background to concentrate on a close subject, called bokeh in Japanese. Perhaps you want to take a "streak" photo of lights on a Ferris wheel, and want motion blur. But if your goal is a sharp image from foreground to background, use the narrowest aperture (before diffraction limitation) and slowest shutter speed, with a tripod. Group f/64 (Ansel Adams, Imogen Cunningham, John Paul Edwards, Sonya Noskowiak, Henry Swift, Willard Van Dyke, Edward Weston and others) shared that esthetic of absolute sharpness; see their work.