TCampbell
Been spending a lot of time on here!
- Joined
- Mar 31, 2012
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"sharpness" is an elusive term... it's actually not a technical term.
amolitor mentioned that f/11 will provide a greater focused range (more "depth of field"). But that's different than resolving power. Lenses are judged on their ability to resolve fine amounts of detail -- much in the same way you might be asked to read the increasingly tinier rows of text on the chart when visiting an eye-doctor.
Suppose our "eye-chart" has 10 rows of text... each row using a smaller and smaller font (btw, test charts for lens MTF scores don't use the eye-chart used by an optometrist... but the analogy works). Let's say a ludicrously good lens can read all 10 rows, but a very good lens can only read the first 8 rows. Your lens might be able to read the text in the 8th row at f/8... but at f/11 it might only be able to read the first 6 or 7 rows even though a greater range of the image appears to be in focus. Depth of field is based on the focus being "good enough" that it seems reasonable.
The "sweet spot" for most lenses is usually about 2 full stops down from whatever wide-open is, but that's a generalization and the reality will vary from lens to lens. Often it's f/8 or very close to f/8. Most crop-frame cameras start to become slightly diffraction limited at f/11 (another generalization -- it varies by the size of the photo-sites on the sensor.)
After all this... keep in mind that the level of resolving power I'm talking about is generally NOT noticeable unless you have a large output size. E.g. you will not notice this difference in images that you post to photo-sharing sites because the image sizes are far too small to see the detail.
amolitor mentioned that f/11 will provide a greater focused range (more "depth of field"). But that's different than resolving power. Lenses are judged on their ability to resolve fine amounts of detail -- much in the same way you might be asked to read the increasingly tinier rows of text on the chart when visiting an eye-doctor.
Suppose our "eye-chart" has 10 rows of text... each row using a smaller and smaller font (btw, test charts for lens MTF scores don't use the eye-chart used by an optometrist... but the analogy works). Let's say a ludicrously good lens can read all 10 rows, but a very good lens can only read the first 8 rows. Your lens might be able to read the text in the 8th row at f/8... but at f/11 it might only be able to read the first 6 or 7 rows even though a greater range of the image appears to be in focus. Depth of field is based on the focus being "good enough" that it seems reasonable.
The "sweet spot" for most lenses is usually about 2 full stops down from whatever wide-open is, but that's a generalization and the reality will vary from lens to lens. Often it's f/8 or very close to f/8. Most crop-frame cameras start to become slightly diffraction limited at f/11 (another generalization -- it varies by the size of the photo-sites on the sensor.)
After all this... keep in mind that the level of resolving power I'm talking about is generally NOT noticeable unless you have a large output size. E.g. you will not notice this difference in images that you post to photo-sharing sites because the image sizes are far too small to see the detail.
