What aperture and focus setting will keep 1–2m and distant subjects sharp with a Canon 550D and Sigma 8mm?
Asked 10/4/2011
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I use a Canon 550D with a Sigma 8mm lens, usually around f/8 and manual focus set near 1m. I want both nearby subjects (about 1–2m away) and distant subjects (around 10m and beyond) to look sharp. What aperture and focus setting should I use for the best balance of sharpness and depth of field?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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For achieving maximum sharpness, your aperture must be
- stopped down about a stop or two from maximum aperture for optimal optical resolution;
- stopped down enough so that depth of field would be sufficient;
- open enough so that shutter time would not be long enough to introduce camera shake and motion of subject would not be visible;
- open enough so that diffraction would not cause blurriness.
According to MTF50 chart in review by Photodo, the lens has best optical resolution from about f/4.5 to about f/6.3.
Since diffraction limit for Canon 550d is about f/6.8, so the smallest selectable aperture (for largest DOF) without diffraction is also f/6.3.
If using f/6.3 results in a shutter speed too low to avoid camera or subject motion, you might get a sharper result using a larger aperture or higher ISO. Or you might have to close down further if depth of field is insufficient. At f/6.3, hyperfocal distance is half a meter, so the distances you mentioned in your question are well within focus.
Originally by user4390. Source · Licensed CC BY-SA 4.0
user4390
14y ago
0
Generated from our catalog & community — verify before relying on it.
With an 8mm lens, depth of field is already very large, so getting both 1–2m and distant subjects sharp is mainly a balance between lens sharpness, enough depth of field, and avoiding diffraction or motion blur.
From the community answers, this lens appears to be optically strongest around f/4.5 to f/6.3, and on the Canon 550D diffraction starts to become a concern past about f/6.8. That means f/6.3 is a good target if shutter speed is still fast enough.
If you stop down more, you may gain a little depth of field but lose some crispness to diffraction. If you open up more, you may lose depth of field. So start around f/5.6–f/6.3 and keep focus around 1m, then check results.
Also test your own lens-camera combination: mount the camera on a tripod, manually focus, shoot a series through all apertures, and compare the files on a computer. You may find several apertures look essentially the same. If shutter speed gets too slow, use a wider aperture or add light rather than stopping down further.
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