What is the sharpest aperture for Canon 550d with Sigma 8mm?

Question

I usually use F/8 and a manual zoom at 1m. What configuration should I use to assure both objects that are far away(10m) and objects that are close(1-2m) are sharp

Answer 1

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.

Answer 2

This is a case where I would strongly recommend performing an experiment for yourself. With 1/3 stops, the lens only supports 17 aperture settings. Set up a tripod, manually focus the lens, put the camera in Av mode, cover the viewfinder, use a remote release cable, and take a picture at each aperture; it should only take about 5 minutes. Then you can examine the results at whatever zoom level you want on your computer monitor.

You may well find that there's a range of apertures that have indistinguishable sharpness, which would give you more freedom in the future! You'll also learn a little bit more about the characteristics of your lens+camera combination, like any exposure shifts at different apertures.

Finally, if the results aren't sharp enough for you, you may want to add light or post-process the photos to increase the sharpness of your subjects.