70D Focussing with wide aperture

I have been using my 70D with Tamron 28-75 f/2.8 and could see that there is a slight issue with foucssing at f/2.8. The subject doesn't seem to be sharp compared to Canon 40mm f/2.8 STM lens. I read on the internet that many people have the same problem while using the center focus point in Canon 70D with non STM lenses at wide apertures.

Does everyone have this problem? How is everyone overcoming it? Using Live View? Using a different focus point than the center? Using Zone focussing?... etc. What would be the best way to overcome this issue as I see that Canon is not acknowledging it as a problem.

I have done some tests at 40mm f/2.8 by using a focus point just above the center and also with the center zone, but can't tell if there is any improvement compared to the Canon 40mm STM lens.

I don't own either of those lenses, but please note that the sharpness of the Tamron 28-75 is nowhere near as good at f/2.8 as the Canon 40mm, especially away from the center. Have a look at this comparison page for proof of this:
http://www.the-digital-picture.com …omp=0&FLIComp=0​&APIComp=0

So basically, don't expect much out of the 28-75.

The issue discussed on the internet is about a general focusing issue at f/2.8 or wider. The actual lens doesn't make a difference in the context of the issue. They will all focus badly. The fact that your Canon 40mm focuses perfectly means that your camera most likely does not have the issue mentioned on the internet.

Your Tamron 28-75 not giving sharp pictures might mean:
1. The lens just is NOT sharp, as seen in the above comparison.
2. You might need to Micro Focus Adjust the Tamron.

Did you get sharp pictures of the same subject using Live View with the Tamron? Conduct a few more tests using viewfinder and using Live View with both lenses in good light using a tripod and a high contrast subject to narrow the problem down.

I had a Tamron 28-75. Nice, optically adequate, small, light lens. My copy just sucked at 28mm, wide-open. At f/3.5 is was really good. I only had issues with the wide end. It was good at other focal lengths, at any aperture. Perhaps you are perceiving a bit of softness as OOF?

Have you tried micro focus adjustment for the lens?

Thanks everyone for the responses. I have now compared some old pictures I shot with my T1i using the same lens. With T1i, I agree that there wase some softness at f/2.8 but it seems to be a bit worse in 70D. I have not tried micro adjustments before. I will give this a try.

Just read about microadjustments in the manual and think this will not resolve my issue. I have a problem only at f/2.8 and get sharp pictures at other f stops. Microadjustments will effect the focus at all focal lengths. I will check the results with live view. If those are better, I will stick with live view for f/2.8 and viewfinder for remaining focal lengths.

SAPSTAR wrote in post #17647096
Thanks everyone for the responses. I have now compared some old pictures I shot with my T1i using the same lens. With T1i, I agree that there wase some softness at f/2.8 but it seems to be a bit worse in 70D. I have not tried micro adjustments before. I will give this a try.

Understand that the same lens may operate slightly differently on various bodies, even on various copies of the same model camera. This problem is often more obvious with third party lenses like Tamron and Sigma. For instance, on my Canon 60D all my Canon lenses work just fine and continue to do so on my new 7D Mark II. My Sigms 18-35 f/1.8 however worked fine on my 60D but needs to be adjusted on my new 7D II, and also has focus issues on my T2i, but not on my T3i! This is all due to manufacturing tolerances between camera bodies and lenses. That's one of the main reasons why most upscale camera bodies have MFA capability in the first place. Its not just to fix lenses with back/front focus issues, but to compensate for these manufacturing tolerances between cameras and lenses.

SAPSTAR wrote in post #17647113
Just read about microadjustments in the manual and think this will not resolve my issue. I have a problem only at f/2.8 and get sharp pictures at other f stops. Microadjustments will effect the focus at all focal lengths. I will check the results with live view. If those are better, I will stick with live view for f/2.8 and viewfinder for remaining focal lengths.

I would definitely do the micro adjust.
It's clearly the most noticeable wide open, which is why you do the adjustment with the lens wide open.

My Canon 50mm 1.4 had significant front focus issues at 1.4 on a 70D. After doing the MFA, I'm happy with the lens again.

I have done microadjustments today and below are the results. All shots are taken at ISO-100, 1/250 shutter speed at f/2.8. The distance from the camera to the subject is about 3 feet.

Canon 40mm f/2.8.

Instead of trying various adjustments, you may want to try DotTune method described here https://m.youtube.com/​watch?v=7zE50jCUPhM. It is more reliable than trying to compare shots.

Please remember that it is recommended to set up a distance of about 50 times the local length, which will be about 4 meters or 12 feet.

Thanks very much for the information. I will definitely give this a try.