Low light focusing

[Mtnman]

I have a   7D and a EOS 3.  When I put them on a tripod with the same lens and same low light(or shadows in sun) situation I notice the   7D cannot focus razor sharp like my old EOS 3- only until I shine I very bright light on the subject can I get the 7D to focus very sharp.  Then with the added light it does a great job-the same in bright light situations.  Why does the 7D need so much light to focus sharp?  Is the 5DIII much better than the 7D in low light focusing?  I have set up the AF microadjustments in the 7D, but that should not influence the fact that more light always is needed to get sharp picture focus.  This really bugs me.  I see soft pictures all the time on photographers sites and that just irks me.  Manual focus is out of the question with my vision.

[Mtnman]

Thank you amfoto 1. You are quite right about the canon flashes being able to pinpoint my focus much better.  I have noticed this, but did not know I could get them to not fire and still send out the IR beam to focus. I have  the Ex 420 and Ex 580 II. I will look into the ST-E2 .That will help alot.  I am still thinking hard about upgrading to the 5DIII for the full-frame , performance ISO boost, and better low-light capabilities.  I am thinking that when you do the AF microadjustment you notice how wide a range the camera considers in-focus to be.  On my 70-200 it went from -6 to +8 and I guarantee that when it locks in satisfied at any point near the outer limits the picture will probably come out softer. 

[amfoto1 · Accepted solution]

---

 I am thinking that when you do the AF microadjustment you notice how wide a range the camera considers in-focus to be.  On my 70-200 it went from -6 to +8 and I guarantee that when it locks in satisfied at any point near the outer limits the picture will probably come out softer. 

---

It is difficult to MFA zooms with the type of MFA the 7D has (also 50D, 5DII and others... I call it "MFA 1.0" ) . Your adjustment can be right at one end of the zoom and incorrect at the other. But with the amount of variation you're seeing, I wonder if the lens might need work.... Maybe something is worn inside, or just out of calibration?

With modern zooms you also have to consider whether the lens is parfocal or varifocal. However, I believe all the Canon EF 70-200s are parfocal, so should maintain focus fairly accurately when zoomed.  

5DIII, 6D, 70D all have an improved form of MFA that might be a big help. It's increased in capacity from 20 lenses to 40. It also has lens-specific MFA... by which I mean that if you use two different EF 50/1.4 lenses on the camera, you can have different adjustments set up for each of them, based upon the lens serial number. The older style MFA was lens-model-specific, if you set up an adjustment for an EF 50/1.4 the same would be applied to all EF 50/1.4 you used on that camera. That was the best MFA 1.0 could do. 

But perhaps the biggest improvement with MFA 2.0 is that it allows for two adjustments with zooms... one at each end of the zoom range.

***********
Alan Myers

San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & PRINTROOM