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.

[amfoto1 · Accepted solution]

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 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. 

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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.

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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 

[amfoto1]

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@Skirball wrote:

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No, the 1DX and 5D3 are rated at -2 EV, and I don't think - though I'm not sure - that this applies to all focus points.  True, the 6D only has the 1 sensitive point, but it's a full stop more sensitive than any other model.

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You are right. That's what's being shown now. And it doesn't apply to all the AF points. If memory serves, the two cameras with 61 AF points have up to 41 of them that act as the "better" cross or dual-axis type, but this number will vary depending upon what lens is being used. There are specifics about the lenses and their effects somewhere in the docs for those two cameras.

I've noticed different specification claims on these, depending upon where and when I've looked. Initially -3EV was claimed for 5d3 etc. and 7D was rated to -1EV... but this seems to have been scaled back a little bit to -2EV and -0.5EV, respectively. Also these claimed specifications have a bit of fudge factor, I know from using 7D and 5D2 side by side. They are rated the same, but I've always found the 5D2 able to focus in a wee bit lower light than the 7D. Not a lot and the 5D2 focus slows quite a bit, but I've seen some difference even though they are rated exactly the same. I imagine it's next to impossible to make a hard and fast claim with respect to AF performance because there are so many variables in gear combinations, users and shooting conditions.

I guess the fairest, or perhaps safest thing to say is that the three current full frame models are good for approx. one or two stops better AF performance in low light, compared to all the crop sensor models and the older FF models, but Your Mileage May Vary depending upon a bunch of factors. Even so, the difference is significant, and makes perfect sense considering those current FF cameras' low light image quality capabilities too.

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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