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.

[Skirball]

The way you describe it (“razor sharp”), it sounds like it’s locking focus in the region of where you’re aiming, but not precisely on target?  As opposed to simply not locking focus in that area at all.  The latter is what I would expect most the time, it’ll either make focus or it won’t, and if it does it should be “razor sharp”.  However, I could imagine that in low light situation the camera will hunt a bit for a contrast point, and if it finds one in the area of the focus point, but not directly in the center, it’ll take that for focus.  

That said, we can’t rule out technique without seeing you shoot.  Are you shooting ultra thin depth of field shots?  Are you using outer focus points or focus and recompose?  Are you using a slow shutter speed due to lack of light and what you’re seeing is actually camera shake?

I can’t speak directly for the 5d3 over the 7D, but I’ll tell you that the 6D absolutely astounds me on how it’s able to focus in practically dark situations, and the 5d3 should be similar.

[Mtnman]

I do not know about camera shake on the tripod.  And how that relates to the EOS 3 being able in the same scenario getting it razor sharp with center single point focus.  Tested the 7D against the EOS 3 on a tripod, lowlight, 24-70mm at 4.0 and 400 ISO.  But  those settings have nothing to do with the camera being able to find focus in low light.  Taped a new dollar bill to the wall at 8 ft on 50mm. in low light.  In order for the 7D to find focus as sharp as the EOS 3 I have to shine a bright light on the bill while back button focusing then I take my shot(without the light of course) and it will be as good as the EOS 3 (without a need for a bright light added at the time of focusing).

[Skirball]

Sorry, didn’t pay attention to the tripod part.

So is the 7D not locking focus at all (without added light), or does it hunt and lock?  And are the images completely out of focus or just soft?

[Mtnman]

Just soft usually,(it does lock focus and does not hunt, but I need to be able to see eyelashes in sharp focus when I target the nose and eye area) until the light source  comes to a bright enough point that it finds pinpoint focusing.  With enough light focus is razor sharp no matter the iso, lens, or aperture.  Was the EOS 3 made that much better when it came to focusing- I know film days have long been gone, but that was/is a great camera body.   Is this just a function of the 7D infrared focusing mechanism not up to par to some other camera bodies.

[Skirball]

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@Mtnman wrote:

... but I need to be able to see eyelashes in sharp focus when I target the nose and eye area...

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As you should.  I wouldn't call this normal behavior, if it locks focus, it should be on focus.  The 7D may not excel at low light focus compared to some of the current models, but that shouldn't matter.  If it's too low light to lock focus it'll hunt, but once it locks it should be true.  Especially on a stationary flat subject.  Are you on one-shot (e.g. focus priority)?  I guess I could see it missing with AI servo...

[Mtnman]

I shoot Al Servo alot(pet photography and kids) but my tests were done on "one shot"  and "single frame"  since I noticed this behavior.  That is where adding the extra light allowed the camera to lock focus a little better to get that sharp focus.  I ran this test over and over.  Every time I added light at the focusing time I could get a sharp image but others times it is hit or miss with a soft image.  Thanks for your time.  I can soften my images later as you know, but I cannot make them crisp in post software and it look like our eyes see it.  Soft is at 11x14 but that is not acceptable, because with enough light my images are sharp at 16x20.  Will the 5DIII correct this issue with this 7D?  I hate too much culling with this camera body.

[amfoto1]

I honestly don't know how low light the EOS-3 is rated for.... However, it does have the first iteration of the very sophisticated 45 point AF system used in the 1V and most of the 1D-series cameras that came later.

The 7D is rated to be able to focus down to -1 or -0.5 EV. This is fairly typical of current Canon models other than the 6D, 5DIII and 1DX. Those are rated to be able to focus down to -3 EV (which is approx. moonlight). The 6D is limited to the center AF point only (out of 11 total), that will focus in such low light. The other two models can focus that low with more of their AF points (Both have 61 AF points total, but the number that act as more sensitive cross-type varies depending upon what lens is being used.)

So, yes a 5D Mark III would be able to focus well past what your 7D can. I've also found the 5D Mark II able to keep focusing, albeit center point only and rather slowly, in a little lower light than 7D too, even though Canon rates the two the same.  

Something you might want to try, to get the 7D to focus better in low light, is use one of the Canon flashes to emit a Focus Assist beam. That's a grid of near IR light projected onto the subject and really does help the camera focus, wihtout being obnoxious or effecting exposure. You can set up so that the flash won't fire, if you prefer. This will work on most of the Canon cameras (all that I'm aware of). There are some third party flashes that have this feature, too. Or, if you would rather use something smaller, the Canon ST-E2 flash controller module can be used instead. It sits in the camera's hotshoe just like a flash, but is much smaller.  

***********
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 

[Skirball]

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@amfoto1 wrote:

The 7D is rated to be able to focus down to -1 or -0.5 EV. This is fairly typical of current Canon models other than the 6D, 5DIII and 1DX. Those are rated to be able to focus down to -3 EV (which is approx. moonlight). The 6D is limited to the center AF point only (out of 11 total), that will focus in such low light. The other two models can focus that low with more of their AF points

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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.

Regardless, this doesn't sound like a sensitivity issue.  If the 7D thinks it has focus, it should be focused.  Something is amiss, and while a new camera would "fix" it, the 7D should be capable of this. Either something is not working properly or his camera is set to some sort of shutter priority.

[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.

***********
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