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Thread started 13 May 2007 (Sunday) 11:04
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Strange focus/depth-of-field issue. Help!

 
Ian ­ Trott
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May 13, 2007 11:04 |  #1

Here is an image shot by my assistant at my daughter's wedding. Canon EOS 20D, Canon 70-200 f2.8 L lens (non-IS version.) Exposure - 400 ISO, f.2.8 @ 1/250. Other than ACR processing, sizing for this forum, this image is as-is. Not certain if she used multiple focus points or a single center or other focus point. Does this image not look strange? D on the left is the most out of focus with S in the middle a bit less, etc.. Am I missing something here, besides a few follicles? I've come across other similar pics from the same assistant (her gear) so we're trying to figure out what might be wrong before contacting Canon - if indeed something is out of whack!?


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cdifoto
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May 13, 2007 11:05 |  #2

At f/2.8 you aren't going to get everyone in focus. Nothing is out of whack, and there's no reason to call Canon.


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Ian ­ Trott
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May 13, 2007 12:02 as a reply to  @ cdifoto's post |  #3

Looking at the arrangement of the 4 subjects, you can see that the fellow in the rear and the one on the far right are much more in focus. BUT, and herein lies the problem, the little guy in the middle appears (to me) to be on the same plane as the far right subject and in front of the fellow in the rear and consequently should be in focus. This doesn't appear (to me) to be the usual shallow depth-of-field at f2.8.




  
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MALI
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May 13, 2007 12:33 |  #4

Yeah, the kid in the front and the man behind him should both be in focus but the kid is not while the other guy is. One possibility is that kid moved a little. My suggestion increase the shutter speed even more and repeat similar shots and see if it keeps happening. You can also try A-DEP mode just to make sure. If it happens even there; then you have a problem.


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cdifoto
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May 13, 2007 12:34 |  #5

MALI wrote in post #3199007 (external link)
My suggestion increase the shutter speed even more and repeat similar shots and see if it keeps happening. You can also try A-DEP mode just to make sure. If it happens even there; then you have a problem.

My suggestion: f/8


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Ian ­ Trott
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May 13, 2007 15:04 |  #6

cdifoto wrote in post #3199011 (external link)
My suggestion: f/8

f8.0 is not the solution. The image clearly shows there is more underfoot than this just being a simple too little depth-of-field issue.




  
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Ian ­ Trott
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May 13, 2007 15:11 |  #7

MALI wrote in post #3199007 (external link)
Yeah, the kid in the front and the man behind him should both be in focus but the kid is not while the other guy is. One possibility is that kid moved a little. My suggestion increase the shutter speed even more and repeat similar shots and see if it keeps happening. You can also try A-DEP mode just to make sure. If it happens even there; then you have a problem.

I'm thinking it isn't a movement issue as a shutter speed of 1/250 with flash should freeze minor subject motion. I'm going to have my assistant repeat a similar shot with the particular camera/lens combo in question and see if she can repeat the results. I like your suggestion of A-DEP. Will pass that on to my assistant. Thanks for the suggestions.

I'm wondering if an element inside the lens could be out of position, ever so slightly? Is such a thing even possible?




  
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strmrdr
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May 13, 2007 15:15 |  #8

run a focus test to make sure it isnt back focusing.
if you do a search on back focusing or focus targets you will get the instructions to do it.


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Ian ­ Trott
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May 13, 2007 15:38 |  #9

strmrdr wrote in post #3199547 (external link)
run a focus test to make sure it isnt back focusing.
if you do a search on back focusing or focus targets you will get the instructions to do it.

Not sure I'm making myself clear about the problem.

If you look carefully at all 4 subjects and their relationship to each other within the image, it's pretty obvious (at least I think it is) that if Steve in the middle-back is in focus and Chris on the right is in focus, then Seth in the middle-front and Dwayne on the far left would have to be in focus too because they are all within the same plane of focus. Chris appears to be the front-most subject while Steve (rear-middle) is the furthest back. Hence, the 2 that are out of focus are actually between them.

Now, if back focusing was the issue it would be back-focused evenly throughout, not just the left side.

If a lens element were out of kilter, then I'm surmising that could cause a left to right focus issue, but I don't know enough about this topic to know if that is the case or could be the case!?

I quess what I'm looking for is someone to shed some light on that topic. I understand all about f stops, exposures, front and back focusing - I'm an early adopter of digital photography (started out with 2 famously back focusing EOS 10Ds) and my profession is wedding and portrait photography.

BTW, thanks to all who respond to this thread.




  
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Box ­ Brownie
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May 13, 2007 16:46 |  #10

I wonder? I do recall reading a goodly while (sorry I can remember which forum) about lenses have issues right to left and vice versa but this was evidenced on the egdes of the frame not in the centre and may have actually been a misaligned sensor.

So, and sorry if I am stating the obvious or repeating anything above ~ at f2.8 the DoF will be quite shallow (teaching you how to suck eggs, eh!) and granted what you say about the relative position of each group member in the same 'planes of focus' the effect makes me think of focus & recompose and for such a tight group shot with a wide open lens........just possible, maybe.

Hmmm! looking at the image more critically I think the picture frames behind the central figure are just about as sharp as that central figure. Was she dead square in front of the group or could she have been (as viewing the image) slightly off to the left & as such pointing the camera at angle across the group thus excerbating the shallowness of the f2.8 DoF, indeed with the tilting verticals on the wall and frames I really do wonder if all this is is a DoF issue made worse by poor shooting technique. One last thought was this the IS lens, if so does she always wait the the half second IS takes to stabilise.

Re: focus points have you checked in ZoomBrowser?

Oh for the record I would never want to shoot weddings and if you cannot pinpoint a hardware reason I wonder in the nicest possible way if you should be checking your assistants' cameracraft & techniques a litle more critically. Please do not take any of my comments as judgemental, just analysing what you have shown with the info given.

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Ian ­ Trott
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May 13, 2007 17:44 as a reply to  @ Box Brownie's post |  #11

Thanks for the insight BB. I had thought initially about that idea - center focusing on the far right subject, then recomposing, thus putting the plane of focus off of the other 2 subjects. However, knowing pretty much where the shooter is standing makes me think if that were the case, the out of focus area should be to camera right rather than left.

The angle the shooter is standing at in relation to her subjects is minimal. The far right subject is almost against a wall on his left and the shooter is likely against the wall herself opposite him or just a bit off center. Could be a combo of things as well. Off kilter lens, sensor, AND technique?

It shouldn't be too difficult to re-enact, shooting absolutely straight on and altering angle after each test shot. I also hadn't thought about the possibility of a sensor being ever so slightly out of alignment. So slight perhaps only exhibiting this flaw at certain angles, apertures, etc., etc., etc..

By the way, her 70-200 isn't an image stabilized version. I have the good fortune to owning that gem.




  
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StewartR
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May 14, 2007 10:12 |  #12

Of course it's hard to tell from such a small picture, but I don't think any of them look as sharp as I'd expect with a 70-200 lens.

Also, bearing in mind that we don't really know how this photo was taken - including important aspects like whether the person or the camera decided what to focus on - then it would seem imprudent to jump to conclusions regarding the equipment based on just this one photo.

From what I've seen on this forum, human error is responsible for far more focussing problems than equipment error. If there are doubts about your assistant's technique, as there seem to be, then I know where I'd look first.


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Ian ­ Trott
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May 14, 2007 12:11 as a reply to  @ StewartR's post |  #13

Thanks everyone for the feedback on this topic, most precisely regarding this questionable photo. In researching the topic of focus and re-composing I've come across an excellent article from BobAtkins.com. Here's a tidbit quote from the article ...

Quote - " If you run through all the numbers and make assumptions about typical subject distances, apertures and angles likely to be swung through when focusing and recomposing you find that the vast majority of the time it shouldn't give you a significant loss of sharpness. In fact most of the time it probably won't be detectable.

But will it matter? Lets say we're shooting wide open at f2. What's the depth of field at 10m. The answer is the depth of field will extend from 8.7m to 11.7m (see my Depth of field (external link) article and calculator). So even at f2 the amount of defocus will be very small compared to the depth of field and "focus and recompose" should result in a sharp image. So when won't it be OK? Well, the greater the angle though which the camera is turned, the greater the focus difference, so the wider the angle of view of the lens, the greater the possible focus difference will be. The faster the lens and the closer the focus, the smaller the depth of field will be. So the worst case would be a close focused fast wideangle lens, shot wide open. " - end quote.

This brings me back to my original point. If 2 of the subjects are framed within the space (or plane) of the other 2 subjects, i.e. Seth and Dwayne (left and front-middle) are actually in-between Steve (rear-middle) and Chris (far right,) then all four should be pretty much equally focused or equally out of focus because I beleive all 4 subjects are within the depth-of-field for this shot. But in this photo there is a substantial difference between the 2 groups and I'm questioning whether it's a mechanical malfunction rather than the shooter's angle and technique.

I'm reposting a sharpened version of the photo so you can better see the difference.

Also, my assistant is a bit unsure of her angle to the subjects so if anyone viewing the photo can provide their interpretation of the shooters angle, that might be helpful.




  
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Titus213
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May 15, 2007 20:21 |  #14

I don't see a big difference here - but my eyes are old and the image is small. I would definitely try to reproduce the problem under controlled/known conditions.

Perhaps it is the individual. I have a couple of grandchildren who are evidently going to go thru life out of focus...:lol:


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Yeoer
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May 16, 2007 07:31 |  #15

I can't tell did-illy squot from that image as its too small and over compressed.


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Strange focus/depth-of-field issue. Help!
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