front/Back Focussing

nickjohnwatson

nickjohnwatson

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Nick
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Hiya everyone, i wonder if you might be able to help.

I bought a 50mm 1.4 USM from the classifieds and i'm not sure if its back focussing or not. I've never had this problem before so am unsure.
I think its still covered by warranty with WEX but ill need to go through the seller for it but dont wanna do all that if its not what i think it is.

This picture was taken with it, using centre point and single shot focus. It looks "ok" as a small file for use on facebook or whatever but i had it printed for them and the focus is way off, if you look at the big file on flickr you can see the issue. I've noticed it on a few images. I'm worried that if its not the lens then it could be an issue with my cameras autofocus, although I've not noticed it with any other lenses.....



seems to be fine sometimes as in the next 2 its fine





Sorry for the rambling post but does anyone have an idea if it could be that?

Nick
 
Any chance of a link to the RAW so we can see where the focus point is and what it covers? I'd be guessing you're looking at a DoF of around 10-15cm minimum with those settings and distance so would be interesting to see what the camera thought it was focussing on.
 
Ignore that - just seen you used centre point and presumably re-composed. One obvious difference between the three images, assuming you used the same technique, is the centre point is much closer in the latter two to what was presumably the focal point (the eyes), so your movement when re-composing would have been less.

At shallow DoF, the 'error' introduced by Focus and re-compose is not insignificant if you're not careful.

To keep the shallow DoF, but get a sharper image, try selecting a focus point nearer to the focal point you want so you have to move the camera less, or better still, not at all.
 
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Furtim said:
Ignore that - just seen you used centre point and presumably re-composed. One obvious difference between the three images, assuming you used the same technique, is the centre point is much closer in the latter two to that was presumably the focal point (the eyes), so your movement when re-composing would have been less.

At shallow DoF, the 'error' introduced by Focus and re-compose is not insignificant if you're not careful.
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Yeah i always focus and recompose if needs be, dont think i moved forwards or backwards though for the group one as we were all stood stationary. it seems to lock focus sometimes but then sometimes seems to hunt a little. not sure if its user error or an issue with my gear, there was a few from the day that are the same. seems very wierd
 
One thing ive noticed when manual focussing is that when you wind the focus ring slowly out to focus on something, if you go a little bit too far, you have to come back to before it was focussed and then go to it again, you cant just come back to focus if that makes sense. (sounds like it makes sense in my head)
 
nickjohnwatson said:
Yeah i always focus and recompose if needs be, dont think i moved forwards or backwards though for the group one as we were all stood stationary. it seems to lock focus sometimes but then sometimes seems to hunt a little. not sure if its user error or an issue with my gear, there was a few from the day that are the same. seems very wierd
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It's not about backwards or forward so much as rotating the focal plane (effectively parallel to the sensor) that causes the issue.

If, for example, you focus with the centre point on the head, and then move the camera so the centre point is now basically in the centre of the shot, no matter how careful you are, you're probably twisting the top of the camera forward slightly. In doing so, you're going to push the top edge of the focal plane behind the head of the guy.

The affect is only really noticeable at wide apertures because the DoF is so shallow, and is more evident the further away the intended focal point is from the centre of the frame. Combine this with any other accidental / unavoidable movement, and it's very easy get soft images.
 
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true_focus_illust.png
 
nickjohnwatson said:
Maybe i could try using one of the outer focus points for this kind of thing to see if it makes a difference, your explanation is great and makes a lot of sense, that picture explains it well too :) maybe i hadnt realised how much difference it could actually make?
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I would try it and if possible, shoot with the focus point over the focal point - shot a little wider and crop if necessary.

The only limitation might be that on the 550D only the centre point is a 'cross' point, the others are all linear - in simple terms, the centre focus point is the most accurate as it evaluates focus on two axis (up and across), where as the other points only use either vertical or horizontal. So if the subject you are trying to focus on itself is not that distinct (ie. High Contrast), the outer points may struggle more than the centre point to nail it.

3907096987_c8d3e97f23.jpg
 
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It's close

Using your distances - and let's say you shift the focal plane by 15 degrees, using the formula above, you'd be looking at the focal point moving around 14cm behind the head. If, based on your calc, 21cm 'in front of subject' was acceptable sharp, this is going to reduce it to around 7cm all other things being equal.

The subject swaying back and forward, you swaying back and forward, each by only 2-3cm, if timed badly would wipe that out. You've still got a DoF of 45cm, but a good 44cm of it might well be behind his head!

Also remember, DoF refers to that is adequately in focus, it's not an absolute and also depends on how the image is presented and the distance from which it is viewed. When you're getting to the limits of the DoF defined, it's going to start getting soft, but not too soft!

Obviously this is speculation - angles, distances etc are all estimated, and the formula I've used probably needs to take into account distance from centre point, but basically the closer the subject, the more greater the effect. The more of an angle you turn the focal plane through, the greater the effect - that's just maths. So if you want to shoot wide open, if you can move less of an angle by using a closer focal point the better. Another option is to increase the distance between you and the people, and perhaps crop in post, but then again, you could always just close the aperture down a stop or two. While you won't get the same DoF, you will usually find that the bits that are in focus are even more in focus as very few lenses are that sharp wide open.
 
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Sorry - one more thing on a group shot like that one - you have to make sure that the whole group is in the same focal plane too at such a narrow DoF or someone will be 'soft'.
 
I don't think this is a focus-recompose problem, more likely a one-off error that could have several causes, or an unlucky combination of them.

David, your focus-recompose diagram shows a greatly exaggerated and very unusual situation, basically depicting a full-length figure shot from about one metre distance, using something like a 14mm lens on 1.6x format. The focus-recompose error shown (12cm-ish?) equates to DoF of an impossibly large aperture, like f/0.7!

Reality is that focus-recompose technique is only a problem when three critical factors combine - unusually close distance, focused point a long way off-centre, and very shallow DoF. That's not the case here, as the OP is using a 50/1.4 lens from about 5m (my estimate, Exif focusing distances are only approximate) with total DoF of over 50cm. Using focus-recompose technique, the angular shift is around 8 degrees only.
 
On other comment on the 50 f1.4 USM, I have found this lens to be a bit unreliable focusing on my 7D, I think the micro motor USM (not ring USM as found in the more expensive lenses) is not up tot job of focusing the f1.4 accurately.

I see you shot this at f1.8, the effect of the out of focus background is nice but you will need to pay attention to the focusing, try using live view to see how the camera is doing.

HTH

David
 
Mine sometimes goes a bit funny with my 7D, I've set in on +5 in the micro adjustment as it seems the best fit for it. Anything at f1.4 is going to be tricky quite a lot of the time.
 
AndyHayball said:
Had this exact problem. Up close was sharp and from 2 meters it was soft. Sent it to canon and came back sharp as a razor at 1.4-1.8+ across all ranges. Send it in with the body.


www.andyhayballphoto.com
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I think I might do. I've just bought it and putting off using it as it's doing my head in lol
 
nickjohnwatson said:
Spoke to wex and got a return label so hopefully it'll come back nice and sharp and working properly so I can enjoy using it!!
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Good job! Please update us with the results as I have a great suspicion it will come back great. I left the body and lens with them so they could calibrate the lens to the body. I'd suggest the same if you can be without camera for a few weeks.


www.andyhayballphoto.com
 
AndyHayball said:
Good job! Please update us with the results as I have a great suspicion it will come back great. I left the body and lens with them so they could calibrate the lens to the body. I'd suggest the same if you can be without camera for a few weeks.


www.andyhayballphoto.com
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Is that how long it will take lol? Hope it doesn't take ages! !!!!
 
At F1.4 the depth of field is really fine, just because it goes that low don't need to be on it all the time, around F2 will still give similar background defocus.
Micro adjustment can help but tends to need a different setting for close and far subjects!
When focus is critical and subject still enough there's no substitute for manual focusing and live view.

My Sigma 50mm is just the same but rewarding when you get it right.
Used around F4 it's a lovely sharp easy to use lens.
 
nickjohnwatson said:
It's back at wex now to get repaired and checked. The focus ring wasn't accurate I tested it in manual focus too. Hope it'll be ok once ita back :)
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Don't be surprised if it's not. This may be the sort of thing where xx0D AF-system will really struggle at longer distances. Only the centre point is half decent, and you can't really do much recomposing at f/1.4.
 
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