Focussing Issues

[lisa_hynes]

<p>Hi, this is my first post.<br>

I am experiencing focusing problems with certain shots, for example, in a full length shot of a bride with the train laid out I often find the bride's head is out of focus. I shoot with center point focusing which I use to zoom in, focus my shot, then zoom out and re-frame the shot. For whatever reason this technique sometimes does not seem to work. I am always careful to use an appropriate fstop and I seem to have this problem with and without flash.<br>

I use a Canon 5DII and have this problem with the 24-70mm f2.8 L.<br>

Can anyone advise me on a focusing technique that will help this problem?<br>

Many thanks<br>

Lisa</p>

[fotografz]

<p>You really need to learn how depth of field (DOF) works. </p>

<p>As you stop down the lens aperture, the area that is in acceptable focus increases. Generally it is 1/3rd in front, 2/3s behind the focus point. </p>

<p>Most lenses have a depth of field scale marked on the barrel which shows how deep the total DOF is at any given aperture setting.</p>

<p>An easier way to actually see if everything you want in focus actually is ... you can use your camera's "stop down" button. </p>

<p>Google "depth of field" for a deeper understanding how it works.</p>

<p> </p>

[william-porter]

<p>Lisa, </p>

<p>In addition to learning more about depth of field, you may also need to read about the problems that can be caused by using focus and recompose. These problems manifest themselves mainly when (a) you're using a very large aperture like f/2.8 or f/1.7 and (b) you're close to the subject. But when those conditions are both present, then if you focus on (say) the bride's eyes and then recompose the shot, you may, by recomposing, change the distance between the camera and the bride's eyes and thus throw them out of focus. </p>

<p><a href="http://blog.duncandavidson.com/2008/09/focus-and-recompose-exposed.html">http://blog.duncandavidson.com/2008/09/focus-and-recompose-exposed.html</a></p>

<p><a href="http://visual-vacations.com/Photography/focus-recompose_sucks.htm">http://visual-vacations.com/Photography/focus-recompose_sucks.htm</a></p>

<p>Google "focus and recompose" for more info.</p>

<p>Will</p>

[pete_s.]

<p>On a full length shot the focus & recompose error is small and the dof is large (more than +/- 1 feet @ f/2.8) . There are a lot of elements in the lens moving when you zoom so the focus may shift a little while you zoom. So it's not actually a good way to operate like you do. Best way is to zoom to the right focal length and frame then select a focus point on the brides face or as close as possible and then shoot.</p>

<p>Here is a technical article that shows that in real life focus recompose works pretty good:<br>

http://www.bobatkins.com/photography/technical/focus_recompose.html</p>

[pete_s.]

<p>Forgot to add that you may have some problem with your particular lens as well, both mechnically and optically. You have to run some controlled tests to determine if it's okay or not.</p>

[neilambrose]

<p>Don't zoom in, focus and then zoom out. It doesn't work.</p>

<p>Focus is accurate only at a given focal length. If you focus and then change the focal length then the point of focus will change as well. Often it's subtle and you can't always tell that it has moved in the viewfinder, but it's usually enough to cause unwanted front or rear focusing.</p>

[lisa_hynes]

<p><a href="/photodb/user?user_id=453741">Marc Williams - Franklin/Mich</a> - Thanks but I do know how depth of field works, I have been a professional photographer for 6 years!<br>

<a href="/photodb/user?user_id=3825929">William Porter</a> - Thanks for the links, very helpful<br>

<a href="/photodb/user?user_id=2237443">Pete S</a> - I borrowed a friend's identical lens and did a few tests by tripod mounting my camera and shooting then switching lenses and shooting the same thing. I then layered the shots in photoshop to compare them to see if my lens is at fault. It seems to be fine. Still, I might have it seviced just in case. Many thanks for the link also.<br>

<a href="/photodb/user?user_id=2220488">Neil Ambrose</a> Can you recommend an alternative focusing technique? The zoom in and focus thing is something I've always done but I'd appreciate knowing any other method that might work better. Should I be manually focusing?</p>

[rt_jones]

<p>Stay out of AI Servo mode for shots like these.</p>

[neilambrose]

<blockquote>

<p>I'd appreciate knowing any other method that might work better. Should I be manually focusing?</p>

</blockquote>

<p>Not sure what you're asking here. If you've got a zoom lens, you zoom to the focal length you want to use and then focus - either AF or manually, whatever you prefer. Done.</p>

<p>Where you're going wrong is in attempting to use the lens as a kind of telephoto focusing aid. That's not what it's for, and leads to the problems you've described. If you absolutely need some form of focus magnification then consider using a camera with live view, and focusing in on the rear screen with the built-in image zoom function which is designed for focus checking.</p>

<p>If you focus and then later change the focal length of the lens you change the point of focus at the same time. Makes no difference if you use AF or manual focusing. The same thing occurs in both cases because you've moved the lens elements from the position where they were correctly focused.</p>

[john_deerfield]

<p>Are you using a UV filter? True story: Friend takes AA & AAA baseball pics primarily with a Nikon 70-200. Some pics were fine, some weren't as sharp. The lens was sent to Nikon for service and they said the lens was fine. But the problem persisted. I told him to remove his UV filter for one game. Result, every image was take sharp. If using a UV filter I suggest shooting one wedding without it and see what you think. </p>

[picturesque]

<p>What focusing mode are you using--One Shot, AI Focus, or AI Servo? Are you using the back button to focus or the shutter button? Have you found the plane of focus in the shots where the bride's face is out of focus, and if so, where is it? Why is it a problem now and not before, and did you just get a new piece of gear--if so, which one is it? What were you using before when the zoom in method worked? What f stops and focal lengths are you using (when this happens)?</p>

[buffdr_rasouliyan]

<p>To confirm your focus, can't you simply use your live view, then go 5x to pull your subject in then you can see for sure before you take the pictures? Try this, set your camera to One shot focus, crank down on the aperture to F-8, compose, focus and then shoot. BTW I gave up on my 24-70 2.8 as I couldn't get a clean shot at 2.8. I always got great stuff at 4.0 so I sold it and now I use my 24-105. v/r Buffdr</p>

[savagesax]

It's possible the lens is off. I've heard of a few 24-70mm lenses having this focus issue. Maybe go to a quality camera store and try another lens. Are you using a UV filter or something like that? If so take it off and see if there is any difference. Lastly if you bumped the 5D 2 maybe the mirror is slightly off.

 

Another way to check the lens and use a tripod and shoot an object at different lengths, such as 24mm, 50mm, and 70mm. Try different Fstops. This will help you determine if it's the lens or the body.

 

Can you post an image or 2? Often just looking at the image will tell us what the problem is. This will rule out human error VS a camera problem.

[stephaniaw]

<p>Lisa,<br>

It seems that your issue could be in the zoom in/out portion of the steps you listed. Try using one shot mode and rotating your focal points so the the point you focus on is as close to the bride's eyes as possible. Start out by adjusting the lens' focal length, then frame, focus and shoot. As pp said, you are not going to get the desired result by zooming in, focusing, then zooming out. Hope that helps!</p>

[arie_vandervelden1]

<p><em>zoom in, focus my shot, then zoom out</em></p>

<p>You're counting on the lens being parfocal - that is, maintaining focus as you zoom in and out. The 24-70 is supposed to be parfocal but some folks say it's not (varifocal) at closer focusing distances: e.g.<br>

http://www.photo.net/canon-eos-digital-camera-forum/00GqsK</p>

<p>Maybe experiment a bit with the camera on a tripod and a brick wall. If you lens is varifocal then you must refocus every time after zooming.</p>

[davebell]

<p>I think zooming after focussing is the issue, and DOF is a big factor too (yes, you have been a professional tog for 6 years....) but this does seem to be a potential issue as well.</p>

[wedding-photography-denver]

<p>I used to have the 24-70 L. Had it for two years and only used it a handful of times as it would mis focus regularly.</p>

<p>Having said that, I would say the zooming is probably your cuase in this case.</p>

[francie_baltazar]

<p>This lens is really really fickel. I have off issues with it on occasion - I never ever use the techique you described with this lens - it's just to sensistive. Change your point of focus and you should not have this issue. I have found that unless you are shooting at 4.0 or above you have to use a single point of focus. Also this lens requires a really steady hand. I take a breath and hold it before I shoot with this lens... It's really a pain lens - however, I LOVE the effect so it's worth the hassel. I also have mine serviced two times a year to make sure it's not out of since with itself. I use this lens to photograph toddlers on the run - to practice moving around a lot and still getting the shot. hope this helps...</p>

[William Michael]

<p ><strong ><em >"I shoot with centre point focusing which I use to zoom in, focus my shot, then zoom out and re-frame the shot. For whatever reason this technique sometimes does not seem to work."</em></strong></p>

<p > </p>

<p >(Ditto Neil's answer.)</p>

<p > </p>

<p >It will not work. </p>

<p > </p>

<p >There are very few (not one I know) zoom lenses which hold Focus across the Zoom Range. Zoom Lenses are NOT designed to do that. </p>

<p > </p>

<p >Also: after focussing and then zooming out, often you will be recomposing the shot, such that the centre point, is <em >no longer the centre point.</em> </p>

<p > </p>

<p >Not that this would make much difference most of the time, but there still will be a difference because of the Parallax Error, after recomposing. </p>

<p > </p>

<p >Parallax Error would be exacerbated in <em >tighter shots</em> and or using <em >wider apertures</em>.</p>

<p > </p>

<p >*** </p>

<p > </p>

<p ><strong ><em >"Can you recommend an alternative focusing technique? The zoom in and focus thing is something I've always done but I'd appreciate knowing any other method that might work better."</em></strong></p>

<p > </p>

<p >(Ditto Neil again.) </p>

<p > </p>

<p >The shot must be first framed with the zoom, and then Focus must be achieved – by whatever method considered the most appropriate. </p>

<p > </p>

<p >It is not so much a method of “working better”, it is simply the correct way of using the tool, and how the tool was designed and made, to be used.</p>

<p > </p>

<p >WW</p>

<p > </p>

<p > </p>

 

[William Michael]

<p>I should have been clearer with my answer. <br>

<br>

<em><strong>There might also be other issues. </strong></em><br>

<br>

I was outlining the correct method of using the tool and mentioning Parallax apropos Focus and Recompose – <em>as these two issues were obvious candidates for causation, from the information supplied.</em><br>

<br />Precise answers to Nadine's list of questions will serve to systematically analyze the situation and confirm or deny other causes of the problem.<br>

<br>

WW<br>

<br>

</p>

<p > </p>

 

[markonestudios]

Whereas you have described that you focus -> zoom -> recompose, the correct way to do it is zoom -> focus -> recompose :]

<p>Since you say that you tested an identical lens with identical settings and got decent results, I believe this is user error rather than lens malfunction...

[johan_ingles_le_nobel]

<p>Have you considered purchasing a split-screen focusing screen for your camera? I purchased one when I moved to a dslr and it makes a fair old difference with my focusing</p>

[prinosphotography]

<p>My 24-70L needed to be calibrated as it did not focus well at the wider focal lenghts. After calibration it works well, but it sometimes to be a bit off. <br /> I would be careful about focus & recompose with any fast wide angle, it's easy to alter the plane of focus and introduce error that way (e.g if you center focus on the head, then pan down to recompose, you've just tilted the focus plane away such that the subject's head is in front of the focus point). With a 24/1.4 this is even more evident than with the 24-70.<br /> Others in this thread have commented on the focus shift during zoom. My 17-40/4, 24-70/2.8 and 70-200/2.8 hold focus through the zoom range in static tests that I've done, though the end results are better when you focus at the long end and zoom out vs. the other way around. I think these constant aperture L lenses are effectively parfocal. Non of the variable aperture canon zooms I've tried are even close to parfocal. <br /> Here was Canon's Chuck Westfall on the topic a while back (I think this was a discussion on the 24-70 and 24-105L):</p>

 

<blockquote>

<p>There is no official answer, because Canon Inc. won't comment on it. Based on actual product testing, though, both of these lenses are parfocal as long as you start out by focusing sharply at the maximum focal length and then zoom back to a shorter focal length <strong>without refocusing</strong>. Look at the files on a computer monitor at 100% magnification to verify. <br /><br />If you autofocus after zooming out, the camera may drive the lens to a slightly different distance. This is possible because depth of field is greater at the shorter focal lengths of the zoom lens, assuming a fixed subject distance. But if the image remains sharp after focusing accurately at maximum focal length and then zooming to minimum focal length without refocusing, the lens is parfocal.<br /><br />Best Regards,<br /><br />Chuck Westfall<br />Director/Media & Customer Relationship<br />Camera Marketing Group/Canon U.S.A., Inc.</p>

</blockquote>

<p>Chris</p>

[William Michael]

<p>I have read the first paragraph (or someting very similar by Chuck), before. I note that Chuck is <em>Director/Media & Customer Relationship.</em><em></em><br /><br />As this technical (maths) matter and it bears on the topic, then allow a slight indulgence of precise argument.<br /><br /><strong><em>"My 17-40/4, 24-70/2.8 and 70-200/2.8 hold focus through the zoom range in static tests that I've done, though the end results are better when you focus at the long end and zoom out vs. the other way around."</em></strong> <br /><br />One cannot have it both ways the lens either holds focus or it does not. <br /><br />One confirmed solid NO answer in the test results: is a NO answer for the test. Therefore the lenses do not hold focus and should not be expected so to do.<br /><br />*** <br /><strong><em></em></strong><br /><strong><em>"I think these constant aperture L lenses are effectively parfocal."</em></strong><br /><br />Yes I understand the point being made - what it means is "if I use the lens this way, I can be alomst 100% sure to expect this result" which is what Chuck was saying also, basically.<br /><br />However I am not convinced that the lenses (when zoomed from full telephoto to full wide angle) actually do hold the same Plane of Sharp Focus.<br /><br />Consider that (on a 5D) at 105mm an Half Shot of a Person at 15 ft at F/4 has about 12 inches DoF. <br /><br />At 100% magnification it is reasonably easy to convince me we could find the Plane of Sharp focus – within perhaps ½ inch <br /><br />Zoom the lens to 24mm and the DoF is 63ft – find the Plane of Sharp focus in that bush even at 1000% magnification. So as a test “looking at 100% magnification to verify” is a nonsense, IMO. <br /><br />Even at 50mm the DoF is about 5ft or 6ft roughly – and I reckon at 100% we could establish the Plane of Sharp Focus within a few inches – but not within ½ inch.<br /><br />Notwithstanding this DoF issue, Auto Focus is NOT an exact machine, by any means - it has tolerances.<br /><br />That’s why, in practical terms the use of the zoom from Telephoto to W/A can be leveraged for use in the field “to hold focus”, if you will. I have no argument with that. <br /><br />But don’t confuse a practical application and extrapolate it as scientific test results and therefore as a fact, is my point, and this is my argument with Chuck’s answer – and also the personal tests which have been cited.<br /><br />***<br /><br />I have done similar tests with the 16-35L and the 70 to 200F/2.8L; and both EF-S kit lenses, and for practical purposes for those four lenses, I concur with the general statement, with my additions:<br /><br /><strong><em>“I think these [two] constant aperture L lenses [operate effectively as "parfocal" for practical purposes, when zoomed telephoto to wide]. </em></strong><br>

<strong><em>None of the variable aperture canon zooms I've tried are even close [to the same results].”</em></strong><strong><em> </em></strong><br /><br />I had a 35 to 350L for a day or two, and although I did not stringently test it – it seemed <strong><em>not</em></strong> OK in this regard to holding Focus. Even as I recall it seemed sloppy during incremental FL comparisons such as 100 to 200; 200 to 300 and then the reverse, etc.<br /><br />I only have had a quick play with the 100 to 400L – not enough time to establish anything of worth, to make a comment re its ability or not to hold focus for practical "in the field" purposes.<br /><br />***<br /><br />I reiterate: <strong><em>There might also be other issues, other than zooming after obtaining Focus. </em></strong><br />Precise answers to other questions asked will serve to systematically analyze this aprticular situation and confirm or deny other causes of the problems the OP has with the 24 70 in question.<br />WW</p>

[prinosphotography]

<blockquote>

<p>One cannot have it both ways the lens either holds focus or it does not. </p>

</blockquote>

<p>I don't think I said they don't hold focus, just that the technique of zooming after focus works better from going tele to wide. With the lenses I have, going wide to tele does indeed hold focus, but the focus is not as accurate to start with at the wide end (or there is just a bigger tolerance for error), so if it starts slightly front- or back-focused when wide, it's still back or front focused when zoomed in. Being parfocal and having correct focus are different. My 24-70 is less accurate at the wide end I would say (though much better after being calibrated). My 17-40 seems fine (but it's depth of field is much greater and it's only f4), and my 70-200 is spot on throughout it's range, forward or backward. </p>

<p>Whether they are designed to be parfocal or not, I'm not sure (Canon doesn't mention this in there specs that I can tell), but the ones I have are <em>effectively</em> parfocal. Let's say that we had a zoom that was spec'd and known to be parfocal. I still would not expect better results by focusing wide first, then zooming.</p>

<p>My primary comment to the OP was simply that based on my own experience and that of others I have heard over the years, the 24-70 can sometimes benefit from recalibration of focus. I also think that can be exercerbated by the fact that it's focussing accuracy is probably not as good at it's wide end compared to it's tele end.</p>