Auto-Focus Problems with Canon 40D - 17-55 2.8 Setup

<p>Hello! I have shot two weddings now with my Canon 40D and 17-55 2.8 (ultrasonic) lens setup. As the night goes on, and the available light goes away, my camera really struggles to find focus. I manually move my focus points and use the "AF-ON button" to achieve auto-focus (I have disabled the half-press shutter focus). This problem is most exaggerated in dark ballrooms during the reception and the last shot of the couple in the limo. I have to take 7-10 frames, in hopes that 1 or 2 will be in focus. Is this a limitation of my gear, or do I need to have it serviced? I plan to move up to the Canon 7D ASAP and I hear that the auto-focus is excellent on that body but I have to find an interim solution for my 40D! Please help!</p>

<p>Are you using a Canon Flash and the Focus Assist Beam?<br>

Is this problem happening mostly at FL's, at the wide angle?<br>

Is this problem happening more at all AF points OTHER than the middle AF point?<br>

Is this problem happening more often when the background has Bright Lights? (noted Ballrooms and Limo comment)</p>

<p> . . . my first thought is the problem is not knowing in detail how the AF works; how best to use AF, in really low light . . . answering the questions above will assist.</p>

<p>WW</p>

 

<p>I am using the Canon 580EXII flash and have not changed the factory settings of the flash for ETTL although I am using a PocketWizard FlexTT5 on my hotshoe, and the 580EXII on top of that (I have another strobe off camera for receptions). <br>

It is happening at all focal lengths. <br>

I rarely use the middle AF point so I'm not sure, but it happens equally between the other AF points that I use. <br>

I wouldn't say that I've noticed it more when the background has bright lights. The ballrooms and limo's that I have been shooting in have not had bright lights in the background, they are just really dark. <br>

My understanding of the Canon AF is that is uses areas of contrast to find focus. That being the case, when the camera doesn't have enough light to distinguish areas of contrast, the camera fishes for focus. Is that correct? I'm all ears for any advice on how to best use the AF system!</p>

 

<p>1. Any camera is going to struggle with focus in dim light. The 7D will, as well. In these situations, use the flash's focus assist. If you don't use flash, you can set it up so that you just get the focus assist but no flash.</p>

<p>2. If your flash is now sitting higher than if it were on the hotshoe, your focus assist pattern may be missing the subject. Go into a dark room and activate all the different focus points to see what kind of pattern they make. They are different. If you do this you can easily see how the pattern might be missing.</p>

<p>3. Be sure you are using One shot--AI Servo does not activate the focus assist.</p>

<p>4. Research autofocusing with crop sensor cameras, particularly Canons.</p>

<p>Thanks for the feedback: Refining the advice . . .</p>

<blockquote>

<p><em><strong>"I rarely use the middle AF point so I'm not sure, but it happens equally between the other AF points that I use."</strong></em></p>

</blockquote>

<p>Then I would (I do) only use the centre AF point in really dark situations. (Well, actaully 90% of the time I use the middle point it is a technique thing - i.e employing as few "techniques" as possible for maximum output).</p>

<p>Note Point #2 Nadine makes - using AF Assist for camera mounted Flash units - you need to know where it points and at what distances.</p>

<p>Yes the Canon system uses CONTRAST differences - but the centre AF point is the most sensitive.<br />With practice Focus and Recompose can be an asset. I find that the line of the man's (white) shirt against the Black Jacket or the line of the White Gown against the Bride's Bust-line is mostly always useful.</p>

<p>Manual Focus - over focusing and pulling back to RE-confirmation with the Red Square is a technique I use also.</p>

<p>I use Prime lenses much more than zooms - though F/2.8 is pretty fast - F/1.4 is brighter.</p>

<p>There are lots of past threads on this topic also.</p>

<p>But in the main my modified advice is that you are using the outer AF points and you should use the middle one, only.</p>

<p>WW</p>

<p>"If your flash is now sitting higher than if it were on the hotshoe, your focus assist pattern may be missing the subject." The FlexTT5 is very low profile and only raises the flash 1.4" but I will test the patterns in a dark room like you recommend. <br>

"Be sure you are using One shot--AI Servo does not activate the focus assist." I did not know that! Ever since I started using the "AF-ON" button for focus instead of the shutter half-press, I have also been using AI Focus (which I assume in this case also disables the focus assist?) This may be the whole problem right here!<br>

"Yes the Canon system uses CONTRAST differences - but the centre AF point is the most sensitive." I think I have heard that before, but didn't remember it in the field! I'll have to try that next time I'm in a bind.<br>

"With practice Focus and Recompose can be an asset," I have had trouble with this in the past, especially shooting at 2.8 or 3.2 because the DOF is so thin that the recomposition throws off the focus, which is why I started manually moving my focus points. But, in a bind, I'd rather have the shot in focus and the subject in the center than to have a well composed out of focus shot! lol.</p>

 

<blockquote>

<p><em>"With practice Focus and Recompose can be an asset," I have had trouble with this in the past, especially shooting at 2.8 or 3.2 because the DOF is so thin that the recomposition throws off the focus, which is why I started manually moving my focus points.”</em><br>

<em> </em></p>

</blockquote>

<p>It depends.<br>

One can F&R accurately @ F/2, if one knows the rules and the limits:<br>

For example – you are shooting a tight half shot of a Bride with a 50mm lens on a 5D – at about 8ft Shooting Distance.<br>

If you nail focus on her bust line (as I mentioned) and then tilt the camera upwards to recompose, in most cases (most female body shapes), there will be very little parallax error and you will nail the eyes in focus for all front on, to ¼ Profile shots.<br>

The trick is to swivel the camera upwards, without moving the camera away from, or toward the subject.<br>

At f/2 for a tight half shot using a 5D, you have about 9” or 10” DoF.<br>

***<br>

Even tighter:<br>

This is an example of F&R @ F1/8, using centre AF on the bridge of her nose (sharp contrast line from widow light).<br>

Knowing it would be OK, because the Recompose was only a small movement across the frame: the key element – keeping MY head locked in place – not swaying it backwards or forwards before I released the shutter: <a href="../photo/10963088&size=lg">http://www.photo.net/photo/10963088&size=lg</a><br>

***<br>

On the other hand – if the only place you can nail focus is on the shine from the shoes against the polished wooden floor (very useful sometimes) then for the same Half Shot I mentioned above, you need to pull an aperture of about F/8 to be safe, if you are going to F&R from a standing position. . . OR you need to get to ground level to nail focus on the shoe and then stand upright – and f/4 will suffice, with practice.<br>

***<br>

Also, I question as a general practice, the need to use large apertures in dark lighting scenarios <strong><em>when one is using Flash:</em></strong> correct Flash exposure usually delivers adequately the emphasis on the subject – narrow DoF is often not necessary, as an addition.<br>

***</p>

<blockquote>

<p><em>"But, in a bind, I'd rather have the shot in focus and the subject in the center than to have a well composed out of focus shot"</em></p>

</blockquote>

<p>Obviously, wide apertures and more necessary when shooting Sans Flash – in these instances, <strong><em>if it is really dark</em></strong> and you are at the absolute limit - then IMO, the best ploy is to centre the subject, as you mentioned, BUT <em><strong>shoot wide and crop later to the framing required.</strong></em><br>

This set is all F&R to some extent – <strong><em>but all are shot a little wide </em></strong>(to restrict the amount of Recompose) and then cropped later.<br>

It was always the intention to shoot for a square format final product, though: <a href="../photodb/folder?folder_id=964622">http://www.photo.net/photodb/folder?folder_id=964622</a></p>

<p>WW </p>

<p>All the advice given is great. To add to what has been said, my wife is a "zoom" shooter and @ dark venues even she will switch to fast(er) primes to help the AF achieve a faster more accurate focus. Also, and especially in dim rooms, a UV filter can throw off the AF.</p>

<p>Be sure to test the focus assist patterns for both horizontal and vertical at various distances. In dim/dark light levels, this is the only 'quick' way to autofocus. Otherwise, you have to give the autofocus some time.</p>

<p>Do not use AI Focus at any time. This may be your issue right here. AI Focus will interpret any movement of the camera as subject movement and focus on whatever contrasty thing is in the background. It will also tend to get snagged on moving objects in the background.</p>

<p>Did you do your research on Canon autofocus? Looking at past threads in which William W. has written can be useful...</p>

<p>Here's a tip for you. Use on of the Custom Settings (C1, C2, C3) for your no flash/high ISO setting, so you can switch instantly to high ISO and not have the flash fire. The focus assist will still work though and will help you focus. If you set the mode (in the Custom setting) to something like TV or AV, you can easily change a factor--shutter speed, aperture or ISO--also pretty quickly).</p>

<p>@Nadine - "AI Focus will interpret any movement of the camera as subject movement and focus on whatever contrasty thing is in the background" I am not finding this to be the case. The lens "searches" in and then out and comes to rest without achieving focus on anything in the frame. <br>

Are you saying that you shoot in One Shot mode all of the time? Even with moving subjects (dancing, walking closer to or farther away from you, kids on the move, etc)? If so, how do you ensure that your subject has not moved outside of the focal plane that was achieved before pressing the shutter when shooting at or around 2.8?<br>

I have read about the Canon autofocus system before but I need to spend more time researching and reading William W's posts. <br>

Great tip on the Custom Settings!</p>

<p>Tom,</p>

<p>While the 7D will indeed do better, in a dark room, they are all going to struggle. I found that when it get truly dark, I resort to manual focus. I prefer not to use a flash as I find it ruins the look I'm after. Try using manual, or following some of the good advice above for keeping with the AF.</p>

<p>Tom--that behavior is when the camera just plain has trouble focusing on anything--because the light is too dim, etc. When I first got my 20D, I experimented with AI Focus for weddings, and found it to be very unsuitable. For instance, when shooting fast dancing, the focus went straight for moving subjects in the background.</p>

<p>Didn't you post a thread asking about the 24-70mm autofocus? All of the pitfalls of Canon autofocus are in that thread, perhaps some through links. Maybe re-read that thread?</p>

<p>Yes, I use One Shot all the time, mostly the center focus point, focus/recompose all the time. The only time I use AI Servo is if I have a moving subject coming toward or away from me, in good light. I never use auto focus points.</p>

<p>If using One Shot, with or without focus assist, and the subject is moving, I will employ different techniques. For fast dancing, I will set an appropriate DOF through aperture/distance/f stop, focus on something approximately the same distance away as the subject, and wait for the action I want--press the shutter.</p>

<p>For moving subjects in processionals, etc., I use a technique that Marc Williams described, where you focus and press the shutter button as one movement. It helps if you focus on the bouquet or closer subject and if you have focus assist. An old technique is to focus on a pew, and when the subjects get to that pew, you press the shutter button. Again--you need to rely on some DOF.</p>

<p>Many beginners mistakenly believe that it is good to just shoot everything at f2.8 (on a zoom) and f1.8 (or widest, on a prime) for maximum blurry background. This is a misconception. You can get nice blurry backgrounds on teles using f5.6 or f8, even. And while narrow DOF works well for single subjects, when you have multiple subjects, ask yourself whether having just one subject in focus is always best? Narrow DOF has it's place, but in many images, having a bit more DOF can a) save you from blurry images and b) allow more than one subject to be in focus.</p>

<p>I just saw a processional image taken using f1.4. It was in nice focus, taken by a highly experienced wedding photographer who was using f1.4 because he was not using any flash. The background (guests in pews) and foreground were all nicely OOF. By the way, I would not try this with a 40D (or maybe even 7D)--higher end cameras with superb autofocus plus practice--would be needed here. Back to the image--the fact that the background and foreground were so nicely OOF probably will not impress anyone other than wedding photographers. In fact, clients typically (at least, my clients) like to see the expressions on the guests around the subjects, and don't appreciate all that bokeh at all.</p>

<p>Study DOF and how it applies to what you shoot.</p>

<blockquote>

<p><strong><em>"Many beginners mistakenly believe that it is good to just shoot everything at f2.8 (on a zoom) and f1.8 (or widest, on a prime) for maximum blurry background. This is a misconception. You can get nice blurry backgrounds on teles using f5.6 or f8, even."</em></strong></p>

</blockquote>

<p>Shallow DoF at F/8: <a href="../photo/11199251&size=lg">http://www.photo.net/photo/11199251&size=lg</a><br>

The set (included in my portfolio to address exposure when shooting whites in sunlight) is here: <a href="../photodb/folder?folder_id=972502">http://www.photo.net/photodb/folder?folder_id=972502</a><br>

Even the First Shot of the three, which is fairly wide by comparison with the other two, displays adequate Shallow DoF, because of: <strong><em>distance of the background relative to the Subject.</em></strong><br>

<em> </em><br>

Shallow DoF at F/5: <a href="../photo/9272173&size=lg">http://www.photo.net/photo/9272173&size=lg</a> and <a href="../photo/9289383&size=lg">http://www.photo.net/photo/9289383&size=lg</a> <br>

<br>

Shallow DoF at F/3.5: <a href="../photo/9193714&size=lg">http://www.photo.net/photo/9193714&size=lg</a><br>

<strong><em>F/3.5 was chosen, because it was very dark and I was unsure of F&R</em></strong> . . . I was at ISO3200 HH with an 85mm lens at 1/125s, even so, it is NOT the sharpest photo I have ever taken but better to use F/3.5 than choose F/2.4 in the desire to “save grain” and shoot at ISO1600 – or worst choose F/1.8 in the desire to use ISO800 and risk a complete mess of OoF nothingness.</p>

<p>As in the previous sample in the post above, I do shoot my 85mm lens at F/1.8, and other lenses at F/1.4, but usually only when I am absolutely certain.</p>

<p>WW</p>

<p>We use exclusively the same combination -- always lock focus with the rear button and the recompose -- but a strobe of some power > is fired into the image somewhere. Can not see why to ever turn your flash off in almost any situation >> & never use a ISO above 400.</p>

<p>The flash sends out a focus assist -- in One Shot mode .... Even if you just place the flash into a bounce card or wall/ceiling ~~ it will still help you focus in low light . The 2.8 aperture works fine > just prefocus -lock - recompose - I try to keep the aperture at 3.5 min</p>