Frustrating issues with auto-focus. Need advice.

<p>Hi there!</p>

<p>So I have a D7000 paired with a sigma 17-50 2.8. Im on a trip in Canada and the only lens I brought is that sigma, as my main all-rounder.<br /> I took these photos aiming always at her face. AF-S always. Handheld. Very calmly, not rushed pics. Aperture mode, f2.8.<br>

<br /><img src="http://i40.tinypic.com/f9lff5.jpg" alt="" /><br />http://i41.tinypic.com/k1cnpl.jpg<br>

<img src="http://i41.tinypic.com/k1cnpl.jpg" alt="" /><br>

I tried taking both pictures with the focus square moved to the side she was, as well as having the focusing square at the middle, focused her face, half press, recomposed, press all the way.<br /> I have to say that I have tested the lens with the coin on the ground thing, looking for bad focusing issues and at the time, all went good. Im not sure I did the test in the right way though.<br /> Also, with my other lenses, like the stock 18-105, I didnt remember having something similar.</p>

<p>What is the deal? Camera defective? Lens defective? Do I need to tweak any settings? Is my technique bad?</p>

<p>Try shooting at f8 and report back. At f2.8 your depth of field is remarkable shallow. Also are you in single or continuous focus? Single is what you want.</p>

<p>What Nick said...</p>

<p>I see you were in AF-S. My apologies.</p>

<p>Yes, im on AF-S. And I wanted the DOF to be shallow, actually!</p>

<p>I wanted the background to blur. What Im pissed is that the camera chose to focus the background instead of her, even when I pointed the focus square at her face.<br>

As you can see, the background is pretty crisp.</p>

<p>Ah, one thing I forgot to mention is that I could only solve the matter when I zoomed in a bit. Like 35mm instead of 17mm. Which is not really a "solution". ;/</p>

<p>On one of my cameras, the half press does not work and the camera focusses again after I reframe. I have moved the "focus" command to another button, and I think this is possible on all or most cameras.</p>

<p>Mukul - that shouldn't happen if you're in AF-S mode; it would happen if AF-C, or in an auto-detecting mode.<br />

<br />

Allan: There can be issues with AF fine tuning being different between focal distances and zoom lengths, though it's surprising that it's far enough out that we can see it in an image this size! Do you know whether this lens always behaved that way, or is it recent?<br />

<br />

As a slightly better workaround, given that you're taking your time over the shot anyway, could you make do with live view focus until you get the chance to get everything checked out?</p>

<p>Hey Andrew, that lens is actually pretty new. 4 months since I bought it but this is the first time im really taking it in for some action.</p>

<p>What is the difference between liveview focus and viewfinder foucus?</p>

While it possible that a mechanical problem caused this, perhaps on a few shots you inadvertently lifted your finger off the shutter and then when you recomposed, the camera focused on the background. Also note that the actual AF sensor is usually somewhat larger than what is displayed in the viewfinder, so it is possible that even if you filled the AF sensor with your subject's face, the background had more contrast than your subject and the sensor focused on that instead of your subject. Note that at 17mm / f2.8, it is very difficult to blur the background, unless you are very close to your subject. For those images, I would have probably used F5.6 or f8

<blockquote>What is the difference between liveview focus and viewfinder foucus?</blockquote>

 

<p>Focus using the viewfinder relies on autofocus sensors in the base of the camera. They use "phase detect", which means they are fast and know how to move the lens quickly (in part because they know how out of focus it is). However, to get to them, the light leaves the back of the lens at an angle which depends somewhat on the lens design, passes through the main mirror (which is half-silvered), bounces off a secondary mirror, and gets deflected down to the sensor. When taking a photo, the whole mirror arrangement moves out of the way, and you're relying on the lens position that achieved the best focus according to these sensors being the right position for perfect focus on the sensor. Even so, it usually works pretty well - it's the only mechanism available on film DSLRs, obviously.<br />

<br />

Live view keeps the mirror out of the way, and uses the same sensor you use to make the photograph to capture the intermediate image. This intermediate image is used for contrast-detect autofocus - the lens is moved until there's a maximum of local contrast detected by the sensor. Because the light path is the same as the final photo, it's guaranteed to be accurate (unless you point at something which confuses the sensor). Because it doesn't instantly know where to move the lens to compared with its current position, it's somewhat slower than phase detect - all it can do is keep checking the sensor to see whether the image is getting sharper or softer. Compact cameras use this mechanism as well; phase detect is why DSLRs are good at sports and compact cameras aren't.<br />

<br />

If you're deliberately taking your time about the shots, and if you don't trust the other autofocus behaviour, I'd try live view - I don't think you'll lose anything (other than that you're looking at the LCD rather than the finder) and it might help. The camera can be tuned to make its phase detect autofocus work better with the lens, but if you're seeing very different behaviour at different distances, you might need to send the lens for a service. (Newer Sigmas can actually be tuned electronically to compensate for variation like this, but I don't think your one is one of the designs that allows it, at least for the user. They may be able to do it at the factory.) If the lens <i>was</i> okay and has started playing up, something inside may have shifted; if it's always been like that, warranty may cover calibrating it. In any case, I wouldn't be trying to analyze this in too much detail while you're away from home, so I hope the live view is a work-around. Good luck, and have a good trip!</p>

<p>When I use AF, I try to make sure the camera doesn't see some line or object that is easier to focus on than my subject that is very near the subject. <br>

For example: In the first shot, the line of the water and the distant trees is a nice high contrast line that could grab the AF attention. It may also be within the focus point , even if you put in on her face. </p>

<p>Example 2: You have two sign posts, the horizontal black lines on the bus, right at head level, and the nice sharp lines of the bench. Compared to those things, a human is a soft blob. If you're not careful the camera will defect those and tell you " I'm in focus now ! " </p>

<p> </p>

<p>To clarify, Allan - are you finding it <i>always</i> (or at least usually) does this, at longer focal lengths, or were these isolated incidents? All AF systems occasionally get confused (and thumbs occasionally slip). I should also check that you weren't accidentally nudging the focus ring and engaging manual override (back focus would correspond to shoving it towards infinity). But it doesn't sound like that's the case to me, if you were taking your time. I agree that in the second image the camera might manage to pick up the background, but for the first image I'd really be surprised if the AF system was so lax - especially on a D7000 - that, pointing at a sunlit face containing sunglasses, it was trying to look at anything else. Never rule out operator error, but if Allan is convinced he was being careful, it <i>is</i> possible that it's the lens.</p>

<p>Andrew, my three cameras are on S-AF, and in one of them the half press does not lock focus. Switches do go bad, after all.</p>

<p>My understanding is that the controlling mechanism for auto focus is in the camera body, not in the lens. The lens has no sensors but only obeys orders. The obvious thing to do here is to test the body with another lens.</p>

<p>Im not sure Andrew, really.</p>

<p>But from what everyone mentioned, it might be really a mix of bad technique and lack of understanding my camera. If the autofocus range is really larger than what the viewfinder shoes me, and it was the size of her face when I took the shot, it might really have focused the background. </p>

<p>Im gonna try to focus on the chest next time. Or maybe zoom in, lock focus and zoom out!</p>

<p>I retract my DoF comments. With a 17mm at f2.8 and a subject at about 10 feet, you have about 18 feet of DoF, adequate to cover your subject. The comments about sensor error are worth testing.<br>

Also, I believe you can fine tune the Auto Focusing of your lenses with the D7000 so it is more accurate. (I shoot a D2X, so that is all just a dream to me!). <br>

I have always set up my cameras so that the focusing is done by the AF-On switch on the back of the camera body, and exposure is controlled by the shutter release button. This way it is so very easy to lock focus---assuming it is set accurately. </p>

<p>Mukul - you're quite right. Forgive me, I'm a software engineer, and I'm slow to blame the hardware. :-)<br />

<br />

The controlling mechanism is in the body, but different lenses have different behaviour when it comes to light exiting the rear. This is why many recent cameras (including Allan's D7000) have AF fine tuning in the menu. It's also not uncommon for the zoom and focus distance to affect things. Canon have added the ability to use different AF fine tune settings at different zooms; Sigma's recent programmable dock mechanism for their lenses allows both focus distance and zoom length to be adjusted (and that <i>is</i> in the lens - there is some electronic feedback between the two, including, I believe, some lenses adjusting for focus shift on stopping down). I don't know the protocol enough to know exactly what can be controlled. As another sample point, my 80-200 AF-D is beyond the AF fine-tune adjustment ability of my camera at short focus distances, but fine at long range; supposedly this is a known artifact of the varying telecentricity of the light leaving that lens design. There's some manufacturing tolerance in both lens and camera - it may be Allan can tune the problem away, but if not, I'd hope that either Sigma or a Nikon service centre could help. Testing the lens on another body or the body with another lens would potentially help tell whether there was something seriously wrong with either.<br />

<br />

Allan: The AF area <i>is</i> larger than the viewfinder square, but it's not huge (on a D7000). It <i>may</i> be the problem, but for now I'd zoom in on the images on the camera after I took them and make sure they're okay - don't trust them for now! Live view is independent of the lens and camera being out of whack - unless the lens is actually soft, that's your back-up option. In case my paranoia is correct, be careful you're not nudging the lens's focus ring. I <i>wouldn't</i> zoom in, lock, zoom out - I can't speak for your lens, but on a lot of lenses this causes a small shift in focus position. And... good luck explaining to the young lady where you're pointing the lens. :-) (Honestly, I'd expect it to cope with the face. Newer Nikons actually seek out faces to focus on.)</p>

<p>There is a command when viewing the picture on the camera LCD that will show the focus point, I think also available with Capture NX. I had a similar problem and found I was not focused on what I thought I was. If you haven't deleted them from the card, take a look.</p>

<p>Don't know if it was mentioned already but did you set your focus metering to spot?</p>

<p>I rarely have focus problems as you're having with my 18-55mm Pentax kit lens & 6 year old K100D DSLR by setting my focus metering to spot and assigning AF to my "OK" button on the back.</p>

<p>Various camera models and brands exhibit different focusing behaviors. My Pentax's focusing is VERY precise but SLOW to get there evident by my pressing the "OK" button and hearing the initial quick zip sound as it locks in focus. Scene in viewfinder goes crispy.</p>

<p>I press it again without moving the camera off the intended subject and I can hear tiny scratch sounds that are micro tweaks as the camera meters again on top of the more sharper view. Scene in viewfinder gets even more crispier. </p>

<p>Other camera brands favor focus speed over that kind of precision with some having thicker anti-aliasing filters built on top of the sensor that may hinder a certain level of precision focusing. My Pentax has a much thinner AA filter and it shows shooting Raw. Sometimes it's SO sharp I get moire patterns.</p>

<p>To give you an example I just got my first HDtv and I'm just in love with the picture after I eyeball calibrated it that I'm taking photos of it 7 feet away at 55mm, 1/30th, f/5.6, ISO 800 handheld. Problem is if I don't spot focus on the subject (i.e. HSN sales people) like on the edge of their eyes or hairline, the second hit to the "OK" button induces the micro tweaks and causes the meter to focus on the 740p pixel grid MY EYES CAN'T SEE!</p>

<p>You might want to become VERY familiar with the behavior of your camera model's focusing and level of precision in comparison to speed. </p><div></div>

<blockquote>

<p>There is a command when viewing the picture on the camera LCD that will show the focus point, I think also available with Capture NX. I had a similar problem and found I was not focused on what I thought I was. If you haven't deleted them from the card, take a look.</p>

</blockquote>

<p>The problem with using focus and recompose is that NX2 does not show where the <strong>original focus point</strong> was focused--only where the focus point indicator ended up in the final composition.</p>

Depth of field isn't he issue. The camera either focuses on the intended object or it doesn't. Or as some have mentioned,

your finger might have lost contact long enough to let the previously acquired focus be rest.

 

Are you certain that both the lens and the camera body were set to auto focus?

 

My D800 has at least ten AF parameters. It's a complicated mechanism. In addition to verifying that you were in AF-S

mode as you have, you might want to check the AF-S priority selection setting. I would recommend that it be set to Focus

rather than Release for this situation. Also, verify whether your AF points are being selected automatically or whether you

can select single points.

 

Check these settings and keep practicing. If your model is standing still, you can also try autofocus in live view mode. It's

slow but it works reliably.

<p>The software that ships with Canon cameras can display the focus point used for a shot. This can be very helpful when trying to diagnose these sorts of issues. Does any of the software that ships with Nikon cameras do the same?</p>

<p>Yes, Dan Nikon Capture NX2 and View NX2 both show the focus point, but as I noted above it is not accurate with focus and recompose situations.</p>

<p>What Dan said is very true: these higher-end AF systems are relatively complicated. AF-S is a nice point of departure, but then still: how is the camera set up to behave in AF-S mode? There are many options. Lenses as the 18-105VR may have covered the problems more thanks to more DoF (slower aperture); at these wider apertures, it becomes more critical to understand what does what.</p>

<p>Did you use "focus and recompose", that is use the central focus point to focus, and then reframed the image to 'move' the main subject to the right? Focus and recompose may not work with shallow depth-of-field... as the field of focus is absolutely flat, but the movement you make is actually circular, so you are moving everything out of focus. If not in a rush, it is simply better to select one of the outer AF points manually, and focus like that.</p>

<p>So, I would really start by first performing a series of shots where you focus, do not move the camera with this lens and see if it is focussed properly or not - start to exclude the likelihood of a hardware problem. To be blunt, operator error is not unlikely as the D7000 AF module is deceivingly complex.</p>

<p>"Focus and recompose" when you've got umpteen suitable peripheral AF points to put over the subject, makes no sense to me at all...</p>

<p>I have two suggestions ... get a camera which has a small target area, I am glad I shoot with a Panasonic G and GH which has this option because it makes AF a consistant viable option.<br>

Second ... since you don't then there is the technque I used back in film days and this was to focus on something at the same distance as the subject.<br>

I wonder if when each time you focused the woman snapped into focus... if it did then obviously you have a problem with the gear or how you are using it [ how it is set up ... mode etc] I use AF all the time and the point of my focus always snaps into focus and it is immediately obvious that AF has done its job.</p>