Questions on Focus

[trevor_newman1]

<p>I recently completed my first "photo shoot" of a friend. I used my d90 and 50mm 1.8. Wanting to keep the eyes in focus but still blur the background I shot at f/3.5 from a distance of around 6-10 feet on a sunny day using only natural light. Since the AF sensors don't actually fall on the eye when I compose my shot I would use the outer sensor closest to the eyes, lock exposure and focus and recompose ever so slightly (generally pointing the camera a degree or so down) and take the shot. Later when I reviewed the images I noticed the area under the focus point indicator was rarely ever in sharp focus and that the eyes were not sharp either. Granted at a moderate size...(5x7) the shots look tack sharp but once I blow them up 50% and 100% it becomes clear I missed the focus. Some of the shots (generally the front lit ones) came out perfect.<br>

The other day i went around walking with my camera and did some tests and many of my shots came out looking soft or the part I focused on was not in focus with some space in front of or behind it in focus. This really concerned me so I set my camera up on a tripod and shot a couple of images at f/2.8 with an extremely narrow depth of field and the self timer and they came out perfect with the area under the AF sensor perfectly in focus. I then took the camera off the tripod and took the same shot to find it slighly blurry and out of focus. Now I realize I do move a little while taking pictures but I don't feel like I move through the whole foot/half foot depth of field necessary to put my main subject out of the circle of confusion especially at 1/80th and above...but perhaps I do? I never experienced this when I shot film but I realized last night that I never blew up larger than 8x10 and you can't see the focus indicators on a negative. Perhaps I have had this problem all along and never realized it until I had the capability to blow up to 100% and actually test it?<br>

It could also just be my technique of course. Normally I wouldn't care about images looking perfectly focused at 100% but I would really like to get better at portraits as my reputation for a good eye and good lighting have been garnering me requests for portraits. Any tips, advice, or clarification would be greatly appreicated. I am self taught so I am always looking for hints and tips that might help. Thank you all so much.</p>

[blumesan]

<p>If the images are sharp when using a tripod and become blurred when hand held, it sounds like your hands are not holding the camera steadily enough. It's not the back and forth movement that will put the subject out of the focal depth; it's the side to side (up/down) movement. The usual cure is to increase the shutter speed.</p>

[tobey_bilek]

<p>Digi cams are quite difficult to focus.<br>

<a href="http://diglloyd.com/diglloyd/free/index.html#Technical">http://diglloyd.com/diglloyd/free/index.html#Technical</a></p>

<p>Basically they are set up for 5.6. Same with confirmation light.<br>

Manual focus if you can using the provided screen or a Katz Eye. Split image helps some people, but for technical reasons they are set up for 4/ 5.6 lenses and lose accuracy of 2.0 1.4.</p>

<p>Some use live view . </p>

 

[Dieter Schaefer]

<p>Since the images taken with a tripod came out tack sharp, front- or backfocusing of the lens/camera combo can be excluded - so I guess, it is you moving too much. However, I assume your test shots were of static subjects whereas I consider portraits "moving" subjects (if ever so slightly, but with a narrow DOF there isn't much room for error). Even if the eye is within the DOF - it only looks tack sharp - especially at 100% - when it is in the plane of focus. Another problem though could be that there isn't something for the outer AF area to focus on - I would try the same portrait shots again using the central sensor (the only cross-type on the D90) for focusing. Though locking focus and then recomposing always carries the danger that things change and focus isn't were it was assumed to be. Especially if you tilt the camera, the plane of focus tilts with it, and what has been in focus before now no longer is. Naturally, you still need to eliminate camera shake as the cause for the blurry images.</p>

[Matt Laur]

<p>I just happened to have this cheesy graphic handy from a different focus-related thread. But I'll pollute this thread with it, too:<br /><img src="http://static.photo.net/attachments/bboard/00T/00TU0p-138337684.jpg" alt="" width="574" height="600" /><br /><br />If you combine then re-compose problem with a bit of upper body movement on your part and the possibility of a little subject movement, it's very easy to get the important stuff less than tack sharp from 6 feet at f/3.5 using a 50mm lens. In ideal conditions, you've got only a couple of inches on either side of the precise focal plane. Less than half that if you shift the plane during recomposing. And then you're down to only an inch of tolerance for movement by both you and your subject. <br /><br />I catch myself drifting like that all the time on handheld shots - especially if I've been walking hard or am a little fatigued after a long day.</p>

[Dieter Schaefer]

<p>Matt, nice graphical explanation what I tried to say above. Portraits are one area where I really love the D300 - always one AF area to position exactly were I want it. I also keep the camera on AF-C; AF-S is pretty much reserved for anything that really doesn't move (at least not on a relevant time scale). Can't remember when I ever used the focus lock...</p>

[trevor_newman1]

<p>Thanks for the graphic Matt! I was trying to picture it in my head but that really makes it much easier. Thank you for all your comments everybody. I will try to keep practicing my focus technique. I really believe this ability to conveniently blow up every image so large has made me much more cognizant of even the smallest focusing erros.</p>

[hans_janssen]

<p>I always set the nearest af-point on an eye and focus(AF-C) and do my composing by moving back, left, right and keep the eye in focus. I use the af-on for focusing and I believe you can set the ae/af l knob for that too.</p>

[pete_s.]

<p>"The focus recompose trap" is not the problem here.<br /> From 6-10 ft with a 50mm on a 1.5 crop camera it is not an effect you could notice even shooting wide open. The angle is too small.</p>

<p>No, the focus error in this case comes from either moving the camera (body, hands, heads etc) while recomposing or from moving the camera too soon after focus is locked (this is only in effect when doing focusing and recomposing fast). It could also be that the AF sensors are not where one think they are (they are usually larger). If the entire image is blurry then it's a handholding problem with a too slow shutter speed.</p>

<p> </p>

[rarmstrong]

<p>Trevor, I guess that I would have to agree with Pete on this one. Using your D90 with a 50 mm lens set at f3.5 at 8 feet from your subject would give you a DOF of more than one foot...<br>

<a href="http://www.dofmaster.com/dofjs.html">http://www.dofmaster.com/dofjs.html</a><br>

So, while Matt's diagram is correct and can explain what can happen by changing the angle to your subject, the actual distance from the camera to your subject's eye or tip of the nose will be possibly one inch. It is much more likely that your lateral motion is responsible for the focus problem.</p>

[Matt Laur]

<p>Just to be clear, I'm postulating based on the short end (six feet) of the "6-10" distance Trevor mentioned (and that leaves you with about 7 inches of DoF). And I'm considering a the focus-recompose issue as just one factor that might contribute, along with other factors, to a higher ratio of focus problems. I know from my own handheld experience that my movement, subject movement, and recompose plane tilt can all combine to easily move the eyeball the 3.5 inches needed to land it outside the sharpness zone (remember, if you have a foot of DoF and have placed the eye, for example, right in the middle of it or a third of the way back in that range... you're still only a few inches from being out of focus). Just sayin'! </p>

[rarmstrong]

<p>Agree, Matt...that works:-)</p>

[Dieter Schaefer]

<p>Agree with Matt too - shot some portraits recently, with a manual focus 105/2.5 fully open. Since I was a bit farther away, DOF was about the same as for the 50 at f/3.5 and 6' distance. Lot's of OOF images - though they looked sharp in the viewfinder. Poor manual focusing capabilities of the current AF viewing screens was one reason, the other was simply my own and the subjects movements - really didn't take much to render the images OOF.</p>

[rarmstrong]

<p>I've had the same experience recently with the Sigma 30 mm at f2. It doesn't take much movement of the subject or the photographer to throw that crisp eye focus that we all want, off a hair. And, what you see in the viewfinder or on the LCD may appear to be pretty good until you put it up full size on the monitor. Good discussion and I think we are all getting at similar points...it takes a very small amount of motion or change in focus angle or distance to the subject to throw off that perfect shot.</p>

[blumesan]

<p>Occurs to me that it is easy to distinguish between focussing error and motion blur due to camera movement. Just pick a subject (e.g. your front lawn) and take a few shots focussing on an object 6 - 8 ft. distant. If it's a focussing problem, something will be sharp. If it's camera movement, nothing will be sharp.</p>

[glenn_c1]

<p>Should the plane of sharp focus for a lens be a true, flat plane or a sphere? It makes more sense to me that it should be a sphere - something at the same distance but 45 degrees away should be in sharp focus, not something 45 degrees away and 1.4x farther away, for example, which is what it would have to be to be in the same plane.</p>

<p>This is an area of lens performance I do not well understand. Lens reviews often mention deviations from perfect "flat plane" focus characteristics, but it seems to me that a flat plane doesn't make sense either optically or as a design goal.</p>

<p>In any case it has major implications for "focus and recompose" shooting, which is my usual MO.</p>

 

[Matt Laur]

<p>It is a plane, Glenn. Otherwise, shooting things like rows of people or paintings on walls would be nigh on impossible.<br /><br />The planar area that's in focus is parallel to the film or sensor that's in the back of the camera.<br /><br />Obviously, there are always fidelity issues. Something has to give when you bend the light more aggressively at the edges of the frame than you do in the center... which is one of the reasons why you often hear about how some lenses are less happy in the corners when they're wide open, or why CA gets uglier away from the center of the image.</p>

[pete_s.]

<p>Glenn, the plane of focus should be flat because otherwise a flat object would make a distorted image. So curvature of field is one of the lens aberrations and lens designers try to make it flat field. On lenses like macro lenses used for reproduction it is even more important so these lenses are better than regular lenses in this regard.</p>

[dan_south]

<p>Trevor, what was the shutter speed on those handheld shots? Can you tell us what ISO setting you were using and what the lighting conditions were?</p>

<p>If you shoot in dim lighting conditions with a handheld camera, you might have to increase your ISO setting to force the camera to shoot at a faster shutter speed. If your shutter speed was lower that 1/125th for a 50mm lens, camera shake might be the problem.</p>

<p>If your shutter speed was /125th or higher, then you might have moved toward or away from your subject while recomposiing. Perhaps try using continuous autofocus with a focus point that focuses on the subject's eye. If this doesn't give you the composition you like, shoot a slightly wider shot and crop it on the computer to match you desired composition.</p>