rob_piontek Posted April 7, 2008 Share Posted April 7, 2008 I have long thought that my D50 and 50mm 1.8 do not autofocus very welltogether. Normally I'm using the single point AF mode, put it on my subject,focus, reframe, take the picutre. The faces of my subjects, where I put the AFpoint, often seem softer than they should be. It's not really bad, butconsitently bad enough to make me think that somethings not quite right. Especially when occasionally the stars align and I get a really sharp image withthis lens. I know that there are many things that go into making a sharp image,i.e. wide open most lenses aren't that great, especially the closer you get tothe edges, very small depth of field wide open for an f1.8 lens, not shooting ona tripod, etc.... so it's always just been a suspicion. Last night I finally did some tests to try to figure this out. I didn'tcompletely convince myself, but what seems to be the case is that the center AFpoint works better than the others. When I use the other AF points, the focuspoint the camera chooses seems to be a bit farther away than it should be. Itseemed to be pretty consistent. I'm curious if anyone else has noticed similarissues. Link to comment Share on other sites More sharing options...
dcphotography Posted April 7, 2008 Share Posted April 7, 2008 Yep, normal. Center AF point is more sensitive than the other points. Its also known as a cross-type sensor, it can detect vertical and horizontal lines, and the others are vertical-type sensors which detect horizontal lines.(yeah, horizontal) Link to comment Share on other sites More sharing options...
albert_smith Posted April 7, 2008 Share Posted April 7, 2008 <I>I'm curious if anyone else has noticed similar issues.</I><P> Yes, but only in the worst-case situations. When I am in very close and wide-open, I get the same results that you have experienced. I don't know if this is from a conflict in the accuracy of the AF points, or it is from the lens itself, which may have some field curvature (while close-focused and wide-open), but I have been able to see this effect while I was testing my 50mm f/1.8 AF on a tripod with a multiple distance subjects.<P> From my test, the center is sharper than the edges when trusting the AF points to confirm focus.<P> In the real world, at moderate distances, hand-held, 3D sunjects, this is mostly a non-issue. I have many good shots with the lens used wide-open for off-center composed subjects.<div></div> Link to comment Share on other sites More sharing options...
albert_smith Posted April 7, 2008 Share Posted April 7, 2008 ... And center point. The CD case sticker is very sharp, and can be read with ease when viewed at 100% magnification.<div></div> Link to comment Share on other sites More sharing options...
niccoury Posted April 7, 2008 Share Posted April 7, 2008 I notice that too with my D50, The outside points hunt more so than the center one. It's a bummer, but not much can be done. Link to comment Share on other sites More sharing options...
blumesan Posted April 7, 2008 Share Posted April 7, 2008 <<I'm using the single point AF mode, put it on my subject, focus, reframe, take the picutre.>> Possibly your problem may be due to focus and then reframe. Reframing can place the focus point at a significantly different distance from the frame center. Link to comment Share on other sites More sharing options...
rob_piontek Posted April 8, 2008 Author Share Posted April 8, 2008 Albert - nice test! The center CD definitely looks sharper, but it should be easier to focus on because there is more depth of field at the larger distance, right? Also, the right hand CD seems to be on an angle, so it's not clear to me what should actually be in focus. Mike - that's definitely true, but the camera - subject distance does not change, so the only issue would be whether the lens gets significantly softer as you move away from the center. Isn't this a standard technique of focusing? Link to comment Share on other sites More sharing options...
albert_smith Posted April 8, 2008 Share Posted April 8, 2008 <I>...should be easier to focus on because there is more depth of field at the larger distance, right?</I><P> Well, not posted was the shot of the rear/left most CD which was also not as sharp as the center one. I tested all three points across the frame, but only posted the two above to make my point without clogging the thread with photos.<P> <I>Also, the right hand CD seems to be on an angle,...</I><P> The CD case may appear to be angled, but when I set up the test, I made sure that all of the case fronts were at the same angle, square to the film plane. You may be looking at the spine of the case which is angled, since the "V" shape of the slightly open case allows the case to stand. The front of all of the cases are all square to the camera.<P> Again, this is all basically overthinking the potential (or limitations)of the lens. In the real world, we don't usually set up this perfect positioning of a subject. The 50mm f/1.8 is a fine lens for real subjects in an environment. Link to comment Share on other sites More sharing options...
george_borden Posted April 8, 2008 Share Posted April 8, 2008 Lenses are sharper in the center than nearer the borders. Take a look at the test results on Photozone for this lens as an illustration, especially wide open. Perhaps what you're seeing is this loss of sharpness rather than incorrect focus? Just a thought. Link to comment Share on other sites More sharing options...
lindsay_schoon1 Posted April 12, 2008 Share Posted April 12, 2008 I have this same setup.. D50 and a 50mm 1.8. I have to say this lens is the best one I own. The pictures are a little soft, but not out of focus soft. See below. I just attribute this to the higher ISO I tend to use and that I always have this thing on 1.8. I wonder if yours might be defective, honestly, but do first try focusing with the center focus spot and reframing -- that's what I always do. I've been satisfied. <br> <img src="http://www.eyelikelindsay.com/sitebuildercontent/sitebuilderpictures/Weddings/weds__2_.jpg"></img> Link to comment Share on other sites More sharing options...
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