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Photo Forum / Digital Photography / DSLR Cameras / September 2006Canon 30D focus autofocus accuracy with f/1.8 lens
I got the 50 mm f/1.8 AF lens for my new Canaon 30D. It is much sharper than the 18-55 zoom that comes with the camera, at f/3 to f/8 or so. The autofocus works good enough on three dimensional objects. I'm using only the central focus spot. But for flat subjects like books or walls, or more importantly focusing on a subject all of which is effectively at infinity, it is nowhere near good enough. Is this to be expected? I have to manually focus, which is not exactly easy with the flat matte screen with no focus aids. What good is autofocus if it does not work? Why don't they have visual focusing aids like in my old Minolta X-700? Doug McDonald > I got the 50 mm f/1.8 AF lens for my new Canaon 30D. > [quoted text clipped - 12 lines] > What good is autofocus if it does not work? Why don't they have visual > focusing aids like in my old Minolta X-700? Because the focusing screen is relatively transparent, manual focus can be unreliable, as can depth of field preview. Most DSLR AF does not work especially well unless custom calibrated. Fixation in London will do this for the 30D - it involves setting up the sensor, AF module, mirror and screen so they are all perfectly positioned, and reprogramming the CPU of the camera to tell it this. As delivered, the camera contains instructions to modify focus points to compensate for small errors. These only show up, normally, at infinity with short focus or wide aperture lenses used wide open. You would want a camera set up like for aerial work, for example. Fixation custom-tune most of the UK national press and news agency cameras routinely. They are 'good enough' out of the box, but sometimes good enough isn't good enough :-) David I recently bought the 350D with the 18-55 kit lens. I too am disappointed with the auto focus results, far too many shots are fuzzy and are simply not in focus. I've been looking at some of the pics taken with my old point and shoot Sony...hate to admit it but they compare favourably. I'm reluctant to splash out on a lens in case matters don't improve. > I got the 50 mm f/1.8 AF lens for my new Canaon 30D. > [quoted text clipped - 14 lines] > > Doug McDonald > I recently bought the 350D with the 18-55 kit lens. I too am > disappointed with the auto focus results, far too many shots are fuzzy > and are simply not in focus. I've been looking at some of the pics > taken with my old point and shoot Sony...hate to admit it but they > compare favourably. I'm reluctant to splash out on a lens in case > matters don't improve. Canon 50/1.8 Canon 85/1.8 Canon 28-135/3.5-5.6 IS Canon 70-300/4-5.6 IS http://digitcamera.tripod.com/#slr > I got the 50 mm f/1.8 AF lens for my new Canaon 30D. > [quoted text clipped - 12 lines] > What good is autofocus if it does not work? Why don't they have visual > focusing aids like in my old Minolta X-700? When are your going to give up your life as a troll? Greg  Signature"All my time I spent in heaven Revelries of dance and wine Waking to the sound of laughter Up I'd rise and kiss the sky" - The Mekons > > I got the 50 mm f/1.8 AF lens for my new Canaon 30D. .... > > What good is autofocus if it does not work? Why don't they have visual > > focusing aids like in my old Minolta X-700? > > When are your going to give up your life as a troll? It may well be a bit trollish - not owning a 30D I have no idea, but I'd be surprised if someone can't easily focus a 1.8 on a decent matte screen... I used to *hate* focussing aids.. (Just for the record it is worth noting that this *isn't* the Douglas MAcDonald with the mutiple personality complex, paranoia and ridiculous enlarging claims, in case that is what you meant.. If it isn't what you meant, then just ignore me... (O;) >> > I got the 50 mm f/1.8 AF lens for my new Canaon 30D. > .... [quoted text clipped - 6 lines] > I'd be surprised if someone can't easily focus a 1.8 on a decent matte > screen... I used to *hate* focussing aids.. And I really missed the split image that I had on my OM-1... SignatureJørn Dahl-Stamnes http://www.dahl-stamnes.net/Foto/ >>>I got the 50 mm f/1.8 AF lens for my new Canaon 30D. > [quoted text clipped - 13 lines] > enlarging claims, in case that is what you meant.. If it isn't what > you meant, then just ignore me... (O;) Interesting. Not only is his name disturbingly similar but he has the same writing style. I thought it was an ever bigger troll since the last we heard from the Douglas he was incessantly bashing Canon. Greg Signature"All my time I spent in heaven Revelries of dance and wine Waking to the sound of laughter Up I'd rise and kiss the sky" - The Mekons >>> I got the 50 mm f/1.8 AF lens for my new Canaon 30D. > .... [quoted text clipped - 6 lines] > I'd be surprised if someone can't easily focus a 1.8 on a decent matte > screen... I used to *hate* focussing aids.. I can focus the thing just fine on the matte screen ... but it takes time, care, and thought. And it probably would not be good enough if the thing were diffraction limited at f/2.8. With my old focus-aid Minolta it was trivial ... just focus until the "shimmer" disappeared. Doug McDonald >> I got the 50 mm f/1.8 AF lens for my new Canaon 30D. >> [quoted text clipped - 16 lines] > > Greg This was not a troll! It was a question ... a real genuine question, "Is this to be expected?" IS IT? Doug McDonald > I got the 50 mm f/1.8 AF lens for my new Canaon 30D. > [quoted text clipped - 3 lines] > The autofocus works good enough on three dimensional > objects. I'm using only the central focus spot. Try enabling all the AF points. http://digitcamera.tripod.com/#slr >> I got the 50 mm f/1.8 AF lens for my new Canaon 30D. >> [quoted text clipped - 5 lines] > > Try enabling all the AF points. Please explain why. The manual says that only the center one works properly with f/2.8 lenses. In any case, of course, if you have s somewhat three-dimensional subject, you need to use a single-point focus if the desired plane of focus is not at the nearest point. Doug McDonald > >> I got the 50 mm f/1.8 AF lens for my new Canaon 30D. > >> [quoted text clipped - 8 lines] > Please explain why. The manual says that only the center one > works properly with f/2.8 lenses. Not that only the center can work properly, but the center can work better. > In any case, of course, if you have s somewhat > three-dimensional subject, you need to use a single-point > focus if the desired plane of focus is not at the nearest > point. If you need that. BTW, I suspect the camera does not use the nearest point everytime, but it tries to be more clever in picking a focus point. But anyway, you were talking about flat scene. With all the AF points enabled, maybe the camera can ignore a few points that have AF error, and uses the majority that have good focus at a same distance. http://digitcamera.tripod.com/#slr > *From:* Doug McDonald <mcdonald@SnPoAM_scs.uiuc.edu> > *Date:* Wed, 30 Aug 2006 09:27:16 -0500 [quoted text clipped - 17 lines] > > Doug McDonald What's important for AF is contrast. So if you want to focus on something flat, focus on the edge. If there's effectively no edge, like a plain flat wall with no colour changes, well that would have been a very boring picture anyway! >> But for flat subjects like books or walls, or more >> importantly focusing on a subject all of which is [quoted text clipped - 13 lines] > If there's effectively no edge, like a plain flat wall with no colour > changes, well that would have been a very boring picture anyway! By flat I meant physically ... not contrast-wise "flat". I'm talking things like books, posters, or scenics all of which are at least a couple of hundred lens focal lengths away. They have plenty of contrast. I've made more tests and the results are bizarre. If I hand focus correctly, then carefully switch to autofocus mode and do an autofocus, the camera gets the correct result. Otherwise, it is usually off. This is not, for me, a killer fault, since for such scenes I'm a nervous settings fiddler anyway, and will of course use the camera's closest zoom view to check things anyway. I am delighted how well the camera does for most three dimensional scenes. Doug McDonald |
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