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Photo Forum / Digital Photography / DSLR Cameras / February 2007Focus discrepancies in Rebel XT and XTi, manual and auto focus
I have noticed the most puzzling and disturbing focus discrepancies in both my Rebel XT and my XTi. Please do the following tests with yours (and please report back!): Set your camera on a tripod Set up a scene with some sharp black-on-white writing in the center Set your lens to auto-focus and press the shutter button half way down until the lens focuses Adjust your eyepiece dioptric to your eyesight Shoot a picture at wide aperture (f2.8 will do) Now, without disturbing anything, set your lens to manual focus Move the focusing ring to dislodge the previous focus setting Re-focus carefully on your viewing screen (do not re-adjust your diopters!) Shoot a picture at the same f-stop Now upload your pictures to your computer and compare the two side-by-side by enlarging the center to the maximum (try "actual pixels" with Photoshop) I've repeated these tests with every conceivable variation of lenses, cameras, with glasses and without, with additional supplementary diopter correction lenses, etcetera. INVARIABLY, the picture done in auto-focus is right on, but the one done manually shows a pronounced focus shift! You can readily see the difference in sharpness between the two. I sent the XTi to Canon and it was returned with the same problem, after thorough "inspection and adjusting". What is going on? Do you see the same discrepancies in your own tests? > I sent the XTi to Canon and it was returned with the same problem, > after thorough "inspection and adjusting". They didn't find a problem since this is normal. > What is going on? The focusing screen in your camera is not optimized for manual focus. So, every time you see an image that looks perfectly focused you have too much tolerance for it not to be perfectly focused. > Do you see the same discrepancies in your own tests? No need to. If you want to use this body with MF lenses you should invest in a KatzEye screen if they make one for that model. Rita Rita ? Berkowitz wrote: > > I sent the XTi to Canon and it was returned with the same problem, > > after thorough "inspection and adjusting". > > They didn't find a problem since this is normal. If that's the case it's a shame. I can't really believe it. Do other brands show the same problem? And why is not this a problem with film auto-focus Canon SLRs? Or is it? And why isn't there a warning anywhere in the instruction manual? I personally like to set my lenses to manual focus, I hate to press a "hold" button and refocus if I don't agree with the camera's chosen point. That renders the Canon Rebel XT and XTi near useless to me. BTW, I spent two hours on the phone with various Canon technicians this afternoon. They seem to think the problem is with my eyes, that I need to adjust the eyepiece diopter. Never a word that this baffling camera behavior is "normal". That's why I'd like to know if other users are getting the same results from the tests. > BTW, I spent two hours on the phone with various Canon technicians this > afternoon. [quoted text clipped - 6 lines] > That's why I'd like to know if other users are getting the same results > from the tests. My problem was DEFINITELY fixed with the diopter. Shooting through a viewfinder only, I can never trust my own judgement as to whether I've manually focused correctly or not. I would not be a good subject for your proposed test... dwight >> BTW, I spent two hours on the phone with various Canon technicians this >> afternoon. [quoted text clipped - 8 lines] > > My problem was DEFINITELY fixed with the diopter. Which brings up a question I've been meaning to ask for some time. How accurate is the built-in diopter adjustment on a low end Canon? I have a Rebel XT and I'm -.5 in one eye and -.75 in the other. And if I start doing more manual focus I will definitely get a Katz Eye, I've been thinking about it for quite some time. Greg > And if I start doing more manual focus I will definitely get a Katz Eye, > I've been thinking about it for quite some time. > > Greg A Katz Eye will not correct an inherent focus discrepancy, should you have one. These are screens whose only virtue is that they're brighter and more contrasty and thereby facilitate focusing, PROVIDED you have a perfectly aligned system to begin with. >> And if I start doing more manual focus I will definitely get a Katz Eye, >> I've been thinking about it for quite some time. [quoted text clipped - 7 lines] > contrasty and thereby facilitate focusing, PROVIDED you have a > perfectly aligned system to begin with. Yes, but as others have pointed out the lack of a decent focusing screen on current autofocus DSLRs is probably your problem with manual focus, not an inherent problem with your Rebels. Greg > > A Katz Eye will not correct an inherent focus discrepancy, should you > > have one. [quoted text clipped - 8 lines] > > Greg And by what mechanism do you suspect the focusing error occurs? As I have explained, in the test procedure the diopter is adjusted after the camera has autofocussed the lens. The lens is then refocused manually until the image, already adjusted for my eyesight, again appears tack sharp. The second picture is then taken. How do you explain the fact that the image focused on auto is right on but the image focused manually shows a very perceptible focus shift? It is quite obvious to me, and I fail to see why it isn't to everybody, that the focusing screen brightness plays absolutely no role in the test procedure. Please explain to me then how the focus discrepancy occurs. Better yet, do the test on your own Rebel XT, it won't take you 10 minutes. PS - I have also observed that the manual focused image INVARIABLY shows 1/3 of a stop more exposure than the autofocus image. Explain that one also. >> > A Katz Eye will not correct an inherent focus discrepancy, should you >> > have one. [quoted text clipped - 18 lines] > How do you explain the fact that the image focused on auto is right on > but the image focused manually shows a very perceptible focus shift? As "Rita" said: "The focusing screen in your camera is not optimized for manual focus. So, every time you see an image that looks perfectly focused you have too much tolerance for it not to be perfectly focused." I'm not going to do the test because I don't trust the combo of my eyes and my Rebel XT's screen. > PS - I have also observed that the manual focused image INVARIABLY > shows 1/3 of a stop more exposure than the autofocus image. Explain > that one also. PEBSAC? Can't think of any other reason. How do you explain it? Greg >> > A Katz Eye will not correct an inherent focus discrepancy, should you >> > have one. [quoted text clipped - 22 lines] >Better yet, do the test on your own Rebel XT, it won't take you 10 >minutes. In the procedure you are applying you should be adjusting the diopter until the image is tack sharp, not adjusting the diopter then changing the focus to accomodate your misadjustment of the diopter. Of course it may be that you have the focusing screen in upside down. >PS - I have also observed that the manual focused image INVARIABLY >shows 1/3 of a stop more exposure than the autofocus image. Explain >that one also. If you can see 1/3 stop more exposure you've a better eye than the lot of us I suspect. > >PS - I have also observed that the manual focused image INVARIABLY > >shows 1/3 of a stop more exposure than the autofocus image. Explain > >that one also. > > If you can see 1/3 stop more exposure you've a better eye than the lot > of us I suspect. I can just barely see it in the image - but I can see the shutter speed in the camera below the screen change from 1/20 sec to 1/15 sec (I've been shooting on "A") at the same aperture. I'm told by a knowledgeable friend that these cameras use two different systems to focus (and probably to determine exposure thereby) - one system is independent of the viewing screen and uses intersecting beams (!?) and I don't know what to focus the image; the other uses the traditional groundglass screen and top prism to focus and possibly also to determine exposure. >> >PS - I have also observed that the manual focused image INVARIABLY >> >shows 1/3 of a stop more exposure than the autofocus image. Explain [quoted text clipped - 13 lines] >traditional groundglass screen and top prism to focus and possibly also >to determine exposure. I've never seen such a description of Canon's AF and AE systems. I think the AF is done in the mirror box, and has nothing to do with the "prism" (the XT doesn't have one).  SignatureMiss New Jersey USA said she's pregnant Monday, after Miss Nevada USA resigned over nude photos. Miss USA just got out of coke rehab. Miss Congeniality won her title with her presentation on how to tell undercover vice cops from the real Johns. > In the procedure you are applying you should be adjusting the diopter > until the image is tack sharp, not adjusting the diopter then changing > the focus to accomodate your misadjustment of the diopter. > > Of course it may be that you have the focusing screen in upside down. I've done it both ways with the same focus discrepancies. I've done it with my glasses on and off, I've done it using supplementary diopter lenses (I have a small collection), I've now done it with three cameras, two Rebel XT and my new out-of-the box XTi, all with the same results.On a few occasions, very few, focus points have matched exactly on the images. I urge readers to do tests yourselves - you might be in for a shock. PS - the 17-85 Canon kit lens is a dog at 85mm and short range! >> In the procedure you are applying you should be adjusting the diopter >> until the image is tack sharp, not adjusting the diopter then [quoted text clipped - 12 lines] > > PS - the 17-85 Canon kit lens is a dog at 85mm and short range! Err... 18-55 or 17-85 "kit" lens? SignaturePhil > > PS - the 17-85 Canon kit lens is a dog at 85mm and short range! > > Err... 18-55 or 17-85 "kit" lens? > > -- > Phil It's the cheapie that comes in the kit, maybe it's the 18-55 (I rarely use it). If it is, I revise my statement: it's a dog at 55mm in the short distance range. >>>A Katz Eye will not correct an inherent focus discrepancy, should you >>>have one. [quoted text clipped - 22 lines] > Better yet, do the test on your own Rebel XT, it won't take you 10 > minutes. The effect you are observing is a property of your eye and the human visual system. As you change the focus of the lens, and as the focus gets close, your eye will change focus and lock on to the image plane faster than you are changing focus. To overcome this effect, you can try and simultaneously look at a focus point and the image, trying to make both appear as sharp as possible, but it is not easy (training may help). This is a known effect in astronomical instrumentation and has been known for decades. The work-around is strongly ground glass (which loses light and detail) or a microprism assembly which we see in old film cameras, or other methods. For example, I used to use a camera with a clear glass screen and a small semi-circular spot which formed a knife edge. The clear glass was great for seeing faint stars in a telescope, and the knife edge was used for a Foucault test for precise focus. But the clear glass, even with fiducial crosses, was poor for focusing. You could buy the right angle finder for canon cameras: it has some magnification that will help focusing, but it doesn't make the situation perfect either. If in a studio, connect to a PC and check focus on the pc. > PS - I have also observed that the manual focused image INVARIABLY > shows 1/3 of a stop more exposure than the autofocus image. Explain > that one also. Not sure on that one. Roger >> And if I start doing more manual focus I will definitely get a Katz Eye, >> I've been thinking about it for quite some time. [quoted text clipped - 7 lines] >contrasty and thereby facilitate focusing, PROVIDED you have a >perfectly aligned system to begin with. And have a split prism and a microprism ring. As for an "inherent focus discrepancy", how many SLRs have you encountered that actually have one of these? On every SLR I've taken apart you're going to to have to bend the primary structure to get an "inherent focus discrepancy" . On a Leica the rangefinder can get misaligned without a whole lot of difficulty, on an SLR you have to quite a lot of damage. > >A Katz Eye will not correct an inherent focus discrepancy, should you > >have one. [quoted text clipped - 11 lines] > misaligned without a whole lot of difficulty, on an SLR you have to > quite a lot of damage. I know little about the inner workings of an SLR. In the olden times (read Nikon F times) I suppose placement of the viewing screen was done with extreme precision. I would be leary of having some third party outfit replace the factory screen with another - you'd think it would lead to focusing errors. But, apropos my comment above about the two different focusing systems, I suspect it would not affect auto-focusing. >> >A Katz Eye will not correct an inherent focus discrepancy, should you >> >have one. [quoted text clipped - 18 lines] >lead to focusing errors. But, apropos my comment above about the two >different focusing systems, I suspect it would not affect auto-focusing. The viewing screen rests on the structure of the body and is held against it by a spring. There's no magic involved in getting it into the correct position. Doing it without getting dust specks behind it now . . . In Nikon F times the viewing screen was intended to be changed by the photographer. >>> BTW, I spent two hours on the phone with various Canon technicians this >>> afternoon. [quoted text clipped - 17 lines] > >Greg with the camera turned off, can you see the focusing squared clearly? Watch them while you adjust the diopter correction. > with the camera turned off, can you see the focusing squared clearly? > Watch them while you adjust the diopter correction. Yes, after adjusting the diopter on the autofocussed image I can then see the viewfinder perfectly with the camera turned off. I don't see why the reverse would help any - I probably did adjust it that way when the camera came out of the box, which, BTW, was when I first noticed that the screen was a tad off when focussed at infinity and what started me on this unpleasant voyage. >> with the camera turned off, can you see the focusing squared clearly? >> Watch them while you adjust the diopter correction. [quoted text clipped - 5 lines] >first noticed that the screen was a tad off when focussed at infinity >and what started me on this unpleasant voyage. The screen could be in the wrong place, such that when the image is focused properly on the viewfinder screen it is not focused properly on the imaging sensor. One of the Photo magazines, either Pop or Modern ran an article on this problem many years ago, back in the days of film. since what is seen in the viewfinder is not an aerial image I can't figure how the diopter or bad eye could make a mis-focused image look good. there's a lot of things I can't figure out, this is just another one of them. > My problem was DEFINITELY fixed with the diopter. Shooting through a > viewfinder only, I can never trust my own judgement as to whether I've > manually focused correctly or not. > > I would not be a good subject for your proposed test... My test removes the possibility of a diopter problem. Notice that in my test the diopter is calibrated after the camera has autofocussed Using manual focus then should give the exact same point of focus, since the eyepiece has been corrected for your eyesight already. >> They didn't find a problem since this is normal. > > If that's the case it's a shame. I can't really believe it. Do other > brands show the same problem? I think the current trend is most people use AF and don't play around with MF much. Yes, other brands have the same problem to more or a lesser degree. Your camera is working within 100% of factory-designed specs. I found the website for KatzEye. http://www.katzeyeoptics.com/ > And why is not this a problem with film auto-focus Canon SLRs? Or is > it? It depends on the factory installed focusing screen. Plus, higher end cameras give you a choice of screens. > And why isn't there a warning anywhere in the instruction manual? It would confuse people since most people use AF. It's kinda like the debates we have here about IS/VR were some people feel this feature cures everything from the common cold to gout. > I personally like to set my lenses to manual focus, I hate to press a > "hold" button and refocus if I don't agree with the camera's chosen > point. I know you don't want to hear this, but if you plan on using MF a lot you will need to shell out $100+ and get the KatzEye. > That renders the Canon Rebel XT and XTi near useless to me. You'll get used to AF since everybody does sooner or later. Rita >>> They didn't find a problem since this is normal. >> [quoted text clipped - 5 lines] > lesser degree. Your camera is working within 100% of > factory-designed specs. I found the website for KatzEye. Agreed. I've always had MF problems with my D30/D60/DRebel. Amazingly, I can live life normally like you and most of us. > http://www.katzeyeoptics.com/ > [quoted text clipped - 20 lines] > > You'll get used to AF since everybody does sooner or later. I saw the light and am giving up on Sigma and Tamron permanently due to the superior AF on Canon lenses. SignaturePhil :I have noticed the most puzzling and disturbing focus discrepancies in : both my Rebel XT and my XTi. [quoted text clipped - 32 lines] : : Do you see the same discrepancies in your own tests? Listen to what Rita says here Bob. Your camera is an Auto Focus camera. If you can't get a sharp pic on AF, it has a back focus error. This is not a Professional level camera and you can't expect it to perform like one. If you truly do want to manually focus this camera, get a manual focus screen for it. You may have a problem then with detecting correct AF but it's all a compromise on these cheap SLRs. SignatureAustralian Wedding Photography between Kempsy, NSW and Sunshine Coast. http://www.photosbydouglas.com Digital photos enlarged and printed on Canvas http://canvas.photosbydouglas.com |
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