fisher Posted June 6, 2005 Share Posted June 6, 2005 I own a 300D and a 70-200 f4 L lens. I discovered some situationswhere the lens constanlty back focuses. <ahref="http://fisher.hu/ft/">Here are the examples</a>I spent about a month testing the lens in different sitations. MostlyI was not sure about the defective, I had blurred (due to the camerashake) images, out ouf ones (probably due I moved the camera) but nowI am sure something is wrong.I did some google and found many people experiencing weird focusingwith this lens, especially on digital bodies, but found no final wordabout it. Please check <a href="http://fisher.hu/ft/">the images</a>and tell what do you think. Link to comment Share on other sites More sharing options...
jeff medkeff anchorage, a Posted June 6, 2005 Share Posted June 6, 2005 It looks to me as though the camera is focusing on the background and not on your subject. Are you picking your own focus point, or letting the camera do so? If the former, are you putting the focus point over a high-contrast line on the subject before focusing? Link to comment Share on other sites More sharing options...
jurgen_konig Posted June 6, 2005 Share Posted June 6, 2005 Welcome to the "70-200 F4 Back focusing club"! I've got the same lens and while it's very sharp at 200mm, it back focuses badly between 70-135. I'll be sending mine in to Canon to be calibrated under warranty. If you do a search on the net, you'll find quite a few people with the same problem. Wonder if it's a design issue or just poor QA ? Link to comment Share on other sites More sharing options...
fisher Posted June 6, 2005 Author Share Posted June 6, 2005 In bot examples I have used the center focusing point. I have noticed that the subjects are having relatively low contrast/pattern compared to the background. Link to comment Share on other sites More sharing options...
chiswick_john Posted June 6, 2005 Share Posted June 6, 2005 Are you aware that the actual focus area of the sensors are much bigger than that indicated on the screen and therefore any higher contrast area which is outside these marks will be used by the AF to focus on. Link to comment Share on other sites More sharing options...
fisher Posted June 6, 2005 Author Share Posted June 6, 2005 Yes, I am. Thats why I posted the full images (dowscaled hoever). Does anybody have actual information regarding the AF sensor size of the 300D? I can see the sensor holes in the body but not sure about the active area. Link to comment Share on other sites More sharing options...
christopher_bibbs Posted June 6, 2005 Share Posted June 6, 2005 In order to confirm you have back focus and not some other problem, you'll need to run a controlled test. A flat, high contrast subject that takes up the full frame. Anything less has too many variables to make anything certain. Link to comment Share on other sites More sharing options...
chiswick_john Posted June 6, 2005 Share Posted June 6, 2005 You can test the area yourself - mark a small x on a white peice of paper and then see how far off the X the AF actuates and locks on - you may also find the AF area is not central with the screen mark as well. Link to comment Share on other sites More sharing options...
pjmeade Posted June 6, 2005 Share Posted June 6, 2005 I'm no expert and I've only had my 70-200/f4 for a month, but with that wealth of experience, I'm going to side with those who say this is a result you get when trying to focus on a low contrast subject in front of a hight contrast background. If you use f4, it's more obvious because of the shallow dof. I think the lens is excellent, but does have some minor limitations. Once we recognise that and learn accomodate them in the way we compose and check the focus, we can get excellent results. I hope this helps. Best wishes. Pete Link to comment Share on other sites More sharing options...
digitmstr Posted June 6, 2005 Share Posted June 6, 2005 If you look at the picture fo the two men on horseback you'll notice that the hors head you posted is on the left. yet, you said you used the center AF point... The picture of the girl sitting with the building on the background tells me that the subject is so small compared to the backgrouond that would require very carefully aiming of the AF point in order to actually target the girl and NOT the wall. In each case there's ample room for user's error. That is not to say that some lens/body combos require calibration. Canon will calibrate your camera and lenses. They do this by first testing your camera with a lens they use for refercence. Once they adjust the camera body they test each of your lenses and adjust the focusing (including the AF motor on the lens itself) until they obtain the same consistent results of their reference lens (which is usually a 50 f/1.8 If you feel you have a backfocus issue send everything to Canon and they'll take care of it. It won't be free though... Link to comment Share on other sites More sharing options...
fisher Posted June 6, 2005 Author Share Posted June 6, 2005 Regarding the horsehead: the crop shows the focused leaves and the out of focus horsehead not the actual focusing area. Reagrding the sitting girl: I had enough time to take the shots carefully. I have about 5-6 pics with the same result. I even tires to focus on the ground before girl but without any success. I am quite sure this focusing problem - on these examples - not because of my error. Link to comment Share on other sites More sharing options...
chiswick_john Posted June 6, 2005 Share Posted June 6, 2005 "Please check the images and tell what do you think" - we have - so go and get it fixed if you are convinced you are right in identifying equipment error. Link to comment Share on other sites More sharing options...
david8 Posted June 6, 2005 Share Posted June 6, 2005 Just sent in my 20D for focus repair... A. Check the files in the canon software the see what focus points were used.. B. As presented once here, take a ruler and place it standing up on a table. At a sharp angle to the ruler, Using the center focus point (only), focus on the center number and take a shot. Repeat the test at a number of focal lenghts, then review the data. Is the center number sharp? If not, check the focus point data. On My 20D, it indicated the focus point was over the number "8" (as it should have been) on my ruler, yet the number 7 was in focus with the number 8 blurred. C. Try another lens to determin if the problem is the lens or the camera. FWIW Link to comment Share on other sites More sharing options...
chiswick_john Posted June 6, 2005 Share Posted June 6, 2005 Photographing an angled scale is usless as the wider the focus area the nearer the sensor will focus. Link to comment Share on other sites More sharing options...
peter_rowe Posted June 6, 2005 Share Posted June 6, 2005 Hi Fisher,The 70-200mm f4 L is a very good lens and Canon does a very good QA job with "L" class lenses. It is very unlikely that there is a problem with the lens if you bought it new. If it has been dropped or something else then possibly. In your photo's, are you using a focus and recompose? I suspect you are since the centre of the photos (where the centre AF point should be)is not on the foreground subject. If so I would strongly recommend using automatic AF -- just as a test. Just go take 50 shots that way. Please keep in mind the extremely shallow DOF with the 70-200mm. Angled tests are notoriouly unreliable. The best way to test your lens is as a previous poster mentioned with a fully flat subject, like a newspaper if you are zoomed in, or a block wall with good contrast. Just make sure it is as exactly perpendicular as possible. If there is a problem my guess would be that it a lot more likely to be the camera than the lens. Does the camera work perfect with other lenses? Try a non focus-and-recompose on a toally flat sunject that fills the viewfinder and see what happens. This is not camera shake so you are doing a great job there since some of the image is in focus. I have taken over 3,000 shots with my 70-200mm f4 and I am only now getting to where I can get reliably focused shots consistently. --Peter Link to comment Share on other sites More sharing options...
fisher Posted June 6, 2005 Author Share Posted June 6, 2005 For the easier evaluation I just put marks on the pics. I have used the center focus point only. I also uploaded a new picture of a dog. Link to comment Share on other sites More sharing options...
fisher Posted June 6, 2005 Author Share Posted June 6, 2005 ...and uploaded a <a href="http://fisher.hu/ft/img_2835_good.jpg">example without any problem</a>. Shot with a 100 f2 Link to comment Share on other sites More sharing options...
david8 Posted June 6, 2005 Share Posted June 6, 2005 hi john the ruler worked for me... using center focus point only to isolate and insure what I was focusing on. you may be right, but it clearly showed me that the camera was indeed locked at the point desired (via image-focus point review), yet the image was focused further from the camera. Canon seems to agree that there is a problem and is repairing. Dave Link to comment Share on other sites More sharing options...
mike_r1 Posted June 19, 2005 Share Posted June 19, 2005 My 70-200 f 4 L which was purchased from Calumet/San Diego, back focuses by at least 50mm from 4 meters away. The longer the subject to lens distance the more it backfocuses. other primes 85mm & 105mm on 20D body do not. Ive also confirmed the problem exisits with this lens on 10D, 20D and I had also the chance to check it on a 1D MK2, sadly they all showed the same back focusing effect. - Lens is booked in with Canon Service for a rapid quality control checkup & realignment. -Mike. Link to comment Share on other sites More sharing options...
mike_r1 Posted June 20, 2005 Share Posted June 20, 2005 Before and After shots. EOS 20D Body was checked and found to be normal, Lens was adjusted for focus error problem. http://ohidunno.com/gal/index.php?list=29 I recommend you to get the lens checked with your local canon service center. Link to comment Share on other sites More sharing options...
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