If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
Focus flange distance....
I have a Sigma 100-300 f4 EX. When I first got it, it focused fine. Now
however it has gone soft on me. At f4 it will focus fine at 135 and 200mm producing a very sharp image but not so at 100, 250 and 300 with 100 being very blurry. Stopping down improves the situation a bit. I am using it on a 20D and a D60. It behaves virtually the same on both. I contacted Sigma and was advised to have my cameras' 'focus flange distance' (FFD) checked. Am I right in thinking that if this FFD were wrong, then it would not focus at any focal length? Also that it is highly unlikely both cameras would have FFD errors, suddenly. |
#2
|
|||
|
|||
"LCD" wrote:
I have a Sigma 100-300 f4 EX. When I first got it, it focused fine. Now however it has gone soft on me. At f4 it will focus fine at 135 and 200mm producing a very sharp image but not so at 100, 250 and 300 with 100 being very blurry. Stopping down improves the situation a bit. I am using it on a 20D and a D60. It behaves virtually the same on both. I contacted Sigma and was advised to have my cameras' 'focus flange distance' (FFD) checked. Am I right in thinking that if this FFD were wrong, then it would not focus at any focal length? Also that it is highly unlikely both cameras would have FFD errors, suddenly. You're right. The problem is almost certainly with the lens. You should first test the lens using manual focusing. This is a quick and easy test. If you can focus manually at all focal lengths, there could be a problem with the electronic interface that is causing the camera's processor to think the lens has been correctly focused when it has not. With autofocus, you should use single shot AF for your tests. There are known problems with Sigma lenses and predictive autofocus systems used by several camera manufacturers including Canon. Depending on the subjects you shoot, this may mean that you should avoid using continuous AF mode and stick to single shot AF where possible. If the problem is still there in manual and single shot AF modes, I suggest that the cams that move the lens groups when zooming have probably become worn - or possibly they may have been damaged. Has the lens had extensive use? Have you ever dropped it or subjected it to mechanical or thermal shock? Whether as a result of wear or damage, this can result in the lens groups being out of position for some focal lengths. It is perfectly possible that they are still in the correct position for other focal lengths, which could explain why the lens can focus perfectly at some focal lengths but not at others. I would recommend that you should ask a competent camera repair technician to look at the lens to diagnose the problem. It should not cost a lot of money to identify what is wrong. Possibly a CLA (clean, lubricate, adjust) is all that is needed, but replacement of the cams could be quite expensive. It might be cheaper to replace the lens, so ask only for an estimate at this stage. At the end of the day, this is a lot of lens for a very low price. Externally, it appears well made. It gets good ratings for its optics. It has the HSM feature. However, to keep costs down, money has to be saved somewhere. With Canon L glass, you can be sure that the optical and build quality is of a high standard. Sigma lenses cost much less money to buy and even less to make, so something has to give. With Sigma, the saving is usually made in the internal build quality, which often leaves a lot to be desired, but is invisible to the buyer. Good luck. Let us know how what happens. |
#3
|
|||
|
|||
In article , LCD
writes I have a Sigma 100-300 f4 EX. When I first got it, it focused fine. Now however it has gone soft on me. At f4 it will focus fine at 135 and 200mm producing a very sharp image but not so at 100, 250 and 300 with 100 being very blurry. Stopping down improves the situation a bit. I am using it on a 20D and a D60. It behaves virtually the same on both. I contacted Sigma and was advised to have my cameras' 'focus flange distance' (FFD) checked. Am I right in thinking that if this FFD were wrong, then it would not focus at any focal length? Also that it is highly unlikely both cameras would have FFD errors, suddenly. It is highly unlikely that the camera's focus flange distance could change. Either the bayonet flange would have to move, or the sensor would have to move. It is theoretically possible. As was discussed recently in another thread, the depth of focus (the range of film/sensor positions over which the image will be sharp) is a small fraction of a mm*. However, if this did happen, it would show up on all lenses used with that body. Theoretically possible, though extremely unlikely, for one body. The odds against it happening simultaneously to two bodies are probably something like the odds of you winning the lottery two weeks in a row. It is IMO *far* more likely that some element in the lens has become displaced, or one of the cams which move the various groups around within the lens has become flaky. From my limited experience of Sigma lenses (1 out of 3 has crapped out with a faulty diaphragm) and from a much greater reported base, this would come as no great surprise. It is likely that Sigma are just giving you the run-around. The improvement on stopping down is not much diagnostic use, as both problems would show this. The fact that it varies with focal length is however strongly indicative that it is the lens which is at fault and not the body. If you can show that (a) other lenses work fine on the bodies, even wide open, or (b) that the lens shows the same fault on a third body, you should have them (as the UK saying is) "banged to rights". *For a subject at "infinity" (i.e. for this purpose a distance very long compared with lens focal length) then depth of focus t is given approximately by t = 2CN, where C is the acceptable circle of confusion, say 0.02mm for a 35mm film or DSLR camera, and N is the f-number. Thus at f/4 this would be 2x0.02x4 or 0.16mm. David -- David Littlewood |
#4
|
|||
|
|||
"LCD" wrote in message
... I have a Sigma 100-300 f4 EX. When I first got it, it focused fine. Now however it has gone soft on me. At f4 it will focus fine at 135 and 200mm producing a very sharp image but not so at 100, 250 and 300 with 100 being very blurry. Stopping down improves the situation a bit. I am using it on a 20D and a D60. It behaves virtually the same on both. I contacted Sigma and was advised to have my cameras' 'focus flange distance' (FFD) checked. Am I right in thinking that if this FFD were wrong, then it would not focus at any focal length? Also that it is highly unlikely both cameras would have FFD errors, suddenly. Not only that, but the problem would manifest itself with any and all lenses you have, too. If this is not the case, it is undoubtedly the lens that is the culprit. -- Skip Middleton http://www.shadowcatcherimagery.com |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Distance scale on Nikon 18-70mm zoom lens | Gary Jones | Digital SLR Cameras | 8 | February 4th 05 01:41 AM |
Nikon announces new flagship **FILM** SLR – the F6! | Peter Lawrence | 35mm Photo Equipment | 228 | October 15th 04 12:40 AM |
Nikon 35mm SLR: manual focus vs. auto focus | S. S. | 35mm Photo Equipment | 69 | September 24th 04 05:35 PM |
how close is infinite focus? | Lazarus Long | Digital Photography | 74 | August 13th 04 06:55 PM |
hyperfocal distance | leo | Digital Photography | 74 | July 8th 04 12:25 AM |