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Old Dec 7, 2005, 5:48 AM   #1
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I have just got this lens from e-bay and it seems to be focusing short of where it should be. To give you an idea, if you were to look at a computer keyboard from about 2' away at a 45 degree angle from the left.... when focusing on the H key, the G key is the one that is sharp.

Is there anything that I can do or should I be sending it back.

Someone did mention that it could be the camera but that this lens is the first that is 'sharp' enough to pick it up.

Thanks,

Mark.
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Old Dec 7, 2005, 10:56 AM   #2
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What camera is it (5D or 7D)?

Your test method is probably flawed. I never have trusted those AF charts either (trying to get the angle just right, etc.).

Also, are you using a tripod? You may be leaning justa tad after locking focus. In addition, at around 2 feet, you're getting pretty darn close to the minimum focus distance (around 18 inches if memory serves), so that may be throwing it off a tad shooting wide open.

Ditto if the contrast isn't quite high enough in the lighting you're in.

Also, some lenses are actually sharper just outside of center at wide open apertures which can skew your results. The 50mm f/1.7 is probably not one of these lenses, but I haven't looked at an MTF chart to tell. I've got a Tamron 35-105mm f/2.8 with this characteristic (it's sharper just outside of center at some focal lengths, according to MTF charts).

Try a different test, and make sure to use a tripod. All I did was a quick test using some books on a table with a 100mm f/2 (one of my better lenses) to see if my 5D worked OK. I didn't even take my own advise (no tripod used). But, if you lean ANY, it's going to throw off your results without a tripod. Try something like this (or use DVD Cases, etc. to get a similar setup):

http://www.pbase.com/jcockfield/image/51015183/original

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Old Dec 7, 2005, 11:55 AM   #3
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Hi Jim,

The example of the keyboard was just to give a picture of what I meant. I took lots or 'random' shots around the office and in my hotel room (nothing like that!!!) as it arrived at my work yesterday. I have a tripod and DVDs with me (I like to be entertained), so will set something up in the hotel room tonight. I will also try different apertures to see if that makes a difference.

I have the 5D same as you. If there is a problem, what can I do?

Thanks... Mark.
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Old Dec 7, 2005, 1:17 PM   #4
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Mark:

If it's actually front focusing, I'd try to determine if it's the lens or the camera.

It's very rare to see anyone with alegitimateAF calibration problem with the 5D.

With the 7D, backfocus (not front focus)was a very common issue.Apparently, the entire problem boiled down to defective adhesive used on a number of cameras (speculation, but it matches what some users reported finding).

The 7D has a piece of rubber on the bottom of the camera that covers up some screws. There are 3 hex screws that apparently control the AF sensor alignment. Users found that turning the screws clockwise around 1/4 turn on average corrected bad backfocus problems. Apparently a small change in the screws makes a huge difference.

Some users also reported that some white adhesive on the hex screws was either coming off or missing when they looked at their cameras to calibrate them (hence my thoughts that the entire problem was defective adhesive, with the screws controlling AF sensor alignment working themselves out some over time.).

I do not know if the 5D has a similar setup, and I'm not inclined to try and remove theplate I see at the bottom of my 5D to see if I can find any adjustment screws, since my AFworks just fine.;-) If it is similar, then front focus would imply the screws arein too far (need to be backed out slightly).

Also, KM may frown upon someone doing this (for warranty purposes), and you would risk damaging the camera.

So, your best bet is to figure out if it's really front focusing by doing some more tests. Then, try to narrow it down to the lens or the camera. If you've got another bright prime, I'd use it. If not, you may be able to find a camera shop with lenses that could assist you.

Then, I'd contact KM if you have a problems and send it back to them. They'd have the equipment and expertise to calibrate it properly, without you risking voiding your warranty by doing it yourself. If it's the lens, they should also be able to fix it, too.

Turnaround time in the U.S. for AF calibration has been reportedly quite good (many users said it only took about a week). Outside of the U.S., turnaround time is usually worse (I don't know what to tell you to expect in the UK, but KMshould be able to give you an estimate if you find you need to send it in).


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Old Dec 7, 2005, 1:27 PM   #5
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P.S.

As for your random shots, make sure it's not technique.

When I first got this camera, I tended to use the Center AF point only, then reframe before shooting.

After using it a while, I found that this tended to introduce some AF errors using brighter lenses. So, I now tend to select my focus point with the controller, trying to minimize reframing after I get focus lock. I read an article a while back explaining why too much reframing can introduce severe focus errors with a bright prime, but I'd have to dig around to find it.

Of course, my assumption is that you're also confirming that you are getting AF lock (watching for it in the viewfinder with a half press of the shutter button).


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Old Dec 7, 2005, 1:58 PM   #6
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Hi Jim,

No I made sure that I was focusing on the exact spot where I released the shutter and I also have locked the camera so it will not release unless I am in focus.

I will update you tomorrow as I am off to the hotel to run the tests.
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Old Dec 10, 2005, 1:38 PM   #7
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Tests done on this lens and since posting the start of this problem I have been got a Sigma 70-200 2.8 APO EX (not the new DG version). This is having the same problem. I have taken many test shots with both of these lenses using DVD boxes and the 'ruler test'. Both lenses are focusing short so started to think that it could be a camera problem, but I was not able to get the same error with my Tamron 70-300 4-5.6. Today I have been into the shop where I purchased my camera from and tried the Minolta 50 1.7 and Sigma 70-200 2.8 on a 7D (they did not have a 5D in). I was able to get the same problem with both lenses on the 7D so what next...... ????

Are both lenses faulty, or is it something else?

The guy in the shop did suggest that 'older' lenses can have problems on newer bodies. Admittedly the Minolta 50 is quite old but the Sigma EX is quite recent. He was saying that there have been quite a few issues where the lenses need re-chipping. Is this something that anyone has heard of. Does anyone have a Sigma 70-200 2.8 that was used on film OK, but when moving to digital has got this problem. Are there any differences between the AF on older lenses/bodys? Surely Minolta would not change anything?!?!?

One thing that I do notice happens with these lenses (and a little on the kit lens and the Tamron), is that a split second after the camera flashes/bleeps to show focus lock, the lens then moves to a very slightly shorter focus..... is this something that happens on other peoples cameras?

Please help......

Mark
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Old Dec 11, 2005, 7:05 AM   #8
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"He was saying that there have been quite a few issues where the lenses need re-chipping"

The only re-chipping issue, with Sigma lenses, that I have heard of or experienced first hand were older lenses that did not autofocus at all or gave error messages (err 99 on Canon gear). Re-chipping would not solve anyfront focus problems.

All autofocus cameras need a certain amount of contrast to focus automatically and are designed to focus within a certain percentage of the DOF (I'm no expert on this subject by any means). From what you have described, I think your probably focusing within those specifications. My 50mm f/1.7 front focuses by a small amount when wide open (f/1.7) also but does not present a problem in normal use.



John




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Old Dec 28, 2005, 11:03 AM   #9
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I know this is an old topic but Im new to the forum and I hope I can reopen the debate.

I have a 7D and just got a Minolta 17-35mm D lens to go with it, I am getting exactly the same problem as described. If I set the focus manually, its perfect every time but the AF cant get it right. I occasionaly get similar issues with my old 35mm lenses but this new lens is so bad, I dont think I can use it. I have written to Minolta for their advice as I don't see the point in wasting time with Jessops (Uk Photo Retailer) but I am convinced its this calibration issue.

Cheers

Tony
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Old Dec 28, 2005, 1:00 PM   #10
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i am an 11 month 7D owner

i am (was) a 40 year minolta fan

i just sent my 7D to minolta after shooting my 2005 xmas shots the same way i have been shooting xmas for 30+years - aperture priority set f1.7 - center spot AF

every single shot came out 'defocussed', but could not tell on 'shitty' lcd

after setting up a test rig (using a 50mm f1.7 minolta)i determined that (at wide open apertures with multiple lenses) the 7D was back focussing 2" at a tripod mounted measured range of 4'

tripod looking down at 45 degree angle at yardstick laid out on well lit table away from camera with a lens cap alongside to mark 18" - spot AF on 18", got confirmation 'blink', result = 18" blurred, 20" razor sharp, 21" soft, 22' blurred

tried with other lenses up to and including my 80-200 f2.8 APO (shot at 8' range) with comparable results

IMO: ALL the 7Ds probably come poorly calibrated (to say the least) which is probably the reason most people have so much PP even after setting the sharpness to +2
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