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FORUMS Canon Cameras, Lenses & Accessories Canon EF and EF-S Lenses 
Thread started 24 Mar 2009 (Tuesday) 07:14
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70-200 f2.8L IS image quality different from left to right

 
kit
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Mar 24, 2009 07:14 |  #1

Picked up at 70-200 f2.8L IS today.. so will shortly be selling my 70-200 F4L IS.

anyway.. i haven't had time to take real world shots yet (pls don't flame me) but decided to do take a couple of focus test shots for interest's sake.. after all this lens is not cheap.

i found that focus was pretty much spot on.. and as expected, wide open at 200mm the lens was a lil soft.

but interesting thing I found was that contrast and sharpness seemed to be significantly better on the right side than the left side of the image at all focal lengths. Its really sharp in the middle though.

is this a common occurance?

Camera is a 5D2 so its full frame.


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bohdank
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Mar 24, 2009 07:24 |  #2

Sounds like a decentered element.

Exchange it or send it in to Canon for repair.


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kit
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Mar 24, 2009 07:25 |  #3

Here are the images I'm talking about. 100% crop.
f2.8, 200mm, ISO100, 1/8th sec, mirror lockup, tripod.

Left:

IMG NOTICE: [NOT AN IMAGE URL, NOT RENDERED INLINE]

Middle (focus point):
IMG NOTICE: [NOT AN IMAGE URL, NOT RENDERED INLINE]

Right:
IMG NOTICE: [NOT AN IMAGE URL, NOT RENDERED INLINE]

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Bendel
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Mar 24, 2009 07:28 |  #4

It looks to me like your focus chart is at an angle? Maybe the two sides aren't on the same focus plane.


Brandon
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wimg
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Mar 24, 2009 07:30 |  #5

Ok, this does sound a little like a decentering problem, which can be fixed by Canon. You'll need to do some more testing, however, to make sure.

Align the camera with a wall, i.e., sensor perfectly parallel to the wall, either a wall made of bricks with a fair amount of structure, or a wall with some newspaper pages affixed to it, and take test shots at different FLs and apertures, in a structured way, and check if the sharpness changes with aperture as well. It should, under normal circumstances and (light) decentering. Also check at which FLs it is worst.

This way you can make sure it is decentering, and give Canon some extra information with regard to any fixes required. I'd like it if you could post a few samples as well, at the worst focal length, at different apertures, to help you in determining exactly what is going on.

Please do use a sturdy tripod, a remote switch, and MLU to make these test shots.

Kind regards, Wim


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kit
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Mar 24, 2009 07:43 |  #6

yeah, I noticed the focus chart was at an angle, :oops: with a minimum focussing distance of 1.4m it was difficult to know it was lined up properly :) but even so, if it was at a slight angle I would still expect the plane across the centre of the image to be same level of sharpness and contrast, and in this case the contrast is quite different.

wimg: i'll take a few more test shots and post up my results.. thanks :)


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wimg
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Mar 24, 2009 07:51 |  #7

kit wrote in post #7586944 (external link)
yeah, I noticed the focus chart was at an angle, with a minimum focussing distance of 1.4m it was difficult to know it was lined up properly :) but even so, if it was at a slight angle I would still expect the plane across the image to be same level of sharpness and contrast, and in this case the contrast is quite different.

wimg: i'll take a few more test shots and post up my results.. thanks :)

Just do make sure everything is very well aligned. A slightly angled test chart, because it is rather close, may give this impression too. Most of the time, decentering will not be right at the centre left or right, often it is offset, anywhere, f.e between centre left and corner left bottom. Another giveaway is that at large apertures with decentering you either have the soft spot sharp, and nothing else, or everything but the soft spot, with a fairly rapid transition from soft to sharp in the image.

With a good lens, I'd expect sharpness to fall off evenly going from centre to all corners, so that is what I am looking for in the suggested test shots.

Kind regards, Wim


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kit
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Mar 25, 2009 07:21 |  #8

I did some further test and I can confirm that contrast and sharpness was noticably better on the right than the left of the image. This included some more focus test shots and a newspaper on the wall scenario.
I did back to back shots with my 70-200mm f4 IS @f4 on the same body and it displayed no such problem, IQ was consistant on both sides, and its true what everyone says that the f4 is sharper than the f2.8... significantly too!

at 200mm, f2.8 the image was soft as I expected, but interestingly enough, when I stopped it down to f8, the focus test chart showed that it was actually back focussing by around 14mm! and at 14mm it was really sharp, where as the focus point was OOF...
maybe this is common on all of these lenses and hence why everyone says 200mm @ f2.8 is no good.


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bohdank
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Mar 25, 2009 08:01 |  #9

You should also try manually focusing usng Liveview, then without touching the focusing ring, AF and see if the lens changes focus (watch the distance scale for any movement). This will help to tell you if the AF is off. You can also then look at Liveview and see if the focus has now changed.


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kit
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Mar 25, 2009 08:25 as a reply to  @ bohdank's post |  #10

yeah good suggestion bohdank... will try that!


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vadim_c
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Mar 25, 2009 08:56 as a reply to  @ kit's post |  #11
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I had similar paranoya when I bough mine. After all you are right it is not cheap.
So... Take a measure tape. If you do not have it buy at home depot or lowes, very useful and cheap thing. Dont try a substitute for example a rope.

Now put some target against the the wal, for example a couple of newspapers, use an electric tape or scotch to fix it.

Take the measure tape and find exact middle point between outer edges of the taget. Do it very precisely, do not skip.

Set the camera on a tripod approximately perpendicular to the wall and against the center that you found. Take the measure tape again and make sure the ditance from the tip of the lens to the edges is strictly equal. Move the tripod as needed. The lens should always be aimed at the center.

Once you do that take a shot is inspect the right and left areas, The sharpness should be the same and most probably it will be. If not send the lens to Canon, they should realight it very easily.
I must stress once again: Do not trust your eyes on that. Take measures very precisely.


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70-200 f2.8L IS image quality different from left to right
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