70-200L 4IS - shifting minimum focus distance

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Archive 2008 · 70-200L 4IS - shifting minimum focus distance

Daan B
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p.3 #1 · 70-200L 4IS - shifting minimum focus distance

DynoMoHum wrote:
I do not believe these issues are totally related to mis-focus. It's easy to see that there is a loss of contrast, you can also see that even at the point of actual focus, the contrast is not what it is when focusing at further distances. I haven't done it yet, but it should be easy with live view to get around any issues of AF mis-focus and then verify that the contrast is NOT as good, regardless of how accurate the focus is or isn't.

It could be... But the loss of contrast could also be a side effect of a possible backfocus. When shooting 3D subjects at 200mm MFD I can see a backfocus very clearly. Where it is in focus, I don't see a loss of contrast (besides the normal loss of contrast when shooting wide open).

Feb 22, 2008 at 05:07 PM

Dawei Ye
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p.3 #2 · 70-200L 4IS - shifting minimum focus distance

I haven't had mine calibrated, but you could be onto something, I have noticed better AF performance stopped down and I usually shoot at 5.6

The thing is my copy is so sharp otherwise, that I don't want Canon fudging it up if I send it in. I may have to because my 40D is going in eventually (I've been saying this for 3 months) to get its issues sorted out, so we'll see how it is afterwards. I'm tempted to throw all my Canon gear in, but then I don't want them coming back misfocusing with my Tamron 17-50

Over on POTN, a poster mentioned that the MFD issue only affects a handful of units and is not within spec, but reading Mfurman's links it looks like there are many users of this lens who find this issue.

Feb 22, 2008 at 11:04 PM

Dawei Ye
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p.3 #3 · 70-200L 4IS - shifting minimum focus distance

Actually after searching and reading through so much, I still have NOT found 1 acceptably sharp pic from anyone at MFD and 200mm. Unfortunately Jammy's thread's pics are offline now

Feb 22, 2008 at 11:22 PM

mfurman
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p.3 #4 · 70-200L 4IS - shifting minimum focus distance

DynoMoHum:
I do not believe these issues are totally related to mis-focus. It's easy to see that there is a loss of contrast

I agree. It is not misfocus but some impact of focus shift associated with residual spherical aberrations.

I also think that at f/4.0 and MFD there is no "masking effect" of misfocus. especially at 135 mm.

Feb 23, 2008 at 12:35 AM

DynoMoHum
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p.3 #5 · 70-200L 4IS - shifting minimum focus distance

I've done more testing, I'm pretty convinced my camera does have this issue. However, I'm also pretty convinced that it's a design issue and that it's probably pretty much like that for all of these lenses. I have no way to know that for sure but that's what my gut tells me. I haven't decided yet if this issue is enough to throw the baby out with the bath water so to speak... The lens as a whole seems to be quite nice, and if this is it's only flaw, then it's still overall a very good lens. I also do not know if there is any other zoom of this focal range that is any better. Now, if this issue is something that can be fixed or it is possible to get a copy that does not have this issue, then perhaps the lens would be more or less perfect. I personally would hate to think that my lens is less then it should be, that is... if it can be made to work better, then I would like to have it work better. If this is as good as any of them ever get... I'm not sure if I will be willing to give up on all the other good things that come with it...

Here are crops from two different shots taken at 200mm f4 MFD, the first was a attempt to use autofocus, the second I used my Canon 40D's Liveview and 10x zoom to manually focus. In the autofocused shot, not a single point in the photo of my work cube's wall was in focus, in the manually focused shot, about half was the other half was not, this being due to the fact that the camera's sensor was not perfectly parallel to the wall(user error, not the camera's fault). The second crop is clearly much more in focus then the other... The point of this is to show that if the lens does get focused correctly at MFD, it can be reasonably sharp... (perhaps not perfectly sharp, but reasonable none the less)... The problem... AF doesn't seem to be able to focus that well at MFD and 200mm.

I should also add... that assuming all of this is related to SA, it's probably virtually impossible for the AF system to correctly AF given the SA problem.

This image is copyrighted by the owner

Edited on Feb 23, 2008 at 03:06 AM

Feb 23, 2008 at 02:56 AM

Daan B
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p.3 #6 · 70-200L 4IS - shifting minimum focus distance

Thanks DynoMoHum for posting these pics. It clearly shows that (reasonably) sharp pics can be made at MFD 200mm. This is what I've experienced also. The fact that you can achieve this by manually focusing instead of AF points towards a possible AF problem. My first thought then would be that there is a back/front focus issue. But if this is a non correctable design issue (focus shift/RSA) or something that can simply be solved by calibration is not yet clear to me. Like I said, nobody with MFD problems had their lens calibrated yet (at least who participates in this thread up till now). I hope to get some answers after my visit to Canon Service coming Tuesday

Of course it does not help that every user has a subjective opinion on what is sharp or not. Maybe, the level of sharpness that you've achieved by focusin manually is within specs.

If the MFD problem isn't solvable, I will keep the lens anyway. I only came across this problem by doing test shots on flat surfaces. In real world shooting, doing portraits and other distant stuff, it performs excellent. It is very sharp, has nice contrast, neutral bokeh, killer IS, etc... So, I agree with you, if this MFD flaw is it's only weakness... well, there are worse things in life

I am done with it for now... It's time to enjoy this lens

Edited on Feb 23, 2008 at 07:22 AM

Feb 23, 2008 at 07:07 AM

DynoMoHum
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p.3 #7 · 70-200L 4IS - shifting minimum focus distance

Keep us posted on what happens... I'll be very interested to know if Canon can make 200mm MFD work well on these lenses.

Feb 23, 2008 at 12:24 PM

Daan B
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p.3 #8 · 70-200L 4IS - shifting minimum focus distance

DynoMoHum wrote:
Keep us posted on what happens... I'll be very interested to know if Canon can make 200mm MFD work well on these lenses.

Will do

Feb 23, 2008 at 12:40 PM

tmr4
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p.3 #9 · 70-200L 4IS - shifting minimum focus distance

This thread reminded me of the 50L focus shift discussions. In the case of the 50L the plane of focus shifts backward up to a couple cm when shooting close in and stopped down a few stops (my copy is worse at about 3 ft and f/2.8). The sharpness of the 50L is just fine in this case it’s just that the actual focal plane is behind the intended focal plane. Popular belief is that uncorrected spherical aberration is the cause and that this was deliberate in the design to increase the quality of the bokeh.

It seems as if something similar is happening here. I don't think the MFD is shifting between 135 and 200mm but that perhaps uncorrected spherical aberration is degrading the image quality close in at the upper end of the zoom range. At least with my copy the lens focuses fine at the MFD at all FL but as with others the IQ is reduced at 200mm compared to 135mm. Thus I guess one could say that the focus distance needs to be greater at 200mm to get the same sharpness as at 135mm.

I didn't test this directly but with my lens the sharpness at 200mm is fairly close to that at 135mm by 6 ft and a bit better than that at 10 ft. I did do a quick test for posting here at MFD to show that the IQ is reduced at 200mm compared to 135mm. Here is my set up (upper region in center is a flat focus target with a lower angled region to show the DOF):

This image is copyrighted by the owner

Here are two 100% crops at about the MFD, both sharpened to better show the IQ differences. They show that IQ is indeed degraded at 200mm at MFD compared to 135mm. These were manually focused and with the standard testing precautions. A link to the unaltered original (other than conversion from raw with 0 sharpening) is below each crop.

70-200 f/4 IS @ 135mm and 51" FD

This image is copyrighted by the owner

http://www.pbase.com/tmr4/image/93383214/large

70-200 f/4 IS @ 200mm and 51" FD

This image is copyrighted by the owner

http://www.pbase.com/tmr4/image/93382699/large

I did do a few additional shots with this setup to see if the IQ at 200mm improved as the focal plane was moved backward. Within the limits of the FOV for this test (just a backward shift of several inches) I didn't notice any difference in the image quality. Based on my observations above I would say that the improvement in IQ at 200mm occurs gradually from MFD through 6ft with some increase still beyond this.

Feb 25, 2008 at 01:27 AM

mfurman
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p.3 #10 · 70-200L 4IS - shifting minimum focus distance

DynoMoHum:
The lens as a whole seems to be quite nice, and if this is it's only flaw, then it's still overall a very good lens.

Completely agree. The lens is excellent. Canon should not have listed 0.21 as maximum magnification though.

Feb 25, 2008 at 01:38 AM

mfurman
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p.3 #11 · 70-200L 4IS - shifting minimum focus distance

tmr4:
Based on my observations above I would say that the improvement in IQ at 200mm occurs gradually from MFD through 6ft with some increase still beyond this.

Nice test. I still see some contrast degradation though. In my experiments, the focusing distance of 6 feet, was definitely showing better results than at MFD but I usually needed at least 7 feet to accept the image for print. At slightly more than 8 feet, things get to be normal but I usually (like you) set the focus limiter to 3m to avoid any problems at 200 mm.

Feb 25, 2008 at 01:47 AM

mh2000
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p.3 #12 · 70-200L 4IS - shifting minimum focus distance

I've suspected similar behavior from my non-IS version... the annoying thing is that you get an AF confirmation and think you're good. Anywhere away from MFD my lens is stellar, but for me, this reduces the usefulness. It's most annoying when you are just kind at the edge with anotherwise nice, but not quite sharp enough image... now all of a sudden the cheap primes blow away the L.

Feb 25, 2008 at 02:39 AM

Daan B
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p.3 #13 · 70-200L 4IS - shifting minimum focus distance

Ok, today I had Canon Service take a look at my 70-200L 4IS.

First the tech measured focus at a distance of 2.5m. He said the lens had a backfocus from 135mm to 200mm. Then he measured focus at 3.5m. No backfocus at all. He couldn't explain the difference to me. For that he would have to investigate the lens further. He could calibrate the lens to perfect focus at MFD over the entire range. But that would probably mean a performance penalty at other focus distances. So I didn't have my lens calibrated. He also said that the lens was focusing according to Canon's specifications. Although with a very high tolerance. I asked him if he came across this MFD issue in the past. He wouldn't answer directly. He only said that people didn't use this lens at 200mm MFD very often. Unfortunately not a new insight, only a confirmation of what I had concluded myself already. So much for that.

For me it doesn't really matter. I am not going to use this lens at 165mm-200mm MFD. I know it's limitations. I also know that this lens is designed this way. I can get acceptable results at 165mm-200mm MFD for certain purposes. Furthermore, I haven't seen one pic shot at 165mm-200mm MFD that is sharper than what my copy can produce.

Edited on Feb 26, 2008 at 02:09 PM

Feb 26, 2008 at 02:08 PM

DynoMoHum
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p.3 #14 · 70-200L 4IS - shifting minimum focus distance

Thanks for the update... It's pretty much what I figured... This basically is a design flaw it would seem... that is, focus at 200mm MFD is not going to be as good as it is at other distances...

[Edit] I do not know the exact distance to this face in the memorial... Originally I was thinking it was indeed very close to about 5 feet, but the more I think about it, it must be further away then that... Perhaps sometime soon I can go back and figure out exactly how far away it is... I'm all but certain it's no more then 7 or 8 feet away]

For what it's worth... the photos taken by me with the 70-200mm f4 IS, in the following post I made at dpreview were made at very close to MFD, probbably like 5 feet away, and this was/is at 200mm f4.5 At least the ones showing the face in the memorial that are taken straight on... the ones taken from the side showing focus accuracy were more like 7 or 8 feet away from the focus point...

http://forums.dpreview.com/forums/read.asp?forum=1029&message=26921865

Bottom line... this really isn't much of a problem for me, I never really expected this lens to be used for macro photography.

Edited on Feb 26, 2008 at 02:56 PM

Feb 26, 2008 at 02:24 PM

tmr4
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p.3 #15 · 70-200L 4IS - shifting minimum focus distance

I would term this more of a design compromise than a design flaw. It is too bad that comprehensive performance characteristics for all lenses aren't available so one knows what to expect prior to purchase. If they were then everyone would know ahead of time what they were getting into. I suppose that would result in fewer lens sales though.

I have used the lens at around MFD and 200mm. This is a very tight head shot on a crop body. I've gotten acceptable sharpness for my purposes. Of course this might not work for everyone where critical sharpness is needed. Still for me this is one of my sharpest lenses wide open. The 135L gives a somewhat similar FOV at its MFD and it is sharper than 70-200 f/4 IS wide open and at MFD. I find I use it more close in that the 70-200.

Feb 27, 2008 at 05:32 AM

Yakim Peled
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p.3 #16 · 70-200L 4IS - shifting minimum focus distance

Leon Noel wrote:
Ok Daan I ran a few simple test shots (IS off, ~ MFD, on tripod, triggered with wired remote), here's my findings (bear in mind I believe my copy backfocuses a little, a few mm for short focus distances)

- At 70mm it beeps if I half press the shutter on remote. Picture came out sharp.
- At 100mm it flashes but no beeping, shutter would not trip (no AF obtained), so I moved the tripod back away from the subject, about 1-2cm, then it beeped. So there was a tiny shift here. Picture came out sharp.
- At 135mm it beeped again, sharp result.
- At 200mm it beeped also, sharp result.

So the only MFD shift I experienced was from 70mm jumping to 100mm, and even so it was only 1-2cm. I don't know why Canon says MFD at 200mm can be about 3.9m !!!

Hope that helps

, good luck and happy shooting.

Mine was the same. Tack sharp at MFD at all focal lengths. I would have kept it but the aperture issue bugged me. Not enough background blurriness at f/4 and slow AF in low light.

Happy shooting,
Yakim.

Apr 01, 2008 at 11:31 AM

agedbriar
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p.3 #17 · 70-200L 4IS - shifting minimum focus distance

In his article "33 Techniques for Creating Sharp Images", under par. 24., Ariel Bravy mentions this 70-200 f4 IS issue as being a known flaw.

http://www.wonderfulphotos.com/articles/cameras/sharp_photos/

I also didn't know anything about it when I purchased the lens thinking that I would be able to use it for near-macros of flowers as well. Of course I'm happy with the 70-200 f4 IS, but I did buy a true macro lens after learning that those soft close-ups of mine were not operator errors but rather were due to a design limitation of the zoom.

Apr 12, 2008 at 11:40 PM


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