focus loss using polarizer on telphoto

[lightrae]

I have encountered a problem with focus when using a polarizer on my telephoto lens.
To be more specific: my 35-350mm Canon L zoom lens can not be focused through a linear polarizer at magnifications greater than about 135mm; neither manual or auto focus.
I've tried several polarizers, all of which work perfectly on my 28-135 IS and on the 35-350 at magnifications <135mm.
I'm using a Canon D60 body, usually shutter priority, jpeg large or raw.
I've never encountered this problem with my 35mm Pentex film body, at magnifications up to 1000mm.
Any and all info would be helpful.
Ralph

[marxz]

I have encountered a problem with focus when using a polarizer on my telephoto lens.
To be more specific: my 35-350mm Canon L zoom lens can not be focused through a linear polarizer at magnifications greater than about 135mm; neither manual or auto focus.
I've tried several polarizers, all of which work perfectly on my 28-135 IS and on the 35-350 at magnifications <135mm.
I'm using a Canon D60 body, usually shutter priority, jpeg large or raw.
I've never encountered this problem with my 35mm Pentex film body, at magnifications up to 1000mm.
Any and all info would be helpful.
Ralph

well if your using a linear polariser then that's your main problem... autofocus and exposure accuracy will sometimes suffer depending on how the camera focus/exposure sensor works

D60's are not very good with their auto focus compared to later models they have a lot of low light issues and suffer a lot from slow lens and/or using linear polarisers

... also, even if you are using a circular polariser as you're also effectively stopping down the lens around 1 stop then the D60 is going to start to have serious problems auto focusing I don't know about the 30-300 but if it's wide open aperture is around 4.5 at 135mm of zoom then your effective aperture (stop wise as far as the autofocus system is concerned) has already crept over the recommended 5.6
I have this problem with mine when using a polariser with my 50-500 Sigma on my my D60 - anything from around 200 just hunts focus for a while by 300 it's simply faster to manually focus.

[lightrae]

Thanks MARXZ, for taking the time to reply.
I recognize the issuses with f-stops, hunting auto-focus, ect. that you raised. However, if I understand you correctly, you can achieve manual focus.
But I'm talkimg about IMPOSSIBLE to focus at high magnification, neither autofocus nor manual focus.

Yesterday I shot the same scene at 35, 100, 135, 200 and 350 mm.
I used 4 different polarizers (3 linear & 1 circular).
No exposures were under f-16 and every shot was done twice; manual and autofocus.
Every shot at 135mm and under was perfectly focused... no hunting, no delay......
But very shot at 200 and 350mm was out of focus.
There was no unusual delay or hunting with autofocus, it simply stops out-of-focus. Switching to manual was no help. Impossible to focus with a polarizer in place.
I should mention, too, that this only happens with a polarizer. I can use other filters and I have never encountered any problem with focus before, with any filter.

I can live without a polarizer at the higher magnifications, but it's frustrating to constantly mount and un-mount the filter as I change magnification.
I know that I can (and eventually will) change equipment and problem will disappear. But I feel as if I'm missing something very fundimental here and I should very much like to understand the basic cause(s).
Both body and lens have been serviced by Canon within the past 6 months and work perfectly in all other respects.

I would appreciate any additional thoughs, no matter if they are "off-the-wall". It's looking like a unique problem so any explanation could be unique as well.

Ralph

[jeffryz]

Without having enough infomation, I am wondering if you were too close to your subject for the camera to focus "at high magnification". A longer lens has a farther minimum focusing distance. Also the fact that you were shooting at f/16 is reducing the amount of light able to get to the focusing sensor and at the longer zoom you are cropping out more light. If minimum distance was not the problem maybe you need to lower your f/ stop.

[lightrae]

Thanks jeffryz. Your points are valid, but in this case I have more than ample distance.
Also, my aperture isn't the problem. I only mentioned used f-16 to emphasis that I was shooting well within the lense' range, under bright sunshine.

Perhaps I'm not clear enough about the nature of the problem.

I have a lens (35-350mm, f3.5-f5.6) that works perfectly. I add a polarizer and assume that I've lost about two f stops, effectively leaving me with an f-11 lens. So on high magnification, I shoot with f-16 or better .....just to be safe.

But as I've stated before, I can't achieve a focus at magnifications greater than 135mm; not on manual or auto focus. The polarizer works fine at the lower magnifications.

[dang]

With most modern automatic DSLR's, the f/stop shound NOT matter, since the lens is wide open during focusing and only stops down once the shutter is pressed. However. you must use a Circular Polarizer, with Digtals. Standard ones will not focus properly regardless of what you do, as far as I'm aware.

There may also be another problem with how you're using the camera it's self. I'd suggest using only the center point to focus, and once you've half pressed the shutter and focused, keep holding the shutter at half way, and reframe your shots. Using multi point focus can be confused, and tends to focus on the brightest point with the most contrast.

Alos, to achieve sharpest results, it's usually best to stop down 2 to 3 f/stops from wide open. And depending on what lens you're using, it could be the lens it's self is just soft at the longer end. More information, with links to example shots showing the exif would help a lot.

On a side note, be sure also that your shutter speed is around double the focal length of the lens you're using, unless you have very steady hands or use a tripod. Many times people confuse motion blur as "out of focus", when in fact they're two separate things. And if you're shooting at f/16 at 300mm plus using a CPL filter, it's going to take a LOT of light to keep the shutter speed high enough.

[lightrae]

As with the previous responses, Dang, you have hit some valid points but none help in this case because I have already eliminated those potential causes.
I'm fairly new to Digital but shot 35mm for some 34 years. Much of my photography has been "wildlife type" with lenses of 200mm to 1000mm so I'm aware of the problems of shake, low light et cetera.
My test shots used every technique I ever learned including remote release, heavy tripod, full sunlight, centre weighted metering, manual and auto focus, both linear & circular polarizers, ect.
Thanks everyone. It looks as if this is just a oddity about this lens.

[dang]

That's always a high possibility that the lens is just soft, and why I was asking for more information. With quality control becoming less persistent these days, you could easily have a bad copy. And also, don't forget to lock up the mirror, even if using a remote cable, since the shake from mirror slap can be exaggerated greatly with long exposures. Hope you get it figure out, sometimes things of this type can really be frustrating.

Good luck, and happy shooting.

[madlights]

All I can say is this is really strange. I, like others, can understand that because of dropping down the light wouldn't focus well in autofocus (or at all in auto) but why in manual?...and if a lens element was out of whack you'd see it without the polarizer. Maybe just ask Canon...cause this is strange...could it be you're boosting your ISO up to compensate for the Fstop penalty and getting noise which would amplify any irregularities or softness of the lens on the long end? Or even so if your exposures are darker it might add noise even without cranking the ISO up? Kind of grabbing at straws here...but it sure seems it shouldn't do that. I remember with a long film lens I had and a 2x converter (manual focus) if I'd use 400 film my pics would seem out of focus or soft because of noise..,but really mostly the higher speed just made it harder to tell. But then you're a very experienced photographer and you'd know the difference..this is strange. Yeah I'd ask someone at Canon. But that noise thing could just put a somewhat out of whack lens over the edge? Problems like this aren't fun eh? It's the manual focus thing that puts the wrench in the gears.

[dang]

I thought more on this too, and with a 10X zoom, it's not going to be as sharp at the shortest or longest mm as toward the center. But my question is, are you positive there's not a backfocus issue with the body it's self? I don't recall anything specific with the D60, but do remember the D10's had a focus problem. It might be good to have it checked though, since Canon has never been a company to admit to problems, even when they're aware of them. So here's my thoughts...

Since depth of filed will become more shallow at the longer mm, try using adding a couple of extra f/stops to help maximize it. Use a tripod, and mirror lock up for some test shots and see what happens. When doing your test shots, auto focus first, take a shot. Repeat autofocus, but switch to manual and focus slightly over, repeat the procedure and focus slightly closer then review your results. If you do several series in the same order, you should be able to tell if there's some consistency to the problem which you can manually compensate for. Be sure and do each series at different distance to subject also. This will also allow you to speak with Canon, and send them test shots proving there's a problem.

If you wouldn't mind posting the results, I'd like to see them, and I'm sure others would too, just in case we run across this type thing again.

Good luck, hope you get it figured out.

[lightrae]

Again, good thoughts, Dang.
I'm away from home for the next few weeks, but when I return I'll have access to a second D60 and a D30, as well as detailed tech info on the lens. I'll run some more tests then with the suggestions in mind and let you know the results.

Thanks again,
Ralph

[marxz]

- OK just a suggestion I can understand the autofocus with a high aperture ... kind of

but if this is happening in manual focus (and I guess it looks sharp in the view finder) have you checked your diopter adjuster on your eyepiece?
the D60's was (errr still is) a right PItA and I keep knocking mine all the time. I end up just putting a thin slice of gaffa tape over it.



- Dang but the MAXIMUM f stop of a lens does as wide open with a filter that might knock a couple of effective f stops off the already dim view may take it the wrong side of the sensors minimum F value for focusing the DOES happen on my Bigma at full extension it's 6.3 (theoretically already over the 5.6 minimum aperture rating for Canon EOS's) than whacking a polarizer on it it just hunts and never finds focus unless I lock it on to a really strong contrast edge (something with 4 or more stops difference along a sharp edge)

[lightrae]

Thanks MARXZ, I don't have any problem with accidental diopter changes. I use a 1.5 adapter under my eyecup. Everything sticks out far enough to prevent anything from getting at the diopter adjustment knob.
However, you did hit one thing that I failed to mention or at least failed to clearly explain.
When I say that I can't achieve focus at full extension with a polarizer mounted, I mean exactly that. What I see is what I capture. What I capture is what I see.
It doesn't back focus or front focus. It isn't a "mis-focus". It isn't slightly out of focus. NOTHING IN THE SCENE APPEARS TO BE IN FOCUS. The entire image is completely blurred. At NO point in the stop-to-stop, manual or auto rotation of the focus ring does ANYTHING ever come into focus; not in the viewfinder and not in the captured image. Furthermore, I'm using a great, long,south-facing, white picket fence that runs away at an angle for my subject. I can easily see my centre-of-focus to within +/- 6 inches over about a 100 foot DOF. Everything checks out perfectly without the polarizer and with the polarizer, up to 135mm.

The closest analogy that I can make is trying (unsuccessfully) to focus through a window.
I'm FAIRLY convinced that one (or both) of the filter glass layers is having a slight lensing-effect and distorting the image at high magnification. The exact how & why of the interaction would likely require an optics lab to determine, but this seems possible. Nothing else makes sense to me.
The ONLY variable, other than the presence or absence of the filter, is the physical position of the lens elements relative to each other and relative to the filter glass.

Does this make sense? Or is my thinking completely wrong-headed?

Ralph.
P.S.
This lensing effect seems even more likely when you consider that the macro for the Canon 35-350L is 135mm, the point where trouble starts.

[marxz]

OK now that IS weird.... ummm how about you take your lens to your nearest friendly camera shop and compare it head to head with another identical lens.....

but yeh that's weird

[madlights]

Ok if this is changing the optical path of the light. Say the lens or an element of the lens is focusing on the polarizer or being even influenced in some way by it...then adding a step ring or combination thereof between the front element and a polarizer ougtta change the optics...since it would change the distance from the polarizer to the lens? You could maybe at least get an indication that way. If it were the case, it might miss focus at a different distance or give some other indication? Good luck.