My TC seems to be front focusing - any ideas?

I attached my new (well, bought used a few months ago) Canon 1.4x TC mk1 to my 70-200 f4L and 1D, and it seems to be front focusing. Without the TC, all pictures are sharp where I expect them to be sharp. With the TC, the further away the object, the further the focus plane seems.

Shots can be seen at:
http://picasaweb.googl​e.com/ztross/TestShots

For example, on the car mirror and lamp post, they are reasonably sharp (although note on the car mirror the focus plane is slightly in front of the mirror), but then on the picture with the house the focus is on the fences and the house is OOF (I was definitely aiming and watching the AF points illuminate on the house). The license plate shot shows that it OOF completely.

Wide open (ie, f5.6), high shutter speeds (about 1/400-1/500), ISO200.

Hmm, this is interesting, because I've just experienced the same thing.

Took out my 200 f2.8L to take a few pics and then put on the Canon 1.4x TC (Mk II) when I saw some birds. When I downloaded the pics later, I realized that those pics taken with the 1.4TC were all front focused - not a great extent, otherwise they were sharp with just the 200mm.

Don't mean to hijack your thread, just adding my experience here as well.

Perhaps someone out there already had similar issues as these and would offer some feedback.

(BTW, I posted a bird shot in the birds forum here, the front focus is clearly evident in the pic... I'll post another here if requested).

I think you just have your focus point off. go out and practice more and see what you come up with.

hi godzakka,

I did a quick test today to see what, if any difference, a combination of lenses, filters would make on the results.... my front focus issue seems to be solved when I take off the front filter (B&W UV Haze F-Pro). The image is much clearer and sharper without it (when using a 1.4xTC with my 200L f/2.8 ). The filter seems to have no effect on a straight 200mm.

Here are my results:
https://photography-on-the.net/forum/showthre​ad.php?t=315392

That sounds interesting. Is the F-Pro a multicoat filter or a single coat?
The current B&W multicoats are all labelled MRC.

kolok - thanks for the tip. I will try taking my filter off and take a few practice shots and let you know how it goes. Your thread gives me hope.

rklepper - while I appreciate the insightful comment, I have been doing this for several years and haven't had issues with my focus points before, so I highly doubt that each and everytime I use a TC I am misfocusing and when not using a TC I have returned to focusing correctly.

Using a TC on a 70-200 will affect IQ and reduces your margin of error for focus. Lose the filter.

Lester -- I'm really not sure! I suppose it's just a single coat, no MRC tag on it, just says B+W UV Haze F-Pro...

godzakka - looking forward to your results - as I mentioned my other thread, try a number of shots in sequence of the same subject (everything else being equal) first just the 70-200 with filter, then without; then 70-200 with TC with filter, then the same without filter ... If your results mirror those of my thread then it's safe to assume that the TC doesn't like a filter on the front of a lens... which is a bit strange because I've never had such issues shooting slide on my Elan7e. Should check out other bodies if this applies as well... or other filters for that matter.

I've got two 67mm filters, so I will perform the tests tomorrow during a break at work or when I get home, and I will try both filters (not sure what they are, but one was on my dad's pentax 6x7 lenses, so it's OLD!). Will report back tomorrow night.

kolok wrote in post #3163194
Lester -- I'm really not sure! I suppose it's just a single coat, no MRC tag on it, just says B+W UV Haze F-Pro...

godzakka - looking forward to your results - as I mentioned my other thread, try a number of shots in sequence of the same subject (everything else being equal) first just the 70-200 with filter, then without; then 70-200 with TC with filter, then the same without filter ... If your results mirror those of my thread then it's safe to assume that the TC doesn't like a filter on the front of a lens... which is a bit strange because I've never had such issues shooting slide on my Elan7e. Should check out other bodies if this applies as well... or other filters for that matter.

If you are going to this test (with and without filter) remember a few procedural points:

a) You will need to refocus after removing/adding the filter.
b) There will be a statistical spread of AF precicision , IE it is not perfect. So you can't just compare two shots with and without filter.

My test for this involved taking 10 shots with and 10 shots without filter with a new AF action between each shot.

To measure sharpness and focus without subjective effects I then used software to measure the MTF 50% point and looked at the statistics of this. If you can't do this you will have to rely on subjective judgement.

I tested several lenses and found no impact from a good multicoat filter, I have not checked a TC yet but I would not expect this to change anything.

Results are here http://www.zen20934.ze​n.co.uk …Tests/Filter_AF​/index.htm

"I have not checked a TC yet but I would not expect this to change anything."

That's exactly the point kolok and I are bringing up - we wouldn't expect anything to change, but as kolok said, it does for him. I hope it does for me. As soon as lunch comes I plan on going outdoors to try this out and prove it right or wrong (at least with my equipment). I will take multiple shots, though, so thanks for the advice Lester.

godzakka wrote in post #3164738
"I have not checked a TC yet but I would not expect this to change anything."

That's exactly the point kolok and I are bringing up - we wouldn't expect anything to change, but as kolok said, it does for him. I hope it does for me. As soon as lunch comes I plan on going outdoors to try this out and prove it right or wrong (at least with my equipment). I will take multiple shots, though, so thanks for the advice Lester.

OK the reason I have said this is I have tested the 200mm with both the 1.4X and 2X on and a filter, I don't think I would have got the excellent results I did is this filter had a significant effect. Another reason is it is difficult to see what effect of physics could cause a problem with a TC added when it does not on the lens alone.

But I am not prepared to say it does not have an effect with a TC as I have not specifically tested for that at this point.

Even though it is a single coat filter rather than a top end filter I am still surprised there appears to be an effect.

Let me stress that when I tested for this I had to use enormous care because the differences between with and without filter were less (undetectable) compared to the variation in AF from shot to shot.

Hello,

Lester, you're right about doing a comprehensive test. However, the results as I posted in the other thread are very consistent from one shot to another. Seems that as soon as I introduce a filter, the lens/tele combo front focuses. I'll need to find a B+W MRC filter to see to what extent, if at all, that would fool the AF on my cam.

Also looking forward to seeing godzakka's initial test results.

Cheers for the input!

I'll post this in both threads on this subject for completeness.

OK I have tested the 200mm + the EF 1.4X and not found a significant variation in sharpness due to the filter.

Interestingly there was much more variation in sharpness with the teleconvertor than without. But the variation was, within statistical limits, the same with and without the filter. Also the variation was small enough to not be easily detectable in the 100% crops and probably requires software analysis to spot.

I have updated the writeup on the tests here, this page also includes a link to a page with the 100% crops.

Well...

It seems to help in some shots, but I hastily took my shots today and not anywhere near as scientific as Lester would've liked. I will post them to my site here: http://picasaweb.googl​e.com/ztross/TestShots​2

And try again tomorrow, but with better shots. The first set are 200mm, filter, no TC. The second set are 200mm, filter, w/TC. The last set is 200mm, no filter, w/TC. Again, probably not much use in posting them, but you are welcome to look at them and if need be I will put up full crops later.