Filter shifts focus

[kphoto99]

Here is a first for me, a filter shifts the focus.

I have 70D and 70-200 IS 4 lens. I calibrated the AFMA to be +3 for the tele end, everything works fine.

I put a (cheap) filter on the lens and didn't think anything about it. Went to shoot some presentation on stage. I was about 50 feet from the stage. Used the lens at f4.0 200mm. After I went home and examined the photos, I could see that the camera was front focusing by about a foot. The microphone in front of the speaker was in perfect focus and sharp.

I figured that I need to calibrate the AFMA, so I setup a dummy head outside in front of a brick wall and the camera 50 feet away. First I focus in live view, turn the AF switch on the lens to MF. Can't find any AFMA setting that will give me focus confirmation. So I examine the photo taken with live view and I see that the wall is in focus, not the dummy. Try again, same result. At this point I figured that the camera and lens will have to go to Canon for some serious calibration.

For some reason I decided to remove the filter and try again. Now everything works perfectly, live view focus on the dummy and PDAF with AFMA of +3 focuses perfectly also.

Conclusion, this filter makes PDAF to front focus by about a foot, and DPAF back focus by a foot. Other then the shift in focus the images are sharp.

This is very weird.

 

[YuengLinger]

Care to share the brand of filter?

What happens when you hold the filter up in front of your eyes and look at things close, middle distance, and farther away? Do you see any strange optical effects?

I wonder how often this happens and we blame AF?

I think B+W has had price drops, especially on the smaller filters, such as 67mm and below.

 

[kphoto99]

Care to share the brand of filter?

What happens when you hold the filter up in front of your eyes and look at things close, middle distance, and farther away? Do you see any strange optical effects?

I wonder how often this happens and we blame AF?

I think B+W has had price drops, especially on the smaller filters, such as 67mm and below.

The filter is branded Monoprice, if you are familiar with them, they are very good for cables and other stuff. This filter was on clearance for about $5.

Now I have tried the looking though it with my eyes and for far away objects the image shifts when I move the filter. I think there is a curvature to the surface.

I'm going to look at the B+H filters now.