front AND back focusing related to previous focus point

Hi All,
I have been having issues with my 70-200 f2.8 II missing focus quite a lot lately. I've painstakingly set, reset the MA but have ben frustrated with it missing focus. Last week I was trying to get it right again when I noticed if I focused behind the target then focused on the target it would back focus. Same thing would happen if i focused in front of the subject, the focus plane would shift forward. Not as badly as the back though.

You can see that the two focus planes don't meet.

Is this the lens having problems or could it be the body?

Here's a couple of pics. these are ~100% crops and sharpened up to about 90. These are both shot at 200mm and I was about 9meters from the target. The driveway slopes up from where I was shooting and the target was parallel to the camera. The target was vertical so was not sloping forward or backward. I used spot focus in one shot mode and focused on the bottom right hand corner of the target, just ups and left of the focal logo.

Thanks for any feedback.

Focusing behind first.

You might find the area of the focus sensor actually goes onto the drive & picks up the contrast line in the pavers - I'd be curious to see what results you get if you put your focus point smack in the middle of the focal target instead of on it's edge.

Actually, the top one was in the middle of the target. This is a screen snap from DPP.

swoffa wrote in post #17273916
Actually, the top one was in the middle of the target. This is a screen snap from DPP.

thanks - it's often hard to tell how much of the pics are actually covered by the focus sensor & a lot of folks assume it's exactly the same size as the red box (which it isn't)
Still doesn't help your problem though - sorry!

Anybody have any clues if this would be a lens or body issue?

You might want to send both to a Canon service center for alignment. I'm detecting that this combination is the only one you're unhappy with, so it's probably the lens. Sending both in should resolve this to your satisfaction. Good luck.

One of the MFA issues I found when using FoCal was that while my target was about 20 feet away, most of my photography is done from 50-150 feet away. What resulted was the handful of shots I shot from, say, 15-25 feet were focused right where I wanted them to be, anything beyond that distance were just a tad soft.

So, I purchased a slanted-ruler type of MFA tool and set it up in my back yard on a sunny day, about 85feet from tripod to target. The results were noticeably better. I haven't touched the MFA since.

As for perceived problems with the 70-200, I'm guessing you are shooting at a fairly close in target and setting your MFA accordingly. You may want to re-do the MFA from a longer distance. Also, does your camera allow 'wide' and 'tele' MFA settings for that lens? I set both ends on my 5Diii for all 3 of my zooms (16-35, 24-105 and 80-200) in my back yard that day.

Also, be sure to use something like f5-f8 or so when doing MFA. Most lenses are not at their sharpest at or near wide-open or stopped down to the max.

Thanks for the response.

I have the 5D3 as well and have set both wide/tele as you have done. Although I have purchased the focal software I don't use it. As you may know Focal is not automatic with the 5D3. I seem to get better results when using the dot method( I think that what is known as). I just use the Focal template printed on A3 paper.

Anyway, when I first purchased the 70-200 it was amazingly sharp at any distance. I could zoom way in on images and eyelashes etc were as if they were stuck to the monitor. Gobsmackingly brilliant. Nowadays it seems everything is just off focus, sometimes a foot or two out. It pains me so.

I did shoot these at 2.8 so I might have another go at f5 or f8 as you suggest. Though I do wonder if the resultant plane will be where f2.8 is.

DotTune.

I would give MA a try once again at a distance just beyond infinity focus at the long end. I recall reading while learning about DotTune method that this provides the most reliable reading.

I'm not sure a narrow aperture will benefit much, it seems to me you'd want the narrowest DOF. Doesn't the AF system calculate focus by taking its reading wide open anyway, regardless of set aperture?

If the lens worked well before and now is showing problems I'll go back to my original thought, it may he time for a service center visit.