Canon 70-300 DO Autofocus has become inaccurate

[Mel_P]

Hello - I am not aware that I have dropped this or damaged it but I went out with this lens to test my new Kenko teleconverter on my Canon 5D MkIII and my 7D MkI and the images were all slightly blurred. I haven't used my 70-300 DO f4.5-5.6 IS USM for a couple of months.
(PS it's not the teleconverter - this is perfectly sharp with my 100mm f2.8 IS USM macro lens).

At home afterwards I scratched my head as to why my images of the Egret were blurred so I did some testing at home.

- if I do an autofocus and then switch to live view the image is not quite sharp - I have to manually focus slightly nearer to get sharp.
- if I do a manual focus in live view and then switch off live view and change to Autofocus the focus changes to slghtly further away but is not sharp
- manually focussed images are sharp.
- same results on 5D and 7D so it's definitely the lens.
- auto focussed images with my other lens are in focus.

So it seems an issue with my 70-300 DO lens - I have tried the AF microadjustment on the 5D and adjusting to -20 improves the focus but is not quite there. If I send it back to canon can they do some adjustment / repair (can't see how damaged - works perfectly in other respects)?

Anybody with knoweldge about these things please?

 

[D.P.]

I me pretty sure you will find the 70-300 DO is not approved for using teleconverters.

Also tele converters can often require a different AF microadjustment. If you had a canon TC you could send it with your body to canon for tuning. You can send the body and lens to canon but if you say it works fine without the TC then it is unlikely anything will be changed by much.


Also, I don't think either of your bodies support auto focusing at f/8?
EDIT: just checking your 7D definitely doesn't support AF at f/8 and your 5Dmk3 does but only with upgraded firmware. Either way don't expect focusing at f/8 to be fast or accurate, plus you will only get a small subset of the focusing points working at f/8 on the 5Dmk3


If you need more reach I would return the TC, sell the 70-300 DO and save up for the new canon 100-400mm L

 

[Mel_P]

Perhaps I haven't explained enough.

I just happened to notice the problem when I was using the teleconverter - my first reaction was to blame the teleconverter - but it's not that causing the issue. It seems to be the 70-300 lens itself.

- The same behaviour is observed with or without the teleconverter
- the 5D Mk III happily autofocusses at f8 - I have the new firmware

HOWEVER, whether it autofocusses at f8 is not the issue.

Auto focus works both with and without the teleconverter but the autofocus is inaccurate in both cases. After auto focus is achieved, with or without the teleconverter, the image is not in sharp focus but needs an extra manual "tweek" to focus the image sharply. Not a big tweek but enough for the captured image to be slightly out of focus. Same effect seen on 2 camera bodies.

PS - the 7D is not supposed to autofocus at f8 but with the kenko it actually does - I didn't expect it to!

Canon teleconverters are very restrictive in their use - only on expensive L lenses - As I suspect only then can they guarantee no drop in quality - but the kenko one works fine for me and others (from a web search before buying).

 

[D.P.]

I would send both your bodies and the lens to canon for tuning then.

As for the f/8 AF on the 7D, no camera purposely inhibits AF at smaller apertures but the phase detection sensors just don't work well below that. Even camera that can focus at F/8 performance is very poor compared to faster apertures. There is just very little information for the focus system to use in order to find focus because the light rays become very close to parrellel.


I also stand by what I said, if you find 300mm too short then adding a TC to such a lens won't help you resolve any more detail. You are better off just cropping the image and using the better autofocus and low ISO that the wider aperture brings.

 

[R B Customs]

Your bullet points about live-view just make it sound like a simple back-focus issue set on buy using the TC. The viewfinder and AF system are on a slightly different plane to the sensor (live view) this is a fairly typical side effect of using most kinds of converters. I have to admit though, I didn't think a TC would give you so much grief.

 

[D.P.]

Your bullet points about live-view just make it sound like a simple back-focus issue set on buy using the TC. The viewfinder and AF system are on a slightly different plane to the sensor (live view) this is a fairly typical side effect of using most kinds of converters. I have to admit though, I didn't think a TC would give you so much grief.

Everything is built within a tolerance, the mount from the sense is like 46mm +/- 0.5mm. The AF sensors are also placed within +/- 0.5mm say. Then lens is built within another tolerance. The dimensions may all be within the tolerance level but if they all have the same sign then they add up.

Adding a TC adds one more link in the chain that has its own tolerance. plus this is a Kenko TC whose tolerances are likely higher than Canon's.

This is why a naked lens can be fine but adding a TC gives back focus issues. Sometimes lenses are slightly out with no TC but adding a TC makes it really bad. Plus the change in perspective makes the same kind of focus error look much worse even if technically the difference is the same


In general you should avoid TCs wherever possible and putting on lenses that aren't design for TCs can only cause more problems.

 

[Participant]

Everything is built within a tolerance, the mount from the sense is like 46mm +/- 0.5mm. The AF sensors are also placed within +/- 0.5mm say. Then lens is built within another tolerance. The dimensions may all be within the tolerance level but if they all have the same sign then they add up.

Adding a TC adds one more link in the chain that has its own tolerance. plus this is a Kenko TC whose tolerances are likely higher than Canon's.

This is why a naked lens can be fine but adding a TC gives back focus issues. Sometimes lenses are slightly out with no TC but adding a TC makes it really bad. Plus the change in perspective makes the same kind of focus error look much worse even if technically the difference is the same


In general you should avoid TCs wherever possible and putting on lenses that aren't design for TCs can only cause more problems.

He said that the same problem happens with or without the TC? I suspect there's more going on here than tolerances.

 

[D.P.]

He said that the same problem happens with or without the TC? I suspect there's more going on here than tolerances.

If the lens has back focus issues due to cumulative tolerance errors then this can be made worse when adding a TC. The focus issue will still be there when naked but may be even worse with the TC.


From the OP's description the lens definitely needs to be tuned and can be sent off to Canon with his bodies.

Eitherway I stand by my advice that it is a wate of time and money to use a TC on a 70-300mm f/5.6 lens, even if it is the DO or L version. The only exception would be if tripod mounted and stopped down to f/11 or so, otherwise it doesn't make much sense.

TCs only make sense if the lens is significantly out-resolving the sensor. For this to happen the lens must be very good. Beyond that you loose A stop of light so increasing the ISO by another stop will just rob a load of detail. Unfortunately since you are at f/8 then it is liekly the ISO is already pushed high so the results just won't be worth it. Then there is the fact that the longer focal length will need a faster shutter speed by 1 sto, which will require raising the ISO again, see previous sentence. With the increased focal left everything then becomes more sensitive to vibrations, movement, focus errors. You will see weaknesses in your technique, limits in the tripod or head etc. The focus speed and accuracy will take a hit so your chances of a good sharp photo are reduced.

Easier and better just to crop the image without using a TC.

 

[Mel_P]

He said that the same problem happens with or without the TC? I suspect there's more going on here than tolerances.

I fear you are right - but it's only this lens - I don't really want Canon to change either body as they are both spot on with 4 other lenses! I hope I can just return the 70-300 lens. I have emailed canon - omitting an mention of a TC which has only served to confuse the issue - the problem is there with both bodies and the lens on its own.

Perhaps I will return the TC!!

Mel