300 f/2.8 Non IS - Sharp without but soft with both converters??? Anyone had this???

I have a 300 f/2.8 non IS. By itself it is tac sharp and great to use yet the minute i put either converter on (1.4 or 2x) every shot appears soft, I have used the 1.4 extensvely on my 300mm f/4L IS before I had this lens and it was sharp all the way through. I've even tried stopping down but to no avail.

Any ideas what could cause this? I literally have hundreds of shots that all look the same. All taken on 2 different 40D bodies, I tried my old 400D and got the same results.

I've posted example with both converters - All shots are untouched except for the 300 shot example by itself which is cropped and sharpened.

SHOT WITH 1.4X CONVERTER UNCROPPED

HTTP response: 404 | MIME changed to 'image/gif' | Byte size: ZERO | PHOTOBUCKET ERROR IMAGE

A wild guess would be that the TC adds a bit of "softening". Perhaps by simply degrading the IQ b/c the extra glass in the optical path or maybe the TC screws up the focusing? The last pic sure looks tack sharp, nothing wrong with the lens itself. Have you tried the TC on any other lens to see if those shots also get affected the same way?

I have used the 1.4x extensivley on my 300mm f/4L IS, is has been great and sharp wide open too. Here's an example of it from about 3 weeks ago. Obviously I cannot use a 2x on the f/4L

I noticed that I get some softness/out-of-focus when using my TC's. I had to dial in +10 and +15 micro focus adjustment to get them sharp.

jdando wrote in post #8946161
I noticed that I get some softness/out-of-focus when using my TC's. I had to dial in +10 and +15 micro focus adjustment to get them sharp.

I was wondering if micro adjust would help ..... unfortunately I only have 40D's for now but I won't forever I guess.

To me, it looks like a bit of front-focusing once you've got that TC on.

Collin85 wrote in post #8946204
To me, it looks like a bit of front-focusing once you've got that TC on.

I scoured through as many shots as I could looking for either front or back focussing, I think you may be right, if anythingit seem slightly to the front, I wonder if thats in thens but only increased by TC's? it seems funny it would only happen with TC's so I'm guessing it may be minescule in the lens itself.

Would micro adjust overcome it or would it be too great?

butcha27 wrote in post #8946267
I scoured through as many shots as I could looking for either front or back focussing, I think you may be right, if anythingit seem slightly to the front, I wonder if thats in thens but only increased by TC's? it seems funny it would only happen with TC's so I'm guessing it may be minescule in the lens itself.

Would micro adjust overcome it or would it be too great?

Not sure if MA could fix a focus discrepancy that bad, but worth a try. You should know that other glass causing AF difficulties is not uncommon. Some users even report that (quality) UV filters cause front/back focusing on some of their lenses.

these are stacked converter shots ... 1DmkIII 300mm F2.8 (non IS) + 2x + 1.4x
No sharpness problems here. On the other hand I have had to micro adjust every lens I own to make sure it is focusing accurately.

Nice shots, I guess the only way to try is to give it a go on someones body with the micro adjust feature and see what happens.

For what it's worth, my 300mm f2.8L IS is very sharp. After microadjusting with the 1.4x, the results were very little loss of IQ. However, with the 2x converter, even with microadjustment, I've never been able to get sharp results. The optimal focus is still soft. Since I've seen others not have this problem, I once asked if it's possible to get a bad copy of the 2x (or 1.4x, for that matter) or if they can be calibrated. I never got an answer to that.

can you give us the exif for these images please

cwood wrote in post #8946443
these are stacked converter shots ... 1DmkIII 300mm F2.8 (non IS) + 2x + 1.4x
No sharpness problems here. On the other hand I have had to micro adjust every lens I own to make sure it is focusing accurately.

Canon recommends MA with and without an extender.
My 300 f4L is sharp on both my 40D and IDmk3 (without MA), if I add the 1.4 converter it is a little soft on the 40D and 1Dmk3, fortunately I have MA adjusted this set-up on my 1D and it works fine. Unfortunately the 40D does not have MA.

But it's not hard to determine whether it's a focusing issue though - just shoot along a fence or wall and check the sharpest section.

Thanks for all the input, much appreciated