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Minolta AF 100mm 2.8 Macro RS - Back Focusing

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edom31 View Drop Down
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Post Options Post Options   Quote edom31 Quote  Post ReplyReply Direct Link To This Post Topic: Minolta AF 100mm 2.8 Macro RS - Back Focusing
    Posted: 28 May 2021 at 19:47
Hello

I wonder if anyone ran into this with a MAF lens... All my other MAFs are focusing great (two different copies of the 50/2.8 Macro; 50/1.7; beercan...)

My new-to-me MAF 100/2.8 Macro RS is basically focusing behind my subject. I notes this in random shots and then decided to do a test:

lined 5 batteries at a 45 degree angle (no mathematical meassurements). At tripod, set AF to the middle battery = I get the two batteries behind in focus and the one in the middle out of focus (at f2.8).

I shoot with an A35 STL, so likely no inner camera changes I can make...

Anyone has had this eperience? Anyone services these? I have not tried (well, I have but do not have the film developed) it on film...

thanks
 



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Phil Wood View Drop Down
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Post Options Post Options   Quote Phil Wood Quote  Post ReplyReply Direct Link To This Post Posted: 19 June 2021 at 16:33
I have lenses (50/1.7 & 28/2.8) that back focus and are only really of full use on camera bodies with microfocus adjustment. At least with something like a 28 you can take sharp shots in most circumstances because of the wide DOF, just what you don't have with a macro shot. However, macro is usually best done with MF.

Some lenses have shims inserted between the mount and the lens body - these must be there to adjust something, I have often wondered if one could use shims to adjust for off-focus issues?

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Post Options Post Options   Quote Phil Wood Quote  Post ReplyReply Direct Link To This Post Posted: 19 June 2021 at 17:55
Had a look in the service manual for the 100 Macro (1985 variant) which is pretty much the same as the RS. The shims are known as 'back washers' and come in six thicknesses for adjusting the 'flange back' - which seems to be about infinity focus.
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pegelli View Drop Down
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Post Options Post Options   Quote pegelli Quote  Post ReplyReply Direct Link To This Post Posted: 19 June 2021 at 18:27
As far as I understand back- or frontfocussing is caused by slightly off-centered lens elements thereby causing the rays at the edges of the lens causing a slight deviation in the phase detect. Most of the time moving the lens forward or backward (with the shims behind the mount) don't change that.
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Phil Wood View Drop Down
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Post Options Post Options   Quote Phil Wood Quote  Post ReplyReply Direct Link To This Post Posted: 19 June 2021 at 22:45
Thanks Pieter - that's the first credible explanation I've come across of the cause.
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Post Options Post Options   Quote sybersitizen Quote  Post ReplyReply Direct Link To This Post Posted: 19 June 2021 at 23:48
Originally posted by edom31 edom31 wrote:

... I shoot with an A35 STL, so likely no inner camera changes I can make...

There is an AF adjustment that can be made in the A55, so there might be a similar one in the A35. I don't know. However, it would affect every lens you use.
 



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Post Options Post Options   Quote Phil Wood Quote  Post ReplyReply Direct Link To This Post Posted: 20 June 2021 at 11:02
Originally posted by sybersitizen sybersitizen wrote:

Originally posted by edom31 edom31 wrote:

... I shoot with an A35 STL, so likely no inner camera changes I can make...

There is an AF adjustment that can be made in the A55, so there might be a similar one in the A35. I don't know. However, it would affect every lens you use.


Sadly AF micro-adjustment is not available in the A35 or the A55. However, where available, it does not affect every lens, just lenses with the same lens ID. This can be a problem if you have two copies of a particular lens, or a third party lens that has 'borrowed' its ID from a Sony/Minolta lens you also have.

The A68, A77, A77ii, A850, A900, A99 & A99ii have the feature and can record adjustments for up to 30 different lenses. You can use more than 30 lenses, those without stored adjustments are used unadjusted.
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Post Options Post Options   Quote Phil Wood Quote  Post ReplyReply Direct Link To This Post Posted: 20 June 2021 at 11:08
I should add that the LA-EA2 & LA-EA4 also support the adjustments.
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amrep View Drop Down
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Post Options Post Options   Quote amrep Quote  Post ReplyReply Direct Link To This Post Posted: 20 June 2021 at 11:53
Syber didn't claim there is software micro AF adjustment i A55 and A35. He mentioned the adjustment screws under the flash. They are easily accessible on the A55 and probably also on the A35.
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Post Options Post Options   Quote sybersitizen Quote  Post ReplyReply Direct Link To This Post Posted: 21 June 2021 at 06:26
Originally posted by amrep amrep wrote:

Syber didn't claim there is software micro AF adjustment i A55 and A35. He mentioned the adjustment screws under the flash. They are easily accessible on the A55 and probably also on the A35.

Correct. I actually performed that adjustment on an A55 purchased from eBay that had an AF problem. Got it working perfectly afterwards. However, it can't really help if just one lens is out of whack.

Also, I fully understand how AF micro adjustment works. I've used it with my A77.
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