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Old Jul 23, 2011, 10:52 AM   #1
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Default Can the lens mounts be swapped?

For some reason Sigma has killed the 120-300 f2.8 APO for the Sony/A mount. There has been some speculation as to why with references made to a lawsuit brought against Sigma. No matter the cause there is no current plan by Sigma to bring this lens to the Sony users.

I am wondering if a person could take the same lens in a different mount and convert it to the Sony/A mount? The mount would have to be swapped and the pin assignments for the electronics would have to be altered (most likely).

I know there is going to be more to the swap than just changing mounts. I am hoping techies of the group might chime in share their thoughts. A feasibility study of sorts.

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Old Jul 23, 2011, 11:25 AM   #2
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Canon's EF mount has a flange distance of 44mm, while Sony's flange distance is 44.5mm, so converting a Canon mount to a Sony mount isn't possible. You won't be able to focus at infinity.

Nikon's flange distance is 46.5mm, and the throat diameter is smaller than Sony's, so, physically, this shouldn't be tough.

Probably the best thing would be to use scavenge an A-Mount Sigma for the parts you'd need to convert an F-Mount Sigma to A-Mount.

But remember that the Sigma 120-300mm f/2.8 APO is a big, heavy lens. I'd be very careful about handling it.
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Old Jul 23, 2011, 11:57 AM   #3
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Not quite as simple as swapping the lens mount and moving the pin assignments.

The electronics in the lens would need to understand the language the Sony camera is speaking.
Every manufacturer has their own propriatery method of communicating with the lenses.

So at a minimum you would need to change the lens mount, alter the contact pin positions and re-chip the lens to understand Sony's control language.

Older Sigma lenses sometimes had to be sent back to Sigma for re-chipping (free service) just to understand the language newer bodies of the same manufacturer were using.
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Old Jul 23, 2011, 1:40 PM   #4
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Thank you for the replies. This is what I was hoping to see.

Probably the best thing would be to use scavenge an A-Mount Sigma for the parts you'd need to convert an F-Mount Sigma to A-Mount.

Will the flange and throat differences that you mention create any unique issues that would affect final performance? I guess I am asking if the thing could be expected to work properly or will just always be close but not quite right? It sounds like this should work from a mechanical standpoint.

So at a minimum you would need to change the lens mount, alter the contact pin positions and re-chip the lens to understand Sony's control language.


You are correct PeterP. From what I have seen thus far this seems to only apply to the SLT equipped cameras.

Can I kill two birds with one stone and use the chip and mount from the same donor lens or are the chips lens specific? Would it matter which lens was chosen as the donor? Same type, series, that sort of thing?

Last edited by Old Boat Guy; Jul 23, 2011 at 1:42 PM.
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Old Jul 23, 2011, 2:40 PM   #5
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Will the flange and throat differences that you mention create any unique issues that would affect final performance? I guess I am asking if the thing could be expected to work properly or will just always be close but not quite right? It sounds like this should work from a mechanical standpoint.
The flange distance is from the image sensor to the lens mount. If it's not right you can't focus at infinity or you could focus past infinity. The throat diameter is the diameter of the mount that protrudes into the body. Too large a throat diameter on the lens, and it won't fit inside your body. You can scavenge parts from one Sigma lens and be reasonably certain that they'll fit on another Sigma lens, but not necessarily the lens you've got and the lens you want. This presumes that, inside every Sigma lens is a substructure that everything else attaches to. I think that's pretty safe, but I think that presuming that substructure is common to more than a few different lenses is leaving a little too much to fate.

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So at a minimum you would need to change the lens mount, alter the contact pin positions and re-chip the lens to understand Sony's control language.

You are correct PeterP. From what I have seen thus far this seems to only apply to the SLT equipped cameras.
No. This kind of thing has been going on for years, and with different mounts.

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Can I kill two birds with one stone and use the chip and mount from the same donor lens or are the chips lens specific? Would it matter which lens was chosen as the donor? Same type, series, that sort of thing?
Possibly, but the donor lens should have a similar zoom range at least, and the chip might not know tha tthe lens it's attached to is capable setting an aperture of f/2.8.

If you've got a lot of time, money, patience, and skill, this might be an interesting intellectual exercise, but I don't think you should expect too much in the way of a useable device.
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Old Jul 23, 2011, 5:19 PM   #6
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From the admittedly little I know on the subject, I would agree that it might serve as a good learning exercise, but wouldn't expect a fully functional lens. If you really feel the need to try it, I would suggest picking up a couple cheap, used lenses to practice with first.
I think I would wait to pick up what I wanted on a used equipment site, or auction, if new stock isn't available.

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Old Jul 24, 2011, 9:39 AM   #7
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Thank you for the replies. While the answers are not what I had hoped to hear I cannot say that I am surprised.

VTphotog: I could not agree with you more. Had Sigma followed through with production I would have already done the trading,selling necessary to buy one. I am always up for an intellectual exercise but not for an end result that does not function as it should. Never build what you can buy unless the product you can buy does not suit the intended purpose.

As far as cost is concerned this looked like an attractive idea. The non-OS variant of the lens in question sells used for less than half the price of the OS model. The purchase of a second similar lens for the parts lens becomes a problem when said lens does not exist. I have a Sigma 70-200 f2.8 APO lens ($550.00 second hand) that could have been scrapped for the necessary parts.

Up front the idea sounded promising.

Steve

TCav: I know you are not big on taking credit but your technical knowledge of this topic never ceases to amaze me.

Last edited by Old Boat Guy; Jul 24, 2011 at 9:49 AM.
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Old Jul 24, 2011, 11:58 AM   #8
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Rather than altering that 120-300 f/2.8 lens if you really want one.
Might be easier and even cheaper (and guaranteed to work) to locate a used nikon/canon mount version.
Then pick up an older canon/nikon dslr.
I picked up an old Rebel xti for 200$ on ebay for my wife a while back.
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Old Jul 24, 2011, 12:13 PM   #9
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TCav: I know you are not big on taking credit but your technical knowledge of this topic never ceases to amaze me.
Thank you. You are very kind.

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Old Jul 24, 2011, 4:55 PM   #10
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Quote:
Originally Posted by PeterP View Post
Rather than altering that 120-300 f/2.8 lens if you really want one. Might be easier and even cheaper (and guaranteed to work) to locate a used nikon/canon mount version. Then pick up an older canon/nikon dslr.
I picked up an old Rebel xti for 200$ on ebay for my wife a while back.
If I was only using it for a single purpose this could be an option but one that would never make me happy. Another problem comes in when you look at the almost $3,000 + price tag of the OS version for Nikon or Cannon. I have a monster investment in the Minolta A mount and I am not going anywhere else right now.

On the other hand, dad walked away from the camera store with most every Minolta lens with duplicates on all of the expensive stuff (the high dollar shooter did Cannon back then). I could sell everything and swap brands.

problem with that is I would probably switch to Pentax.

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Last edited by Old Boat Guy; Jul 24, 2011 at 4:57 PM.
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