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Bob_dodds
Tinkerer
Username: Bob_dodds
Post Number: 1
Registered: 09-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, September 19, 2007 - 06:28 pm: | 
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Do you think these lenses can be collimated by their regular adjustment for moving the rear lens back 2mm toward the focal plane? That would allow conversion to Sigma dslr.
I know somebody who does that with 55mm by milling off 2mm, but 58mm does not have an extra 2mm to mill off.
I can find 0.5mm of that 2mm on 58mm by sinking the back of the lens 0.5mm into the back of the Sigma plate. That still leaves 1.5mm for the regular collimating adjustment mechanism or some other way.
The rear of the lens will be tight against the frame around the Sigma IR/dust filter. |
Glenn
Tinkerer
Username: Glenn
Post Number: 178
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, September 23, 2007 - 09:40 am: |
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Bob,
I am somewhat puzzled by what you are trying to actually achieve.
Obviously if you can move the 'Total" lens group assembly into the correct position, then the optics will produce an image in the focal plane. The problem is the optics are held within the lens barrel, the optical cell does not slide up/down within the helix assembly. The inner Sigma bayonet is basically Pentax K, which is not compatible with the Canon mount, so how are you attaching the lens to the Sigma body. I have made many adapters to fit Canon FD macro equipment to the Sigma SD9/10, but have never looked at fitting Canon FL/FD lenses due to the mount incompatibility. I must admit that I have never looked at using the Sigma outer mount. The Sigma dust shield does not offer enough clearance for any lenses that would be worth using in my opinion.
The Sigma 30mm/f1.4 offers far superior optical performance, when compared to the old FL lenses, so I cannot see that all the work involved in the conversion would be worth it. A conversion has got to really work and offer real advantages in my book. The f1.2 is not a real enough advantage, considering the snags. |
Bob_dodds
Tinkerer
Username: Bob_dodds
Post Number: 4
Registered: 09-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, September 26, 2007 - 06:36 pm: |
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People have adapted Canon FL 55mm f1.2 to Sigma SA by milling off 2mm from the lens base. First the Canon mount plate is removed, minus 2mm, and then 2mm more milled off, then the Sigma SA plate is put on adding back 2mm, net loss 2mm overall. 58mm does not have 2mm to spare for milling off so I would like to collimate away 2mm. With the extra 2mm in place, it focuses at three or four feet away. 55mm and 58mm cannot scratch the dust filter because the frame around the dust filter prevents.
Sharp is necessary, but sharpest is then not a priority for portrait work. 30mm is not the most popular portrait focal length, either. Sigma 50mm and 70mm are also extremely sharp, but not the best for portrait work. I will have Sigma 30mm, 70mm macro and maybe 50mm macro for ultimate sharpness, 18-200 for OS and AF in sports action, and old manual lenses for portrait work.
Example of why FL 55mm f1.2:
http://forums.dpreview.com/forums/read.asp?forum=1027&message=24272316
...sharp and great bokeh. |
Bob_dodds
Tinkerer
Username: Bob_dodds
Post Number: 5
Registered: 09-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, September 26, 2007 - 06:44 pm: |
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" if you can move the 'Total" lens group assembly into the correct position, then the optics will produce an image in the focal plane. The problem is the optics are held within the lens barrel, the optical cell does not slide up/down within the helix assembly" -Glenn
Does the collimation adjustment allow the helix to move farther or move everything back in the barrel?
If the brass guide tees are removed, it seems possible to remount them with different location screw holes to get more rearward travel of rear element, but without them I couldn't manipulate any better infinity focus to simulate such a method. I think that experiment fails because it does not emulate the relationships as if the guides were in place. The rear element seems to come back a lot more, maybe 2mm, but infinity focus is still three feet. |
Glenn
Tinkerer
Username: Glenn
Post Number: 180
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, September 27, 2007 - 07:01 pm: |
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Bob,
OK, I now see where you are going on this. Did not appreciate that you were just replacing the Canon lens mount with a Sigma SA unit. Yes these lenses do have great bokeh and will make good manual lenses The part of the lens that you need to modify is the infinity limit stop. Now I have no knowledge of the FL f1.2 internal workings. You will have to examine the lens and see what actually limits the movement of the helix. Sometimes it is just a limit screw in the focus ring or a simple machined 'slot'.
The brass guides should not be moved, with the limit stop removed you should be able to get proper infinity focus. Now this depends on how the actual range of helix movement is limited on this lens, as I said I have never stripped these FL lenses out. I also wonder if by your comments on removing/repositioning the guides, that you have noticed that they physically stop the rearward travel. In many lenses the helix does not play a major part in collimation, accurate machining and shims are the major players in locating the optical cells in the inner barrel.
Sorry I cannot be of more help. |
Bob_dodds
Tinkerer
Username: Bob_dodds
Post Number: 8
Registered: 09-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, October 08, 2007 - 12:33 am: |
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I think I need to put the guides back, and find which 180 degree cycle they go on. Only two possibilities I think.
I can put the brass guide tee's back in then. To do that I think it easiest to hold the lens upside down, place the guide in place with curved tweezers, poke something up through a hole to register a guide's hole over the lens base hole, hook the guide in that position from above, stick a screw up in there and tighten it.
Then I go looking for stops.
There could be limit screws behind the front facing with writing on it. There might be a c-ring threaded in the outer tube behind the helical, visible, could turn with a wrench.
I have some easier Sigma conversions to do first. |
Bob_dodds
Tinkerer
Username: Bob_dodds
Post Number: 9
Registered: 09-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, October 08, 2007 - 12:34 am: |
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Using two hooks maybe I don't need to hold the lens upside down to put the guide tees back. |
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