I love this lens but it just barely focus on infinity and any photo taken beyond 5m (15ft) causes the mirror to get stuck (on my 5D2). It's the upper part of the ring (that holds the rear element) that needs some grinding. I can see small marks on the ring where the mirror hits.
Also, looking into the camera house, the upper part above the sensor is where the rear element of the lens first hits.
So with luck I will achieve both infinity focus and mirror clearance with some grinding. The idea is to remove the ring from the lens and grind it on a sandpaper (wet) taped to a table - and holding/moving the ring upon that surface.
Since the mirror have been able to leave small marks on the ring, I recon the metal is not of the hardest sort.
Any idea what grinding paper I should use? In this part of the world they're marked with grain density like 240 (fine), 400 (extra fine) and there are also papers available with 600 and 1200 (super fine). Grateful for suggestions if you have done anything similar!
wfrank wrote:
Grateful for suggestions if you have done anything similar!
In the photo, the top thin lip on the left, representing 12 o'clock position mounted, I usually don't reduce past 10 thousands inch. The ring is made of brass and the lip could split off, even though the lower rim is intact.
If you remove the ring use a good spanner wrench, mark the 12 o'clock position on the ring first. I use a metal disc as a jig to hold the ring while sanding, so as not to bend the ring out of round and to aid in accurate positioning. I have screws in the jig to adjust the angle of sanding, but you could tape a toothpick under the edge to get an angle. Brass is soft and will be removed quick enough with wet/dry 400 paper.
The tolerances are really close. You may get to the point of apparent full mirror return at infinity setting, but the lower 15% of the frame appears dark. This is because the mirror is slowed down by ring contact and blocks the image while the shutter is open. You may sand more, and get full frame exposure at f/1.2, but at f/4.0 the lower darkness returns. This is because the longer exposure stopped down acts as a 2 stage mirror break, instead of a quicker shutter action.
At this point, if handheld infinity scenics is required, a trick would be to back off infinity adjustment where wide open hard stop infinity is just short of sharpness, to increase the mirror clearance, and stopped down to f/4.0, infinity sharpness is retained.
After that, Liveview or tripods are options, etc, etc. Good Luck!
I did it a couple of times and would like to tell you how.
I always removed the ring, instead of working on it while it was on the lens. I believe this is the more conservative way to go. I also see no sense in shortening it in an asymmetrical manner, i.e. only on the upper side.
I recommend to sand it down evenly. Until it is thin all around, indeed quite thin. This way, you can end up with the middle of the glass element being the rearmost part, if you want.
Just pay attention when you attach it again. IMO this is the only moment when the modified part ever will have to withstand a certain amount of strain. In operation, there is no significant force on this ring.
In case you screw it up somehow, you could still epoxy the glass element back in place (that's the way Dim.ka did it in the past, if I recall correctly).
By the way, make sure not to turn the last glas element by mistake (rear surface NOT inside)!
I was able to file my two Rokkor 58/1.2s using Paul's (Cogitech) excellent conversion directions.
Filed retaining ring without removing it. Adds to the excitement, being careful not to scratch rear element.
I'll go for some 240 paper and carefully grind it to see how it turns out, sounds promising.
Jim - knowing your long experience of this (and you probably provided the original parts for this EOS converted lens) - I dont really follow your last paragraph about the "trick". Would you mind clarify that?
p.1 #10 · Grinding the retainer ring/Rokkor 58/1.2
JimBuchanan wrote:
At this point, if handheld infinity scenics is required, a trick would be to back off infinity adjustment where wide open hard stop infinity is just short of sharpness, to increase the mirror clearance, and stopped down to f/4.0, infinity sharpness is retained.
This "trick" is based on the fact that, 1. Moving the lens assembly away from the mirror will give more mirror swing clearance, but not quite reach infinity at wide open aperture, but stopped down to f/4 for example sharpens up infinity objects. 2. handheld infinity scenic type shots are stopped down 2 or 3 stops, typically, for increased sharpness and contrast.
Said another way, you could infinity adjust the lens for f/4, not f/1.2. The process would be to move the lens assembly back away from the sensor until sharpness starts to suffer, then stop. This is done during your infinity adjustment procedure. PM me if you need a procedure reference.
When I take scenic infinity type shots, I use a tripod and mirror lockup, or liveview. This trick was suggested to me by a lens user that felt full mirror clearance (no physical contact at all) was important at all costs. I thought I would just pass it along.
