Odd Canon 100mm f3.5 focus issue

[Goody]

I have an odd focus issue with a new to me Canon 100mm f3.5 (silver and black) lens. It focuses fine on my Canon P but on my Canon 7 the rangefinder image in the viewfinder will not converge at infinity looking at a distant object. The focus scale reads the correct distance on the P if I focus on an object at a known distance (for example 10 feet) while on the 7 the scale and the RF focussing don't match. But weirdly with either of my 50mm lenses (a Nikon f2 LTM and a Canon 1.8) both rangefinders work correctly.

 

[Mackinaw]

Have you taken any pictures with this lens on your P or 7?

Jim B.

 

[Goody]

I am finishing up a roll on each. But the fact that the focus with the rangefinder is so off from the actual distance on the 7 with this lens - not true of the other lenses I have used on the 7 - makes me rather doubtful that it's focussing correctly with the 7. On the P everything is as expected.

 

[Robert Lai]

I bet it's the rear Canon lens cap.

I bet it's the rear Canon lens cap.

Do you have the original Canon rear cap?
The original rear cap is SO SHALLOW that the lens MUST BE PUT AT THE MINIMUM FOCUS DISTANCE BEFORE YOU CAP IT. Otherwise, the rangefinder coupling cam on the rear of the lens will jam up against the inside of the cap.

I had this problem with a 50mm f/1.4 LTM Canon lens that I bought from Japan, complete with original 48mm Canon UV filter, front push on metal cap, and the original rear cap. When placed on my newly acquired Canon 7s, infinity was way off and could not be "achieved" by the rangefinder.

Both camera and lens were sent off for overhaul.
The repairman said that both the camera and the lens were off. The camera needed a general overhaul, which it got.
The lens cam was displaced due to cumulative collisions with the shallow rear cap. This is a common vintage Canon problem.

Anyway, when I got it back I shot the lens and camera at 1m, f/1.4. The focus was tack sharp, right on the bull's eye!
Infinity was also correct.

The original Canon 48mm filter was thrown away (has no front threads, what good is it?), as well as that accursed rear cap. A 48-49mm step up ring allows me to use the more common 49mm filter size and hoods.

 

[Spavinaw]

Robert--Just checked my Canon 100mm f3.5 black and chrome lens and bakelite cap. The overall cap thickness is 9mm, the internal depth is 8mm, and the lens focusing tab at infinity sticks out 7.4mm. Inside it is identified as: A Canon. If you still have the the cap that gave you problems, I was wondering what the internal depth was and if it had any identification marking.

 

[Mackinaw]

My Canon “Made in Occupied Japan” rear cap fits onto any Canon LTM lens I have with no interference with the focusing cam (outside of the 25mm, which requires a special deep cap).

Canon LTM lenses did use a variety of focusing cams. The 100/3.5 uses a “tongue type” that sticks out the back of the lens. Canon did recommend that when screwing this type of lens on or off the camera, to close-focus the lens so the cam wouldn’t bang into the camera’s rangefinder mechanism while focusing. Presumably you could damage the rangefinder if you didn’t do this.

Jim B.

 

[Robert Lai]

I don't have the original cap. I threw it away several years ago, in anger. It was shallow, made of bakelite or some other plastic. I think it did say "A" on the inside, but I'm not absolutely certain of that.
Measuring my other LTM caps, they are about 12mm in depth externally, with an internal depth of 10mm.

You can test this yourself. Put the lens to minimum focus distance, then cap the rear of the lens. With the lens capped, start moving the focusing ring towards infinity. If you start to feel resistance, then STOP. The RF cam is hitting the inside of the cap.

If the lens goes all the way to infinity without resistance, then you're in the clear.

Note, I got mixed up in my original post.
You need to have the lens at minimum focus distance before capping with the original cap. I'll go back and correct that post if I can.