Canon 25mm 3.5 LTM lens and finder questions

[largedrink]

I was lucky to purchase a nice example of the 25mm Canon f3.5 LTM lens last week, along with the accessory viewfinder, both with their leather cases. I am looking forward to using them on my Canon P but there are a couple of things I'd like to fix first and hoping the forum can help.

Firstly, the lens is very clean but there are traces of fungus behind the front element that I would like to clean. Can anyone advise the easiest way to access the front element on this lens? I am familiar with dismantling other lenses from the front, and want to make a sure I follow the correct procedure with this one.

I managed to dismantle the viewfinder and clean it throughout, it's very clear now. It's exactly the same as the example shown for sale at this link: https://sunrise-camera.com/product/canon-25mm-f3-5-w-28mm-finder-leica-ltm-l39/

But with mine there are a couple of missing screws on the base. When looking from underneath it's missing the shiny silver ball-shaped screw that is forward of the brass-coloured screw under the accessory shoe (see photo 4/8 in the link above). And when viewed from the side (photo 3/8 in the link above) it's missing the rear screw that goes across the pivot to the other side of the shoe.

It still mounts correctly, and it pivots in both directions (forwards and backwards). I'd like to know whether I need to replace these items? I understand that this model has auto-parallax correction with the V-series Canon rangefinders, but with my Canon P I intend to use a Canon Finder-Coupler to compensate for parallax manually. Hoping I can get away without needing further repairs to use it accurately with the finder-coupler.

Thanks for any advice and also more than happy to see examples of photos with this lens.

 

[Mackinaw]

I had fungus in my Canon 25/3.5, but it was in the rear group. I came in from behind to access. For you, looking at the front trim ring, I see notches for a lens spanner. It should easily loosen. This lens is assembled very “logically’ and is easy to work on.

Can’t help you with the viewfinder. My 25/3.5 finder is for Canon V and VI-series cameras with automatic parallax correction. The finder for the P needs the manual parallax version. I’d take a mess of pics with your finder in place and see if you’re happy with framing and composition. It may be that the finder you have is more than good enough for most situations.

Jim B.

 

[largedrink]

Thanks for the advice Jim, good idea to test the finder and you're probably right that it will be good enough for most situations.

I think the notches you can see on the example lens in the link I posted are from the skylight filter that has been screwed on. On my lens there are no obvious notches. I am guessing the first step is to use a rubber tool (or rubber glove / plastic lid combination in my case) to remove the front trim ring.

 

[davhill]

I concur: the visible notches are the filter. Probably friction fit to unscrew the trim ring, and the same for the cone. It looks like a serrated rim on the lens element which may then pull the front pair. This lens has 5 elements in three groups: two front, two mid, and a singleton rear element.

“A super wide angle lens with a diagonal angle of view 82°, developed based on Topogon (manufactured by Carl Zeiss, Germany. Angle of view: 100°. f/6.3). This lens has a unique optical system: a spherical lens element with a very high refraction index is used as the first lens element and a special optical glass lens element with the infinite radius of curvature is used as the rearmost lens element. This lens was the fastest lens in the world as a 25mm lens at the time.” -Canon Camera Museum

What’s “infinite radius”? Is that a way of saying “flat”?

 

[Doug]

...What’s “infinite radius”? Is that a way of saying “flat”?

IMO it would have to be a fancy way of saying "flat"! 😀

 

[largedrink]

Yes the rear element is definitely flat, that Canon Museum description also contains a block diagram of the lens:

I did try unscrewing the name plate with friction but unsuccessful so far. I'm also a bit worried about how close those first two elements are on the diagram and whether they can be easily reassembled.

 

[Mackinaw]

.....I did try unscrewing the name plate with friction but unsuccessful so far. I'm also a bit worried about how close those first two elements are on the diagram and whether they can be easily reassembled.

Looking at the diagram, the fungus I had on my 25/3.5 was between the third and fourth elements. Canon uses a "go, no-go" type of assembly. The elements drop into preformed "wells." There's no chance of not putting the elements back into their proper position.

Jim B.

 

[largedrink]

I have made some progress, but still haven't found an easy way to get behind the front element. The fungus is not too bad and doesn't seem to affect images, but would be good to clean away to prevent it spreading.

I found that the name ring from the front seems very reluctant to undo. I did undo the smaller notched ring that surrounds the front element, thinking that this would release the element, but this ring seems to be for trim only. The front element remained in the lens and wouldn't pop out.

I also undid a black notched ring from the back of the lens, which from reading about other LTM lenses is a common way to get at the elements. Removing this allowed the entire focus assembly to be separated from the lens assembly, but looked to be complicated to reach the front element still.

 

[davhill]

You can get quite far from the rear of many of these lenses. Once you’ve got the focus block out (don’t lose those shims!!), the rear elements may unscrew, then the middle pair of elements. Then you’re at the diaphragm, and by simply opening it fully you may be able to clean the rear of the front pair. This suggestion is COMPLETELY off the top of my head—I do NOT have a 25 or any ultrawide to fiddle with.

 

[largedrink]

Thought I'd post a few shots from the first roll of film using the Canon P and the 25mm lens, and grainy expired TriX... no evidence of fungus. The next roll will be using fresh FP4...

Signs
by Hugh B, on Flickr

Crossing the road
by Hugh B, on Flickr

Arrow
by Hugh B, on Flickr

Old bike
by Hugh B, on Flickr