Letus Extreme infinity focus point question

Essami

Essami

Well-known member
hi,

Im finding that the infinity focus on all my Canon FD lenses is way before the infinity mark on the lens. The beginning goes well and the closest focus point is usually pretty much accurate. But the further I focus the less the focusing markings on the lens and the real distance correlate. On my 50mm lens the infinity focus is at about 1.5m and beyond that its just blur.

I can get everything in focus all right but was just wondering why it behaves like this?

Sami
 
Hi Essami,
Sounds like the backfocus is out, you will probably notice that it is worse with wider lenses. Have you tried adjusting it? I don't have a Canon mount, but should be similar to my Nikkor mount - Here is how to do it...If the procedures below still do not allow proper back focus, there is a way to adjust it by going internal, but you should call the MFG first. Unfortunately, mine was WAY out and I had to do some surgery. See Below first.

J


1. Setup your camera with the Letus35 Extreme / Economy using the preceding instructions, and attach a 50mm lens. (Any lens will do, but a 50mm is the most helpful.)
2. Place a Siemens star in front of the lens at a predetermined distance. It will be more helpful to have the distance be closer to the lens, and that the distance corresponds to a marking on the 35mm lens (4 feet or 1 meter is a beginning point). To determine the distance the star is at, you will need to measure from the film plane out to the star. (See above for how to measure focus.)
3. Using the SLR lens, turn it until it reads the distance that the Siemens star is at. (4 feet / 1 meter for example.) The image should be in focus at this point. If the image is not in focus, you will need to adjust the back focus.
4. To adjust the back focus of the adapter, keep every thing set up as it is currently. Hold the lens with one hand to prevent it from falling out of the adapter. Gently loosen the screws holding the lens mount in the Extreme / Economy.
5. Once the lens mount is loosened, gently and slowly pull the lens mount forward in 1mm increments. Each time you make an adjustment, watch the resulting image (preferbly on a preview monitor). You will reach a point where the image will be in focus at the reading marked on the lens. Once you have found this location, tighten the lens mount screws.
6. Make sure that when you are tightening the lens mount screws that the lens mount is square with the adapter. If it is not perfectly square, you may find that parts of your resulting image are not in focus when other parts are.
7. Now that your lens is set correctly you can double check it by setting the star at different distances, measuring it, and then setting the lens accordingly. The measurements should hold up through this process. If it doesn’t, then you need to adjust the back focus some more.
 
Sami,
Hopefully your GG is close to correct (7.5mm). If not, you have to take the front end apart and adjust the GG's rubber feet / aluminum posts (an it must stay as square as possible). Took me about 3 hours guessing and checking before getting it right. I ended up using the rail mount screw as a 'feeler gauge' to align the GG to the right specs. Adaptor looks crisp now and Focus points lineup correctly.


J
 
hey,

can anyone tell where exactly the film plane is on the Letus Extreme. I have the support bracket and its attached to rails. I have the A4 "manual" that came with LEX for backfocus adjustment and it says there should be a filmplane marking but I dont see any.

I got the 1 meter mark in focus OK but I think now my infinity is not quite in focus. I dont have enough reach to get there (the lens focus ring stops just 1mm before I get there I think). Whats the best way to check infinity focus? Wait for the moon to appear?

Im thinking I might be using the wrong point as film plane... At the moment Im using the point where (looking from front of the camera) the letus bracket ends. About halfway on the letus "nose".
 
Found it from letus support site:

"The film plane mark is aligned with the back of the rods support bracket that attaches to the Extreme. If you take your focus measurements off the back of that, you will be aligned with the imaging plane."
 
Only problem is... I'm not using the rods support bracket because my LetusExtreme came as a bundle from Zacuto, using Zacuto's custom setup for the Letus. They should have put the film plane marking on the adapter itself, not on an optional attachment. Ugh...
 
I had my Letus apart. The GG is roughly 13/16" from the prism housing (that is, 13/16" towards the lens).

I put my own mark there.

The rails bracket on my Letus is 11/16", not the 13/16" reality.


J
 
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