Letus35 FE w/ Canon EF Lens not focusing up close. Any Help!? at DVinfo.net

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Old June 27th, 2007, 12:22 AM   #1
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Letus35 FE w/ Canon EF Lens not focusing up close. Any Help!?

I currently have a V1U with letus35 FE and a Canon EF mount. On all my lenses (ef 28mm 1.8, 50mm 1.4, 85mm 1.8) I am experiencing the same issue. The closest I can focus is 3 feet. Anyone have the same problem with other lenses? I would love to shoot closer (with the 28mm) but cannot get anything in focus closer than 3 ft. With my canon rebel I an shoot, with the same lens) about a foot away.

here are my questions:

1. Why is there a difference in the Canon rebel and Letus35 FE
2. How can I get the letus35 to focus closer?
3. Is it just me or do all lenses do this?


Anyone's help is greatly appreciated.

thanks!
-wam
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Old June 28th, 2007, 04:28 PM   #2
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Have you tried zooming in your focus ring closer on your camera? If you have at least a 105mm or higher lens then you can get a closer depth of field..but with digital lenses it might not be possible like you would with a manual lens. I usually use a nikon lens but from my understanding for the nikon type cameras and lenses it is 1.5x magnified when using manual lenses on a digital camera because it is not accurate when using the manual film cameras. That might be the reason why you cannot get as close because the letus only see's your lens as a manual lens and not have a digital compensation. For example a 50mm lens manual lens is seen as a 75mm on a digital camera. And i suppose it is the same the other way..a 50mm digital lenses is seen on a letus as around 33mm which is more of a wide angle type lenses and not great for closeup shots. You might look for some old FD manual lenses, that will give you a more accurate depiction of what you want to shoot.
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Last edited by Cary Lee; June 28th, 2007 at 04:40 PM. Reason: ADDENDUM
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Old June 28th, 2007, 04:45 PM   #3
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I think he is talking about the prime lens not being able to focus. Maybe I misunderstand, but if that is true, the zoom on the cameras lens won't make a difference, except if it is zoomed in too much, in which case, all focus is lost.

The first question is what is the close focusing capabilities of the lenses normally. Some longer lenses only focus no more than 3 feet, so check out all your lenses.

Second, if you think its still a problem, increasing the distance between the rear element of the prime lens, and the ground glass by lossening the and sliding the lens mount away from the ground glass will give you closer focus. This has the same effect as an extension tube would on a 35 mm camera. Its also possible the ground glass mount can be slipped farther way from the lens by adjustment. I don't have FE, but on the pegs holding the the 35a ground glass, I can make some adjustment. You shouldn't need a whole lot.
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Old June 28th, 2007, 04:54 PM   #4
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Backfocus adjustment.

From what I've seen on the FE the only way to adjust is to move the Nikon/Canon mount forward about 1-2mm to adjust the back focus. The pegs on the FE cannot be adjusted forward or back unless you physically cut the pegs. ( which I wouldn't even try to do). There was writeup by bob hart some weeks ago on how to adjust using a pvc pipe attachment and shims to get it to 6mm for the correct back focus adjustment. Unfortunately there is only one screw holding the lens adaptor to the body so you might need to add another 2 screws to keep it from getting out of alignment when focusing the lens. Maybe Bob would chime in and better explain how to do it again.
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Old June 28th, 2007, 05:07 PM   #5
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My fixed Nikon 50mm shows closest focusing of 1.5 ft. I m guessing the Canon 50 in normal array would be similar. If it only focuses at 3 ft, I'm betting that only a slight movement forward will be required.
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Old June 29th, 2007, 12:17 AM   #6
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Unfortunately, I only had a play with the Nikon end of the reversable mount, not with Quyen's dedicated EF mount. How it goes for Canons I cannot vouch for. If the Rebel can see closer with the same lenses, then it seems like the mount could also do with moving forward a little as for the Nikon, though maybe not as much as for the Nikon.

My cut-down piece of PVC I used for the thick shim might not work against the rear face of the EF mount as I don't know how Quyen has machined it. However, an accumulation of cardboard or plastic shims might do the same job.

The early part of this clip -

http://www.youtube.com/watch?v=yugfv4ZVXgg

- illustrates where I added the two lower compression screws. They were not on 120 degree radial centres. If you add them, you have to be careful to avoid the pillar sockets and motor power supply wiring.

I drilled the holes slightly smaller as normal for threadcutting and tapped the screwthreads into both the outer metal tube and the ABS end-piece.

With a dedicated mount which has a continuous inner shoulder, you may get away with just using the single top thumbscrew with shims behind the mount.

If the thumbscrew has a tendency to walk the mount out, you could try getting another screw of the same thread size, screwing several nuts on the shank until the thread is covered from damage by chuck jaws, securing it head-first inside the chuck of an electric drill.

The screw has to be about 1" or about 25mm long so you can be sure the head goes back far enough inside the chuck to clear the inside of the chuck jaws and the screw shank lays straight in the chuck. All the hex nuts must have their flats aligned by tightening each against its neighbour.

The method is to dress the screw-end into a shallow cone shape by running the drill whilst drawing the end of the screw across a metal file or sharpening stone. If you have a lathe, this is obviously better for the job because getting a cone-end on a screw correctly centered using the drill method is difficult and requires more than one go at it most times. Don't do this to the original Letus35 thumbscrew.

To find the correct backfocus point, set your lens on a numbered focus point. 1 metre or 1.5 metres should be easy enough to find. Tape the lens focus ring to the lens barrel to prevent an accidental reset of the lens focus.

Then measure to a point 40mm rearwards along the tube case of the Letus body, from the front face of the plastic endpiece in the Letus body which carries the lens mount. This point represents the focal plane. Scribe a mark in the metal tube or mark the point with a piece of tape.

Set up a target Siemens chart exactly the same distance from your focal plane mark on the Letus body as indicated on your selected number on the focus ring of your lens. You will need the camera to be on a tripod or a fixed mount and a tape measure to do this properly

Cut yourself a selection of shim washers of various thicknesses and add these behind the mount until you get correct focus for your selected lens setting.

If you want to use the lens in a "macro" mode, you can add even more shims to bring it foward of the correct backfocus position. Be aware though this this will leave you with less of the mount resting inside the Letus body and it may become too unstable.

Last edited by Bob Hart; June 29th, 2007 at 12:36 AM. Reason: error
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Old June 29th, 2007, 12:21 AM   #7
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1-2 mm shim...

Thanks for all the feedback. Any more ideas are welcome.

Ok so I can't move the glass any further back. Its at 44mm from the rear of the lens, which i think is the right length for the canon EF lenses. I have tried to move the glass closer to the lens but the focus get further than the 3ft. distance. So the solution might be an increase in the distance from the lens and the GG so... I think If I can get a 1-2mm shim that might do the trick. Do you know the thread where they talk about adding a shim to the lens adapter?

I did check the lens focusing length and on average the minimum focus length is about plus 1 foot.
-wam
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Old June 29th, 2007, 12:23 AM   #8
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got it.

Thanks Bob. I got the thread. Brilliant. i will give this a shot.
-wam
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Old June 29th, 2007, 12:40 AM   #9
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Winston. Check the correction I just posted as an edit. You can find a downloadable Siemens chart by searching for ---- "focus_pattern.pdf" ----

The URL is published in one of the older Letus35 posts here.
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