Help Solve This! A Fascinating Minolta Infinity-Focus Dilemma

[BrendanLeahy]

...

 

[Jacek Zakowicz]

first check your minolta adapter for proper height
It should be somewhere between 23.15mm to 23.2mm max.
That is overall height from the lens flange to the bottom of the Red lens board.
any local machine shop can do this for you....
Then mount the lens and make sure there is no air gap between the lens mount and the adapter flange. The easiest way is to look against a light source when the lens is mounted

 

[Jacek Zakowicz]

Would you mind clarifying this a bit more?

Any reason why you think this might still show up at infinity focus on the M-X but not the Weapon?

You mount the lens and look at the joint of the lens with the adapter from the side against a light.
any light passing through the joint will tell you that there is an air gap and the lens is not seated properly. Sorry that is the best I can do- it's a pretty standard way of checking it.
As far as the MX I wouldn't worry about that. If I only had got a penny every time I heard that haha.....;-()

 

[Serge Polevitzky]

Brendan -- I had a similar situation with a Leica-M 21mm 'Lux on the Mg Weapon .. when I removed the OLPF. Without an OLPF, it would focus objects very
sharply at about 9' when the 21mm 'Lux was set to infinity. My takeaway is that the OLPF influences the back focus.

Back on topic: Could be a minor deviation as Jacek points to. Could also be that the Mg OLPF is not seated completely right. I noticed on the DSMC2 OLPFs
that I needed to make sure the bottom of the OLPF did not roll up the rubber gasket as I tried to install it. Also, I use a business card on the top, as I then try
to get the top to avoid jamming the rubber gasket. So the process is (1) tilt the OLPF with the top pointed away from the camera, (2) next, slip the bottom of
the OLPF past the rubber gasket at the bottom (still keeping the top pointed away from the camera), (3) next, slip a business card or similar (slick plastic would
be best) between the rubber gasket at the top and the top of the OLPF, (4) last, slowly tilt the top of the OLPF inward, using the business card to finesse past
the gasket and keep the gasket from bunching up.

It's a very long shot, but you are probably bumping up (sorry for the pun) against some very tight tolerances, and any one of them being just a .01mm or so off
could be your root cause.

Last thing: are you sure the Minoltas are not hitting the OLPF & possibly slightly deforming it ? Check the "posterior" of the Minoltas for max extension in mm
(which should occur when they are at infinity) and then the distance from the Minolta mount's rear flange to the OLPF. Also, do you get "expected focal distances"
when the lenses are set at, say, 3' or 5' ... and the "focal wonkiness" (sorry for the technical term) only occurs as your focal setting progresses toward infinity ?

My 2¢,
Serge

Hello fellow redusers!

I purchased a custom-made Minolta mount from a couple of brilliant guys. The mount has allowed me to use my old Minolta lenses on the RED bodies with great results.

I upgraded to the Weapon MG back in March, but it wasn't until a few weeks ago I actually needed to use my Minolta lenses at infinity. Surprisingly, the lenses (21mm and 28mm), when set to infinity, would throw only as far as 8ft. They should technically focus 12ft and beyond, but fall short. This is now true of all of my Minolta glass for infinity, falling short of what they're listed at achieving.

Here are the possible issues I managed to mark off the list:

1. Other mounts: a C7 PL mount with DSO lenses and a RED Canon Al mount with Canon lenses all hit infinity just fine
2. Back focus: RED confirmed back focus is the same across Epic + Weapon bodies regardless of sensor
3. Other bodies: My Epic-X M-X Monochrome with the Minolta mount achieves infinity perfectly fine

Reached out to RED, and they said they can't seem to figure it out (although they can't troubleshoot non-RED mounts anyways).

So, I'm turning to you, REDusers, to help me see what I can keep trying with this. I really don't want to adjust the back-focus since I interchange mounts/lenses regularly. If I shave the mount down, I'll end up over-throwing focus on my Monochrome.

Here's the dropbox link to check out the rudimentary tests I did with three lenses, three mounts (all lenses set to f/4):

1. 28mm Minolta MC and MD (late 1960s vs mid-1970s)
2. 35mm Minolta MC
3. 24-105mm Canon EF set to 28mm and 35mm
4. 38mm DSO lens

If you have any thoughts after this, let me know!

 

[Jacek Zakowicz]

Brendan -- I had a similar situation with a Leica-M 21mm 'Lux on the Mg Weapon .. when I removed the OLPF. Without an OLPF, it would focus objects very
sharply at about 9' when the 21mm 'Lux was set to infinity. My takeaway is that the OLPF influences the back focus.

Back on topic: Could be a minor deviation as Jacek points to. Could also be that the Mg OLPF is not seated completely right. I noticed on the DSMC2 OLPFs
that I needed to make sure the bottom of the OLPF did not roll up the rubber gasket as I tried to install it. Also, I use a business card on the top, as I then try
to get the top to avoid jamming the rubber gasket. So the process is (1) tilt the OLPF with the top pointed away from the camera, (2) next, slip the bottom of
the OLPF past the rubber gasket at the bottom (still keeping the top pointed away from the camera), (3) next, slip a business card or similar (slick plastic would
be best) between the rubber gasket at the top and the top of the OLPF, (4) last, slowly tilt the top of the OLPF inward, using the business card to finesse past
the gasket and keep the gasket from bunching up.

It's a very long shot, but you are probably bumping up (sorry for the pun) against some very tight tolerances, and any one of them being just a .01mm or so off
could be your root cause.

Last thing: are you sure the Minoltas are not hitting the OLPF & possibly slightly deforming it ? Check the "posterior" of the Minoltas for max extension in mm
(which should occur when they are at infinity) and then the distance from the Minolta mount's rear flange to the OLPF. Also, do you get "expected focal distances"
when the lenses are set at, say, 3' or 5' ... and the "focal wonkiness" (sorry for the technical term) only occurs as your focal setting progresses toward infinity ?

My 2¢,
Serge

OK
1. Of course the OLFP is affecting the Flange depth- roughly 1/3 the thickness of the OLFP
2. The location or even tilt of the OLFP has absolutely no influence on the flange depth
3. It (OLFP) can't be deformed because it will crack first as it is made of glass
4. The lenses are DSLR retrofocal lenses so they will easily clear the olfp as they are designed to clear the flip up mirror in the DSLR.

 

[Douglas Underdahl]

I think I understand what you're saying, and it seems to me that the Minolta mount was seating correctly on the Epic MX but not on the Weapon, is that right?

From your description, the Minolta mount is not seating on the camera's flange. It's pushed away from the sensor. This would account for it focusing by eye at 8' when the lens barrel says infinity.

So something is different about the MX and Weapon flange area (although the flange focal distance AKA back focus is identical). This difference is probably tiny, and the factory Red mounts and others that function correctly that you describe have enough relief that the difference does not cause interference. But your Minolta mount doesn't have this relief. My guess (and it is just a guess) is that on the Minolta mount:

A) the holes for the pins are too small or not quite in the right place, or

B) that the surrounding edge is a bit too large to fit into the recess around the Weapon flange area or

C) the pin relief pocket is too small or

D) something else on the Minolta mount is a bit smaller than it should be.

Study the back of your Minolta mount and compare it to the back of a factory mount and find the difference. You may need some digital calipers and the skill to use them to see what's what. Also, you can use a water based marker pen to rub on the back of the Minolta mount, attache it to the Weapon, and see where the ink is transferred to see where the interference is.

signed, Douglas Underdahl OEM Red Mount Supplier

 

[Jacek Zakowicz]

I concur Dr Doug ;-)
However if the adapter hung up on the weapon it would most likely get jammed and be hard to remove. Unless there is a burr or a piece of dirt that prevents it from seating flat...

 

[Les Hillis]

Back-focus on the camera would be checked with something like this - http://www.red.com/store/products/red-focus

Using something like that, along with a verified accurate lens mount, allows the sensor to be positioned so that any back-focus issues are then known to be the result of other (out of spec.) lens mounts or the lenses themselves (if their optics are out of whack).

You can check the camera back-focus yourself by eye-balling it through the view-finder using a wide-open wide-angle lens, but it's not as accurate.

If you do take the camera somewhere to get the back-focus checked, you should probably take the MX one and get that checked as well, just to confirm there are no differences between the camera's there.


Did you thoroughly examine or measure the Minolta lens mount as Douglas suggested? Chances are that's where the tolerances won't be matching up. It's also the one part of the equation that would be easiest (or even possible) to fix/adjust to get things fitting properly.


I do wonder though whether it's possible for the back-focus on your Weapon camera to be adjusted to bring the Minolta lenses and mount into line without throwing your other lenses and mounts out of alignment, and if that's where the back-focus on your MX is already set (you could try doing that yourself beforehand if you're definitely going to get the back-focus checked properly anyway). Maybe the lens experts here can say whether or not some lenses have more back-focus tolerances than others like that.


Edit - Also, can you say where you got the Minolta mount from and how much it was to get it made?

 

[Satsuki Murashige]

What kind of metal is the custom Minolta mount made of? If the lens mount works fine with the DSMC1 body (aluminum?), I wonder if the magnesium body of the Weapon is throwing the mount out of tolerance?

If you don't have a Denz collimator or tool like the Red Focus, I would use the Canon C7 in PL to check the camera's back focus. Set chart to 5', set focal plane to 5', magnify 1:1 on a big HD monitor, zoom in on the lens, set focus, zoom out, check focus. Adjust back focus screw on the camera until the lens is sharp at 5' all through the zoom range. Check marks at a few other distances, including infinity. If they are good, then as long as your other lenses are collimated correctly they should be able to hit infinity. That is assuming all the various mounts are at the correct flange depth spec as well.

 

[Les Hillis]

Glad you got it sorted out Brendan.

So it was a case of the Minolta's needing a more precisely adjusted back-focus on the camera while the other lenses didn't?

Thanks for posting a link to that cinemods site too, I went searching for it once after having seen it in passing and couldn't for the life of me find it :thumbup:.

 

[Satsuki Murashige]

Glad you found the issue and that it was an easy fix. It's worth noting for anyone who may be reading that backfocus on the camera needs to be checked and adjusted on a regular basis to stay within tolerance. Do not blindly trust the 'factory settings' or a previous calibration if it has been awhile. Temperature shifts, bumps in shipping/transit, or small amounts of dust/dirt on the mount can cause the flange distance and thus backfocus to shift. This goes double when using B4 lenses, lens adapters, doublers, etc. That means whomever is prepping the camera package for a job should always check the backfocus as part of their prep (I've had this come back to bite me on a recent job with an Epic MX). Even on film jobs, the 1st AC should have a collimator on the truck and check the FFD regularly.