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that "thorny" infinity focus issue

pellicle

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anyone with much experience with adapted lenses on mirrorless cameras soon finds out that adapter length accuracy (as well as the lens construction accuracy) makes a difference to:
  • is the lens able to reach infinity
  • is the lens throw past infinity
This is more significant with wider angle lenses than it is with normal or telephoto lenses. An entirely different can of worms is opened with lenses which focus by inner element movement (such as later Canon IF lenses and many zooms).

To put a perspective on this when changing focus from (lens indicated) distance of Infinity to 3M
my Canon FD 24f2.8 on the adapter changes length as such:
Infinity = 67.1mm
3m = 67.24mm
so the lens extends 0.14mm

while my Canon FD 50f1.4 is:
Infinity = 65.11
3m = 66.0
extending by 0.89mm or roughly 6 times greater extension

thus .04mm difference in adapter length (from standard) will make a much bigger difference to focus as you experience it on the lens scale with shorter (wider) lenses.

Nothing new here (just setting the basics out first)

My observation comes from my FD24f2.8 when I've been trying to focus on things and had initially found the adapter was faily close to perfect but not quite. I needed the lens to be backed off a teeny bit to focus on infinity.

However on a different day I was unable to reach infinity and I needed to stop down to f8 to even get there (so it did not reach infinity).

The other night I was out shooting the evening skies and found that I needed to move the lens to just the other side of 3m ... let me put this in a diagram:

IMG20181231071421.jpg
   ---            


so the blue line was where I needed to re-position the lens to when I got the adapter, and the green line was where I needed to re-position the lens to when photographing skies.

Hence the data above on extensions and focus points.

So last night I was taking a picture of the skies and found that the lens needed to be set to the green point to get the stars on the hill behind my house. I focused at f2.8 using the magnifier and am sure of my accuracy of focus.

I then left the camera on the back landing (under the cover of a roof) overnight (its as dry as at the moment so rain wasn't going to be an issue) and went back this morning to check it.

Without moving the camera I observed that as the temperature rose (it was 18C ambient last night) the focus shifted and required me to re focus the lens towards infinity to actually focus on infinity. By mid morning (with temperatures around 30C) it became that I was unable to focus on infinity at all (and indeed something about 10 meters away was in perfect focus).

So what's at work here?

I naturally plugged in the temperature into a calculator for thermal expansion and found that the variance in expansion was nowhere near sufficient to account for this (something in the order of 0.15mm in change), so it can't be the metals.

The only other option is that the sensor (or mount) is shifting relative to the lens mount flange and moving the sensor further away or closer. Not knowing the plastics involved in the body I can't comment

I don't "see" any movement of the sensor but then that's bloody hard to measure in this system.

Any thoughts?
 

Tipton

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Rae Leggett
Random late night muddled thoughts:

1) Have you tried a different brand of adapter?
2) Does the particular lens focus to infinity on it's native mount?
3) You're dealing with three different sets of materials (lens, adapter, camera) that probably expand and contract differently under the same temperature/moisture conditions.
4) I haven't heard of that particular brand of adapter

I know I've had a few lenses - cheaper ones, to be sure - that just will not focus to infinity. I don't have native cameras, so I can't test to see if they ever did. I've also had cheap adapters that have an alarming amount of "play" in the mount.
 
  • Agree
Reactions: WNG

bdbits

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My adapted lenses have almost always focused a little past infinity on their respective adapters. I have always understood that was by design, to allow for both lens and temperature variances and ensure you could reach infinity. But I believe when it does not reach infinity the adapter is too long, so there is not a lot you can do in that case.

As far as the mount, I guess I would think Sony would make the sensor mounts out of materials with very low expansion coefficients so as to have any variance relatively immaterial, but I am no engineer or anything close.

In the end, there would not be anything I could do about any of this anyway, other than getting a different adapter that does reach infinity. I just try to ensure that I have focus where I want it at the time of capture, and never rely on the distance scale as more than a general measurement of distance to focal plane.
 

pellicle

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Hi

Random late night muddled thoughts:
:-D

all good

the adapter is one which is made to about the highest precision available and machined from a single billet of Alloy ... which removes any of the vagaries caused by cheapies which are screwed together.

I've used this companies adapters on micro43 (with these lenses) for about 10 years now and never once seen anything like this. M43 having higher magnification (being a crop) would magnify these issues.

Key point is that the system variously reaches infinity very early or not at all ... same lens, same adapter, just left to warm up on the tripod.

I'm concerned that this indicates that either my A7's flange is mobile or the sensor is.
 

pellicle

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UPDATE: I believe that after some experimentation today that the issue is confined to the lens.

If I keep the camera level (instead of angled slightly upwards), dismount the lens and shake it facing down (set to infinity) it focuses at the 3 meter mark but quickly corrects when racking the focus in and out. If I dismount the lens and shake it facing up I'm unable to get infinity but again a few turns in and out returns it more or less.

Perhaps the temperature influenced the "grease" but I now believe that this lens has problems. I believe that this is going to become an increasing symptom for this specific lens as I believe this is the problem mine has (it fits the symptoms)
FD Lens Bushing Repair - Here's a How To for the Canon FD 24mm f2.8: Canon SLR Lens Talk Forum: Digital Photography Review
 

WNG

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I was about to post that it's the lens you have. The nFD 24mm f/2.8 is notorious for a sloppy focusing ring from use and wear. I have one that I picked up this Fall, and it's plagued by this issue.
If the A7 mount precision has your panties in a bind, you can purchase an aftermarket all-metal Tough E-mount kit from FotoDiox for it.
The first gen A7's used a thin sintered metal outer ring and plastic inner spacer ring. The Tough E-mount kit is more rigid, similar to later gen A7's.
 
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pellicle

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If the A7 mount precision has your panties in a bind, you can purchase an aftermarket all-metal Tough E-mount kit from FotoDiox for it.
It doesn't, but thanks for that ref, I stumbled across it recently, but :

its-not-my-bag-baby.jpg
   ---            
 

Reciprocity Failure

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I have several adapters and yes, the infinity issue can be thorny. Best adapter seems to be my M42-Sony adapter which seems to match infinity-to-infinity absolutely perfect, at least based on the very few M42 lenses I actually own. Everything else? Not so much. All the others focus past infinity to more or less degrees.

I also own a Nikon "1" J3 camera and wanted to use adapters on that camera also. Unfortunately, the first one I bought (for Nikon Rangefinder SC mount) wouldn't focus all the way to infinity. Not even close. Thus every lens I tried on it had a very narrow focus range, such as 10 to 20 feet, or 5 to 15, or whatever. Cumbersome and frustrating.
 

sapoeijoek

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I don't have this infinity focus issue with K&F adapter. Even with nFD 20mm.
I think lenses with floating elements eventually will have this focus shift issue, like 24/2.0 or 35/2.0.
 
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sapoeijoek

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I'm not sure if this shot was at f/5.6, scene like this I usually use either 5.6, 8.0 or 16.
No sharpening in the pp, only wb, dehaze, blacks, and whites were adjusted.
39653667593_d84f030405_h.jpg
   ---            


More from Vienna shot.

Rathaus. Canon FD 20/2.8 @ f16
View attachment 85776
 
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