shimming 101

sanmich

sanmich

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Hi all

I have a 50mm lens that focuses about 1 to 2 inches beyond the intended point, when focused at 1m.

First question, does this problem disappears at greater distance (harder to check) and does it mean that sharpness at infinity will not be optimal?

Second question: what shim thickness should I use to correct it?

Last: is there an easily available and precise way of doing the calibration without shooting a film?

Thanks!!🙂
 
http://en.wikipedia.org/wiki/Focal_length

Start with the focal length formula.

1/s1+1/s2=1/f, where s1 is the distance of the lens from the film and s2 is the distance from the object in focus.

so at s2= 1m, and f= 51.6mm, the Leica standard,

1/s1= 1/51.6- 1/1000= 0.01838...

s1= 54.407mm, if your lens was accurate.

BUT, your lens is off by (let's say) 2" at 1m.

It is actually
1/s1+1/1050.8= 1/51.6;
1/s1= 1/51.6-1/1050.8=0.0184281...
s1= 54.265mm

So you need to move the lens out 54.407mm- 54.265mm, 0.1423mm.

And I can even help Nikki with 6th grade math.

For doing the calibration without film, I use a negative with scotch tape across it and a loupe for focus, at the film gate.
 
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Behavior at infinity might be different - depending on what's wrong with the lens. Before you shim, check. For example by comparing a photo taken at infinity, and - of the same subject - with the lens focused slightly before infinity (you can use Brian's formula if you want to know exactly at what distance to set the lens).

Also, what lens is it ? 2" at 1m could be normal focus shift for some lenses.

Roland.
 
Brian,

Thanks!! brilliant! 🙂🙂
Now I know how to do it, no matter what the shift is.
Could you elaborate on your method about the film+scotch?
where do you put the scotch? front or back of the film? (towards the lens or towards the pressure plate?)

Roland, the lens is the heliar 3.5.
NOT something I expect to shift.
It's very cool on a IIIf and apparently able to deliver very good results. Of course being calibrated would help 😉.
The good news are that reaching the shims is extremely easy on this lens...I'll try to get some from Cosina, or else to make some from a disposable aluminum baking plate.
 
Interesting that this lens should be off .... Make sure it's not your camera's RF. The infinity film test will prove this. And far more accurate than checking with a loope and a ground glass (I usually use an old OM focus screen layed over the focus rails and held with scotch tape).

A tip: I cut my shims with nail scissors from an empty coke can. Works well and might fit your needs. Disposable baking pan is probably too thick.

Roland.
 
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Roland,
I have shot this lens against a variety of other lenses both on an M6 and a IIIf.
to cope with the human factor, I have refocused several shots of the same lens, and since the human is me, I did it quite a lot...🙂
thanks for the tip on coke cans...Any idea how thick these are?
I'm not sure I used the right term: I meant disposable baking trays.
I wonder how would it be to use processed film as a shim??

Brian, thanks for the link,
I realize none of my leicas can be used for this, due to their back door configuration. I may try a bessa R.
Another possibility I have thought of:
I will first place different shims between the lens flange and the camera, shoot a film with these different trials, and then determine the right thickness, place the shim in the lens and re-shoot the ruler test to confirm...
 
sanmich said:
Could you elaborate on your method about the film+scotch?
where do you put the scotch? front or back of the film? (towards the lens or towards the pressure plate?)
Click to expand...

You shoot the film, then drink the scotch.
After you have drunk the right amount, it doesn't matter whether it's in focus or not...

Sorry! :angel:
 
icemendicant said:
You shoot the film, then drink the scotch.
After you have drunk the right amount, it doesn't matter whether it's in focus or not...

Sorry! :angel:
Click to expand...

Nice one, Don't be sorry😀
In french, "scotch" is a nickname for adhesive tape.
I guess it has something to do with the company name.
 
Kitchen Aluminum Foil is ~0.07mm. The seal used for plastic coffee containers is a bit thicker, ~0.12mm or so. Coke cans are thicker yet, but I do not remember off-hand.
 
If it's an LTM to M lens, you can make shims out of expired credit or mileage point cards of varying thickness before opening up the lens. To get 2" closer, I think you will need an old NWA Worldperks card cutout, or similar, probably about .2 to .3mm, much less than a thick credit card.

Of course, if that works, you'll have to adjust now for infinity, unless your lens already goes past infinity.

The best deal on micrometers I've seen so far, but haven't actually tried, is about a $20 Harbor Freight one that comes in a wooden box, Made in China, states on the box that it is accurate to .0001"

I'm waiting for a 1/2 off coupon or sale at HF before I get one to try. It would be nice if I could tell folks what thickness shims I'm using instead of what brand credit/mileage card 😀
 
Roland, thanks for the info on the can. I may go this way, but I'll first try with film which is easier to cut (I have three screws going through the shim and this turns things a bit more complex)


Ted, I have not measured infinity yet.
I assume the optical block itself is ok, (meaning that the lens is currently focusing past infinity) and that all is required is to add some distance between the mount and the optical block. Also, the mechanics of the focus seems fine since the lens and camera(s) RF agree on infinity.
I think I will try to work with film.
Ilford spec sheets show 0.11 mm base (no idea how much would be the emulsion) and putting a few pieces of film under the flange of the lens takes it back about 30 degrees, which seems to fit (1mm pitch for full rotation)
My guess is that for a lens with a max aperture of 3.5 and minimum distance of 1m, the shimming is less critical (am I right here??)
 
infinity

infinity

I'm not sure about the mechanics on the Heliar, but I'm thinking that if infinity is spot on right now, with the focus fully racked, then shimming to fix the close focus, may put your focus mechanism just short of infinity?

Although you may be noticing the close-up focus being off more than at inf.

Is there any way you can try other LTM/M adapters? .0x mm thickness is approaching the placebo effect threshold 😀

sanmich said:
Roland, thanks for the info on the can. I may go this way, but I'll first try with film which is easier to cut (I have three screws going through the shim and this turns things a bit more complex)


Ted, I have not measured infinity yet.
I assume the optical block itself is ok, (meaning that the lens is currently focusing past infinity) and that all is required is to add some distance between the mount and the optical block. Also, the mechanics of the focus seems fine since the lens and camera(s) RF agree on infinity.
I think I will try to work with film.
Ilford spec sheets show 0.11 mm base (no idea how much would be the emulsion) and putting a few pieces of film under the flange of the lens takes it back about 30 degrees, which seems to fit (1mm pitch for full rotation)
My guess is that for a lens with a max aperture of 3.5 and minimum distance of 1m, the shimming is less critical (am I right here??)
Click to expand...
 
ampguy said:
I'm not sure about the mechanics on the Heliar, but I'm thinking that if infinity is spot on right now, with the focus fully racked, then shimming to fix the close focus, may put your focus mechanism just short of infinity?

Although you may be noticing the close-up focus being off more than at inf.

Is there any way you can try other LTM/M adapters? .0x mm thickness is approaching the placebo effect threshold 😀
Click to expand...

Ted,
sory, I wasn't clear:
the camera RF and lens mechanics aggree. I have no idea about the optics. In my idea, that does confirm that I don't have too much to do, and hopefully, shimming the optical block as a whole will solve the problem.
The only test I have done was at close focus, and assuming the optical block is ok.
messing with the optical block itself is a whole different story, and I would not do it on such a lens.
(By the way, I did try the lens on two different adapters on my m6 and on a IIIf, although The adapter wasn't supposed to do any difference other than on infinity reaching)
I don't have the skills to agree or disagree about the "homeopathic threshold", I'll just try and see how it goes.
(In fact, the formula brought by Brian should give clear answers on the required shimming precision, given the DOF of the lens at full bore)
 
Hi Michael

Hi Michael

I need to be educated on the Heliar I guess.

Here is my concern: I am assuming your RF, lens, and infinity is working well, optics, focus, etc. are good. Only issue at hand is that at 1m, back focus by 2-3".

So by shimming, we are going to move the optical element out a tad from the film plane right, when the focus is fully racked to 1m or min. focus, right?

So by doing the above, and say that fixes close focus, when racking the focus to inf. are we not now extending the optics away from the film plane by the shim amount, now making infinity a tad off??

sanmich said:
Ted,
sory, I wasn't clear:
the camera RF and lens mechanics aggree. I have no idea about the optics. In my idea, that does confirm that I don't have too much to do, and hopefully, shimming the optical block as a whole will solve the problem.
The only test I have done was at close focus, and assuming the optical block is ok.
messing with the optical block itself is a whole different story, and I would not do it on such a lens.
(By the way, I did try the lens on two different adapters on my m6 and on a IIIf, although The adapter wasn't supposed to do any difference other than on infinity reaching)
I don't have the skills to agree or disagree about the "homeopathic threshold", I'll just try and see how it goes.
(In fact, the formula brought by Brian should give clear answers on the required shimming precision, given the DOF of the lens at full bore)
Click to expand...
 
I think, or let's say, I hope that your assumption of a perfectly focused lens at infinity is wrong.
If the lens is currently good at infinity, and bad at close focus, it's a whole different problem, involving changing the distances between the lens elements. (??)
If this is the case, well, I'm in trouble... because I really don't intend to mess with that kind of issues.
But let's hope the optical block in itself is ok and the helical are correctly machined. All I need is to set the right distance between the lens flange and the optical block and ALL focusing distances will fall right into place...

Let's hope...:angel:
 
Landshark said:
On a 6X6 camera with a 3.5 80mm lens if infinity is considered to be 100m(390') the DOF is over 25M(97'+) with a 35mm camera it's greater.
IE😀on't worry about infinity
Click to expand...

I'm not sure about this.
It seems to me that the same shim/rotational error in focusing could mean centimeters at minimum distance, and hundreds of meters close to infinity.
If the lens is wrongly shimmed, you won't have a good sharpness even at infinity.
 
Hi Michael

Hi Michael

Well, since you'll be testing the shims, we should know soon right?

Once you get the close focus issue fixed, just shoot the moon for practical purposes, and if it shows up aligned in RF, lens is racked to inf. or on inf. mark, and image of moon at say 100% crop is tack sharp, then all is well.

I'm just not seeing how moving an optical cell assembly inside a threaded fixed rotation barrel can let close focus be adjusted away, while infinity distance of element remains constant, with just shimming, and not adjusting some stops or threads.

But let's just wait for the results, if the moon stays good and sharp now, and after at 3.5 or wide open, after you fix the close focus, then it seems all good.


sanmich said:
I think, or let's say, I hope that your assumption of a perfectly focused lens at infinity is wrong.
If the lens is currently good at infinity, and bad at close focus, it's a whole different problem, involving changing the distances between the lens elements. (??)
If this is the case, well, I'm in trouble... because I really don't intend to mess with that kind of issues.
But let's hope the optical block in itself is ok and the helical are correctly machined. All I need is to set the right distance between the lens flange and the optical block and ALL focusing distances will fall right into place...

Let's hope...:angel:
Click to expand...
 
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