Collimators, Autocollimators and Infinity Focus

Mr Bill said:

Yes, I had played with this sort of thing with one of our Gokosha autocollimators (I forget the fl). These had a threaded collar that allowed fine (and precise) adjustment of the objective lens so that different distances could be mimicked, looking into the front of the collector. With the collar set at zero it was an infinity target, and could be verified via a flat mirror in front of the autocollimator. If the film position changes one simply rotates the autocollimator barrel until focus is again achieved (via the autocollimator eyepiece). Then read the distance off the barrel. Using the classic formula 1/f = 1/u + 1/v one can figure how far the film surface moved. Note: f = autocollimator fl, u and f are the conjugate distances of the lens.

As a note the autocollimator is mainly looking only at a small area near the center of the film.

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Yes, and the manual for the c6400 has a chart with the test lens distance scale values already calculated. The c6800 actually has the calculated distance marks engraved on the barrel along side the millimeter marks.

In both cases, however, the numbers only work when the collimator lens is the appropriate distance from the film plane when the measurement is being obtained (120mm or 150mm).

Alex Varas said:

I wonder what kind of glass plate is it used, it would be very handy to know the parallelism is well set with the autocolimator.

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NatCam sold a glass plate on a base. I think it may also have had an adjustment wheel on the base, so it could be aligned.
The Gokoshas in post #6 above are nice because of the adjustable base. The NatCam/Pearl units don't have that, but it is not essential.

reddesert said:

From the Edmund Optics technical documents archive linked above:

https://www.edmundoptics.com/ViewDocument/CollimatorsCollimation.pdf

https://www.edmundoptics.com/ViewDocument/Collimating Systems.pdf

Note that photographers and camera technicians tend to use "collimating" to mean setting the focus distance accurately (so that infinity matches a parallel bundle of rays). While telescope users often use "collimate" to mean aligning an optical system so that the optical axes match, for example getting the optical axes of the primary mirror, secondary mirror, and eyepiece of a telescope lined up in position and angle. Both of these uses are correct, but there is sometimes confusion. They reflect (haha) the different alignment needs of typical photographic vs telescopic systems.

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Yes, very good points. It seems in most fields other than camera/lens repair, autocollimators are used for measuring small deviations from perpendicular.

The National Camera c6400 does not even have a graduated reticle to make an angle measurement (simple cross hairs only), though deviations from perpenducular should be clearly evident. The c6800 reticle has a graduated scale to 15 minutes of a degree, and can make angle measurements.

OAPOli said:

I'm currently building a DIY autocollimator. It's a WIP and I'll make a thread when it's done.

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That is great! Looking forward to it!

OAPOli said:

I think plain glass would work (although maybe with some flatness certification). In the setup illustrated above you would see the main reflection from the mirror at the film rails, as well as fainter reflection(s) from the glass surface(s) lying on top of the flange mount. If the planes are parallel, the reflections will be co-axial. If not, they will be shifted.

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One method of collimating a collimator - that is, making sure your test apparatus is actually producing a parallel bundle of rays - is to use parallax of the aerial image returned from a mirror. It's easier to show this with a picture, so I drew one.



I've drawn it slightly misfocused to show how the test works. This addresses the question of whether the collimating lens is properly focused on the reticle or test screen, not the parallelism of the mirror. In fact, the mirror doesn't have to be perfectly aligned for this test to work, it just has to be fairly flat. You need a reticle or screen with a small clear spot through which you can view the reflected aerial image. If the lens is not correctly focused, then the return image (green line) will be displaced from the object (blue line) and you can see the relative motion between object and image as you shift your eye back and forth. Once this is set correctly, then it is possible to use the collimated beam to test the focus of lenses, alignment of lens and film plane, etc.

There are two types that are useful for service (they shouldn't be mixed with centering setup/machine). Both have their advantages and disadvantages. Simplified description:

1. Adjustable lens autocollimator where there is a graduated lens for focusing. Advantage is easy direct readability, quick use, fixtures aren't required for different lenses but can be used if wanted.
Personally, I use Rollei factory autocollimator of this type. Graduation on it shows tolerance for 75mm TLR lens and everything else would need to be calculated to know the exact displacement.
2. Autocollimator with fixed and calibrated focus. Fixture is required for each lens but micrometric readout for exact displacement of the fixture's mirror can be easily read out without any calculations.
Disadvantage is that a fixture is a must. I use Hilger-Watts collimator of this type. It's not lens specific but more for precise machine alignment with precise angular readout.

Most service manuals specify allowed displacement of the image plane.

Both types have some type of angular readout. Some are more precise and on some it's more of a secondary purpose if they were made for collimation. For example, Leica manual states that parallelism between film gate and lens mount needs to be checked after setting of the focal flange length.
Collimator or Autocollimator can also be assembled on the optical bench with ready made components. Nikon service manuals and some others are showing the basic setup.

Centering machine will center elements on the optical axis. It does contain some type of collimator but it doesn't serve 'collimation' purpose. It measures reflection of the element. Next element in assembly is added and reflection is measured again of that element. The goal is that all elements are centered in relation to each other.

If mirror is used with the collimator, reflection will show levelling of the mirror. Ideally mirror would be leveled and fixture with the mirror would be centered on collimator's optical axis before using it for collimation.
With lens mounted over the fixture with the mirror, angle reading doesn't work anymore as the lens collimates light from any angle within its coverage. In case of lenses with pronounced field curvature this can cause issues since measurement could be taken away from the center of the imaging area.

I've used the often suggested camera with long lens as a collimator and it worked reasonably well. It's not as precise as the collimator setup as there's no readout for any of these values.
Can't be used for angle checking. Had plans to make a proper autocollimator but in meantime I found the factory made ones. They both work very well even though there are features that would make them much better, faster and more precise for camera and lens calibrations.

I have tried with a flat glass in front if the lens of a Rolleicord I was checking and I didn’t see any reflection. I have to mention that my Blight Autocollimator is not very bright and I usually need to have the room without illumination for having a good and contrasty image.

I replaced original lamp with small strong LED chip light and that gives much better illumination and doesn't overheat the lamp housing. Still there are cases where reflection is very faint.
Few times I forgot to remove filter from some older lenses before checking and reflection of an old uncoated filter was almost as visible as if it was coming from the mirror.

Here is a c6400 autocollimator in need of restoration:

Looks like all the correct components of the light module are here. It needs to be cleaned.

Two condensor lenses removed for cleaning:

The lamp has a square fillament:

Light unit cleaned:

What sort of objective lens is used by the autocollimators? I tried an achromatic doublet (close-up no.3, coated) and it wasn't sharp enough on its own. I will try adding a field stop.

The one in post #5 is more of an 'economy' model and has a singlet. The others have doublets. My c6400 has a 200mm doublet about f8 (25mm across). The c6800 is 300mm.

Thanks!

Much time was spent on the focus mechanism. It was essentially frozen. The wheels are taper-fit on the shaft and, through years of slipping on the shaft, one wheel was worn with a hole too big. So, to tighten the wheels, the whole assembly was bound up.

I was able to shim the wheel and get it on tight without binding.

The base was polished with metal polish.

The base contains the connectors for the power cord and lamp, on-off switch, pilot light, fuse and a 6V transformer for the lamp.
I thought I might have to re-wire it, but everythig looks to be in good condition, so I just put the bottom plate back.

After verifying the wiring was all in good condition, I plugged it in and everythng works.

The Siemens star target was in pretty good condition; I didn't mess with it.

I did not want to take the eyepiece apart, but focusing the eyepiece out of the range of the reticle showed some irregularity, likely a lens in the eyepiece is dirty.

Indeed this lens surface in the eyepiece was pretty bad. It did clean up ok.

The beamsplitter/pellicle mirror was examined but not disturbed as it was functioning fine. No need to 'clean' it and damage the delicate 50/50 semi silvererd surface.

The crosshair reticle was examined. I could not see an easy way to adjust it. I left it alone. Adjusting the position of the crosshair reticle is how one adjusts and zeros the vernier scale on the barrel. The vernier scale was alreday was perfect on zero when adjusting it for an infinity object.

After inspection and cleaning, I reassembled it and tried it out!

In this case the camera's shutter is held open with the shutter release cable. The camera is sitting on the front-surface mirror. The mirror is just the right size to fit either on the inner or out rails, depending on its orientation.

In this case I have the mirror on the inner rails.