Collimators, Autocollimators and Infinity Focus

ic-racer · Nov 7, 2024

Before doing any lens adjustments I'm going through my 35mm cameras and lenses that I believe have not never been tampered. So far it seems most are set for infinity focus on the actual film. So, not the outer film rails, and not the inner film rails. The actual film front surface loaded in the camera.
The vernier scale on the Autocollimator is pretty sensitive, so there is a big jump between the three measurements.

OAPOli · Nov 7, 2024

On my sample size of three, the film surface is also slightly behind the film rails.

On one camera, infinity on film matches the lens. On the other two, the lens can focus a little past infinity on film.

ic-racer · Nov 7, 2024

This is what the Rollei SL35 Workshop manual indicates:

Lens flange to infinity focused image = 44.50mm

Camera flange to outer rails = 44.67mm
(Not specified but film should be in here at 44.50mm)
Camera flange to inner rails = 44.45mm

Camera flange to focus screen = 44.50mm

The collimator cannot measure these absolute distances, it can only make relative measurements between two planes.
To measure the lens to focused image distance a lens base micrometer is needed (I don't have that yet).

Setting SLR Mirror Angle With A Collimator

In most all SLRs the mirror pivots at the upper portion of the film frame. Mirror angle has very little effect in that area, as the pivot is not moved with the mirror angle adjustment. In the viewfinder, the top of the film frame is the bottom of the viewfinder image. So, adjusting the mirror...

www.photrio.com

Bill Burk · Nov 7, 2024

I picked up a semi-silver Edmunds mirror with the idea of making an autocollimator. Good to know it’s not critical to position it.

OAPOli · Nov 11, 2024

ic-racer said:
This is one of the few pictures I could find. This one is for a horizontal cine camera/lens collimator.

One does not need this if one has a camera body that is of a known flange-focal distance. In that case, the collimator, after focusing it, will read the offset and then the flange-focal distance can be calculated from the known distance of the camera body.

View attachment 382265

@ic-racer I've been experimenting with a DIY focal plane micrometer of sorts using a micrometer head. The end of the spindle is nicely polished and gives a nice reflection on the autocollimator.

ic-racer · Nov 12, 2024

OAPOli said:
@ic-racer I've been experimenting with a DIY focal plane micrometer of sorts using a micrometer head. The end of the spindle is nicely polished and gives a nice reflection on the autocollimator.

Very clever.

I have seen these on e-bay, but without any adapter for any lens I might have, they want too much money.

ic-racer · Nov 12, 2024

I have measured about 20 cameras/lenses with and without film.

I'm finding way too much variablilty in the film position to make it reliable. Everythng is a compromise.

At this point to adjust lenses, I'm using a set distance in front of the pressure plate. From 0.13mm to 0.17mm.

One thing I have to remember is that with different focal length lenses on the camera, the vernier scale of the Autocollimator will read different when focused 0.13-0.17 above the pressure plate. Each camera lens focal length requires a different calculation.

OAPOli · Nov 12, 2024

@ic-racer how are you measuring the 0.13-0.17mm offset?

ic-racer · Nov 12, 2024

Autocollimator Scale = (Test Lens Focal Length/Collimator Focal Length)^2 * 0.17

The Autocollimator scale has Zero in the middle and counts up or down. In this case, move the head down the desired amount.

For example:

50mm lens and 200mm Autocollimator

Set the Autocollimator to 2.7mm down from zero and adjust the lens infinity so the green pattern is sharp (bouncing off the pressure plate). Now the lens is focusing right at 0.17mm above the pressure plate. That is the "by the book" focus spot for Rolleiflex SL35.

I made a spreadsheet:

ic-racer · Nov 12, 2024

If you have a focusing helicoid you can make a millimeter or micrometer scale for it, in place of the distance scale. You can adjust the infinity focus of your device so that zero is in the middle of the helicoid throw, that way you can measure distances in both directions.

ic-racer · Nov 12, 2024

For example, if you had the camera in this thread (in which case the owner does not know the fixed focus point) and you made a table like below for your Autocollimator focal length, you can answer the question in this thread: https://www.photrio.com/forum/threads/early-box-tengor-focus.210184/

(for 200mm focal length autocollimator only)

ic-racer · Dec 22, 2024

I made this little adapter for my shutter tester so I can use the collimator as a light source for shutter speed testing. This is for cameras where you can't put the sensor behind the film plane or for cameras that won't work with the back open.

Collimators, Autocollimators and Infinity Focus

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