• Welcome to Photrio!
    Registration is fast and free. Join today to unlock search, see fewer ads, and access all forum features.
    Click here to sign up

Collimators, Autocollimators and Infinity Focus

OP
OP

ic-racer

Member
Joined
Feb 25, 2007
Messages
16,693
Location
USA
Format
Multi Format
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

Member
Allowing Ads
Joined
Sep 26, 2022
Messages
801
Location
Toronto
Format
Medium Format
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.
 
OP
OP

ic-racer

Member
Joined
Feb 25, 2007
Messages
16,693
Location
USA
Format
Multi Format
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

Subscriber
Allowing Ads
Joined
Feb 9, 2010
Messages
9,455
Format
4x5 Format
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

Member
Allowing Ads
Joined
Sep 26, 2022
Messages
801
Location
Toronto
Format
Medium Format

@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.
 
OP
OP

ic-racer

Member
Joined
Feb 25, 2007
Messages
16,693
Location
USA
Format
Multi Format
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.
Click to expand...

Very clever.

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

ic-racer

Member
Joined
Feb 25, 2007
Messages
16,693
Location
USA
Format
Multi Format
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.
 
OP
OP

ic-racer

Member
Joined
Feb 25, 2007
Messages
16,693
Location
USA
Format
Multi Format
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:

 
Last edited:
OP
OP

ic-racer

Member
Joined
Feb 25, 2007
Messages
16,693
Location
USA
Format
Multi Format
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.
 
OP
OP

ic-racer

Member
Joined
Feb 25, 2007
Messages
16,693
Location
USA
Format
Multi Format
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.

 
You must log in or register to reply here.
Menu
Log in
Register