Measuring or charting distance from entrance pupil?

[Andy Zou]

I recall somewhere reading that the distance from the adapter to the entrance pupil affects the quality of the combo.  How does one measure the entrance pupil?  Is this only visible in a lens' cross-section breakdown?

[Brian Caldwell]

11 hours ago, Andy Zou said:

I recall somewhere reading that the distance from the adapter to the entrance pupil affects the quality of the combo.  How does one measure the entrance pupil?  Is this only visible in a lens' cross-section breakdown?

Best done on an optical bench with a low-power microscope.  But a macro lens will work OK as long as you focus by moving the whole camera.  1)  stop lens under test (LUT) all the way down; 2)  Focus on the iris diaphragm; 3) move microscope or camera away from the LUT until the front lens surface is in focus - you may have to apply some dust or some Scotch tape in order to see the front surface.  The distance that the microscope or camera moves is the entrance pupil distance, e.g., the distance from the front surface of the LUT to the image of the iris diaphragm.

[Timotheus]

Noob question I guess, but how exactly can you use this measurement? Cheers, Tim

[Andy Zou]

4 hours ago, Brian Caldwell said:

Best done on an optical bench with a low-power microscope.  But a macro lens will work OK as long as you focus by moving the whole camera.  1)  stop lens under test (LUT) all the way down; 2)  Focus on the iris diaphragm; 3) move microscope or camera away from the LUT until the front lens surface is in focus - you may have to apply some dust or some Scotch tape in order to see the front surface.  The distance that the microscope or camera moves is the entrance pupil distance, e.g., the distance from the front surface of the LUT to the image of the iris diaphragm.

Huh!  Fascinating.  So I'd need to get another camera.  I assumed that the optics between the front element and the iris would've thrown off this measurement.

1 hour ago, Timotheus said:

Noob question I guess, but how exactly can you use this measurement? Cheers, Tim

I think I read somewhere here or otherwise that the distance between the rear element of the anamorphic adapter and the entrance pupil is inversely correlated with the quality of the pairing.

[Timotheus]

1 hour ago, Andy Zou said:

I think I read somewhere here or otherwise that the distance between the rear element of the anamorphic adapter and the entrance pupil is inversely correlated with the quality of the pairing.

So pancake lenses rule then?

[Justin Bacle]

9 hours ago, Timotheus said:

So pancake lenses rule then?

The Helios 44 lens sits back and if far away from the rear anamorphic adapter lens. But still gives best results
It depends mainly of what is the "quality" you're looking for.

But yeah, pancakes are great with anamorphic adapters

[Brian Caldwell]

22 hours ago, Andy Zou said:

Huh!  Fascinating.  So I'd need to get another camera.  I assumed that the optics between the front element and the iris would've thrown off this measurement.

I think I read somewhere here or otherwise that the distance between the rear element of the anamorphic adapter and the entrance pupil is inversely correlated with the quality of the pairing.

The optics between the front surface and the iris are what determines the location of the entrance pupil.  "Quality of the pairing" is only indirectly related to entrance pupil distance, but the amount of vignetting is directly related to it.  To minimize vignetting you want the entrance pupil as far forward as possible, and the air gap between an anamorphic adapter and the prime lens to be as small as possible.