Magnification and Subject area for reverse mounted lens?

[baachitraka]

Recently got a reverse mount adapter for Zuiko 50mm f/1.4 lens which indeed works well but I do not know how to find its magnification nor the subject area when reverse mounted.

I can place a graph paper and mark its coverage but is there any other way to calculate it. I may intent to use this lens with an apsc camera there things are slightly different compare to 24x36.

 

[ic-racer]

an apsc camera

Good luck.

 

[Ian C]

We can calculate the magnification as a function of subject-to-film distance. We can also use the 46mm flange distance of Olympus OM-mount lenses (I assumed that this is the lens mount you referred to) to figure out magnification based on focal length, flange distance and measured subject-to-flange distance of the reversed lens.

First Way:

If you measure the subject-to-film distance k (fairly easy to do), the magnification is given as

m = [k + sqrt(k^2 – 4kf)]/2f – 1 (Note we divide the bracketed quantity by 2f and then subtract 1)

Example: Let k = 200mm and f = 50mm. I’ll assume that the lens’s nodal distance is unknown.

m = [200 + sqrt(200^2 -4*200*50)]/2*50 – 1 = 1X = 1:1

This is necessarily somewhat in error due to the unknown nodal distance of the lens. The nodal distance is usually not enough to spoil the result within practical limits. If you know the nodal distance, then it can be accounted for and the result will be exact.


Second Way:

We know that the rear nodal point of a 50mm Olympus OM-mount lens is positioned forward of the mounting flange by f – flange distance = 50mm – 46mm = 4mm. If you reverse the lens with the focusing of the lens set at infinity position this nodal point is now 4mm rearward from the flange and towards the camera.
Now we can calculate the magnification as


m = f/(s – f)

Note: This method is independent of the lens’s nodal distance and should be accurate as is.

The distance s can be measured with a millimeter scale. First measure from the subject to the lens’s flange and adjust for the position of the nodal point. Assume that the lens is placed on extension tubes or bellows so that the distance from the subject to the mounting flange surface of the lens is 96mm. That makes s = 96mm + 4mm = 100mm (the lens is reversed, so the nodal point is 4mm from the flange of the lens towards the camera and must be added to the subject-to-flange distance).

m = 50mm/(100mm – 50mm) = 1X = 1:1

The area recorded should equal the format dimensions divided by the magnification. This assumes the subject is in a plane parallel to the image plane.

 

[baachitraka]

I drew two rectangles, 24x36mm and 18x24mm reactangle overlayed on previous one on a sheet of paper.

Zuiko 50mm mounted with reverse adapter on,

- OM-1n I can see 24x36mm rectangle at around 8cm from the paper to lens.

- with apsc I can see only 24x18mm rectangle at much closer distance than with OM-1n.

This is new for me, so I thought of sharing it here...