Entrance pupil posiiton and focussing

[GMB]

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This is a follow-up threat to a previous threat on the Nodal point, which I learned is not the Nodal point but the entrance pupil (see http://www.l-camera-forum.com/leica-forum/leica-m8-forum/67008-m8-panos-nodal-point-m-lenses.html).

 

The information Leica provides in their data sheets states the position of the entrance pupil by reference to the first lens surface in light direction (for example 25.7 mm for the 50/1.4). Of course, if you focus, the first lens surface moves. My question therefore is the following:

 

Is the position of the entrance pupil at a constant distance to the first entrance point? If that is the case, one would have to place the camera at different positions on a pano head, depending on whether one focusses at infinity or at close up.

 

Or is the position of the entrance pupil at a constant distance to the bayonet (and hence the camera sensor, in which the question would be whether the distances provided by Leica are measured when focused at close up or at infinity?

 

Admittedly, for several lenses the difference is probably of little practical relevance, as the lens movement during focus is minimal, However, for some of the longer lenses the difference could show.

 

Finally, since I am it it, does someone know the position of the entrance pupil for the following discontinued Leica lenses

 

Nocitilux 1/50 (11821)

 

Summilux 1,4/75 (11815)

 

Elmarit 2,8/90 (11807)

 

 

Georg

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[giordano]

Basically the entrance pupil moves with the lens, and, as you say, in theory you have to move the camera back and forward on the pano head as lens extension changes with focus distance (or if you're using a monorail camera you leave the lens where it is and focus by moving the back).

 

But I don't know enough about optics to be sure that the entrance pupil remains in exactly the same plane relative to the front element as the magnification varies from zero (infinity focus) to the macro range. At a guess it doesn't move sigificantly with a conventional lens where the elements don't move in relation to each other, but things may be different when there are floating elements and especially with internal focus lenses (which focus closer by reducing focal length rather than by increasing extension). Any experts here?

[GMB]

Moderator,

No big deal but I wonder why this was moved from the M8 to the customer section. In fact, I deliberately placed it in the M8 section as almost all of the panorama/stiching posts, to which this belongs, are in the M8 section because those who do panorama/stiching do it most with the M8.

 

At the same time, I see a lot of specific lens discussions/reviews in the M8 section, althought they are generally about the lenses and not specifically on the use of the lenses on the M8.

 

As I said, no big deal but still wondering.

 

Georg

[pgk]

Is the position of the entrance pupil at a constant distance to the first entrance point?

 

I'm not an optical expert but I have looked into this (for different reasons!) and I think that except in the case of zooms or a lens with shifting lens groups (some modern macro lenses?), then this is likely to be the case.

If that is the case, one would have to place the camera at different positions on a pano head, depending on whether one focuses at infinity or at close up.

 

Makes good sense - I'd assume that this should be measurable for each distance and adjustment could be made, although you will need a worm drive set-up to do this accurately.

Or is the position of the entrance pupil at a constant distance to the bayonet (and hence the camera sensor, in which the question would be whether the distances provided by Leica are measured when focused at close up or at infinity?

 

No, as the lens fouses closer it extends thus shifting the entrance pupil away from the sensor.

 

Admittedly, for several lenses the difference is probably of little practical relevance, as the lens movement during focus is minimal, However, for some of the longer lenses the difference could show.

I'm not so sure - a close-up panorama may require careful setting up.

 

Finally, since I am it it, does someone know the position of the entrance pupil for the following discontinued Leica lenses

 

Nocitilux 1/50 (11821)

 

Summilux 1,4/75 (11815)

 

Elmarit 2,8/90 (11807)

 

I assume that this data exists somewhere - have you emailed Leica to ask?

 

Lastly, in case this seems exacting and irrelevant, I am modifying a conversion mount to allow a 19mm Elmarit-R to focus at 0.5m when set at infinity and will then use the position of the entrance pupil to align it with another optic for a very specific application - sometimes technical data is of great importance!