DOF for ASPH

[DOUG66]

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Hi 01af

 

Note that I measured the distance of the image of the iris diaphragm to the focal

plane, not the physical distance.

Now consider the case where the plane of the iris diaphragm is considerably

different from the plane of the exit pupil (either in front or behind) then as

the lens is stopped down vignetting will increase.

The only case where vignetting is minimised is where the image plane of the iris

diaphragm is co-planor with ihe image plane of the exit pupil.

I am not using the term vignetting in the normal sense which is due to the oblique

rays reaching the extreme corners of the image. What I am refering to should

perhaps be described as the darkening of the extremes of the image due to an

out of focus image of the iris diaphragm on the focal plane.

 

 

You state that my measurements are wrong, and in your last paragraph that I have

confused the lenses. I refer to your post #51 and my post #56 in which I give my measurements.

 

#51

" in Erwin Puts' Leica Compendium then you can see that the exit pupils' distances for the

Summicron-M 35 mm (IV) and Summicron-M 35 mm Asph indeed are significantly different.

The former's is approx. 48 - 50 mm; the latter's is approx. 34 - 35 mm."

 

#56

"Results:-

Series IV 35mm Summicron 46mm

ASPH 35mm Summicron 36mm"

 

Considering that I have measured the image distance and I do not know whether

Erwin Puts' measurements are of physical or image distance, I think there is

reasonable agreement.

 

 

#57

"Obviously you're confusing the lenses. The distance between the physical aperture diaphragm

and the image plane is approx. 36 mm for the Summicron-M 35 mm (IV) and approx. 46 mm for the

Summicron-M 35 mm Asph—so you got it inverted. In both cases, the exit pupils are not where

the aperture blades are. In the former lens, the exit pupil is before the diaphragm; in the

latter, behind. In any case, the distance between exit pupil and aperture diaphragm has

nothing to do with vignetting."

 

Please read your last paragraph again, I think it is not me that is confusing the lenses.

 

Doug.

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[mjh]

Now consider the case where the plane of the iris diaphragm is considerably different from the plane of the exit pupil (either in front or behind) then as the lens is stopped down vignetting will increase. The only case where vignetting is minimised is where the image plane of the iris diaphragm is co-planor with ihe image plane of the exit pupil.

Maybe I misunderstand what you are getting at but the image of the diaphragm is the exit pupil – that’s how the exit pupil is defined. The exit pupil being in front of or behind the image of the diaphragm is impossible by definition.

[01af]

Note that I measured the distance of the image of the iris diaphragm to the focal plane, not the physical distance.

As Michael "mjh" just pointed out—that is the exit pupil distance.

 

 

Now consider the case where the plane of the iris diaphragm is considerably different from the plane of the exit pupil (either in front or behind) then as the lens is stopped down vignetting will increase.

In most lenses—including the two 35 mm Summicrons in question—the exit pupils and the aperture diaphragms are located at different positions. Still, they don't vignette when stopping down. To the contrary, vignetting usually becomes reduced when stopping down. So what are you talking about?

 

 

The only case where vignetting is minimised is where the image plane of the iris diaphragm is co-planor with ihe image plane of the exit pupil.

The image plane is where the film or sensor is. In the Summicron-M 35 mm (IV), for example, the aperture diaphragm, or iris, is located 36 mm before the image plane (when focus is at infinity). The exit pupil, which is the virtual image of the aperture, is located 48 mm before the image plane, or 12 mm before the iris. In the Summicron-M 35 mm Asph, the exit pupil is located about 10 mm behind the iris (all numbers are approximations, so give or take one or two millimeters). A distance between iris and exit pupil does not cause vignetting.

 

 

I think it is not me that is confusing the lenses.

If you aren't then you're confusing the technical terms.

[DOUG66]

Hi mjh

 

Thank you for your comment I agree with you, the position of image of the iris

diaphragm and the exit pupil are one and the same. This is the point I made

in post #56 which was disputed in post #57 asking me where I got this idea from.

I tried to explain by reasoning but it seems I just confused people.

 

I have since measured the distance of the image of the iris and the exit

pupil and confirmed them to be the same. The method I use is as follows.

 

I use a binocular 10x Microscope. As you probably know this type of microscope

has twin objectives angled so that a steroscopic view of an image is obtained.

By opening and closing the left and right eyes alternativly the image remains

stationary when the object is in focus and if the object is not in the plane

of focus the image shifts laterly when viewed with either eye in turn.

In fact the stereo microscope is acting as a close range rangefinder.

It could be improved by mounting a rotating chopper below the objectives

as an alternative to winking the eyes. Then by running the chopper above the

persistance rate of the eyes as in a cine projector a single image will appear

at in focus and a double image for out of focus.

 

 

Doug.

[01af]

I agree with you, the position of image of the iris

diaphragm and the exit pupil are one and the same.

Not just their positions are the same. "Image of aperture" and "exit pupil" are just two expressions for the very same thing. The exit pupil is the image of the aperture, as seen through the rear element (and just in case someone is wondering ... yes, the entry pupil is the image of the aperture as seen through the front element).

 

 

This is the point I made in post #56 ...

In post #56, you were talking about the iris, not the image thereof.

 

 

I have since measured the distance of the image of the iris and the exit pupil and confirmed them to be the same. The method I use is as follows.

The method you used is entirely irrelevant because you measured something that isn't there in the first place. It's like measuring the distance between your wife and the person you're married to.