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As you know, the entrance and exit pupil is the location of the aperture stop. However as seen from both the front and the rear of the lens, the location of this iris diaphragm is elusive, because you are looking at it though lenses of unknown power. Thus what you are after is the location of the virtual image of the iris.

Try this technique: Set the lens so that it is elevated off the bench, perhaps on a stack of books. Stop the iris down to its smallest diameter. Illuminate the iris with a flashlight beam. Mark the center of the rear and front element lens with wax pencil, make a dot. You might choose to use a scrap of marked cellophane tape, instead.

Procure a magnifying glass, perhaps a 10X loupe (you may need less or more power for this task). Peer through the loupe held to your eye. Focus on the blades of the iris diaphragm, as seen from the rear of the lens. Now back off and focus on the spot you made on the surface of the exit lens. You were forced to move the loupe towards or away from the iris blades to obtain focus. The distance the loupe traveled reveals the location of virtual image of the iris (rear exit pupil). Repeat viewing from the front to discover the location of the entrance pupal.

The above procedure might be more accurate if you substitute a close-focusing SLR. Focus on the blades of the iris, then without changing the camera’s focus, back the camera until the mark of the lens is in-focus. The distance the camera traveled reveals the spacing between the last lens element and the location of the virtual image location of the pupalpupil.
Hope this helps!

As you know, the entrance and exit pupil is the location of the aperture stop. However as seen from both the front and the rear of the lens, the location of this iris diaphragm is elusive, because you are looking at it though lenses of unknown power. Thus what you are after is the location of the virtual image of the iris.

Try this technique: Set the lens so that it is elevated off the bench, perhaps on a stack of books. Stop the iris down to its smallest diameter. Illuminate the iris with a flashlight beam. Mark the center of the rear and front element lens with wax pencil, make a dot. You might choose to use a scrap of marked cellophane tape, instead.

Procure a magnifying glass, perhaps a 10X loupe (you may need less or more power for this task). Peer through the loupe held to your eye. Focus on the blades of the iris diaphragm, as seen from the rear of the lens. Now back off and focus on the spot you made on the surface of the exit lens. You were forced to move the loupe towards or away from the iris blades to obtain focus. The distance the loupe traveled reveals the location of virtual image of the iris (rear exit pupil). Repeat viewing from the front to discover the location of the entrance pupal.

The above procedure might be more accurate if you substitute a close-focusing SLR. Focus on the blades of the iris, then without changing the camera’s focus, back the camera until the mark of the lens is in-focus. The distance the camera traveled reveals the spacing between the last lens element and the location of the virtual image location of the pupal.
Hope this helps!

As you know, the entrance and exit pupil is the location of the aperture stop. However as seen from both the front and the rear of the lens, the location of this iris diaphragm is elusive, because you are looking at it though lenses of unknown power. Thus what you are after is the location of the virtual image of the iris.

Try this technique: Set the lens so that it is elevated off the bench, perhaps on a stack of books. Stop the iris down to its smallest diameter. Illuminate the iris with a flashlight beam. Mark the center of the rear and front element lens with wax pencil, make a dot. You might choose to use a scrap of marked cellophane tape, instead.

Procure a magnifying glass, perhaps a 10X loupe (you may need less or more power for this task). Peer through the loupe held to your eye. Focus on the blades of the iris diaphragm, as seen from the rear of the lens. Now back off and focus on the spot you made on the surface of the exit lens. You were forced to move the loupe towards or away from the iris blades to obtain focus. The distance the loupe traveled reveals the location of virtual image of the iris (rear exit pupil). Repeat viewing from the front to discover the location of the entrance pupal.

The above procedure might be more accurate if you substitute a close-focusing SLR. Focus on the blades of the iris, then without changing the camera’s focus, back the camera until the mark of the lens is in-focus. The distance the camera traveled reveals the spacing between the last lens element and the location of the virtual image location of the pupil.
Hope this helps!

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source | link

As you know, the entrance and exit pupil is the location of the aperture stop. However as seen from both the front and the rear of the lens, the location of this iris diaphragm is elusive, because you are looking at it though lenses of unknown power. Thus what you are after is the location of the virtual image of the iris.

Try this technique: Set the lens so that it is elevated off the bench, perhaps on a stack of books. Stop the iris down to its smallest diameter. Illuminate the iris with a flashlight beam. Mark the center of the rear and front element lens with wax pencil, make a dot. You might choose to use a scrap of marked cellophane tape, instead.

Procure a magnifying glass, perhaps a 10X loupe (you may need less or more power for this task). Peer through the loupe held to your eye. Focus on the blades of the iris diaphragm, as seen from the rear of the lens. Now back off and focus on the spot you made on the surface of the exit lens. You were forced to move the loupe towards or away from the iris blades to obtain focus. The distance the loupe traveled reveals the location of virtual image of the iris (rear exit pupil). Repeat viewing from the front to discover the location of the entrance pupal.

The above procedure might be more accurate if you substitute a close-focusing SLR. Focus on the blades of the iris, then without changing the camera’s focus, back the camera until the mark of the lens is in-focus. The distance the camera traveled reveals the spacing between the last lens element and the location of the virtual image location of the pupal.
Hope this helps!