# Front/rear effective diameter on a camera lens

I'm in the process of trying to understand lenses a bit better so I decided to take a look at the specs of an actual lens I have access to: Kowa LM25HC (datasheet).

Can someone explain the meaning of the front/rear effective diameter? At first, I thought these were the entry/exit pupil diameters, and that exit pupil was the position of the exit pupil, but this doesn't fit in with my current understanding of the concept.

Here is why.

If my understanding is correct, pupil diameter is equal to its distance from the focal point divided by the f-number. This means that, with the sizes given for the effective diameters and the smallest f-number, entry pupil would need to be $21 \cdot 1.4 = 29.4$ mm from the focal point (4.4 mm behind the object-side principal point), and the exit pupil should be $19.4 \cdot 1.4 = 27.16$ mm from the focal point (2.16 mm in front of the image-side principal point).

The drawing in the datasheet shows a point called E.P., which I assume is either the entry or the exit pupil position. It also shows points called H1 and H2, which seem to be object- and image-side principal points, respectively. Whether E.P. is entry or exit pupil, its distance from the principal points doesn't match the numbers calculated above.

If my understanding is correct, pupil diameter is equal to its distance from the focal point divided by the f-number.

This is correct only for the ideal lens, but not for a real photo lens. For example, many real lenses are farther from the film or sensor than their specified focal distance. This is usually done to allow a room for the flipping mirror. A real lens specified as 25/1.4 is the one that would create approximately the same image as the ideal lens with the same specifications. However, the actual sizes and distances in the real lens would be different from ideal.

Effective diameters are simply diameters of the glass portion of the front and back lens.

The E.P. point stands for both entry and exit pupil position, because entry and exit pupil are the diameters of the same diaphragm as visible from the front and the back of the lens. Thus this point shows the position of the diaphragm in the optical center of the lens. Notice that this point is approximately 33 mm from the sensor instead of the specified 25 mm. This is due to the differences between real and ideal lenses mentioned above. In other words, your lens has the actual focal distance of approximately 33 mm, but it creates the same image as the 25-mm ideal lens would create.

The back focus is the distance between the rear glass and the sensor. "Exit Pupil" stated in the table seems to refer to the distance between the front glass (at infinity) and the sensor (not 100% sure, but nothing else seems to fit; perhaps someone else could clarify). I could not find on the diagram the E1 and E1 points mentioned in thus question. Hope this helps.

• I measured the lens and you are right: front and rear effective diameter are the diameters of the actual glass. – relatively_random Jul 16 '18 at 9:09
• For example, many real lenses are farther from the film or sensor than their specified focal distance. Isn't this accomplished by moving the back principal point? I thought that the back principal point is still one focal distance from the sensor, when focused at infinity. At least for this lens, it seems to be. – relatively_random Jul 16 '18 at 9:11
• I could not find on the diagram the E1 and E1 points mentioned in this question. Hope this helps. I messed up. They're called H1 and H2, not E. – relatively_random Jul 16 '18 at 9:11
• I realize that the entry and exit pupils are created by the same diaphragm, but I thought they were never at the same position. If E.P. is the position of the physical aperture, how do I determine the actual positions and sizes of the pupils? I'm getting off-topic now so I'll start another question about that. You answered the actual question here, thanks. – relatively_random Jul 16 '18 at 9:19
• The rear principle plane is inside the glass. Now consider a full frame DSLR lens with the 11 mm focal distance (or 14 mm). The mirror size is closer to 20 mm, but definitely larger than 11 mm or even 14 mm. So the rear principle plane is farther away from the sensor than the focal distance of the lens. – safesphere Jul 16 '18 at 9:29