Measuring the Entrance Pupil Of the Zeiss Loxia Lens Series

This article is to support the entrance pupil measuring technique outlined in Measuring the Entrance Pupil.

Zeiss provides entrance pupil data for the Loxia 21, 35, 50, and 85 mm lenses- see the Zeiss EP data in Table 1. Zeiss gives this data referenced from the image plane, and for the Sony A7, the flange to image plane distance is 18 mm.

I was not surprised to see the calculated focal length to vary from the nominal manufacturer’s value. However, I was surprised that the calculated focal length of the 85 mm lens was 91.8 mm- I repeated these measurements several times.

In the last row of Table 1, the variance between the manufacturer’s data, and the calculated data for the entrance pupil is given. The 21mm lens had the small variance while the 35 mm had the largest.

In conclusion, this method does allow the entrance pupil of a lens to be determined. Furthermore, it demonstrates that the entrance pupil is not fixed for a given lens, but varies as with any other lens parameter.

Finally, Table 1 is interactive; that is, all values in orange are inputs and they can be changed. In this way, you can experiment with the input data. The “Reset” button will return the table to its correct values.

Table 1: Loxia Lenses’ Entrance Pupil Measurement Data

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