Pupil ratio of Sigma 150mm macro lens.

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Jim Ford
Posts: 35
Joined: Sat Aug 13, 2011 11:59 am
Location: Herts., United Kingdom

Pupil ratio of Sigma 150mm macro lens.

Post by Jim Ford »

I thought I'd contact Sigma U.K. to see if they had the pupil ratio for their 150mm f/2.8 EX DG HSM lens.

They were very quick to come back to me, stating that they'd contact their parent company for the information.

The reply came back with the information below, but it isn't very clear to me. This is what was sent:

NODAL POINT PUPIL POINT
ITEM CODE 1st 2nd ENTRANCE EXIT
APO MACRO 150mm
F2.8 EX DG HSM 127.5 146.5 6.8 80.3

It seems to me that the pupil ratio isn't given above, but maybe there is sufficient information for it to be deduced.

I'd appreciate an opinion on this, please.

Jim

mgoodm3
Posts: 273
Joined: Mon Sep 08, 2008 8:50 am
Location: Southern OR

Post by mgoodm3 »

If you know the position of the principal planes and the pupils, you can figure out the pupillary ratio.

I think that the numbers are measured from the back focal point, the detector.

The rear principal plane is at 146.5 - i.e. the focal length of the lens.

The front principal plane is at 127.5 - the planes are crossed, front behind the rear, sometimes seen.

The entrance pupil is at 7 mm - 120.5 mm behind the front principal plane.
The exit pupil is at 80 mm - 66.5 mm behind the rear principal plane.

There are several ways to get to the pupillary ratio from here.

I choose the easiest for me to visualize.

since the entrance pupil is 120 mm behind the front principal plane, it 120.5 + 146.5 behind the front focal point.

The pupillaryposition will be 267/146.5 or about 1.8x focal length. That translates into a pupillary ratio of 1/1.8 or about 0.55.

Can also get the same number by dividing 66.5/120

Or you could just put a ruler up to the lens and measure the pupils by eye.

I think that I have everything right here.

Jim Ford
Posts: 35
Joined: Sat Aug 13, 2011 11:59 am
Location: Herts., United Kingdom

Post by Jim Ford »

Thanks for the reply. It gives me something to work on.

I'll physically check the pupils as near as I can and compare it with the data above.

Jim

mgoodm3
Posts: 273
Joined: Mon Sep 08, 2008 8:50 am
Location: Southern OR

Post by mgoodm3 »

Another thing to think about:

The numbers that Sigma gave you are probably referred to infinity focus.

Since the 150 is an internal focus lens and the planes and pupils will move when the lens is focused closer. The pupillary magnification will probably change.

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