Jump to content


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Importance of exit pupil distance for vignetting - with App

  • Please log in to reply
No replies to this topic

#1 freestar8n


    Vendor - MetaGuide

  • *****
  • Vendors
  • topic starter
  • Posts: 9,620
  • Joined: 12 Oct 2007

Posted 11 August 2018 - 06:49 AM

People often ask about filter size and vignetting - and the main thing referenced is f/ratio.  To some extent f/ratio plays an important role - but in certain common situations, the location of the exit pupil will completely change things.  This is particularly true with DSLR lenses and SCT's with a reducer.


Here is an app that will show how rays actually arrive on a sensor when you know the basic geometry of the setup - including the distance of the exit pupil from the sensor:




Here is a simple example showing how people might think a 15mm f/2 DSLR lens might work with a 30mm sensor and a 31mm filter that is 17mm from the sensor:




Normally people think of the "aperture" as being one focal length away from the sensor - so for 15mm f/2 it will be a 7.5mm disk 15mm away from the sensor, as shown in blue.


But you know this can't be right because the last element of a DSLR lens - including one as short as 15mm fl - is much farther away from the sensor.


In fact, the rays arriving on the sensor are coming from the exit pupil of the lens - and that pupil has a size and location that could be very different from the assumed 7.5 and 15mm.  For DSLR lenses the location of the pupil may be around 90mm - which means at f/2 the size of the exit pupil would be 45mm - and the light arriving from the lens actually looks like this:




This is a completely different situation - and results in vignetting.


For an 8" f/10 sct you might think the pupil is 200mm in diameter and 2000mm from the sensor - like this:




In fact, the exit pupil of a cassegrain is right near the secondary - so in this case it would be about 500mm from the sensor - and its diameter would be 50mm.  So the actual situation would be slightly different:




But where it is particularly important is when you add a reducer to speed it up to f/7.  You might expect the wider cone to be a bigger problem - but the reducer will bring the pupil much closer to the sensor.  I don't know exactly where it is for 8" f/7, but it could be perhaps 200mm from the sensor - and the result would be:




The app includes some links and more descriptions of what is going on - but you should be able to model vignetting more accurately for your system - as long as you know where the exit pupil is.  And unless it is a refractor or Newtonian with no reducing elements - the pupil will likely not be at the distance of one focal length - and that needs to be factored in when considering vignetting.




Woops - the app isn't drawing the layout correctly.  Wait for version 1.1.


OK it should be fixed now.  The diagrams and vignetting were correct but the aspect ratio wasn't drawn 1:1 so the drawing was stretched.  It should be 1:1 now.

Edited by freestar8n, 11 August 2018 - 07:13 AM.

  • bill w and gatsbyiv like this

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Recent Topics

Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics