by Dr. Vladan Blahnik, Carl Zeiss AG


Lens aperture is – in addition to film or image sensor sensitivity – responsible for the degree of irradiance striking the image plane. In a detailed technical article ZEISS expert Dr. Vladan Blahnik shows why lens aperture depends upon object distance and explains the underlying mechanisms for several optical designs of ZEISS lenses.

With many concrete illustrations the reader also learns why a lens should be panned around the entrance pupil, why anti-reflection coatings are so important for image quality, how lens focusing with floating elements is realized or why optical designs strongly depend upon the available space as given by the camera layout.

Download the technical article as PDF: Click here!

Photo: Eric Westpheling

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  2. Darren

    This is amazing! As an aspiring cinematographer and avid photographer, I am fascinated by optics and I love learning about details like this. Thank you so much for putting this together!

  3. Great article, it isn't so easy to understand like Dr Nasse articles but still it explains some of very important connection between physical and functional properties of lens :) Best regards, WZ
    PS. I cannot wait till release of next technical article at Zeiss Lens Blog.

  4. I'm FAR from being an expert, but in the diagram on the bottom of page 6 of the article, aren't the labels for AP and EP reversed? If I'm reading the diagrams correctly, they've marked the Exit Pupil (EP on the left (object) side of the lens and the Entrance Pupil (AP) on the right (film/sensor) side of the lens.

    Can someone please clarify that?


    1. ZEISS Camera Lenses

      Post author

      Hi Mike,

      Thanks for your remark.

      However, if you refer to fig. 8 of the article, the labels are correct.
      The entrance pupil is marked EP (German: Eintrittspupille) and exit pupil AP (German: Austrittspupille).

      Note that in a real lens the position of the entrance pupil may be behind (that is closer to the image plane than) the position of the exit pupil. This is the case for the Planar T* 1,4/85 ZE/ZF.2. It is also the case for the Makro-Planar T* 2/100 ZE/ZF (fig. 26) and many other lenses, especially those with mid and long focal lengths. Schematically the entrance pupil is drawn in front of the exit pupil in textbooks of optics. You see, that this is not necessarily the case in real lenses: the deeper reason behind this is that optical designs of double-Gauss type (Planar) or Sonnar type are very compact but have such a long focal length that the lens stop lies within the focal length of each subpart of the lens in front of and behind the stop -> the entrance and exit pupils are virtual images. That means that their order is reversed.

      Best regards,
      Your ZEISS Lenses Team

      1. Thanks much for taking the time to respond. It didn't make sense to me that the entrance pupil would or could be behind the exit pupil, but I will certainly bow to your expertise!

        Take care,

        1. Alan

          Is the issue simply that EP is for the German "Eintrittspupille" (Entrance Pupil) and not the English "Exit Pupil"?

    2. Arne

      That made me crazy. I'm not an expert, either. But looking into the lens, one does see the exit-pupil, right?

  5. tccin3d

    I have taken same picture with two lenses: 2/100 and 1.4/35 at different distances (to get same framing) and all settings were the same, 2/100 looks darker. With corrected vignetting. What is the reason for darker image?


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