The intricacies and complexity of lens design – interview with designer of the Olympus 25mm f/1.2 lens

Written by admin on April 27th, 2017

If you can get past the Google translate issues, it is worth having a read of this interview with Olympus lens designer published on DC.Watch.

Olympus is embarking on a series of high performance “PRO” level metal, weathersealed, f/1.2 prime Micro Zuiko Digital (micro ZD) lenses with MF clutches, capable of withstanding professional “abuse” for Micro Four Thirds, now that the system has matured and many of the cameras have 1/8000th sec shutter speeds to allow f/1.2 in bright conditions without having to resort to use of ND filters or polarisers.

The following is my summary adopted from this interview.

Lens design is always a compromise!

F/1.2 was chosen instead of a wider aperture in order to keep the lens size down, AF speed fast and cost down – all of which are important for users.

The larger the lens elements, the slower the AF response as it takes more effort from the AF motors to move them.

The current f/1.8 “PREMIUM” range of lenses (17mm, 25mm, 45mm, and 75mm) were optimised for Olympus PEN camera users, but these lenses are not weathersealed.

The Olympus mZD 25mm f/1.2 lens has a an amazing lens design of 19 elements in 14 groups including 1 Super ED lens, 2 ED lenses, 3 HR Lenses, 1 E-HR lens and 1 Aspherical lens with 1 small lens element (the 5th group lens) being used for fast AF that can be moved with high frequency and precision, yet be silent. It uses a retrofocus design in which the front 3 groups produce a negative power which reduce achromatism while the rear groups produce a positive power with the master lens being in front of the AF lens. As aberration fluctuates as the AF lens moves, these variations in aberrations are reduced by increasing the number of lens elements and then controlled by multiple correction lens in the rear groups which also add back in some spherical aberration to improve bokeh and also form a sound barrier to the moving lens helping to create a silent AF system

Design principles included:

  • fast, silent AF optimised for CDAF and PDAF and satisfy the Olympus MSC standard (Movie and Still Compatible)
  • weathersealed
  • robust
  • MF clutch mechanism
  • optimising MTF at low frequency and edge-to-edge high frequency detail to provide contrast and aesthetics, particularly with portraits
  • aim for twice the MTF accuracy of a 35mm full frame lens
  • maintain the shape of Newton rings (An interference fringe of light which is generated when a lens having the opposite curve is superimposed on a certain lens) as constant as possible
  • optimising the quality of blur (bokeh) with aim for blur to “bleed slowly” but maintain resolution
    • use of a front aspherical lens to reduce incoming light to zero spherical aberration
    • adding a lens element to restore some spherical aberration at the expense of some high frequency MTF resolution
  • eliminate coma aberration – avoid use of lenses which sharply bend light as these introduce too much coma aberration – hence the need to use many lens elements to gradually modify the light rays
  • minimise all other aberrations such as distortion to reduce the extent of jpeg in-camera corrections needed but where needed, optical characteristic data corresponding to the aperture value, focus position, and zoom position is sent from the lens to the camera body and corrected by image processing.

 

 

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