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Camera Lens News No. 7
Summer 1999 |
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Carl Zeiss MTF curves are measured – not just calculated
The Modulation Transfer Function (MTF) is nowadays widely used throughout the optical industry as the objective way to clearly represent and evaluate the performance of optical systems, like camera lenses for example.
Trying to trace the usage of MTF in the optical industry we find first roots dating back as far as the year 1940. At that time an early version of MTF was first applied at Carl Zeiss in the Jena factory. Ever since Carl Zeiss pioneered the usage of this method, being convinced that Zeiss optics compared well by any means, MTF included. Other companies in the in-dustry followed decades later. Many refused to publish MTF data of their lenses arguing that it is not as easy as it appears to correctly interpret MTF curves. Today, with the majority of lens manufacturers and many photo magazines publishing MTF curves or similar representations to demonstrate the performance level of camera lenses, we often encounter cases where the curves are misinterpreted by enthusiasts quite grossly. Excellent lenses which earned high reputation amongst professional users are often underestimated by people who have no personal experience with the lens, but rely on their own interpretation of curves only. The opposite can also be seen: Mediocre
optics sell impressive numbers, after stunning MTF data was published, albeit users with personal experience are rather disappointed.
All this leads many seasoned photographers and practitioners to the question: is MTF really trustworthy? Optics designers will be quick to answer “Yes, it is!”. They are the ones who can really interpret MTF curves. They have the professional background and additional information helping them to draw the right conclusions from these curves.
They do know, for instance, that MTF curves can be influenced significantly, if the spectral engergy distribution of the light used for measuring or assumed in calculation changes a bit. So MTF curves of the same lens coming from different sources may vary, even when ”white light” is specified with all these curves. Even ”white light” is not necessarly the exact same thing unless all the weighing factors for the different sections of the spectrum are made sure to be the same.
Lens designers also know that there can be a huge difference between a lens performance designed on a computer and the actual performance of the real lens once in series production. This difference is not simply a fixed amount of loss, something like: ”You design for 95% MTF and you lose 10%, thus getting 85% out of the production. So aim a little higher on the design computer and you will get a little more.” Experienced optics manufacturers know that the opposite is usually true: If you aim for very high MTF values on the computer you create an optical system that is extremely sensitive in manufacturing. It will most likely force you to lower the MTF specification for final acceptance significantly to escape excessive scrapping costs. Thus you may have aimed for 98% MTF and get only 70% on average out of production. At least these lenses show very impressive MTF curves in the catalogues…
Carl Zeiss lens designers may, in a similar case, tend to rather aim for 93% instead of 98 % in order to actually get 90% MTF out of production – with very narrow tolerance bands. The MTF curves that Carl Zeiss publishes, are all measured curves of the first units actually built – not just calculated plots without information about how well they may be executed by the manufacturer.
As far as we can see today Carl Zeiss is still the only manufacturer to print MTF curves which are measured, which describe lenses that can actually be purchased, not just calculated curves of highly ambitious designs which may exceed the manufacturer´s capability of turning them into reality. | |
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