Ever wonder why it's so difficult to focus fast lenses with a DSLR viewfinder?

Started Jan 27, 2013 | Discussions thread
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WillemB
WillemB Contributing Member • Posts: 678
Re: Ever wonder why it's so difficult to focus fast lenses with a DSLR viewfinder?
5

The phenomenon of the 'F2.8 focus screens' has been explained many times the last 15 years...

These screens were developed in the same period as the first consumer zoom lenses, in order to give a bright enough view when zoomed fully in, even when the effective F-stop was F/5.6 or even F/6.3. These matte screens consist of a great many of very tiny asymmetric pyramids, to concentrate the light through the screen onto the spot where the eye pupil is. Physics laws then dictate that the light rays from the outer lenses rim (The F/1.2-2.8 rays) cannot also be brought together towards the eye, so these are missing when you need them for manual focussing a F/1.4 lens...  This is a laymans explanation...

WillemB

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