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Why a subject not in focus in the viewfinder of an analog camera is seen as two misaligned half circles, while in that of a digital camera is evenly blurred?

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    You are comparing two very different cameras and the differences you are seeing has nothing to do with analog vs digital cameras. Please tell us which two cameras you are talking about so we can provide a better answer. – Mike Sowsun Oct 28 at 22:30
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    I have an early EOS film camera with AF and no split prism in the viewfinder. It's not so much "film vs. digital" as it is "Manual focus vs. Autofocus". – Michael C Oct 29 at 11:41
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The analog camera you use most likely has no autofocus and thus has a split prism. These are great for manual focusing, but not so great for auto exposure (AE), as AE is done after the focusing screen.

Since AF usually works fast (and has a confirmation by LEDs), most cameras use plain focusing screens, though some professional cameras offer interchangeable screens.

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    Split prisms and microprisms (both often used in manual-focus cameras) have no tangible effect on automatic exposure (tons of AE cameras have such aids), but they do interfere pretty intensely with autofocus. Not an issue on manual-focus cameras, of course. – Jim MacKenzie Oct 30 at 14:47
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    @ Jim MacKenzie, I agree that it's not related to AE/metering. But it is the requirement to redirect a significant portion of light to the AF module that causes the issue... i.e. autofocus interferes significantly with the use of split/micro prism screens, not the other way around. You can install a split screen in many DSLRs and the autofocus will work just as it did before. – Steven Kersting Oct 30 at 17:44
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Your assumption about analog vs. digital viewfinders is incorrect. Some digital cameras still use "two misaligned half circles" for focusing and some analog film cameras do not.

"two misaligned half circles" for focusing is also called "Split Prism" and was a focusing aid used in most older, Manual Focus cameras. When Auto Focus lenses were developed for film cameras, Split prism quickly went out of fashion as it was no longer needed.

Aftermarket, third party Split Prism focus screens can be fitted to almost any Digital SLR camera for photographers who wish to still use older manual focus lenses.

Some of today's Digital cameras are Manual Focus and use the split image focus method to aid in manual focus. The Leica M8 Rangefinder is an example of a Digital Manual Focus camera that relies on a split image focus aid.

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A split prism finder uses 50% of the available light for each side of the split image on the focus screen. With the advent of autofocus a significant portion of the available light is redirected to the autofocus module; which results in insufficient light for a 50/50 split at the focus screen. I.e. a split screen installed in a modern AF DSLR is prone to blacking out due to insufficient light transmission.

  • How does the split prism finder use up light that does not hit the split prism? It's another matter that accurately imaging focusing screens are darker than eg the 3d-ish type found in TLRs, or the fresnel-enhanced type found in AF DSLRs. Has nothing to do with the split prism. – rackandboneman Oct 29 at 20:31
  • With a film SLR all light is passed through the focus screen. With a modern AF camera a significant portion of the light is redirected to the AF module which makes the viewfinder too dark to work well with split prism screens. And it caused the development of "bright screens" (fresnel) to help compensate for the loss. – Steven Kersting Oct 30 at 17:36

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