How are DSLR viewfinder focus-point indicators displayed?
Asked 5/19/2011
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In DSLR optical viewfinders, how are the red AF point indicators and other overlay marks actually shown? Are they etched into the focusing screen and illuminated, projected from elsewhere, or created with an LCD layer? I’ve also seen claims that some cameras use an LCD layer inside or above the focusing screen. How do these approaches differ, and what are their pros and cons? As a related comparison, is this similar to the overlay system used in hybrid viewfinders like the Fujifilm X100, or is that a different design altogether?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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There are a variety of ways to display highlighted focus point indicators in the viewfinder.
One of the earliest (yet still common) methods is to direct light back through the pentaprism to reflect off the rear-surface of reticles etched on the focusing screen or dedicated "superimpose plate". Displayed information is limited to highlighting the etched indicators.
A relatively recent method has been to place a monochromatic transmissive LCD just above the focusing screen. This allows more information to be displayed (a choice of framing guides and myriad focus points) as required, however the LCD becomes less responsive in cold weather and significantly dims the viewfinder when unpowered.
(Canon EOS 7D)
Another way is to use dichroic prisms between the pentaprism and viewfinder eyepiece lens to reflect an illuminated superimposition display (SI) LCD without affecting the brightness of the viewfinder.
(Canon EOS 1D MkIII)
And finally, the new hybrid viewfinders superimpose a colour LCD display using a half-mirror - however the mirror will darken the optical image.
(Fujifilm FinePix X100)
Originally by user5177. Source · Licensed CC BY-SA 4.0
user5177
15y ago
0
Generated from our catalog & community — verify before relying on it.
DSLRs have used several different ways to show focus-point indicators in the optical viewfinder.
A common older approach is to etch reticles or marks on the focusing screen or a separate superimpose plate, then send light back through the prism so those marks light up. This works well for simple AF-point highlighting, but it limits what can be displayed.
A newer approach is a monochrome transmissive LCD placed in or above the focusing screen. This can show more flexible overlays, such as many AF points or framing guides. The tradeoffs are that LCDs can dim the finder somewhat when unpowered and may respond more slowly in cold weather.
So yes, the statement that some cameras display AF points via an LCD layer is plausible and correct for certain models; other cameras and brands use different methods.
The Fujifilm X100’s hybrid finder is conceptually related in that it overlays information in the viewfinder, but it is more advanced and not the same simple DSLR optical overlay system.
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