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The question Why do the focus points leds appear shadowed? raises another question for me. How are these indicator lights implemented in typical dSLRs? Are they actually small LEDs etched into the focusing screen, and if so, how do they get their power? Or are they projected/reflected from somewhere?

The new Fujifilm X100 has a "hybrid viewfinder" which can show arbitrary heads-up information, including completely switching to electronic viewfinder mode. Is this a much-further-along extension of the same approach taken in dSLRs, or is it different?

In their review of the Nikon D3S, DPReview says:

The AF points are not etched onto the focusing screen, but are displayed on the LCD layer sandwiched inside it.

Is this correct for this model, and do other models and brands follow the same approach? Are there advantages and disadvantages to different designs?

share|improve this question
Just FYI, Nikon DSLRs don't use the red light system - a black bracket appears around the selected focus point. I don't think they can make LEDs small enough to have them actually fixed to the focus screen, I would imagine they are reflected onto it. – ElendilTheTall May 19 '11 at 13:27
@ElendilTheTall - LEDs are P-N junctions like any other diode, or transistor to that matter. As such I don't see why you can't nake them as tiny as a lithographed transistor. – ysap May 19 '11 at 13:53
@ysap: Interesting. @MattDM: The black brackets in my Nikon D5000 viewfinder certainly look like liquid crystal 'images'. – ElendilTheTall May 19 '11 at 14:03
@ysap: There are a couple of reasons. First of all, a red LED will normally be gallium aluminium arsenide. At least TTBOMK, nobody knows how to fab it at nearly as fine if feature sizes as typical silicon CMOS processes do. Second, a smaller feature size means lower power dissipation capability (and therefore lower brightness). A typical LED produces 8-9 lumens/watt. The new higher-power LEDs are quite a bit different, but probably not applicable here. – Jerry Coffin May 19 '11 at 14:35
@JerryCoffin - Thanks. I don't expect the power of miniaturized LEDs to be like this of a regular one. Then, if the problem is the material, then make it in another color. I understand, form what you comment, that it is currently impossible to fab GaAlAs diode in 45nm process, but for this application you don't really need such miniaturization. Nor do you need a great power, given the proximity to the sensor (eye). – ysap May 19 '11 at 14:39
up vote 19 down vote accepted

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.

Rough example of reflective etched-focus points

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 7D viewfinder system (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 1D MkIII cutaway Canon 1D MkIII viewfinder system (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 X100 hybrid viewfinder system (Fujifilm FinePix X100)

share|improve this answer
Very informative. It makes me wonder, though, where did you find the first diagram, with the Photo.SE screenshot reflected? – ysap May 23 '11 at 7:21
I didn't find a decent manufacturer's cutaway showing the simple reflective system, despite it being used on many Nikons and Canons (even "higher-spec" models like the Canon 5D MkII), so created a rough one. – Leorex May 23 '11 at 8:11
Well, it's definitely professionally looks (except for the perspective distortion of the image in the eyepiece...). What did you use to create it? – ysap May 23 '11 at 13:58
For the images you didn't create, are we sure they don't violate the imgur TOS? – Evan Krall May 24 '11 at 4:23
The images come from manufacturer press releases and whitepapers intended for republication. For example, a Canon one I have at hand states, "Excerpts from this material may be quoted in published product reviews and articles." I shall edit the answer and identify the products. – Leorex May 26 '11 at 7:19

To get the definitive answer you can try to find and study the service or repair manuals for some cameras. For the stupid secrecy of the camera makers they are usually removed from the public access, but can be found elsewhere. This is a page from the Pentax K10D service manual; I don't know if you can figure from it how the LEDs themselves are positioned.

Pentax K10D Service Manual: adjust positioning SI-LED (p.26)

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Wow. As far as I can tell, the LEDs are mounted on the elastic PCB strip, mounted in front of a small prism and reflected from the focusing screen, which is in line with @ElendilTheTall speculation. (at least for this specific camera) – ysap May 19 '11 at 14:34
What I don't really understand is how, from what looks to be 7 LEDs they produce 11 focus points? – ysap May 19 '11 at 14:35
@ysap: Why do you think there are 7 LEDs? – Jukka Suomela May 19 '11 at 15:31
@JukkaSuomela - from the diagram of part 0-O170, and from images 6 and 7 it looks like the white parts are the surface-mounted LEDs. As there are 7 of them in the diagram, one can conclude that. It can be that only the five elements that are in a line are actually LEDs, but the same mystery remains. It can also be that I am totally wrong in my speculation... – ysap May 19 '11 at 17:09
Note that at least Pentax and Canon share different methods. You lose the ability to light up focus points when switching screens on Canon, but not Pentax. – Eruditass May 20 '11 at 0:11

Perhaps a closeup of one of these tiny surface-mount LEDs would be interesting:

enter image description here

That's magnified quite a bit -- the package is actually 1.6 mm long. The LED itself is the small square at the center with the gold wire connected to the top. I can't (of course) guarantee that this is exactly the same LED package used in the particular camera in question, but it's probably at least pretty close -- and if there is a difference, it's probably just an even smaller package (which makes little difference since, as I already pointed out, this is already magnified quite a bit).

[FWIW: Alpha 700, Minolta 100mm/f2.8 macro @1:1+68mm of extension tubes, 1 second @ f/8]

share|improve this answer
Are you sure the image doesn't violate the imgur TOS? – Evan Krall May 24 '11 at 4:25
@Evan: Which term of service did you have in mind? Does somebody find it pornographic? – Jerry Coffin May 24 '11 at 15:17
Ah, I didn't see the exposure info at the bottom, so I assumed that you got the picture from somewhere else. – Evan Krall May 25 '11 at 6:22
@Evan: I did. I did the posting in the office, but took the picture out in the breakfast nook -- the light's better there. :-) – Jerry Coffin May 25 '11 at 13:34

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