ADR ( Aperture Direct Readout )
With Jim looking at film 35mm cameras, and as I have one, a few weeks ago I checked out an F2AS for him.
As over the years I progressively graduated from no finder info and no through the lens metering to the current everything included. So it was natural to wonder how Jim would get on with much less than what he gets from his D200 which he is most used to. I thought of this thread then as before the F6 with its display is similar to a digital camera, viewfinder aperture display was by direct viewing of the secondary Aperture Direct Readout (ADR) scale on the lens.
As they say a picture is worth a thousand words, so its easier to explain Aperture Direct Readout (ADR) by posting here with a couple of graphics.
With the exception of the F2 Photomic (DP1 prism), F2S (DP2 prism), F2SB (DP3 prism) where an expensive mechanical coupling was included in the prisms, aperture is not displayed in the viewfinder of any camera using the Pre AI metering system.
In 1977 Nikon introduced Auto Indexing of the lens with the camera's meter, and took advantage of this by adding a window to the prisms of cameras to read and display aperture directly from a secondary engraved scale on the lens. To maintain backwards compatability it was necessary to retain the PreAI meter coupling shoe, but holes were added to it to increase the light reaching the ADR scale. This is fine in good light, but in low light the viewfinde aperture scale can be hard or impossible to see. This is not really a problem as in low light the lens would probably be wide open and even at a couple of stops down its easy to count the clicks as the lens is stopped down. Additional to the AI/AIS range of lenses Nikon introduced the lower cost Series E lenses which were
aimed at users of the new lower cost range of cameras such as the EM and FG.
Viewfinder illumination for low light use, where fitted tended to illuminate the shutter speed only. These were a press and hold the button down to light the bulb type, rather than being on while the shutter button is partially depressed.
Autofocus cameras using cpu chipped lenses changed viewfinder info to what we are used to today. However even the F4 and F5 still use the ADR method with AI/AIS lenses, and this did not change until the F6 and D series which allow for programming an AI/AIS lens so the display is the familiar electronic one.
Autofocus lenses lost the meter coupling shoe so only retained backwards compatability with cameras employing the Auto Indexing system. But as Nikon were still making the most popular AIS lenses until 2005, and the used market had lots of them both camps were and still are well supported.
In the graphic I have used an F3HP as the prism has greater overhang than the F2 so it gives a clearer explanation.
Don
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