Dante_Stella
Rex canum cattorumque
This should be an easy answer, but it's not. I am trying to figure out what makes a lens compatible (or not) with an R screen. Most descriptions describe the R screen as being for lenses with a maximum aperture of f/3.5-5.6. The lenses I am studying for use with it are a 45/2.8 GN Nikkor, an AF 35-105 f/3.5-4.5D, and an AF-S 17-35/2.8D.
Old charts carry the notation that the screen is not suitable for lenses with maximum apertures greater than f/2.8 - but carry this warning with lenses that have maximum apertures of f/2.8. So does the warning apply or not? Did they mean to say "greater than or equal to?"
Newer charts are, to put it mildly, equally self-contradictory.
I understand that different split prism angles correspond to different apertures for blackout purposes (the K blacks out a lot at F/4 for example - I'm guessing because it's optimized to have focusing accuracy with much faster lenses) (though some makes like Katz-Eye claim you can use one angle over a huge range range of apertures). I can't imagine that the R is very optimized for small apertures, since f/5.6 - one stop different from the K - is where blackout starts to be a threat with the R.
The blackout, though, is not the real issue - it's the statement that focusing would not be accurate within the (nonexistent) 12mm circle. Why would that be? I could understand distrusting a particular focusing aid, but no aid is that big (except for the Hs, which are full-field microprisms).
Dante
Old charts carry the notation that the screen is not suitable for lenses with maximum apertures greater than f/2.8 - but carry this warning with lenses that have maximum apertures of f/2.8. So does the warning apply or not? Did they mean to say "greater than or equal to?"
Newer charts are, to put it mildly, equally self-contradictory.
- The blue square means "possible" but that focusing can't be done with a lens with an aperture greater than f/3.5 inside the 12mm reference circle (which, incidentally, the R screen completely lacks).
- They have a blue square for many f/2.8 lenses (but not all - the 18/2.8 and 20/2.8, 105/2.8 Micro, 180/2.8, 300/2.8, and 35-70/2.8D are examples where the R screen is shown as green - perfectly OK).
- The AF 35-105 f/3.5-4.5D has a white (empty) square, despite the fact that all zooms in the same aperture and general focal length range have green squares. This one should work by any account. Why no green square?
I understand that different split prism angles correspond to different apertures for blackout purposes (the K blacks out a lot at F/4 for example - I'm guessing because it's optimized to have focusing accuracy with much faster lenses) (though some makes like Katz-Eye claim you can use one angle over a huge range range of apertures). I can't imagine that the R is very optimized for small apertures, since f/5.6 - one stop different from the K - is where blackout starts to be a threat with the R.
The blackout, though, is not the real issue - it's the statement that focusing would not be accurate within the (nonexistent) 12mm circle. Why would that be? I could understand distrusting a particular focusing aid, but no aid is that big (except for the Hs, which are full-field microprisms).
Dante
Rob-F
Likes Leicas
Well, Dante, that is quite a conundrum! I don't know the answer--I didn't even know it was that complicated. Just as a guess, or a starting point, could it have something to do with the diameter of the rear element? Or possibly with the location of the nodal point?
The chart that I read states that focusing must be done outside the split image (3mm? I forget) circle. I suspect they mean outside the center prism. It's possible that the grain of the matte is different enough, but I doubt it.
I will have to find my "R" screen. I prefer the P screen which does not seem to black-out as much as the "A" and "K". Maybe just the offset angle to the eye.
Rangefinders are easier. Focusing must be done within the central patch.
I will have to find my "R" screen. I prefer the P screen which does not seem to black-out as much as the "A" and "K". Maybe just the offset angle to the eye.
Rangefinders are easier. Focusing must be done within the central patch.
I looked at the back of the R-screen for the Nikon F, which is in one of the gray boxes, and cannot see any difference in the flat-matte portion of the screen going across the 12mm imaginary circle. I should try this screen sometime with the Reflex-Nikkor 500/8.
Dwig
Well-known
Its a simple answer: The angle of the wedge prisms that form the split RF is flatter. This prevents the prisms from blacking out with slower lenses while reducing the focusing precision.
This is somewhat analogous to having a shorter baseline on an RF camera in that the flatter angle, like a shorter baseline, reduces the precision. The precision loss is not an issue with slower lenses, their depth of focus (NOT depth of field) it greater. When used with faster lenses you may not focus accurately enough to shoot at wider apertures if you rely on the split RF prisms.
Dante_Stella
Rex canum cattorumque
That's something that also makes rangefinders less easy for some applications...
D
Dante_Stella
Rex canum cattorumque
Just out of curiosity, why do you think this is a depth of focus issue?
Nikon's screen charts tend to indicate otherwise - the very short very fast lenses are shown as working. Fast wideangles have shallower depth of focus than fast telephotos (which are shown as not working). From this, I would deduct that it is the depth of field that is critical.
The "may not" focus accurately doesn't bother me - but a "will not" focus would...
Cheers
Dante