Split-Screen focus aid

CMoore · Jul 27, 2019

Why is it that half the screen will go black when using a "slower" lens.
I guess it is usually about f/5.6 when most of these screens start to have problems.....but why is that.?
Would it be Difficult/Expensive to make a split screen that works at the smaller apertures.?
I do not think i have any lens that are slower than f/2.0, so it is not a problem for me......but i have always wondered why that happens.?
Thank You

AgX · Jul 27, 2019

The aperture at which one wedge gets dark is dependant on the steepness of the wedges. The more steep, the more pronounced the focusing effect, but the same time the larger the diaphragm opening at which darkening occurs.

AgX · Jul 27, 2019

By the way, the same applies for the microprism grid.

AgX · Jul 27, 2019

CMoore said:
I do not think i have any lens that are slower than f/2.0, so it is not a problem for me.....?

A lucky guy you are...

Chan Tran · Jul 27, 2019

I don't like those split screen. I replace the focusing screens on cameras that can be changed to get rid of them.

AgX · Jul 27, 2019

I grew up with them so to say, and recently exchanged a plain one at a very early Praktica for one with such focusing aid at buying the camera.
A groundglass with a focusing aid is distracting, but then focusing for me is much faster.

AgX · Jul 27, 2019

But there is an alternative as a full-image superimposing finder, but then you lack visualisation of true unsharpness.

benjiboy · Jul 27, 2019

CMoore said:
Why is it that half the screen will go black when using a "slower" lens.
I guess it is usually about f/5.6 when most of these screens start to have problems.....but why is that.?
Would it be Difficult/Expensive to make a split screen that works at the smaller apertures.?
I do not think i have any lens that are slower than f/2.0, so it is not a problem for me......but I have always wondered why that happens.?
Thank You

It depends Chip on the angles that the focusing screens are cut at. I know that Canon made some split image screens for the New F1 that we're cut so they didn't blackout at small apertures, they are the B screens (S B, P B, and AB) giving spot, partial, and average readings respectively. I have an AB screen for my New F1's I bought on Ebay and it works well at small apertures.

CMoore · Jul 27, 2019

benjiboy said:
It depends Chip on the angles that the focusing screens are cut at. I know that Canon made some split image screens for the New F1 that we're cut so they didn't blackout at small apertures, they are the B screens (S B, P B, and AB) giving spot, partial, and average readings respectively. I have an AB screen for my New F1's I bought on Ebay and it works well at small apertures.

Interesting... Thank You.....and thanks to everybody else.

abruzzi · Jul 27, 2019

Supposedly the Nikon FM3a screen doesn’t black out either, or is at least more tolerant. I replaced the screens on my FM2n and FA with the FM3a screen (still available new) I don’t have any really slow lenses, stopping down a faster lens, it doesn’t black out at f11 as long as you are pretty well lined up. At f16 and f22you have to try hard to get centered to un-black both halves, but it is do-able.

reddesert · Jul 29, 2019

A horizontal split image focusing screen works by using the prisms to direct light from the left side and right side of the lens into your eye. When the aerial image formed by the lens is correctly focused, these two beams of light are both making aerial images at the same place, but when the lens is misfocused in front of or behind the correct plane of focus, the aerial images are displaced left-right, splitting the image. The explanation of split-image focusing in wikipedia is surprisingly bad, here's a good explanation: http://dougkerr.net/Pumpkin/articles/Split_Prism.pdf

Because the prisms are directing light at some angle from the left and right sides to your eye, they need to be able to "see" through the aperture, and if the aperture is small enough, it will block one or both beams and the split image will black out. Cutting the prisms at a shallower angle would reduce the black-out but decrease focusing accuracy, as everyone else has said. Back when these were originally designed, I imagine that one of the design goals was to accurately focus a 50/1.4 lens wide-open with its shallow DOF. The 50/1.4 was pretty common, while lenses slower than f/5.6 were not so common.

AgX · Jul 29, 2019

see figures #11 and #12
for the effect of the aperture size

Sirius Glass · Jul 29, 2019

AgX said:
see figures #11 and #12
for the effect of the aperture size

And the brightness or dimness of the light.

CMoore · Jul 29, 2019

I am sure this latest info has helped others as well. But sense i am the OP....... Thanks for this recent link. It is rather interesting.
I am not saying these types of discussions never go over my head.....there is A LOT of physics going on in photography.
But the gist of what was posted here has been enlightening.
Thanks Again

polka · Aug 7, 2019

reddesert said:
Back when these were originally designed, I imagine that one of the design goals was to accurately focus a 50/1.4 lens wide-open with its shallow DOF. The 50/1.4 was pretty common, while lenses slower than f/5.6 were not so common.

May I add that these "split screens" are actually called "stigometer" or "stigmometer" by their french inventor, Lucien Dodin. And that they are generally designed to work down to f/4 stop so that they be usable also with other focals (up to 200mm, if only they meet the condition to open up to f/4 or f/3.5).

You may google for Lucien Dodin, he invented also a lot of other devices related to photography ; he for instance developped a 24x24 35mm reflex camera (the Alsaflex) which inspired the design of the Olympus Pen F.

POLKa

benjiboy · Aug 7, 2019

It's a common misconception that split image focusing screens on S.L.R's are the same as the range finders on range finder cameras they are not, S.L.R. reflex camera optical systems depend on the user having perfect eyesight (either with glasses or without) coincidental rangefinders do not. (I can focus a Leica M accurately without my glasses, I can't do it on my Canon F1-N)

polka · Aug 7, 2019

Indeed, rangefinders and stigmometers (or microprisms) are in their principle totally different devices, but they both rely on parallax effects.

The stigmometer prisms are sending to the SLR viewfinders light coming from two distinct spots of the taking lens. The separation of these spots correspond generally to the opening of the lens at f/4, and this explains why they cannot be comfortably used when the lenses are more closed. But you may try it with lenses of various focals (wide angle, normal, telephoto lenses - even very long focals), if they are closed not more than f/4, the stigmometer is just still functionnal.

What this means is interesting : whereas the base of a rangefinder is fixed, the base of the stigmometric parallax (because designed for up to a constant fstop) thus increases proportionnaly with the focal, and so increases also the criticity of good focusing. So that the precision of your focusing will be independant of the focal you use - but of course, your ability to focuse precisely needs always that you see your groundglass (and its focusing aids) with maximal sharpness... through a well corrected viewfinder !

On the opposite, rangefinders with their fixed base are more precisely focused with wide angle or normal lenses, but get much harder to use with longer focals. And are completely unusable with very long focals (200mm and up).

POLKa

AgX · Aug 7, 2019

polka said:
but of course, your ability to focuse precisely needs always that you see your groundglass (and its focusing aids) with maximal sharpness... through a well corrected viewfinder !

And here the trouble starts... Whereas nearly all Super-8 cameras from the 60's onwards got adjustable eyepieces, such was completely unheard of for still SLR's up to the 90's...

Les Sarile · Aug 7, 2019

I have the split screen that never black out in the Canon New F-1 and the Nikon FM3A. By never I mean I've put both on fully extended bellows, manually stopped down and they will get very dim but not black out. However, for normal use in dim settings, they are very hard to focus because everything looks in focus. This gets worst with wider lenses.

AgX · Aug 11, 2019

benjiboy said:
It depends Chip on the angles that the focusing screens are cut at. I know that Canon made some split image screens for the New F1 that we're cut so they didn't blackout at small apertures.

These screens in effect contain two sets of wedges. Each set has its own slope, thus the screen got two slopes, with as result both, high acutance and resilience against darkening. As pay off the image quality yielded by the hybrid wedges is lesser.
Both wedges are made of rows of prisms that each got two faces, each face forms part of one of the two slopes.

AgX · Aug 15, 2019

-) Was Pentacon the only manufacturer to intruduce a triple-wedge? (From 4th generation Praktica onwards.)

-) Did you experience such as benefitial over a double-wedge?

benjiboy · Aug 15, 2019

AgX said:
-) Was Pentacon the only manufacturer to intruduce a triple-wedge? (From 4th generation Praktica onwards.)

-) Did you experience such as benefitial over a double-wedge?

One of the seventeen focusing screens Canon made the New F1 was one that split the image four ways the "L" screen that split the image horizontally and vertically at the same time., however, I have never used one and think they would take some getting used to.

AgX · Aug 16, 2019

That is something different again. That is a focusing aid employing to my understanding two wedges crossed, making four parts, each forming a quadrant. With the Pentacon triple one you got three wedges in parallel.

But both results in smaller sub-images, unless one makes the wedges spot larger.

AgX · Aug 16, 2019

When was the split image wedge introduced ?

I say Pentacon FM in 1958.
Anyone know of an earlier model?

Chan Tran · Aug 16, 2019

I never use it. On camera with interchangeable screen I put in one without it.

Split-Screen focus aid

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