Split-image focusing for old SLRs - confused

[brainmonster]

I have a Retina Reflex S from the 1950's, and reading the manual it states that you should match the upper and lower halves of the inner circle in the viewfinder to achieve correct focus. However it seems that sometimes the upper half is not visible due to the way that the light refracts.

My question is - can I just focus "normally" on the inner circle halves to achieve focus by judging if they are in/out of focus, or MUST I match the two halves (by judging vertical lines).

The reason I ask is because the ground glass screen surrounding the inner circles seems to be much more sensitive to focus while the inner circle seems to be "more in focus most of the time" than the ground glass.

 

[AgX]

I can't speak for the Retina Reflex, but in general there were/are most various forms of ground glasses. There may even be a version, with a focusing aid at center, surrounded by a clear ring which again is sorrounded by a ground area.

 

[cowanw]

You can use the outer ground glass; you cannot use just I/2 of the inner circle. To use both halfs you may have to move your head slightly so as to centre your eye very slightly up and down or side to side; seeing each half singularly but moving fast enough to compare both as you focus until they appear to match.

 

[John Bragg]

Yes you can. In focus is just that. Sometimes depending on the lens in use, focussing aids can darken considerably. It is fine to just use the ground glass though.

 

[Donald Qualls]

I don't know the Retina Reflex, though I did briefly own the 126 version (in 1981), but the split screen with microprism collar and plain or Fresnel main screen is quite common in SLRs from the 1960s. Assuming that's what you have, it's perfectly fine to focus on any portion of the screen you can see well enough to focus critically. Generally, however, focus will be most accurate on the split, then the microprism, then the plain or Fresnell ground glass.

 

[voceumana]

In SLR cameras, sometimes one half of the split image screen goes dark depending upon the angle with which you view it. You can try moving your eye or the camera up, down, left, right just a little to see if that makes the split image better. In any case, the split image is always easier to see with a bright scene.

 

[brainmonster]

Thanks all for the tips. I will experiment with the focus. The two inner circles are surrounded by a clear ring, which is surrounded by a ground glass for the rest of the screen. I can't guess what the clear ring is for, but I guess it's just for focus that is a little bit clearer than the ground glass to see.

 

[Pieter12]

Split-image focusing aids don't work well in low light or with slow lenses. If the image is sharp in the surrounding area, you should be in focus.

 

[ic-racer]

Ignore it.

 

[Autonerd]

I have a Retina Reflex S from the 1950's, and reading the manual it states that you should match the upper and lower halves of the inner circle in the viewfinder to achieve correct focus. However it seems that sometimes the upper half is not visible due to the way that the light refracts.

I have this problem with some of my 35mm SLRs. Try changing your viewing angle a little.

My question is - can I just focus "normally" on the inner circle halves to achieve focus by judging if they are in/out of focus, or MUST I match the two halves (by judging vertical lines).

I believe you must match. In my experience the halves are sometimes in focus when the image is not.

The reason I ask is because the ground glass screen surrounding the inner circles seems to be much more sensitive to focus while the inner circle seems to be "more in focus most of the time" than the ground glass.

Not familiar with your camera but a lot of cameras have a ring of microprisms around the split circle. They show as dots or granules when the image is out of focus. When the dots disappear, your image is in focus.

My Pentax KX (the camera I have had the longest) has no split-ring, just microprisms in the middle. Maybe it'e because I'm used to it, but I find that a more convenient system than the split ring. Then again, I do get better focus with the ring sometimes...

 

[Europan]

The stigmometre or split-image focusing screen became common knowledge in 1963. Its inventor was Lucien Dodin, 1900–1989.

It should be used with the lens diaphragm open. One has to seek the optical axis with the eye sometimes, that depends on the design of the ocular.

 

[AgX]

In SLR cameras, sometimes one half of the split image screen goes dark depending upon the angle with which you view it. You can try moving your eye or the camera up, down, left, right just a little to see if that makes the split image better. In any case, the split image is always easier to see with a bright scene.

From a certain aperture downwards (aperture depending on the design of the spit-image wedges) one of the two halves goes dark. At leats at the beginning of this darkening one migh move ones sight a bit and see the other half darken instead, by this being able to still use such focusing aid in a primitive way.

 

[Ian Grant]

Split-image focusing aids don't work well in low light or with slow lenses. If the image is sharp in the surrounding area, you should be in focus.

Actually I disagree, split image focussing comes into its own where the camera has a poorer pennta-prism and slower lenses where focussing on the screen is more difficult, I'm thinking of Exa, Retina Reflex, and similar cameras.

Ian

 

[Alan Gales]

Here you go! If you scroll down to viewing and focussing, it tells you that if you can't line up an object on the split screen to just use the ground glass surrounding the circle.

http://kodak.3106.net/download/RetinaReflexGuide.pdf

If you are still unsure that the subject is in focus then I recommend stopping down a bit to increase depth of field.

 

[Donald Qualls]

Wow. The Retina Reflex works like my RB67 (except I don't have a prism finder for mine). Shutter in the lens, non-instant-return mirror.

 

[brainmonster]

Wow. The Retina Reflex works like my RB67 (except I don't have a prism finder for mine). Shutter in the lens, non-instant-return mirror.

It seems like it's got two shutters, one leaf shutter near the lens mount and one metal shutter by the film plane. They synchronize so one is always open, and one is closed except for a brief window which creates the open shutter interval. I think that's why it's called Synchro-Compur shutter, I'm not sure why they did it that way.

I think the Bessamatic works in a similar way, same Synchro-Compur shutter and DKL mount I believe, but the lenses are not intercompatible with the Retina. It also uses Skopar lenses instead of Schneider Xenon lenses.

 

[AgX]

It seems like it's got two shutters, one leaf shutter near the lens mount and one metal shutter by the film plane. They synchronize so one is always open, and one is closed except for a brief window which creates the open shutter interval. I think that's why it's called Synchro-Compur shutter, I'm not sure why they did it that way.

The name Synchro Compur just indicates that it is a shutter of the Compur model, which goes back more than 100 years, but with built in electrical synchronisation contact.

SLRs with modular built lenses, one static rear group and various front assemblies, typically had a leaf shutter and this leaf shutter built behind or in front of the rear group, in any case remaining with the body.
In your case though the leaf shutter is built into the front assemblies.
This again necessitated an accessory shutter to shade the film at changing the front assemblies.

One could have designed instead of the accessory shutter a light tight frame on which the mirror rests.

 

[foc]

This is why autofocus was invented

 

[Donald Qualls]

One could have designed instead of the accessory shutter a light tight frame on which the mirror rests.

That's what the RB67 has -- and from the description I read, the Retina Reflex as well. In the RB, for certain, the mirror platform has foam seals so it's light-tight when it's down. Since you can only remove or mount a lens when the body is cocked (mirror down), this protects the film during lens changes, independent of the (seemingly often missing) dark slide provided for changing film backs.

 

[GarageBoy]

That rear focal plane "shutter" is just to keep the light out when you're focusing with the shutter open - when you press the shutter, the leaf shutter closes, mirror and focal plane flap opens, shutter opens and closes at your selected speed

Microprisms are great when you don't have perfectly vertical lines - turn the focus ring until it stops shimmering. Nikon H screens were all microprism, and $80 a piece new (there were 3 versions that correspond with different focal lengths)

 

[RLangham]

The stigmometre or split-image focusing screen became common knowledge in 1963. Its inventor was Lucien Dodin, 1900–1989.

It should be used with the lens diaphragm open. One has to seek the optical axis with the eye sometimes, that depends on the design of the ocular.

However, before it was common, it was sold with some Rectaflexes in the 50's.

 

[Deleted member 88956]

Just pulled out my Retina Reflex and even with Longar-Xenon 80/f4 attachment I can use split image in quite low a light. It is true that in general splits can darken up for off center viewing (or darker lenses) so trying to adjust how you align your eyeball with it is worth trying.

I will say that Retina Reflex has an excellent implementation of split image. Then again it was a higher class camera to begin with for its day, so that should not be surprise.

The whole ground glass can be used for focusing as is mostly the case in general, but looking at mine you should be able to see split image clearly in a lot of lighting situations.

 

[Deleted member 88956]

That rear focal plane "shutter" is just to keep the light out when you're focusing with the shutter open - when you press the shutter, the leaf shutter closes, mirror and focal plane flap opens, shutter opens and closes at your selected speed

Microprisms are great when you don't have perfectly vertical lines - turn the focus ring until it stops shimmering. Nikon H screens were all microprism, and $80 a piece new (there were 3 versions that correspond with different focal lengths)

Retina Reflex does not have microprisms, at least not one I have. Outer inner circle is clear.

 

[AgX]

I will say that Retina Reflex has an excellent implementation of split image. Then again it was a higher class camera to begin with for its day, so that should not be surprise.

There is an inherent dualism in the principe of split-image focusing aid, which must show in its design: The grade of acutancy and the ability to use lower speed lenses are inverse to each other.
And a attempt to break this dualism, had shortcomings itself. There are laws of physics which are not dependent on "class" of camera.

 

[Deleted member 88956]

There is an inherent dualism in the principe of split-image focusing aid, which must show in its design: The grade of acutancy and the ability to use lower speed lenses are inverse to each other.
And a attempt to break this dualism, had shortcomings itself. There are laws of physics which are not dependent on "class" of camera.

Well, I simply referred to how Retina compares to a lot of its contemporaries. Split circle is BWT quite large in diameter, not sure how that affects darkening threshold.