The Ground Glass & The Focusing Screen

[Gumby]

This "standard situation with medium format" becomes even harder to focus if our eyes are not young and sharp anymore. It goes even harder for handheld manual medium format photography, and can go further hard due to the type of device we are using above the focusing screen.

I'm not sure I agree with this definition of a "standard situation with medium format", but that's an whole other thread. The answer is very simple. The best is what you can focus quickly enough an daccurately enough to caputer the desired image. My expreience with MF is first prefereance for GG (plain screen) with magnifier or magnifying prism, followed by plain screen with microprism. I have used split-image prism with MF but find it an annoyance for the reasons stated earlier in the thread. Hope that helps you.

 

[Silva Lining]

Hi folks all,

This issue of the focusing screen may sound mistakenly as petty minded. With our 35mm rangefinders we enjoy a lot of depht of field unless we are focusing at closest range and widest aperture. Then digital has come making depht of field even greater. But we all know that this is not the standard situation with medium format.

This "standard situation with medium format" becomes even harder to focus if our eyes are not young and sharp anymore. It goes even harder for handheld manual medium format photography, and can go further hard due to the type of device we are using above the focusing screen.

So at each of these stations we may miss a bit, therefore starting from the most advantageous point (the best focusing screen) sounds to my understanding nothing else than reasonable.

Cheers,
Ruben

Surely it all depends on the size of film or sensor as well as the type of lens you are using and the distance to intended point of focus.. My DSLR Canon 5D2 gives me the same depth of field as 35mm and even at f/1.2 the autofocus has a high degree of accuracy. Flip it onto manual focus and it is very hard to focus at wide apertures. In truth, most poorly focused shots come from my moving after the camera has focussed. In the MF Rangefinder world I find the Fuji GW690 to be very accurate - I guess the parallel to your questions would be the Hasselblad for which I have a focussing screen, (rather than a split screen) when I first got it I had a nightmare getting sharp shots, but this was down more to my inexperience with the tool (user error).

I guess what I am trying to say is that for most photography the margin of focus error for hand held shots has more to do with the poise and accuracy of the photographer than the relative accuracy of the particular focussing method.

 

[Gumby]

I guess what I am trying to say is that for most photography the margin of focus error for hand held shots has more to do with the poise and accuracy of the photographer than the relative accuracy of the particular focussing method.

A tripod is the most valuable tool to "level the playing field" in MF photography. As you say, one can have focus error (or focus accuracy, I would add) with any kind of focussing screen... but jitter from hand-holding can ruin even the most accurately focussed MF image.

 

[acheyj]

Hey all, a reality check !
Do some research on optics etc.
Ruben please do not confuse depth of field with depth of focus at the film plane. The later is far more critical as I have been finding at my cost with a Zorki 1. Its the same as the old Leica where one cannot check the focus plane without much effort.
MF/LF is just as critical, mainly the register between the film plane and viewing screen.
Interesting comments on a hair and aerial image, must try it next time I'm under the black cloth !

ron

 

[ruben]

.......Ruben please do not confuse depth of field with depth of focus at the film plane. The later is far more critical as I have been finding at my cost with a Zorki 1. Its the same as the old Leica where one cannot check the focus plane without much effort.
MF/LF is just as critical, mainly the register between the film plane and viewing screen.
Interesting comments on a hair and aerial image, must try it next time I'm under the black cloth !

ron

Hi Ron,

To my understanding I started the thread about the best screen for checking at the film plane, and at my #13 post, following my (mis ?)understanding, I reoriented the thread towards the focusing screen as we normally use it to focus the subject. I hope we are still singing the same song.

Any news from your research about what the quoted "aerial image" is ?

Cheers,
Ruben

 

[acheyj]

My final post on this subject, which has always fascinated me.
Have a look at this .

http://www.luminous-landscape.com/essays/mf-easy.shtml

ron

 

[uhligfd]

Ok, back to MF and TLRs now:

Given a viewing lens of fixed aperture of 1/2.8 ( no way at getting dark at f/8 or so .. ) , which screen will give the best focus aid? GG, simple matte, or with center split and micro prisms. How do we judge this question in view of the briteness of the image?

Case in point; Rolleiflex 2.8f, do I get a Maxwell Hi-Lux brilliant matte or a Maxwell Hi-Lux micro/split RF? Answer: please guide me.

 

[Sonnar Brian]

You get the one that you can focus with the greatest ease. This varies from person to person, and from lens to lens.

I prefer a Nikon P screen for most SLR work. That's what I use with best results. YMMV.

I also have the A, B, C, E, F, J, K, L, R, and H-4 screen for my F and F2. But I like the P screen the best.

 

[ruben]

Hi Brian,

I highly respect your "liberal" (so to speak) attitude, which says we are different - then let's each of us find what suits us best. Specially since I am aware you have a lot of knowledge power, you don't display. I do support your attitude, but you should understand that it works for people of more or less equal knowledge.

At this thread I am in a position of beginer. In fact worse than that as I have invested in the issue and side related issues near 200 bucks, mostly on the basis of my interpretation of the qualities of the split image screen from other forum, and now this thread tells me I have done wrong. OK, it can happen, but then I want to know exactly why.

I think that within our shared liberal approach, there is place for you to explain why do you prefer this or that, beyond your own comfort, or present the different alternatives with their pluses and minuses. You are a man of optical knowledge, and I am sure you could share your knowledge in the most objective and unpersonal way, without offending anyone.

But it seems to me that to hide knowledge for fear of offending touchy people - I am not sure this is the best.

Along the thread several curious things have been thrown to the air that sharply contradict my first hand experience, not just my assumptions. Thus for example I have focused medium format cameras without any central aide. The critical focusing point has been just a compromise, a middle of the way between the diffusion forwards and the diffussion backwards - with a considerable field of sharpness in the middle, that due to my eyes I solved by my hands movement average instead of my eyes.

On the contrary, given harmony of light and minimum aperture, the split image line doesn't forgive. Yes it can be annoying to find it, but that's something else. Now in case this line, unifying both parts of the images is physically wrong (i, e, the split image may be perfectly unifyied but this not producing perfect focus - then kindly explain why. (Of course at this point we assume that all other factors beyond the focusing screen are correctly calibrated.)

Of course, each of us can find easier to focus this or that way, according to many factors, no problem. But I would like to know why, or for what physical reasons, the split image way is the last accurate among the list.


Cheers,
Ruben

 

[Al Kaplan]

I'm currently down to two medium format cameras, a Minolta Autocord and a Rolleiflex T. I use plain ground glass and have no trouble focussing with either one. I always used a plain ground glass when I had the Hasselblad. As I said on a previous post on this thread, the ground glass is the most accurate way to go.

I also have a Visoflex II-s for my Leicas. This is an oddball Visoflex model with the ground glass mounted in a slide-out frame. Supposedly other screens were made for it, like a clear spot with cross hairs. I have both the chimny finder and the prism finder.

What I find really strange is that if I slide out the ground glass panel I can still focus the arial image (and it IS bright!) with no focusing screen. If I then slide in the ground glass screen it shows that my arial image focusing was perfect.

 

[ruben]

hi Pitxu,
The picture you are presenting here is beautifull, no doubt.

Technically is very soft, the softness suiting the subject. We do not know at what distance it has been done, yet whatever it be within a close distance, for an f/3.5 it is remarkable good result. And for sure the little girl, nor the dog were still.

But it is a 35mm camera, not a medium format, and there is a huge margin between both tipes in terms of depht of field. According to the DOF calculator with a 40mm lens at f/3.4 - if you stood at 2 meters from the girl you had 0.5 meter margin. If a 40mm lens is translated to medium square format to a 65mm lens, then at the same aperture your margin would be narrowed to 0.28m - practically half !

Unrelated - but had you used a 135mm lens with a 6x6 medium format, something around a 70 ~75 lens on 35mm format, with an aperture of 3.4 (i.e. the same lighting conditions but with the need to increase the speed to compensate for the larger focal length, unless a tripod was used) then your margin for depht of field would be reduced to only 6cm !!!

Are we starting to understand each other ?

Cheers,
Ruben

 

[Gumby]

But I would like to know why, or for what physical reasons, the split image way is the last accurate among the list.

Hey... I found this on RFF. Perhaps it will answer your question:

http://www.rangefinderforum.com/forums/showpost.php?p=1035898&postcount=14

 

[acheyj]

I'm currently down to two medium format cameras, a Minolta Autocord and a Rolleiflex T. I use plain ground glass and have no trouble focussing with either one. I always used a plain ground glass when I had the Hasselblad. As I said on a previous post on this thread, the ground glass is the most accurate way to go.

I also have a Visoflex II-s for my Leicas. This is an oddball Visoflex model with the ground glass mounted in a slide-out frame. Supposedly other screens were made for it, like a clear spot with cross hairs. I have both the chimny finder and the prism finder.

What I find really strange is that if I slide out the ground glass panel I can still focus the arial image (and it IS bright!) with no focusing screen. If I then slide in the ground glass screen it shows that my arial image focusing was perfect.

Geez u guys this thread has got away !.

Ruben posed the query about checking the film plane register NOT focussing a camera. Ok got that off my chest.
All the same you have all got my attention particularly Al, I would if asked said no image visible, now Al I am not doubting you so will try with a Mamiya C330 and also a Mamiya RB67 both of which have a removable screen.
From time to time I get hung up trying to set up a most frustrating Bertram Press on the GG, now it just occurs to me that also has a GG viewing screen (like a LF ).
So lots of work to do !
Cheers all, shall report in due course.

ron

 

[Gumby]

Geez u guys this thread has got away !.

Ruben posed the query about checking the film plane register NOT focussing a camera. Ok got that off my chest.

Sorry Ron... but not correct. He changed his mind (topic) partway through. We're onto the second topic now. Catch up, buddy! 🙂

 

[acheyj]

Ok my mistake for posting knew I should have shut up and left it to the proper people, shall refain in future.
Someone once told me "Better to remain silent and be thought a fool than open ones mouth and leave no doubt "

Cheers all

 

[ruben]

I have found the following document, explaining the split image screen physics, with the only reservation for the cases in which one the split image screen prisms may blacken. Nothing aobut lack of accuracy:

http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf

Cheers,
Ruben

 

[ruben]

Ruben, please, what is the point of this thread?

At the begining I didn't see the point of prefering a ground glass over a split image screen for checking focusing at the film gate/ film plane.

As the discussion unfolded the split image screen was called into question not just for checking focusing at the film gate/film plane, but also for normal use as focusing screen propper.

Therefore at that point we started to follow the latter discussion, which was of more basic character.

If I understand Al Kaplan, there is no accuracy issue with the split image screen in so far the right lenses are used, but a limitation of the lens you can use with it. And something highly important and new, he says that when the one of the prisms of the split image starts to blacken, the split image starts to misfocus as well.

The latest point if I understood it correctly is of high importance to SLR users in general, since highly pro SLRs come with split image screens, and the blackening process, when happens is progressive - sometimes you have an absolute half black, but many times you have a slightly blackening. Knowing that at this point you shoud stick to the microprism instead of insisting with the split image - this is of great general interest, perhaps the best achievement of this thread so far.

Cheers,
Ruben

PS: It is my opinion that despite the hardships along the road, my knowledge and perhaps the knowledge of many folks as well, has indeed advanced a lot at this thread.
Furthermore, this thread tought me how many things I don't know, how much RFF can be helpfull, and how much I will continue in the future to ask what I do not know till it becomes crystal clear.

Every one is most kindly invited to take part in the process, according to his free will, no one is obliged.

Anybody feeling a thread is dumb, is most kindly invited to do what I do when I feel a trhead is dumb: instead of protesting, molesting or laughing - I just move on to another thread or to another forum. The tremendous variety within and outside RFF is of such scope, that unwillingly sticking to uncomfortable threads, not to speak about publicly disqualifying them, or spreading cynicism, is bizzare. Simple respect for the fellows, please.

 

[Gumby]

Nothing aobut lack of accuracy:

Accuracy of a split-prism is determined by design and manufacturing quality. Both sides have to be identical, in a complimentary manner. If there are either design or manufacturing variances to one half, but not the other... then accuracy becomes a problem. In general, though, this is not a real-world problem or concern.

What Al was talking about, and probably Kerr also but I didn't read the linked material, is 'usability' or 'utility', not accuracy or precision.

 

[ruben]

Accuracy of a split-prism is determined by design and manufacturing quality. Both sides have to be identical, in a complimentary manner. If there are either design or manufacturing variances to one half, but not the other... then accuracy becomes a problem. In general, though, this is not a real-world problem or concern.

What Al was talking about, and probably Kerr also but I didn't read the linked material, is 'usability' or 'utility', not accuracy or precision.

Bold added by me.

Therefore, I conclude that not only Al but Ed as well, agree that for all practical matters, and within the focal length limitations of lens and their minimum aperture, the split image screen (and btw the split image one surrounded by a wide microprism - manufactured by Mamiya for their Tlrs) are physically as accurate as anything else, for normal use in medium format photography, including situations with the most shallow depht of field. Let's say a subject/situation with 1cm DOF is enough for me.

As for the former subject which started this thread, I think that as far as I am concerned, after this thread it can wait.

Cheers,
Ruben

 

[PaulDalex]

I am not sure that I quite understand the orientation of the discussion.
Anyway the split image is a device made embedding in the ground glass (well we should say ground plastic) two wedge prisms oriented in opposite direction (seen by cut it is like a X). If the image is in focus the focused image falls at the center of the X. If not the defocused image is displaced differently by the two wedge prisms.
It is a smart focusing aid, like a loupe and exploit the features of our vision to reduce the typical forth and back rotation of the focusing ring we tend to do when looking on a groundglass.
Naturally all the comment made on the limitation of this method are correct.
Hope it helps