The Ground Glass & The Focusing Screen

R

ruben

Guest
Since I entered RFF I randomly heard about the need of a "ground glass" to check the lens sharpness at film plane, but I never happened to see one personally.

Therefore, whenever I needed to check lens at film plane I used instead an SLR focusing screen, after asking time and again at the forum if it should be ok.

Then the day came in which I needed to check a medium format camera, while the available medium format screens I had at the time were not big enough to sustain it on the film gate, so I found it approriately to ask what is a ground glass at all, and a friend even referred me to try a certain ground glass maker, which by the way proved to be a very nice man and a pro in his craft.

Today the famous ground glass arrived home, spotless work, low cost, usps shipment door to door. But meanwhile, and we are talking about some two to three months (the delay was on my guilt), I happened to buy a supplementary focusing screen made new for Rollei Tlrs - this one I intended to cut for a Mamiya Tlr, to serve as a focusing screen instead of checking the focus at the film plane

Btw, the flickr folks at the "twin lens reflex" there, and Rick Oleson first of all, warned me that not every screen could be used as a focusing screen for the Mamiya tlr since there is an isue of the thickness of the screen, making a focus difference, or error, unless it is according to the specifications.

Good to know, but the split image screen for Rollei happened to be already bought.

So I have now at home both the ground glass and the Rollei split image screen, which one of its sides is of the same length like the ground glass. This means I can use it as a ground glass since I can mount it to the film gate of the medium format camera I intended to measure.

Therefore, my simple question is what advantage has a simple ground glass screen to check focus at film plane over a focusing screen, which in my case is a split image focusing screen, making the work of checking the film plane much easier.

In principle, most focusing screens have different devices to fine focusing against none on the ground glass. Futhermore, a split image aide is held as the most accurate one - So why the talk is about the ground glass instead of focusing screens ?

Cheers,
Ruben
 
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Richard Jenkinson said:
.......

A ground glass and a focusing screen are the same thing. The simple ground glass being the most basic. A focussing screen is a ground glass with either a split image, and/or a circle of micro-prisms cut into it for added convenience. (This is not always "better" than a simple ground glass. For some SLRs plain screens can be fitted for certain applications.)
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Hi Richard,

In case you are technically right, then we should stop recommending ground glass screens, and stop using even the term, and speak about appropriate sized split image focusing sceeens, which are more complex and more accurate devices for measuring

I happen to fall into the whole semantic confusion as a virgin and unless someone tells technically the contrary, I ended as a fool.

Well, it happens.:)

Cheers,
Ruben
 
ruben said:
In case you are technically right, then we should stop recommending ground glass screens, and stop using even the term, and speak about appropriate sized split image focusing sceeens, which are more complex and more accurate devices for measuring
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It may be technically right that GG and a focussing screen perform the same function but htere are two reasons why GG has been the traditional tool since the dawn of photography: 1. It is made of glass, which is hard, and "guaranteed" to lay flat and 2. It can be cut to size easily, and 3. it is geenrally cheap and readily available.

There is no need to be so absolute about recommending GG as the tool for checking image formation at the film gate. That is the traditional tool and considered by most to be the tool that lies flattest and gives the most accurage image. But feel free to use what you have and what works for you. Truth be told (and I think this has been discussed on this forum in the past) a piece of regular Scotch brand tape can be used in a pinch.

I think there are some (if not many) that would be willing to debate the relative accuracy of a split image versus ground glass, especially if one uses a loupe to view the GG image... but to go into that would probably be a futile discussion. In short, though, most don't consider split image to be more accurate than a straight-forward GG, or even microprism.
 
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ruben said:
I happen to fall into the whole semantic confusion as a virgin and unless someone tells technically the contrary, I ended as a fool.
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This does not have to be the situation. If you learned something in the process then you are not necessarily a fool.
 
Also recently pressed into service a screen from a slr (OM 20) and a spare fresnel screen from my 330 pros for mf. Worked really well tho make sure the screen has the microlens the right side !. BUT for small apertures or poor light as Pitxu rightly says they black out.
BTW ground glass screens are easy to make, have made many years ago, sand and water, valve grinding paste, even fine emery paper and elbow grease does the trick, finer the better.

ron
 
I use scotch tape on a strip of 35mm film, and use a loupe with glass near the plane of focus to hold it flat. My rationalization is that film is held in place at the film gate by the pressure plate of the camera. It does not hold as flat as ground glass. so- I'm trying to simulate that with the scotch tape on film strip and the 15x loupe.

Which works best? I don't know, I do what works best for me.
 
Thats got me thinking Brian, sound logical !. Film flatness is a REAL problem with MF and above have never really got good results with a 6x9 camera (Bertram press) using roll film as oposed to sheet film, sadly no longer available. When using a loupe put a pencil mark on the GG side to make sure you focus the loupe on the mark.
ron
 
ruben said:
Therefore, my simple question is what advantage has a simple ground glass screen to check focus at film plane over a focusing screen, which in my case is a split image focusing screen, making the work of checking the film plane much easier.
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1. Ground glass will hold flatter against the film rails.

2. A split image focusing aid on a plastic fresnel lens focusing screen might not work in a rangefinder. It is designed to focus the image from a retrofocus lens and I'm not sure how it would work with a non-retrofocus lens.

Edit:
3. The back-focusing technique, using an SLR, is the most accurate method for adjusting the focus for infinity.
 
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I like Gumby's comments on the difference. I've been shooting a lot of view camera in the last couple of years, and it would be silly to have a split-screen focusing aid in the center of my ground glass. You want to see the whole image as it will appear on film, using a loupe from corner to corner, stopping down and adjusting movements, until everything is sharp.

The split-screen microprism focusing aid was invented for the 'point and shoot' 35mm crowd where focusing had to be quick and essentially at one point in space only.

With the medium format problem, I wouldn't necessarily want a split-screen focusing aid smack in the center of my ground glass. I'd be focusing and then moving the camera for recomposition with every shot. It would be awful.
 
Ok, I think that the thread reached a point in which we should clearly state if the split-image screen is the most accurate means for focusing or not, for the area it is pointed to, when being used as a focusing screen and not as focus check at the film gate, given enough light and type of lens are in harmony as to not blacken the area in question.

Cheers,
Ruben
 
ruben said:
Ok, I think that the thread reached a point in which we should clearly state if the split-image screen is the most accurate means for focusing or not, for the area it is pointed to, when being used as a focusing screen and not as focus check at the film gate, given enough light and type of lens are in harmony as to not blacken the area in question.
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Split-image is NOT the most accurate, even for focusing screens. When it works it is sufficient, and to many people the easiest. The principal benefit is that it is quick to use.

The "most accurate" (and we are thinking at a level of detail down in the nits) the surface upon which an image can be formed that is the "thinnest" is the best. In order: Aerial image is best, then GG, then micro-prism, then split image. Hairs fit somewhere between aerial image an dGG; Scotch tape fits somewhere between GG and microprism (if laid flat). Even within GG there are differences: etched GG is "ever so much better" than sandblasted GG. This is basic physics and is well documented if one scrounges through the various literatures dealing with image formation.
 
Hi Pitxu,

It is not about drama, but about manual focus medium format, which many times will be shot at lowest apertures. Have a tour at flickr for non posed nor tripod medium format images, that you can enlarge there at flickr, and you will be getting me better.

I think we both don't need flickr to know the point, but just in case.

To my feeling, and of course this is highly arguable, the microprism area of a handheld medium format camera forgives to the eyes far beyond what the actual print will show.

So the question still stands is the split image the most accurate means for medium format or what ?

Of course that devices increasing magnification are to be discarded now, since in the same way they will ease for microprism view they will improve split image view.

Cheers,
Ruben
 
Ground glass is darker than a microprism screen, but you could also consider ground glass as a random arrangement of itsy-bitsy prisms, and every one of them will be just a bit different than any other. At any given moment some of those prisms are blacked out. That's why ground glass looks darker than a microprism or rangefinder.

The microprism spot consists of lots of itsy-bitsy little angled prisms just like the split image rangefinder's prisms. When you get in focus it stops shimmering. All well and good, but the angle of those prisms is such that if you stop the lens down too much they start to black out and become useless. Most screens start to do this at about f/5.6. If you have a 500mm f/8 catadioptric (mirror) lens or one of those 400mm f/6.3 "stove pipe" lenses you'll have to focus on the surrounding ground glass.

Those prisms are set to a specific f-stop. Everybody accepts the factthat they black out at small apertures. Nobody tells you that if the prisms are set at the proper angle to "read" f/4 they'll still be reading at f/4 even when the diaphragm is at f/2 or even f/1.2. In theory the "base length" of an SLR's focusing system is the diameter of the lens multiplied by the magnification of the finder. With a rangefinder camera it's the distance between the centers of the two windows multiplied by the eyepiece magnification.

If your SLR's split image or microprism is designed to read f/4 and your eyepiece gives you a 100% (life size) view with a 50mm lens your "rangefinder" has an effective base length of 12.5mm while the darker ground glass image gives you 25mm. Because of the popularity of relatively slow zoom lenses these days SLR cameras are now coming with "dumbed down" rangefinders so they're less likely to black out. of course at f/5.6 your effective base length has been reduced to about 8.9mm. No wonder people are jumping on the autofocus bandwagon! I'll stick with a mechanical rangefinder for everyday shooting and plain ground glass for long lenses and macro.
 
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Al Kaplan said:
......I'll stick with a mechanical rangefinder for everyday shooting and plain ground glass for long lenses and macro.
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Hi Al,

Many thanks for your contribution. What type of screen would you stick to for focusing medium format cameras from moderate wide to moderate tele?

Cheers,
Ruben
 
Richard Jenkinson said:
NO.
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Hi Pitxu,
No reason to loose temper, yet thanks for quoting for me Gumby, who explained latter why I could not enjoy his contribution. In this aspect you helped, but instead of loosing patience I invite you to explain me, or quote Gumby if you like, what is the referred "aerial image", which according to him and you is the No. 1 choice.

Cheers,
Ruben

PS
People should be encouraged to ask as many times as they need. Repressing curiousity on behalf of sacred truths is among the less recommendable practices, enhancing curiousity about the source of such attitude. You could increase the bold size even more and this will not be helpfull at any proportion.
 
ruben said:
Hi Pitxu,
No reason to loose temper, yet thanks for quoting for me Gumby, who explained latter why I could not enjoy his contribution. In this aspect you helped, but instead of loosing patience I invite you to explain me, or quote Gumby if you like, what is the referred "aerial image", which according to him and you is the No. 1 choice.
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Ruben... I can speak for myself. I don't need Pitxu to speak for me or translate my words. If you want to know, please be man enough to ask directly... or ask "your God" for a lesson in "forgiveness".

Don't torture yourself nor abuse RFF - just include me in your ignore list.[/color=#000000]
 
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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
 
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