why is Glass ground ?

[brighamr]

Hi

I'm sure there is an obvious answer but why do we use ground glass instead of clear glass
removing my GG and holding my loupe in place gives such a sharp image
why do I want the GG one

thanks

robin

[Vaughn]

1) hard to see the entire image at one time with a loupe.

2)"holding my loupe in one place" -- which place? The actual place is very important...it needs to be the same place that will be occupied by the film (which is the front of the GG -- side closest to lens). having the image thrown onto the ground part of the GG makes sure you are focusing on the proper plane.

[brighamr]

1) hard to see the entire image at one time with a loupe.

2)"holding my loupe in one place" -- which place? The actual place is very important...it needs to be the same place that will be occupied by the film (which is the front of the GG -- side closest to lens). having the image thrown onto the ground part of the GG makes sure you are focusing on the proper plane.

Hi Vaughn
thanks for your quick reply
If I had a glass made with cross hairs on the lens side/film plane
I could focus the loupe first on that and then check focus thru the lens from there
Am I right in thinking that this is how you set up a loupe in the first place

I'm sure I am missing something but can't figure out what

robin Brigham

London

[John T]

For focusing, yes you are correct. However, how can you compose with a clear glass?

[Nathan Potter]

Clear glass is the same as no glass - only what is called an aerial image is produced at the point of focus. The image is there you just can't see it over the field of view so you can't do a global focus or see the effects of camera movements using only the unaided eye. Grinding ie faceting the glass causes the aerial image to scatter giving the eye a real image to look at. I use a couple of Linhof screens which are finely frosted except for a center spot which is clear with a painted cross at dead center within the clear area. With this arrangement I can do a gross focus using the frosted part, then a critical focus at center using a loupe in the clear area.

Nate Potter, Austin TX.

[SaveBears]

Probably the easiest answer, is try it and see how you do!

Experimentation, has been the hallmark since day one in photography...

[John T]

Caroline,

It is not a silly question. Far from it. It is just something you aren't interested in discussing.

brighamr has a point. Aerial image focusing is a well-established focusing method. Nikon and Olympus (and maybe others) have made them for their SLRs, I have a focusing glass for an 11 x 14 copy camera similar to Nathan's, except that it has multiple clear spots to check focus throughout the image. It seems to be primarily used for astro, micro and copy photography. If the clear area has cross hairs to calibrate the human focus, aerial image focusing is much more precise than ground glass focusing. It just isn't practical for traditional photography.

[brighamr]

Where is Brian Ellis when we need him to explain this sort of thing?

Shouldn't we have a silly questions filter on the forum?

And, . . . then people answer it !!!!!!!!!

Caroline

I joined this forum to get help with things I don't understand

why did you join ?

You don't seem to of asked any questions about LF recently,
You just seem to whine about peoples spelling

Robin

[Anupam]

The M screen on the Nikon F3 is an aerial screen. If you have access to an F series camera, you can try it out yourself quite easily.

And Caroline, what we need is a filter for smartass answers.

[sparq]

Search works quite well, too.
http://www.largeformatphotography.info/forum/showthread.php?t=38116&highlight=ground+glass

[dsphotog]

Why not something a bit less breakable? maybe plastic or acrylic?
David Silva

[John T]

Most plastics that I've seen can work somewhat well. The only potential problem is that they flex quite a bit, thus possibly throwing off the focus in the middle. They do scratch easy, but at least they don't break.

[Daniel_Buck]

and the plastics (from what I have seen anyway) don't seem to be as bright as glass, for some reason.

[aduncanson]

Depending perhaps on the construction of your loupe, there may be a problem with trying to focus on an aerial image. The eye's ability to accommodate, or focus at a variety of distances, is likely to prevent you from noticing that the image is in a plane ahead of or behind the intended film plane.

Early Leicaflex's used a screen with an aerial image, as did a Canon EX-Auto that I bought just to experience the aerial screen. Both used some kind of fresnel or other condenser to bend the rays from the screen corners to the eyepoint so you could see the whole image. The most common examples of an aerial image are the finder on a Brownie Reflex or the tiny reflex finders on many early folding cameras. In those cases the necessary condenser lens caused an obvious barrel distortion of the finder's image.

[Kirk Gittings]

Caroline

I joined this forum to get help with things I don't understand

why did you join ?

You don't seem to of asked any questions about LF recently,
You just seem to whine about peoples spelling

Robin

Robin, this is exactly the place for these kinds of questions. Welcome. Ask on.

[Ash]

Looks like Caroline is a troll.

I can't really add anything to the discussion, but I suck at using clear glass/air to focus. I much prefer something matte to look at the image on.

[Gene McCluney]

While aerial-image focusing is a known method..the ground glass back is not "just" for focusing, rather it serves a double purpose. It works fine for focusing, but it also allows you to view the image area you are wanting to photograph. Without the ground glass, or a suitable substitute, there would be no way to view the "actual" image the lens will project onto the piece of film. Any LF camera with a "viewfinder" like the various Press cameras such as the Crown Graphic can only show you an approximate field of view thru the viewfinders, and then only for a select few lenses.

[Gene McCluney]

Why not something a bit less breakable? maybe plastic or acrylic?
David Silva

Some cameras do have plastic or acrylic "ground" glass screens. Certainly some medium-format and 35mm format SLR's and TLR's use plastic screens, but these are small enough that flex is not a problem. There is no reason why you couldn't "grind" a piece of plexiglass and substitute it for your glass screen, but I am guessing it wouldn't be as bright as a properly made glass focusing screen.

[aduncanson]

While aerial-image focusing is a known method..

The only technique for aerial image focusing that I am aware of is to move your head back and forth while viewing cross hairs and the aerial image on a clear portion of the screen. You must determine if the aerial image moves with your head relative to the cross hairs (in which case the image is closer to the lens than the screen is) or in the direction opposite of your head's motion relative to the cross hair (in which case the aerial image is further from the lens than is the screen.) I have always found this process tiresome and unnerving but some swear by it as the best way to achieve accurate focus.

[Vaughn]

Does it really matter if Caroline is a troll or not? As long as the dicussions generated are civil and informative, who cares?!

When I had a GG with cut corners, I found that I could easily tell what was in the corners via the aerial image. Nice and bright, too.

Vaughn

[cyrus]

I was always curious why we don't have split-screen focusing on the glass, like some SLRs.

[Nathan Potter]

Cyrus, I've always wondered why not split screen viewing for LF screens. I'm guessing that the critical angle formation in the glass is a pretty high tech grind/polish operation and there just is no tradition for such with any of the LF screen suppliers.

Another thing I'm guessing about is the difference between plastic and glass screens. Having ground both myself the glass is much easier do to the hardness of the glass. Using say 200 to 600 grit silicon carbide abrasive slurry in water, glass is an easy and uniform grind (it fracture easily when attacked by a several micron grit particle) Plastic (Lexan, Polycarbonate, etc.) are actually very soft so they don't fracture easily but will have a tendency to smear. Despite that the plastics can be ground using the right technique (higher grind speed lighter force).

Another factor is the difference in index of refraction. Glass (SiO2) index is about 4 while plastics (transparent that is) are generally much lower. So refraction will be greater with glass when a light ray strikes a microchip in the glass. Thus with glass the scattering from the ground surface is greater. I think this contributes to the better viewing angle and slightly brighter image.

Nate Potter

[Struan Gray]

Cyrus, split screens and microprisms don't work very well with slow lenses, when you're using movements, or when you rack the lens away from the film plane for closeups. In all three cases, one or more segments go black because they are pointing too far off the lens axis to receive any light. In 35 mm this is often distracting if you stop down the lens for DOF-preview, in MF it's annoying *nearly* all the time, and in most normal LF situations you are in the black right from the start.

[Nathan Potter]

Of course Struan an obvious caveat that I missed mentioning. But I wonder if the going black phenomena can be mitigated somewhat for longer FL lenses by grinding the flats to a more acute angle to the screen. Of course the "out of focus" split image would be less split but could still be useful.

Nate Potter, Austin TX.

[Sevo]

Of course Struan an obvious caveat that I missed mentioning. But I wonder if the going black phenomena can be mitigated somewhat for longer FL lenses by grinding the flats to a more acute angle to the screen. Of course the "out of focus" split image would be less split but could still be useful.


There were dedicated screens for long lenses for Nikon F's up to F4, with reduced angle splitters - but they were pretty useless on short lenses.

Another issue is that split finders must be centered and aligned towards the axis of the lens - i.e. they black out in the shift/tilt situations typical for view cameras. This also implies that you have to point the optical axis wherever you want to focus, which would mean thowing the camera out of alignment for focusing.

Sevo

[wclavey]

In addition, wouldn't a GG with a split image focusing aid assume that the "in focus" content would be in the middle of the GG, where the circle is? It seems to me that the pictures I am composing all have the "in focus" content off-center - - sometimes so far that my loupe barely fits on the GG.

[Struan Gray]

If you know that you are only going to use one focal length and camera geometry it will always be possible to make some kind of split-image focussing aid. The problem is making them for general use in a wide range of situations.

I have played with focussing onto a diffractive film on which there was a lithographically defined pattern. It was fantastic at snapping into focus at just the right position, but there was very weird colour fringing and other effects in the out of focus portions of the image which made it very hard to judge what the overall composition was going to look like. It would be great in a narrow-view rangefinder though.

A poor-man's split prism rangefinder can be had by placing a mask in front of the lens which splits the front element into two or more sections out near the edge. Supposedly an old pressman trick with TLRs was to put a finger down the middle of the viewing lens. This splits the aperture into two sections, and creates a double image of anything out of focus. With a mask you can get as funky as you like, but also choose to emphasise vertical or horizontal lines, just as with a split-prism.