Maxwell screen: where to focus the loupe on?

[Vic Montaigne]

Hi all!

Last year I bought a Linhof TK 45S from a member of this forum. It came with a "Maxwell HI-LUX Ultra Brilliant Matte 4.7" installed as a replacement for the original Linhof ground glass.

Now I am trying to find the right loupe setup for this camera and me. That includes focusing the loupe.

The usual advice is to focus on the grain of the frosted side of the ground glass. Now with the Maxwell I found that quite difficult. AFAIK there is no real grain on the focus side.
I thought an alternative focus target might be the grid, should it indeed be located on the focus side. There is however conflicting information. Here in the forum Rick Denney describes in a great post from 2009 that the grid is printed onto the protective glass sitting on top (at the photographer's side). The piece of paper that was accompanying the Maxwell screen says however that the grid (if one is ordered) is on the focus side of the actual ground glass.

To me it looks more like it is etched on the focus side of the GG but I cannot say for sure. I could probably feel it with my fingers, but that is out of the question

Could anyone say with certainty where the grid actually is, or if this has changed over time?

If the grid is on the matte side of the GG I could use it to focus my loupe, right?

I am also interested in hearing just how people perceive the "grain" of this ground glass, what exactly do you see that you can focus on?
All I can see is a very small pattern resembling the frozen surface of a puddle, not the typical needle like crystalline surfaces, but the more irregular round lines one can sometimes see. Plus some scattered tiny tiny specks. Would one or the other be the "grain"? I find it pretty hard to focus on that, the grid lines would be more convenient

Thanks a lot for your input.

[Bill_1856]

Call Maxwell. He is extremely helpful. And prepare to spend at least 30 minutes on the phone with him.

[Kirk Gittings]

Focus on an edge that has good contrast.

[Bruce Watson]

It came with a "Maxwell HI-LUX Ultra Brilliant Matte 4.7" installed as a replacement for the original Linhof ground glass.

I used one of these focusing screens for a decade or so. Can't imagine LF without one.

If the focus screen is install correctly, the plane of sharp focus will exactly coincide with the film plane. That's all you really need to know. All you have to do to focus the camera is bring an edge (in the scene you are trying to capture) with good contrast into sharp focus. This edge, in your scene, is what you focus on. You'll see it, believe me.

Other than that, I think you are trying to make a really simple thing unreasonably complicated when you talk about focusing "on the grain". Images in real life don't have grain. At least, none of the thousands I've captured have. It's about the image, not the screen. Focus the image and you'll be fine.

[Kirk Gittings]

There is a kind of graininess to the projected image. I know what he means.

[Vic Montaigne]

Please excuse, I appreciate your input, but my question was not about fresnel screens as such or even the Maxwell screen as such. Nor was it about focusing the image of any outside object.

It was about focusing THE LOUPE, that means, adjusting the adjustable eyepiece of the loupe for individual eyesight and for the varying distances, caused by differing thickness of the ground glass sandwich or protective glass combination, in a way that allows one to see a sharp image of the focusing plane. When there is no lens on the camera the focus plane would show the grain of the ground glass.

In this forum there are tons of threads on this topic, are they all "unreasonably complicating real simple things"? The usual advice given in this context is to point the camera without a lens attached to a bright light and to then adjust the eyepiece of the loupe to focus on the "grain" of the focus plane. This is to be done only once for any given combination of camera/loupe/individual eyesight. My specific question now was how to do that for a Maxwell screen that has hardly any visible grain. And even more specific I wanted to know, if I could instead focus on the grid lines. This would of course only be correct, if the grid is on the focus plane of the Maxwell screen. And I am not sure it indeed is. If someone has adjusted a loupe to the focus plane of the Maxwell screen I'd love to hear about it.

I hope I could make myself better understood this time. Thanks.

[Larry Gebhardt]

I would just zero in on it. First focus on a bright point by eye (with reading glasses if needed). Then focus the loupe so the image is as sharp as you can get it. Then using the loupe adjust the focus of the camera to see if you can make it sharper. A little tweak of the loupe focus should be all you need.

[Vic Montaigne]

Thanks Larry, this sounds like a good workaround.

See, I am not very knowledgable when it comes to optical theory. So I was unsure about how strong an effect regarding critical sharpness would result from small differences in focusing the loupe and thereby getting the plane of focus slightly wrong. Especially with larger prints. I would not want to run into big surprises there.

When following the advice to look at the ground glass itself with a loupe I could discern different focus planes, e.g. the planes where the fine lines of the fresnel go in and out of sharpness. And holding the loupe just millimeters away from the screen made a sharp view immediately unsharp. Now, adjusting the eyepiece of the loupe is doing the same thing, it slightly changes the distance of the loupe's lens to the plane of focus, true? So that is why I was concerned to get the loupe to focus on the right plane.

Your way of focusing the loupe seems a good alternative though, coming from the other end, so to speak . Thanks again.

Out of curiosity I’d still like to know where the grid is placed on the Maxwell screen, front with the focus plane or back on the cover glass…

[Gem Singer]

The Maxwell screen is a piece of plastic that is frosted on one side and has Fresnel grooves on the other side.

The frosted side faces the lens and serves as the "ground glass" where the image is formed

The Fresnel grooved side faces backward toward the viewers eye and is protected by a thin cover glass.

The grid lines are printed on the side of the cover glass that is in contact with the Fresnel grooves.

The cover glass protects the Fresnel's grooved surface from damage when you place your loupe against it to focus.

[Vic Montaigne]

Gem, thank you very much for finally clarifying the grid issue!

I knew about the nature and structure of the Maxwell screen just not where the grid is. Your placing it onto the front side of the protective glass is finally consistent with what I can actually see (and what Rick Denney said in the post linked above). When screwing the eyepiece of the loupe "in", that is clockwise, the grid comes into sharpest focus first, then the Fresnel lines, then the tiny pattern one could maybe call "grain".

This is much more plausible than what is written on the original sheet of paper that came with the Maxwell screen, saying that the grid was on the matte side. And it explains why I cannot really make out a physical either print or etching of lines on the surfaces even using a strong LED-loupe. Of course this is to be expected, when the lines are actually in between. One riddle solved. Thanks.