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 (http://www.largeformatphotography.info/forum/showthread.php?52905-My-New-Maxwell-Screen) 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 :cool:. 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.

[cowanw]

It may be superfluous, but, just to make the point that the focusing of the camera and of the loupe are completely independent. With a unadjusted loupe the correct GG image will still be at its sharpest point when it looks best.
I suggest that you eyeball the focussing of the GG and then use the loupe and adjust the eyepiece to make the image appear as sharp as it can be with the loupe flush to the back surface of the GG. Your loupe will now be adjusted to your eyesight and you can use it to refine your GG.

[Renato Tonelli]

It may be superfluous, but, just to make the point that the focusing of the camera and of the loupe are completely independent. With a unadjusted loupe the correct GG image will still be at its sharpest point when it looks best.
I suggest that you eyeball the focussing of the GG and then use the loupe and adjust the eyepiece to make the image appear as sharp as it can be with the loupe flush to the back surface of the GG. Your loupe will now be adjusted to your eyesight and you can use it to refine your GG.

This is the simplest and most straightforward way (which is also my way...) and it works.

[Bob Salomon]

Point the camera at a light source.
Remove the lens.
Place the loupe on the ground glass and focus its eyepiece till the grainy part of the ground glass is sharp.

You have now focused the loupe to the image forming layer of your ground glass.

[Vic Montaigne]

@cowanW:

It may be superfluous, but, just to make the point that the focusing of the camera and of the loupe are completely independent. With a unadjusted loupe the correct GG image will still be at its sharpest point when it looks best.
Yes, that is clearly so. The loupe is there to project an enlarged portion of the camera image unchanged to the retina, as sharp or unsharp as it may be. And we use the enlargement to make the image still sharper than we could with bare eyes. For this to work the loupe needs to be adjusted correctly.


I suggest that you eyeball the focusing of the GG and then use the loupe and adjust the eyepiece to make the image appear as sharp as it can be with the loupe flush to the back surface of the GG. Your loupe will now be adjusted to your eyesight and you can use it to refine your GG.
If I understand everyone right this seconds pretty much exactly Larry's method.

@Bob
Your advice is where I started from in the archives.:) Just that I did not find so much grain with the Maxwell and therefore hoped I could use the grid instead. But as I now know the grid is not exactly in the focus plain. However, I think I can now manage that tiny "grain" pattern and confirm it by the adjustment found using the second method given here, which many seem to apply in practice.

Thanks everyone!

[Bob Salomon]

@cowanW:

Yes, that is clearly so. The loupe is there to project an enlarged portion of the camera image unchanged to the retina, as sharp or unsharp as it may be. And we use the enlargement to make the image still sharper than we could with bare eyes. For this to work the loupe needs to be adjusted correctly.


If I understand everyone right this seconds pretty much exactly Larry's method.

@Bob
Your advice is where I started from in the archives.:) Just that I did not find so much grain with the Maxwell and therefore hoped I could use the grid instead. But as I now know the grid is not exactly in the focus plain. However, I think I can now manage that tiny "grain" pattern and confirm it by the adjustment found using the second method given here, which many seem to apply in practice.

Thanks everyone!

Vic,

Maybe you need a sharper or a stronger loupe.

[rdenney]

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 (http://www.largeformatphotography.info/forum/showthread.php?52905-My-New-Maxwell-Screen) 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.

Maxwell has done it a couple of ways. With small-format focus screens (and I have one of his screens in my Canon 5D), the grids are etched on the focus-side of the plastic screen. There is no cover glass with these because they are protected inside the camera.

With my 4x5 Hi-Lux screen, the grid is printed on the cover glass, facing the lens, which is in contact with the Fresnel side of the focus screen. The focus surface, however, is on the other side, facing the lens. If you focus on the grid, you'll need to readjust that focus when you bring the image into focus, and that may require some iteration to get both image and loupe perfectly focused. That process takes me about a second or two. But I can also focus the loupe on the texture of the focus side of the Maxwell screen, which can just be made out by looking through the grid and Fresnel patterns. You may have to stop down to see it--it's a very bright screen and that texture washes out at high apertures. I'm not using a 6x tilting loupe with that camera and screen.

It may be that Bill is now or at some times etching grids onto the focus surface as he does with small-format screens. I just don't know.

Rick "who marked 6x12 on the cover-glass backside with a Sharpie pen" Denney

[Vic Montaigne]

Maxwell has done it a couple of ways. With small-format focus screens (and I have one of his screens in my Canon 5D), the grids are etched on the focus-side of the plastic screen. There is no cover glass with these because they are protected inside the camera.

With my 4x5 Hi-Lux screen, the grid is printed on the cover glass, facing the lens, which is in contact with the Fresnel side of the focus screen. The focus surface, however, is on the other side, facing the lens.Thanks for the information. This confirms exactly what Gem Singer reported. As I stated earlier: one riddle solved.


It may be that Bill is now or at some times etching grids onto the focus surface as he does with small-format screens. I just don't know.
Yes that is possible. I do however not believe that the grid is etched on the focus surface in my case. The other version correlates better to what I actually see when looking at the screen with the loupe.


If you focus on the grid, you’ll need to readjust that focus when you bring the image into focus, and that may require some iteration to get both image and loupe perfectly focused. That process takes me about a second or two.Interesting method. I'll give it a try.


But I can also focus the loupe on the texture of the focus side of the Maxwell screen, which can just be made out by looking through the grid and Fresnel patterns. You may have to stop down to see it--it's a very bright screen and that texture washes out at high apertures. Yes, I think you are right. That is probably what happened. I guess the small pattern resembling frozen puddles must then be the "grain" of the Maxwell. In the sequence of coming into focus it follows right after the grid and the Fresnel lines but is hard to definitely focus. I'll see if stopping down can make it clearer.


I'm not using a 6x tilting loupe with that camera and screen.

Just like you I find it more comfortable with that screen to use 4x magnification than 6x or even stronger. I am quite satisfied with the adjustable Rodenstock 4x. I also like the equally adjustable Schneider 6x (not a recent model), both are of impressing clarity and brightness, but - in combination with the Maxwell screen I find 6x just a little too much. Especially in difficult situations the Fresnel lines become somewhat distracting. It works when concentrating, but I find it strenuous.
It is most convenient for my eyes to look through an angled viewer for composition. These tend to use a magnification around 2x, and so does the Linhof. The image snaps nicely in and out of focus. However, from this starting point there is still room for more precise focusing with a stronger loupe. I think for critical focus the loupe is necessary. At least I wouldn't want to give away possibly needed precision for a bit of laziness ;)

I started to look into the focus and loupe issues in more depth when I found it extremely difficult to focus the Schneider XL 110 lens wide open. Now I know (by searching the archives) that it is normal for this lens that the areas near the edges are not projected totally sharp when wide open. And of all possible motives I had chosen one with the near point and the far point both positioned near the edges ;) I went on to shoot the image anyway to the best I could but returned home utterly confused. By what I learned about the structure of the Maxwell and about the Schneider XL 110 I will next time know better how to go about focusing in general and focusing this beast specifically.

[rdenney]

I mistyped. I meant to write that I AM using a 6x tilting loupe. The Maxwell screen permits high magnification better than all other Fresnels I've looked through.

Rick "not=now" Denney

[Keith Fleming]

My way of checking the focus of the loupe is to use a pencil to make an "X" in the center of the frosted side of the ground glass. With no lens on the camera, I point the camera toward a bright light and then adjust the loupe's focus until the "X" is at maximum sharpness. I also use gaffer's tape to tape the loupe so that the proper focus stays where it should. The problem with taping, however, is that I need a dedicated loupe with each camera so that I am not constantly adjusting the loupe.

Keith

[Peter De Smidt]

Using Keith's method would work fine with a regular ground glass. I wouldn't use it on a Maxwell screen, as doing so would likely damage the screen. I have a Maxwell screen on my main camera. I haven't had a problem using loupes from 2 to 8x with it.

[Vic Montaigne]

I mistyped. I meant to write that I AM using a 6x tilting loupe. The Maxwell screen permits high magnification better than all other Fresnels I've looked through.

Wow, so it is just the opposite. Concerning the 6x, last year I found it to work fine for my needs in combination with the Arca Swiss and its great Fresnel, but not with the Chamonix N2. And this year with the TK + Maxwell I am still in the phase of experimenting, right now tending towards 4x for this combination. But then, maybe one gets accustomed to "look through" the Fresnel lines. Or maybe eyes (and their owners) are simply different...


Using Keith's method would work fine with a regular ground glass. I wouldn't use it on a Maxwell screen, as doing so would likely damage the screen. I have a Maxwell screen on my main camera. I haven't had a problem using loupes from 2 to 8x with it. I did not have problems either, just did not know where to focus the loupe on, why I started this thread. Plus, I will find it probably easier to look at the image without Fresnel lines interfering. But this is not a problem at all, as I can always use the very bright adjustable Rodenstock 4x instead.

Thanks again to everyone.