Dim screen after tilting both standards upright on an inclined rail

[Vic Montaigne]

Hello everyone,

I hope this is the right forum section to put my question.

I would be really grateful if someone more knowledgeable than me could explain to me the following phenomenon:

Setup: Linhof TK 45S, Maxwell screen, angled viewer, standard bellows, lens 12.5/450

A lot of rise was needed, and when maximum rise on the front standard was not sufficient I added some indirect rise, pointing the rail upward a little till I got the composition I wanted, roughly judging through the angled viewer. Then I went on to put both standards upright again using the levels. When I looked through the angled viewer again to finetune composition, all at once I saw a nearly black screen. It was so dim that the person accompanying me could see nothing at all and I could make out only very faint outlines suggesting there might be an image there ;)

I removed the angled viewer and resorted to the darkcloth for focusing. With the light available in the situation I would normally not have needed the darkcloth for critical focusing with the loupe. But now it was with some difficulty that I finally managed to focus using darkcloth and loupe. The image was really very dim.

Nothing obstructed the lens, the corners were free. The negatives turned out fine.

I just do not understand why the screen image became so dim after tilting the standards. If I had tilted the rear standard only on a horizontal rail, I would tend to think that it maybe hat to do with angles of light and the fresnel. But I set both standards parallel and vertical. In my understanding this should not be different from a horizontal rail and untilted standards. But obviously I am missing something here.

Thanks for shedding light on this :)

[mdarnton]

Instead of looking directly at the screen from the back, get down and look upwards, through the center of the screen, directly at the lens. The Fresnel concentrates and directs the light in a straight line from the lens through the center of the screen and back, which will, in your case, be downards towards the ground, because of the extreme rise. Your viewer doesn't work because it's looking in straight 90 degrees to the Fresnel, not upward towards the lens.

As you change your view, moving yourself closer to the ground and looking up at the back of the camera, you should suddenly see everything appear all at once on the edge of the screen as the bright spot appears at the edge and then moves towards the center of the screen as you lower yourself more and more. This is somewhat similar to how you have to center your eye in a binocular or you will see nothing: the Fresnel is gathering and projecting the light in a certain direction, and you haven't found the place it's being pointed to, yet.

[Bruce Watson]

Instead of looking directly at the screen from the back, get down and look upwards, through the center of the screen, directly at the lens. The Fresnel concentrates and directs the light in a straight line from the lens through the center of the screen and back, which will, in your case, be downards towards the ground, because of the extreme rise. Your viewer doesn't work because it's looking in straight 90 degrees to the Fresnel, not upward towards the lens.

As you change your view, moving yourself closer to the ground and looking up at the back of the camera, you should suddenly see everything appear all at once on the edge of the screen as the bright spot appears at the edge and then moves towards the center of the screen as you lower yourself more and more. This is somewhat similar to how you have to center your eye in a binocular or you will see nothing: the Fresnel is gathering and projecting the light in a certain direction, and you haven't found the place it's being pointed to, yet.

+1.

[Vic Montaigne]

@mdarnton

Thank you so much for your explanation. As I said, I had suspected it has to do with the fresnel and angles of light, but could not pinpoint that any further. So your exact explanation is very much appreciated. In the situation, while trying to see something, I must have tilted the loupe intuitively, as for seconds light spots appeared just as you predict. Now I have to find out how to do that systematically. I will follow your advice:


As you change your view, moving yourself closer to the ground and looking up at the back of the camera, you should suddenly see everything appear all at once on the edge of the screen as the bright spot appears at the edge and then moves towards the center of the screen as you lower yourself more and more.

...that really sounds like some magic stunt ;) I know it is all physics, but still fascinating...

Let's see if I got your instructions right: So I set the loupe onto the screen near the edge, lower myself until I get a bright spot (like a tilted Silvestri loupe?) and then move the loupe further to the centre while lowering myself further down to look up from an ever steeper angle? Is that correct or somewhere near?

My only problem is that in my memory the screen got dark only after setting the standards upright again. With all the maximum rise and even the inclined rail I could still see quite clear through the angled viewer. But maybe my memory is playing tricks on me, sounds like it.

I am looking forward to put that into practice. Thank you!

[Bob Salomon]

One of the major drawbacks of enhanced screens is a tendency to black out as your eye goes off the optical axis. Try to keep your eye on the axis, even though this may mean that in more extreme cases you can not easily use the reflex viewer.

If you went back to the standard Linhof GG and Fresnel this would be less likely to occur. But the overall gg image will not be as bright.

A second drawback to the enhanced screen is that focus will not snap in and out as easily as it does with the standard gg and Fresnel with wide angle lenses. The wider the lens the more difficult it may be to see the point where it snaps into focus.

[Doremus Scudder]

The blackout effect is usually more pronounced with shorter and longer lenses, since a Fresnel screen has a focal length that is normally optimized for lenses in the middle range. Ideally one should have a Fresnel screen for every focal length, but that, obviously, is impractical.

I use an 8x or 10x loupe designed for viewing slides, and use it reversed, i.e., with the skirt to my eye, instead of resting the skirt on the ground glass. This means I have to hover over the ground glass and find the point of sharp focus all the time, but allows me to tilt the loupe to point toward the "hot spot" for better viewing. I believe some manufacturers (Sylvestri, Peak?) make tilting loupes for exactly this purpose.

A plain ground glass may solve this problem and allow use of your viewer, but be overall less bright. However, I find "blackout" still a problem with extreme movements and focal lengths with a plain ground glass as well.

Best,

Doremus

[Vic Montaigne]

First of all, thank you everyone! All the valuable input really helps sorting out things.

@Bob

That pretty much sums it up regarding the angled viewer. It is a matter of choice really. I tend to use the viewer for composition, and then, as it is easily removed, I go on focusing with a loupe. Thank you for the clear statements. It is good to know the limitations of any tool, be it lens, viewer, fresnel or whatever. That saves fruitless trying and pondering on what might be done wrong, when it is really a matter of natural limitations.

@Doremus


This means I have to hover over the ground glass and find the point of sharp focus all the time, but allows me to tilt the loupe to point toward the "hot spot" for better viewing. I believe some manufacturers (Sylvestri, Peak?) make tilting loupes for exactly this purpose.

Ah, that is exactly what I was asking myself, if I could maybe make use of the Silvestri tilting loupe that I got myself for wide angle lenses (but have not used yet). Maybe it will be good for both, short and long lenses.

At least next time I won't be taken by surprise in a similar situation. Thanks again!