To Fresnel or not to Fresnel.

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soupdragon

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I'm wondering about Fresnel screens and focus accuracy.
More to the point how does anyone know if they are looking at the focused image on the ground glass or the Fresnel?

On all Tachihara cameras the Fresnel is furthest from the lens with the etched side touching the ground glass. By all accounts this is the correct way. However, if I'm concentrating on the Fresnel this must be causing a registration error.

I guess I could get an empirical answer by taking two shots, one with the Fresnel removed but that sounds like hard work to me.

Any guidance?
 
Fresnels don't have a ground surface so it shouldn't really be possible to focus on any of the fresnel surfaces.

Just make sure your loupe is focussing on the ground glass surface and you should be fine. Fresnels don't impact focus if used properly.
 
Normally, a fresnel would go on the viewer-side of the ground glass, so the image focusses on the correct plane at the ground glass.

Unfortunately, some fresnels have obtrusive rings that can distract you.

If you were focussing on the fresnel, then the rings would be sharp through the loupe and the image would be blurry.

Different manufacturers use different registration for the ground glass, eg Ebony's GG sits on the machined wooden frame directly and Linhof use a series of adjuster screws to increase/decrease the position of the groundglass for accurate registration. So depending on the camera will tell you whether your focus is out or not.

Mike
 
All good points.
The problem I'm having is with an f8.0 lens.
The image is quite dark and I can't see the Fresnel lines. All I can do is get best focus and hope it's on the correct plane.

I guess I just answered my own question there.
 
soupdragon said:
All good points.
The problem I'm having is with an f8.0 lens.
The image is quite dark and I can't see the Fresnel lines. All I can do is get best focus and hope it's on the correct plane.

I guess I just answered my own question there.
Click to expand...

Simply ignore the fresnel rings. If the ground side of the GG screen is the closest to the lens, that is where you should be focusing.
 
In the 37 years I've been using my Wista 45DX I have always found the fresnel makes working much easier. First it aids focussing, secondly it increases overall screen brightness very significantly towards the edges and corners.

An exception is more extreme wide angle lenses, in the case of my Wista that's my 65mm f8 Super Angulon. No problems with my 75mm f5.5 Supeer Angulon or 90mm f6.8 Grandagon N.

My Crown Graphic had no fresnel, adding one significantly increased overall screen brightness towards the edges and corners, again making the camera so much easier to use, particularly as it was often used hand held.

If you put two cameras side by side with similar lenses one with the other without a fresnel, you would be surprised just how large a difference the fresnel makes.

Ian
 
I've a fairly new Toyo 45A II which has a built in Fresnel on the lens side, the focus seems fine on that in fact it appears to work better than an old 4x5 Intrepid I had (maybe no surprise there!). I should really test it out at full aperture as a proper test.
 
Focus should be on the ground surface of the ground glass, wherever the fresnel is placed, or if it's absent. If you are having problems, it's almost certainly the loupe that's to blame. You will need to find a high-contrast subject in plenty of light and adjust the loupe to focus on the ground glass surface. If your loupe can't be adjusted, I'm afraid you'll need to get one that can.
It's always possible that a screen has been wrongly placed by a previous owner. The overwhelming arrangement is to have the ground glass surface facing the lans directly with nothing intervening. Then the fresnel screen itself. Usually, the grooves face forward, toward the lens to protect them. Then there is usually some kind of protective sheet of glass after that, as fresnels are very prone to scratching. The thickness of this sandwich doesn't matter, as the loupe is adjusted to take this into account.
There are other kinds of composite screens (like jhotopf's Toyo, above) and you'll need to follow the manufacturer's instructions for these.
Almost any kind of fresnel, like the ones sold for reading or from an old overhead projector will improve the brightness of the corners.
 
jhotopf said:
I've a fairly new Toyo 45A II which has a built in Fresnel on the lens side, the focus seems fine on that in fact it appears to work better than an old 4x5 Intrepid I had (maybe no surprise there!). I should really test it out at full aperture as a proper test.
Click to expand...
Love the Toyo 45A and A2.
The difference is that the Toyo Fresnel as standard is fitted between the lens and the ground glass.
As I understand it, this helps to make the light parallel to the ground surface of the focus screen.

On the Tachihara cameras they are fitted between the user and the ground glass. I'll be honest here. I can see no benefit to a Fresnel if it is used in that configuration.

I may well just remove my Fresnel and have done with it.
 
No, that's not the purpose of a fresnel. (I think you don't mean parallel, but at right-angles)
When light strikes the rough surface of the focusing screen, it's scattered in all directions, as you'd expect from light striking a rough object. You can see that it's scattered, because you can see the image from all around. What happens is that the fresnel lens redirects the light inward to where your eye is situated. The more efficient (and much more expensive) ones concentrate the light so effectively that you have to place your eye in the right place to get the full benefit. This is particularly evident with short-focus fresnels. One by-product of this is that if you use rise, the corresponding point of view moves downward instead of being central. This can lead so some awkward gymnastics in the field.
There may be differences in the micro-geometry of the ground glass itself. The literature for some 35mm cameras seems to hint at this in their references to laser-etching, but I have no information other than vague memories of old advertisements.
The grind size makes a difference. A coarse grind will give a brighter image and a finer grind more detail, but more dimly. A really fine grind begins to approach transparency, which is obviously useless.
Your Toyo screen sounds excellent. Congratulations. Keep it as it is. The Tachihara will work just as well if you remove the fresnel that lies on the eye side of the screen. No reason not to try it and see what you prefer.
One thing to watch is what part of the frame locates the ground surface. The usual arrangement is that the screen is pressed forward by springs so that the ground surface is pressed against the a ledge in the frame.
 
David M said:
No, that's not the purpose of a fresnel. (I think you don't mean parallel, but at right-angles)
When light strikes the rough surface of the focusing screen, it's scattered in all directions, as you'd expect from light striking a rough object. You can see that it's scattered, because you can see the image from all around. What happens is that the fresnel lens redirects the light inward to where your eye is situated. The more efficient (and much more expensive) ones concentrate the light so effectively that you have to place your eye in the right place to get the full benefit. This is particularly evident with short-focus fresnels. One by-product of this is that if you use rise, the corresponding point of view moves downward instead of being central. This can lead so some awkward gymnastics in the field.
There may be differences in the micro-geometry of the ground glass itself. The literature for some 35mm cameras seems to hint at this in their references to laser-etching, but I have no information other than vague memories of old advertisements.
The grind size makes a difference. A coarse grind will give a brighter image and a finer grind more detail, but more dimly. A really fine grind begins to approach transparency, which is obviously useless.
Your Toyo screen sounds excellent. Congratulations. Keep it as it is. The Tachihara will work just as well if you remove the fresnel that lies on the eye side of the screen. No reason not to try it and see what you prefer.
One thing to watch is what part of the frame locates the ground surface. The usual arrangement is that the screen is pressed forward by springs so that the ground surface is pressed against the a ledge in the frame.
Click to expand...
All true and thank you.
That said, I don't own a Toyo any more.
I have two cameras, both Tachihara, one is 8X10 and one is 4X5.

It's the 4X5 I'm struggling with.
I have a 120mm Nikon lens (f8) and the image is not very bright on the screen.
I'm beginning to think I made the wrong choice regarding a suitable lens.
 
soupdragon said:
On the Tachihara cameras they are fitted between the user and the ground glass. I'll be honest here. I can see no benefit to a Fresnel if it is used in that configuration.

I may well just remove my Fresnel and have done with it.
Click to expand...

While it's true that the optimal position of the fresnel is under the GG screen where it corrects the angle of light off axis to the lens and GG, so that the light scatter of the GG screen is more even, Hence the increased brighness of the edges and corners,

However, there is still a huge improvement when a fresnel is fitted to the rear, viewing side. I fitted a fresnel to an MPP with a member here just before Christmas and we put 2 MPPs alongside each other, same lens, mine had a fresnel, his didn't. Luckily, I had a spare fresnel and so he left very happy with one fitted.

Ian
 
Ian Grant said:
While it's true that the optimal position of the fresnel is under the GG screen where it corrects the angle of light off axis to the lens and GG, so that the light scatter of the GG screen is more even, Hence the increased brighness of the edges and corners,

However, there is still a huge improvement when a fresnel is fitted to the rear, viewing side. I fitted a fresnel to an MPP with a member here just before Christmas and we put 2 MPPs alongside each other, same lens, mine had a fresnel, his didn't. Luckily, I had a spare fresnel and so he left very happy with one fitted.

Ian
Click to expand...
So I guess "if it's not broke. don't fix it" is order of the day.
 
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