Fresnel position - pick your brains?

[paul owen]

Hi, just wanted to pick your collective brains!! I've been using an Art Pan 6x17 roll film back for a couple of weeks now and although the screen "brightness" isn't a real problem I have been "testing" this by installing a cheap fresnel ( a cut down A4 page magnifier). The image is easier to see (albeit not dramatically!) but I was looking for some advice re: where to actually place the fresnel. The viewer for the film back comes with a fixed (glued) screen, however, there is a ridge on the frame below the screen (nearest the lens) that allows the fresnel to slide in underneath the glass nicely! As I am not in any way moving the position of the original ground glass, would it make any difference to the focus if I placed the fresnel here, or should I try and attach it to the ground glass screen so that it lies nearer to my eye? i.e. the original screen would be sandwiched between the fresnel and the camera lens? Does this make sense? I always think up these "stupid" questions on a Sunday morning before I have had my required intake of caffeine!! Thanks in advance Paul

[Robert A. Zeichner]

This is not a stupid question by any means. If you place the Fresnel between the ground glass and the lens, you will end up shifting focus toward the film by a distance roughly equal to 1/3 the thickness of the Fresnel. In doing so, you will throw off the ground glass/film plane coincidence and end up with out of focus negatives. If you place the Fresnel between the ground glass and your eyes, your eyes will compensate for the shift and all will be well.

[paul owen]

Hi Robert! Thanks for the input but this is where my "problem" lies!! I understand that if the fresnel was fitted in such a way that it was moving the position of the ground glass then focus/film plane would not be the same. I thought that because the ground glass is remaining in exactly the same position and all I am doing is "placing" a fresnel between it and the lens am I not still focussing on the same plane? i.e. the ground glass has not moved position? All I am relying on the fresnel for is to "spread light" across the glass. Put another way, with the fresnel fitted as I have described (between screen and lens) the film plane and the ground glass remain in the same position/plane. So focus should not be affected, correct?? Despite several strong cups of coffee I STILL can't seem to get my head around this one!!

[Brian Ellis]

It seems to me that if you can place the Fresnel in between the lens and the ground glass without changing the position of the ground glass then your thinking is correct, i.e. that as long as the position of the ground glass isn't changed it shouldn't matter from a focus shift standpoint where the Fresnel is placed. I believe some cameras, e.g. early Techhikas, were designed in such a manner that the Fresnel was required to be placed between the lens and the ground glass . So if the Fresnel worked with those cameras in that position it should likewise work with yours as long as you don't move the ground glass. But I'm cetainly open to being corrected and hopefully someone more knowledgeable than I am will respond to your question.

[Conrad Hoffman]

Maybe this will help- the fresnel, though it is flat, is still a lens. If you put a lens behind your taking lens, the focal length will change. Because of that, you'll focus at a new setting. When you go from ground glass + fresnel, to film, you're back at the old focal length, so your focusing setting is wrong. Now, since the fresnel is flat, and since it's usually right at the ground glass, the actual error is small, but it's still there. For perfection, you have to shift the ground glass slightly. Do your own tests. Use a good magnifier to check focus with and without the fresnel in place. See if you can see a difference. Use an angled target like a yardstick, be sure the lens is wide open, and try not to shift anything during the test.

[N Dhananjay]

As indicated above, the fresnel is a field lens. So, if you place it in front of the GG (i.e., between the GG and the lens), it will refract the rays and shift the focus. Some cameras have been been designed to use a fresnel in front of the ground glass (i.e., the position of the GG is compensated to take this into account) but most cameras are not designed this way. Thus, on most cameras putting a fresnel in front of the GG will result in the GG and film not being in the same plane. Think of it this way, when you are checking focus on the GG, light is passing through the fresnel en route to the GG and you make a decision about which is the point of best focus. However, when you place a filmholder in the camera, you typically pull the entire back (GG + fresnel) back. Light hitting the film is now NOT passing through the fresnel and will focus at a slightly different plane.

In your specific case, I would suggest taping the fresnel behind the GG (i.e., between the GG and your eye).

Cheers, DJ

[Herb Cunningham]

Ok- all the above seem to say leave the ground glass where it is and let the fresnels fall where they can-

that being the case, If I put my Maxwell frosted plastic in front of the clear glass protector he sends with it,
I will be doing the same thing, right? No shimming if the frosted plastic Maxwell is now acting as the ground glass.

[paul owen]

Hooray!! It now makes sense!! Good old caffeine MANY thanks for all the advice! I've decided to reattach the fresnel to the back of the screen (nearest my eyes) using clear self-adhesive photo corners! I am VERY surprised at the improvement in overall screen brightness - right into the corners! Now, off to bonny Scotland to shoot some Provia!!

[Murray]

For DIY-er's like myself, I have been dragging my feet before choosing any generic Fresnel lenses to put on the back side of ground glass because I am not sure whether any random 'found' fresnel panel will have compatible f.l./magnification for a given format.

Does this matter? I thought I recently read something to the effect that the Fresnel f.l. should match the objective lens f.l.

So, the 1.5 x magnification (667mm ?) panels I'm eyeing might not do for a 135 mm lens on 4x5 or longer for 8x10.

Am I off base?

Thanks

Murray

[Struan Gray]

Paul: if you are the Paul Owen who has asked for info on 5x7 Normas on that other forum, bear in mind that the 5x7 Norma Fresnel is one of those that should go between the ground glass and the lens. If your camera comes with a Sinar Fresnel installed, don't go moving it as a result of this thread.

Fresnels hard up against the lens side of the screen shift focus not so much because they are lenses, but because they are a relatively thick slab of dielectric. A sheet of plain perspex or glass with no Fresnel ruling would have the same effect. Such a sheet has no effect on rays impinging perpendicular to it, but slanting rays get diverted away from the front surface a little by refraction. If you imagine the rays from a point that was in focus on the ground glass before you inserted the sheet, they will now meet somewhere behind the ground glass surface. You, the viewer, move the rear standard backwards a little to compensate and the image on the ground glass comes back into focus - but there is no sheet of glass or perspex in front of the film in the holder and so the image on film ends up out of focus.

The shift of one third the thickness of the sheet comes about because most glasses, optical resins and plastics suitable for making Fresnel screens for view cameras have similar indices of refraction of around 1.5.

The 5x7 Norma Fresnel has tabs that sit between the ground glass and the contact points machined on the ground glass frame. These tabs are thinner than the Fresnel (roughly a third as thick :-). They hold the Fresnel in place and at the same time move the ground glass back by the right amount to compensate for the shift they introduce. An image in focus on this displaced ground glass will also be in focus in a standard film holder.