Combining a BosScreen With a Fresnel

In the message a few down from this several people suggested

combining a BosScreen with a Fresnel. I've also seen that suggestion

made in other threads here. However, I don't understand why combining

the two doesn't create plane of focus problems since BosScreens are

made for specific cameras, presumably to maintain the camera's

original spacing between ground glass and film. However, it must not

create a problem since people do it and since Calumet sells a viewing

screen that combines the two. When it's done, is the Fresnel placed

on the lens side of the BosScreen or the photographer side? How are

the two attached to each other and to the camera? I'd like to use

this combination on my Ebony since I have difficulty using a loupe

with the Fresnel that comes with the camera but I'm not anxious to

pay the Calumet price for the one they sell (something like $250 as I

recall) particularly since I already have the Fresnel. This is

obviously a subject I know nothing about so any information from

those who have done it or know how to do it would be appreciated.

My memory isn't what it used to be, Brian.

 

But it seems that the first time I saw a fresnel was on my first Linhof, back in the 60's. I remember it as being mounted permanently on the lens side of the ground glass.

 

Recently I did a catalogue with a friend's Sinar, whose plastic fresnel just snapped on over the outside of the ground glass and could easily be removed for cleaning.

 

Perhaps they can work on either side...

A boss screen is like a ground glass, but it uses wax instead of textured glass. The fresnel should go on the photographer's side. If it goes on the lens side, it will shift the focus and cause a curvature of the focal plane. The shift in focus can be corrected with spacers between the boss screen and fresnel, but the curvature of the focal plane cannot be corrected.

A fresnel should <I>always</I> be on the <I>photographer's</I> side of the ground glass. There, I said it.

With the fresnel on the photographers side, do the ridges point towards the lens or towards the photographer? I have a Wista fresnel/groundglass and it seems pretty bright, but the focusing area gets smaller and round with lens such as F6.8's and slower. Is this normal?

Eric, I have never seen one which did not have the ridges toward the lens. Think of them as tiny prisms.

Brian - I can't help you with the combination Bosscreen/fresnel question, but the Bosscreen page at the Stabilix site is: http://www.stabilix.nl/Bosscreen.htm#top. Looks like one could order direct from them. (I couldn't find a listing at Calumet due to their "improved" user interface.)

 

Alan - FWIW, Toyo mounts the fresnels that are included with several of their 4x5 backs on the lens side of the ground glass, with the ridges of the fresnel against the ground side (facing the photographer) where the image is formed.

All the Graphics (except possibly the Super Graphics) with "Ektalite" screens had the screen mounted in front of the Ground Glass, as does my Technika IV. It probably works better than the more modern way (GG in front of the screen) to equally distribute the light into corners, but the focal plane of the GG is moved forward 1/3 of the thickness of the screen to compensate for optical properties of the screen. I don't understand how a Boss Screen can work unless it is custom fit to each camera back, since the focusing plane is at the wax interface, not at the front of the screen -- causing a focus error equal to the thickness of one layer of glass.

Hmm. Then I guess that means the fresnel on my Cambo is on the wrong side of the GG, as it's nearest the lens. It was like that when I bought the camera though (secondhand). Is there an easy way to verify whether this is right or wrong? Large aperture, close point source like a laser dot, shoot a Polaroid and look at it under a loupe? Maybe not accurate enough though. Any other ideas?

From Mr Wisner:

 

Viewing Screens for Large Format Cameras

 

"Where Does It Go? One subject which comes up often is the correct position of the fresnel lens. In years past, several arrangements have been used, including placing the fresnel behind the ground glass, in front of the ground glass, and incorporated into a ground glass made of plastic. However, there is only one correct arrangement...." contd at:

 

http://www.wisner.com/viewing.htm

 

PS - the best option I've found, on three different cameras, are the Maxwell screens

 

tim

Thanks for all the responses. I guess the answer to combining the two is to treat the Bosscreen as the ground glass and go from there.

Brian,

 

I never got around to trying out the Cambo fresnel with the Bosscreen. Too busy at work, then the sale on the fresnel was over, so I decided to wait. However, I did learn during the investigation that the Cambo fresnel is a snap on/snap off type that fits on the photographer side of the groundglass. It was my impression that the fresnel was cut to match the Cambo camera.

 

I hope this helps.