fresnel on Deardorff back

[mylek]

Hi,

i bought a reducing Deardorff back from 5x7 to 4x5. I was surprised that there is a fresnel installed in front of the ground glass.
I'll like to know what is the proper way to installed it and if it can be used without it as well?

Regards!

[Jim Noel]

The ground surface must be in its original position facing the lens. Therefore, the ground glass needs to be in its original position with the fresnel mounted behind it, nearest the user, with the grooves facing the lens. Care must be used in focusing for it is very easy to focus on the fresnel rings, rather than the ground glass.

[Leonard Robertson]

I think Jim is correct on placing the fresnel behind the ground glass - for modern view cameras. However, I'm reasonably sure when Graflex installed the "Ektalite" screen on their Graflok back, the Ektalite fresnel was in front of the ground glass. It occurs to me that depending on the age of your reducing back, Deardorff may have used the same convention for placement of the fresnel as Graflex did. That may have been considered correct at the time. I was going to suggest measuring the "Depth to film surface" as shown here: http://home.earthlink.net/~eahoo/page8/filmhold.html But then I ran across this thread:
http://www.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=007Ezk
The important sentence is "A Fresnel actually is a lens and as such, shifts the plane of focus rearward by an amount equal to 1/3 of the thickness of the Fresnel". Probably the best thing to do is shoot a test shot at maximum aperture and see if the negative is sharp at the point you know you focused on. Usually this is done with a subject like a fence at a 45 degree angle to the camera, so if the focus is off, you can see which way the sharp focus point is located.

Len

[mylek]

Thanks!

I have a Graflok back installed and the fresnel is in front of it, so i will place the ribs toward the ground glass as it is the other way.
Also, there is a wooden spacer between it and the camera's back.

[Leonard Robertson]

While searching for something else today, I found this thread on Graflok fresnel placement:

http://www.largeformatphotography.info/forum/showthread.php?107443-Replacing-Fresnel-and-Ground-Glass-on-a-Speed

Len

[mylek]

Thanks!!!

[Kevin Crisp]

Is your reducing back a graflok back? Makes a difference.