Fresnel Lens: In Front or Behind the Ground Glass

[ic-racer]

I noticed that the fresnel on my Horseman 6x9 Ground Glass back is behind the ground glass, while the fresnel on my Horseman 4x5 FA is in front of the GG (between the lens and the ground glass).

On the Horseman 4x5 FA the fresnel is smaller and 'set in' so that there is a ledge for the ground glass to rest on the aluminum back. I would think there might be some focus shift with the fresnel between the lens and the ground glass, but on 35mm SLRs the fresnel is also between the lens and the frosted plastic, (or molded on the underside of the frosted plastic).

So my curiosity is: what is the arrangement on other view cameras besides the Horseman. Do most have the fresnel in front or behind the ground glass?

[Donald Qualls]

Usually it depends on whether the Fresnel was designed into the ground glass back, or an after market add-on. You can improve the brightness of plain ground glass, some, by dropping a Fresnel (on the cheap, even one of those "full page" plastic magnifying sheets you can buy for $3) onto the plain back side of a plain ground glass, but if the camera was designed with the ground glass, it's usually on the lens side, because it's more effective there, and is better protected.

Yes, the Fresnel offsets the focus, but by less than its thickness -- the rule of thumb is to set the ground glass surface back by only about 1/3 of the thickness of the Fresnel, which means if you install a front-side Fresnel in a camera not originally made for it, you have to adjust the focusing panel to move the Fresnel's surface closer to the lens than the original ground glass would have been. Conversely, if you have a focusing panel in an old camera with the ground glass missing, and it was originally a Fresnel, you'll have to shim your replacement plain glass or Satin Snow by about 1/3 the thickness of the original Fresnel...

[Glenn Thoreson]

In front, behind, it makes no difference. What does matter is if the focus panel is or isn't milled to accomodate the thickness of the fresnel if it's mounted in the panel and under the GG. If it not mounted in the panel recess itself, it matters not where you put it. However, you will find widely varying opinions on which way the ridges (?) should face. Personally, I would rather use a high quality GG, such as Satin Snow, and just forget about the fresnel.

[Emmanuel BIGLER]

Fresnel lenses are made of molded plastic. Hence they can be easily scratched and their fine grooves catch dust very easily. It is difficult to clean them without scratching them.
So, they get a better mechanical protection betwen the GG and lens, with the grooves facing the GG so that they are not accessible to dust. The GG itself is made of glass and is less sensitive to scratches. The matte side is underneath, the polished side is outside. This polished side of a glass GG is the most resistant part of the whole combination (Fresnel + GG).
As far as the effect of averaging the brigthness over the field, there is no difference whether you place the Fresnel lens under, or over the GG, and the fine circles of the Fresnel will, unfortunately, be equally visible in both cases.

[ic-racer]

Yes, the Fresnel offsets the focus, but by less than its thickness -- the rule of thumb is to set the ground glass surface back by only about 1/3 of the thickness of the Fresnel, which means if you install a front-side Fresnel in a camera not originally made for it, you have to adjust the focusing panel to move the Fresnel's surface closer to the lens than the original ground glass would have been. Conversely, if you have a focusing panel in an old camera with the ground glass missing, and it was originally a Fresnel, you'll have to shim your replacement plain glass or Satin Snow by about 1/3 the thickness of the original Fresnel...

Excellent, thank you!