Fresnel specs (for LF cameras)

[cyberjunkie]

I hope i have choosen the right sub-forum.
I have found a very cheap a Fresnel lens, slightly larger than 8x10", with the following specs:
focal length 250mm
maximum enlargment 2x

It's made of flexible plastic, and i'd like to know if it could be used on top of an 8x10" ground glass or not.
BTW, which are the specs of the fresnel lenses used on LF cameras?

Somebody reported the use of a thin vaseline layer between the ground glass and the fresnel lens, possibly for keeping them in contact, maybe with a flimsy plastic fresnel that was originally made for another use ( for example for easier reading).

have fun

CJ

[Sevo]

Those "reading loupe" fresnels work reasonably well as a on top on the ground glass fresnel. Personally I want to be able to remove fresnels, and would never use vaseline, nor attempt to mount one before the screen...

Sevo

[ic-racer]

http://www.largeformatphotography.in...ad.php?t=64256

If its cheap I'd just try it and see how well it works.

[Emmanuel BIGLER]

The right focal length of a Fresnel lens combined with a ground glass is approx equal to the diagonal of the format, or the standard focal length for the format ; so for 8x10" it is about 300 mm.
250 mm will also work fine for the 8x10" format, a slightly shorter focal length for the Fresnel lens will give a better even-ness of illumination with a moderate wide-angle taking lens... of 250 mm.
With a cheap Fresnel lens that you add behing the ground glass, it is not a big deal if it gets scratched, but beware of the focusing loupe if you touch the Fresnel lens.
Also, if the Fresnel lens has visible grooves on one side, I would put the Fresnel with its grooves facing the ground glass, so that they are somewhat protected. The grooved side is more easily scrached and retains more dirt and dust that the flat side.

[Steve M Hostetter]

I believe the only time you use vaseline is when your using it on the front of a filter for soft focus effect or to aid in the sale of arca swiss and sinar products..

seriously,, you just hold the fresnel against the gg to help compose a brighter image but not to focus with.