fresnel lense focal length

[robc]

just wondering what the typical focal length of a fresnel lense on a 4x5 gg would/should be. (fitted on outside).

[Graeme Hird]

???

Wouldn't that depend on the focal length of the Fresnel lens? They are made with many different specifications, so there is no single answer to this question.

Can you re-phrase the question? What are you trying to do with the Fresnel?

[Doremus Scudder]

Sounds like Rob is trying to cobble together his own Fresnel viewing screen. Fresnel lenses do come in different focal lengths, and the one chosen influences which focal length lenses work best with the viewing screen. Ideally, you would have a Fresnel for each focal length lens you own. Most are compromises, and work well with "normal" focal length lenses and those close by, but can be a pita with very long or short lenses. I believe some companies make Fresnel lenses optimized for wide-angle (i.e. short focal length) viewing.

It seems to me, although I am not completely sure, that the ideal focal length for a Fresnel would be equal to the focal length of the lens used, i.e. 210mm for a 201mm lens, etc.

I'm sure some of the more erudite optical experts on the forum will confirm, expand, or correct this. At any rate Rob, let us know what you are up to and how it turns out.

Best,

[robc]

Yes I was thinking of having a go at making my own GG with a fresnel and I came across this web site which is selling very cheap fresnel lenses.

www.3dlens.com/enter.html (http://www.3dlens.com/enter.html)

my thinking is that I need to bend the light back far enough towards straight out from the GG to be able to view it with a loupe or reflex viewer.

the 95x135mm sheets are 280mm focal length and 3x magnification. What do you think? Would they be suitable. They are certainly cheap enough to try out but pointless if they wouldn't work.

[David A. Goldfarb]

I'm not one of those erudite optical experts, but I do happen to have a spare Linhof 4x5" fresnel, and holding it up to the wall opposite the window while pointing it toward infinity and measuring the focal length with a ruler, I'd say it's 150mm, which would agree with Doremus Scudder's speculation, since that's the "normal" focal length for 4x5".

[robc]

I think there has to be two options. Either you want the image to focus at a distance from the GG which is comfortable for viewing the whole glass back for composition or you want the light to be directly out (perpendicular) to the ground glass for optimum brightness when focusing with a loupe. The two are not necessarily the same. On another recent thread I did a rough approximation that the light from 72mm lens would strike the corners of the a GG at approx 45deg when focused at infinity. So would a fresnel with a focal length of 72mm bend the light that much? I guess there must be a formula for this somewhere but you would need to know what all the angles were and what the comfortable viewing distance is. I suspect using the same focal length as the lens will bring the light approx perpendicular to the gg.

[Struan Gray]

I too am only speculating, but my feeling is that the Fresnel is performing the same role and the condensors in an enlarger. Directing as much light as possible from the exit pupil of the lens into your eye is essentially the same problem as directing as much light as possible from a light bulb onto the entrance pupil of an enlarging lens.

If your eye is a distance D from the ground glass, and your lens of focal length f(l) is focussed on infinity, then to a reasonable approximation the 'ideal' Fresnel will have a focal length f(F) given by the Gaussian lens formula:

1/f(F) = 1/D + 1/f(l)

For my old eyes and a 150 mm lens that works out as about 100 mm or 2.5x.

I actually use a cheap page magnifier sold as having a magnification of 'about 2x'. It has ugly grooves, but helps to even out the light with my 90 mm and 150 mm lenses, so I blu-tak it in place when I'm using them. With longer lenses I don't bother, despite the fact that the ones I habitually use are f9 or slower.