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View Full Version : fresnel lens, cheap and fast



Michael Nagl
6-Jan-2008, 13:42
Dear colleagues,

my 8x10 has arrived, and I need a fresnel. I found out that the Linhof 8x10 fresnel costs about 400 $, and the Beattie about 400 , and here: http://www.3dlens.com/shop/largefresnellens.php ---- fifteen dollars!
Tell me why not. 5 grooves per millimeter is not overwhelming, sure, but -- it`s free!
What would be the appropriate focal length? Does this depend on the lenses I use? (Mine are 150 & 240mm.)

Thanks
Michael

Todor Georgiev
6-Jan-2008, 14:04
Sounds good to me.

For a different purpose, I am interested to find which is the highest resolution Fresnel lens 8X10 or 4X5 available today.

Todor

Peter K
6-Jan-2008, 14:43
Fresnel-lenses for focussing screens are special ones. This lenses have two different focal-lenghts combined by a special ring pattern. Also the groove pitch is 0.1mm (254 grooves per inch).

Peter K

Dave Parker
6-Jan-2008, 14:53
I have tried various fresnels that were not specifically designed for photography work and I have found, they really don't work very well, you have to understand a fresnel is a lens in itself and is specifically made for certain focal lengths, to get the maximum amount of light intensification...I have several 8.5 x 11 page magnifiers around here that I have tested and for the most part, in a photographic application, they are pretty much useless.

Dave

Rob Vinnedge
6-Jan-2008, 15:16
Tracy Storer recently posted some info about a new fresnel source. He also uses them and would have a lot of advice.

aduncanson
6-Jan-2008, 17:12
I have a little experience with selecting an inexpensive fresnel for photographic use and think that with care you can select one that will be useful. It is my experience that 5 grooves per millimeter would be plenty fine.

You can calculate the required focal length using the lens equation:

1/(focal length) = 1/(image distance) + 1/(object distance)

Some interpretation is necessary. Assuming you are trying to collect the light originating at the rear nodal point of the taking lens and direct it to your cornea, then the image distance is the focal length of your taking lens and the object distance is the distance of your eye behind the ground glass. For your 240mm lens, if your eye point is 240mm behind the ground glass (achieved either through young, strong vision of by use of eyeglasses) then the formula yields 120mm for the focal length of the fresnel. This will give you something of a sweet spot centered behind the ground glass.

Alternately if the fresnel focal length is equal to that of the taking lens, the eye distance becomes infinite, meaning that the light from the ground glass will directed (only generally) straight back. Then there will no sweet spot for your eye, but the portion of the screen directly in front of your eye will always be brightest. Note also that with a view camera, the location of the rear nodal point of the taking lens is variable in all three dimensions so expecting mathematical perfection in the real world is a mistake.

A good comprise for you might be to take the fresnel focal length to be equal to the shorter taking lens (150mm), giving the infinite eye distance behavior with that lens and something of a sweet spot (albeit at 400mm or 15.75 inches) behavior with your 240mm lens.

Don't be very concerned about the precision of these calculations. Remember that you have a ground glass in the light path that will spread the light quite broadly around the calculated nominal eye point. You will notice that it is not the usual practice to change the fresnel every time you change taking lenses.

Michael Nagl
7-Jan-2008, 01:46
Thanks for that in-depth-answer. I`ll let you know how the story will end, if it ever will.

Gordon Moat
7-Jan-2008, 11:22
I use to have an ultra cheap fresnel over the ground glass on my 4x5. Barnes & Noble bookstores have these page reader magnifiers made from plastic, and with a little trimming matches nicely over the ground glass. The downside is when you place a loupe over this for closer focus, the lines intrude a bit on getting the focus dialed in. Now I have a Satin Snow (http://www.satinsnow.com) on my 4x5, which is just as bright (if not more so) and much more evenly illuminated.

Ciao!

Gordon Moat Photography (http://www.gordonmoat.com)

h2oman
8-Jan-2008, 10:24
I have some related questions. I am new to LF and just bought a used Osaka. I understand that it is essentially the same as a Tachihara, and that the newer Tachiharas have Fresnel lenses standard. So here are my questions:

1. How can I tell if my camera has a Fresnel lens?

2. When I look at the ground glass from the back it looks like there is a thin piece of clear glass with square corners, and something frosted with clipped corners in front of that. What am I seeing?

3. Am I to understand from the Satin Snow web site that I could get their 4x5 ground glass for $17 including shipping? That's not much to pay if it would give me more light for focusing...

I'm just looking to give myself every advantage (without breaking the bank!) when I start using the camera, any suggestions are welcome!

By the way, I see right now a Tecnikardan Fresnel gg on e-bay. Current bids are not too high. I'm not ready to make a decision in the time left for bidding, but maybe someone else is.

Gregg

LFdelux
11-Jan-2008, 20:10
"How can I tell if my camera has a Fresnel lens?"

If you have to squint or use dark cloth to see the image, then you do not have fresnel. the fresnel looks like a acrylic page magnifier.