Fresnel lenses and ground glass focusing screens your worries answered

[Emmanuel BIGLER]

Could someone explain to me that if a fresnel lens has to have a focal length related to the focal length of the camera lens

Hello from France.

Actually you have two different ways to understand how Fresnel lenses work in combination with a ground glass.
The first idea is to consider that the Fresnel lens "bends" peripheral rays to force them to enter into the photographer's pupil.
See this diagram
https://www.flickr.com/photos/43175600@N00/36006774246

Now which is the proper setting in terms of focal length for the Fresnel lens?

Imagine that there is no ground glass (GG) and that you simply want most camera rays to enter your eye's pupil.
Like in a telescope, or in binoculars, where your pupil is centered at the center of an eyepiece.
Then you would chose a Fresnel lens with the appropriate focal length able to transfer the image of the exit pupil of the camera lens into your eye's pupil. In technical terms, the Fresnel lens acts as a field lens; catches all rays emitted by the camera lens exit pupil, and forces them all to pass through an image, which is the smallest size of the beam, this image of the camera lens exit pupil should be located at the operators' pupil, in principle.

But for a view camera, this is not the right choice, because you want to be able to see the whole image at all points of the ground glass.

Hence the usual setting for the Fresnel lens is to take a focal lens equal to the distance between the exit pupil of the camera lens and the image plane.
In this situation, rays sent by the exit pupil of the lens exit approximately parallel to the optical axis, hence illuminating the GG as evenly as possible.
The other, and more scientific explanation is to look where the image of the exit pupil of the camera lens, seen through the Fresnel lens, is located. If the exit pupil is set at the focal point of the Fresnel lens, the image will be sent to infinity, but this is hard to understand Imagine that the image of the exit pupil is a large white circle located far away, and of a very large size!

Mission: impossible, since we change lenses all the time and re-focus lenses for all distances!
You would need an adjustable zoom Fresnel lens!!!

Well, actually the setting which is chosen is a compromise, the Fresnel's focal length is close to ... the camera lens focal length. This works quite well.

Why does it work?
Actually most LF lenses except telephotos are quasi-symmetric and the distance between the exit pupil and the focal plane of the camera lens is equal to the focal length, and most of the time we focus to object distances much larger than 2 focal lengths, the sharp image is located close somewhere behind the focal plane. Hence the distance between the exit pupil and the image plane is, say, between 100% and 130% of the camera lens focal length (for a quasi-symmetrical design).

Fresnel lenses with a focal length equal to their diameter, or to their diagonal, are standard.
In other terms, most Fresnel lenses are F/1 single element lenses, something almost unconceivable with a real thick glass lens!
And will be perfectly suited for use with a standard focal lens e.g. 150 mm (6") in 4x5" image, diagonal = 150 mm (6"), focusing at large distances, focal length of the Fresnel lens = 150 mm.
In principle with wide angle lenses you would need a shorter focal length for your Fresnel. f/0.7 Fresnel lenses do exist, but are more difficult to find and are somewhat at the limit of present technology.

So most of the time, f/1 Fresnel lenses in combination with the GG, are chosen, as follows
6x9 cm (2X3") - Fresnel = 100mm (4") focal length
4x5" - Fresnel = 150mm (6") focal length
5x7" - Fresnel = 210mm (8"1/4) focal length
8x10" - Fresnel = 300 to 360 mm (12" to 15") focal length

100mm f/1 Fresnel lenses are easy to find. Same for 300-360 mm, under the form of an A4 or letter-size reading loupe. All overhead projectors have a Fresnel lens in the range of 300 to 450 mm focal length.
150 mm Fresnel lenses are more specialized to LF cameras and so far I did not find any cheap one, outside the specialized market of LF accessories.

Actually, the constraints are the availability or sub-f/1 Fresnel lenses for properly illuminating a ground glass when using wide-angle lenses of quasi-symmetrical design.
Fresnel lenses are more useful with WA angles, but even if the focal length is too long, there will be an improvement.
And you do not want to change your Fresnel lens any time you change your camera lens!

I am not aware of specialized ultra-short Fresnel lenses for large format cameras, but I have in mind at least one example for medium-format cameras.
For use with the Hasselblad SWC and its 38mm biogon lens, there was a GG accessory that you would substitute to the film back, to focus and compose like with a view camera. The standard focal length for Fresnel lens in 6x6 cameras is about 80 mm [difficult to say, for example in the post-1958 Rolleiflex, the GG is a single piece of molded plastic combining the function of a Fresnel lens and a GG, impossible to check the focal length easily!], hence for the 38 mm Biogon, a quasi symmetrical lens design, you would need a much shorter focal length that 80. Ideally = 38 mm, distance between the exit pupil and the focal point, but this would imply a f/0.5 lens ...

Regarding the proper assembly of a GG + Fresnel combination, the one I have on my Arca Swiss is as follows, from the lens exit lens to the photographer's eye

Fresnel polished side
Fresnel corrugated side
Ground glass ground (frosted) side
Ground glass polished side

This combination requires a proper placement of the GG, according to this diagram, the image being pushed backward by about 1/3 of the thickness of the Fresnel lens. This offset is built-in the springback design.
https://www.flickr.com/photos/43175600@N00/30929837496

This combination has the following advantages:
The polished side of the glass is the only one accessible to dust and operator's finger prints. Glass is harder than plastic and will resist better to scratches when manipulating a loupe.
The corrugated side of the Fresnel lens as well as the frosted side of the GG are protected, laying one against each other. The frosted side of the GG is more sensitive to dust and scratches than the polished glass side.
The flat side of the Fresnel lens is less prone to catch dust than the corrugated side. The Fresnel lens in plastic is more fragile than the glass GG and is located inside the camera, and cannot easily be damaged ... except if you collapse the bellows to a point where the lens touches the Fresnel lens.

All combinations where you simply lay the Fresnel on top of the GG are fine, no focus shift at all, but doing so the Fresnel Lens is more easily scratched.
However, I have found 310 mm focal length, A4-size plastic Fresnel lenses for €4.90 (e.g. here), taking into account the price of a single sheet of 8x10" film ;-)


Hope this long explanation helps!

[xkaes]

Mission: impossible, since we change lenses all the time and re-focus lenses for all distances!
You would need an adjustable zoom Fresnel lens!!!

All combinations where you simply lay the Fresnel on top of the GG are fine, no focus shift at all, but doing so the Fresnel Lens is more easily scratched.

An even crazier idea would be to have to carry a different Fresnel for each lens. Maybe some perfectionists have the time and money to do this -- as well as the pack animals.

As to the latter point, #1, we can just be more careful, or #2 purchase a "groundglass cover" -- which in this case is a "Fresnel cover".

Your long explanation has cleared a lot of dust -- I hope!!

[Emmanuel BIGLER]

An even crazier idea would be to have to carry a different Fresnel for each lens.

Yes, but in an era where everybody tells you "I simply take pictures with my mobile phone", accumulating ultra-specialized gear like, a whole set of high-quality Fresnel lenses of all sizes and all focal lengths and all possible f/numbers (including rare f/0.5 models), and preferably stored in a elegant real-leather porfolio, is the distinctive sign of the discerning LF photographer who refuses to follow the mainstrem of ordinary photographers

[Peter Collins]

Coucou 'manuel! Merci beaucoup!
Peter

[xkaes]

Yes, but in an era where everybody tells you "I simply take pictures with my mobile phone", accumulating ultra-specialized gear like, a whole set of high-quality Fresnel lenses of all sizes and all focal lengths and all possible f/numbers (including rare f/0.5 models), and preferably stored in a elegant real-leather porfolio, is the distinctive sign of the discerning LF photographer who refuses to follow the mainstrem of ordinary photographers

Hey if it was good enough for W. H. Jackson, it's good enough for me! My guess is that he was a real chick magnet in his day!

[Emmanuel BIGLER]

Coming back to rare f/0.5 Fresnel lenses, I found one from Edmund Optics that would be well-suited for wide-angle lenses in 4"x5"

https://www.edmundoptics.com/optics/...-fresnel-lens/

Effective diameter = 6" = 150 mm, focal length = 3" = 76 mm. This is a f/0.5 lens !
$89 ... not cheap.
If you look at Edmund's offer, most of Fresnel lenses they have on catalog are in the range of f/1 - f/2, only few of them are f/0.7 or even f/0.5.

This one is a f/0.7, 5" [127 mm] square, diagonal 7", focal length 5" [127 mm] could be a good candidate for a general-purpose Fresnel lens in 4"x5". Still $50, though.
https://www.edmundoptics.com/optics/...-fresnel-lens/

[John Layton]

I actually find that a fresnel with a focal length of appx. five inches is, for me, the best compromise for the 5x7 format - with my most-used lenses for this format being 120 and 210, followed by 90 and 305. I'd probably go with something longer if I also used something like a 450 for this format.

I did have to modify my film back in order to place the fresnel between the ground glass and lens...with the cut fresnel field facing backwards (in contact with the ground glass). Forgot exactly how much I needed to recess the assembly into the gg frame...but Bill Maxwell can give you this info.

[EdSawyer]

Emmanuel, great information! Thanks for the detailed write up and sharing of all that data. Very helpful.

[Doremus Scudder]

So let's summarize and clarify for the sake of those possibly misled by the OP's original post:

1. Placing the Fresnel screen between ground glass and camera lens WILL change the focus and result in out-of-focus negatives, so beware! The Fresnel shortens the optical path by ~1/3 of its thickness.

There are two ways to wrongly place the Fresnel between ground glass and lens (assuming here that the camera has a simple ground glass correctly positioned to start with): A. The Fresnel screen is exactly the same size as the ground glass and sits in the recess that normally holds the ground glass; the ground glass is then placed on top of it. This will result in the ground glass being optically spaced too far back by about 2/3 the thickness of the Fresnel screen. If this is a desired configuration, the camera back will usually have to be modified to position the entire sandwich closer to the lens than the original position. B. The ground glass stays where it is originally positioned and a Fresnel screen is (somehow) affixed to it between it and the lens. This results in the optical path being shortened by ~1/3 the thickness of the Fresnel. This latter, of course, could be rectified by shimming the entire sandwich out by the appropriate distance (measure carefully and shim or find someone with an optical bench in both cases).

2. Placing the Fresnel screen between ground glass and eye will NOT change the focus. This is the safest and easiest way to add a Fresnel screen for those that have cameras that originally came with just a ground glass.

Replacing an OEM Fresnel/ground glass sandwich that has the Fresnel positioned between ground glass and lens with a regular ground glass will shift the focus from correct. Better to try and find a factory or after-market dedicated replacement.

3. Cheap Fresnel screens designed for reading help are (thanks Emmanuel) usually in the 300-360mm focal-length range and, therefore, most helpful with 8x10 cameras. Using this focal length Fresnel on a 4x5 (or even 5x7) will work for long lenses but present viewing difficulties with short focal-length lenses. The rule of thumb is that a Fresnel with the focal length of the "normal" lens for any format is close to the ideal compromise, i.e., 150mm for 4x5, etc.


4. There are a lot of readily-available Fresnel screens for 4x5 and larger formats from many manufacturers, both OEM and after-market, that are specifically designed for particular cameras and formats as well as others in various focal lengths that can be utilized by those that have the skill and knowledge to install them correctly. These are vastly superior to the cheap Fresnel screens for reading pages.

Best,

Doremus

[Emmanuel BIGLER]

Thanks to Peter Collins and Ed Sawyer for their appreciation!
Well, I'm happy if we can record here various ideas, experiments, and tests regarding Fresnel lenses and share them with the community!

Thanks to John Layton for sharing his experience with the 5x7" format. Generally speaking, a Fresnel lens with a focal length of 5" covering a diagonal of 8-1/4" [210 mm] for the 5x7" format is roughly a f/0.6 lens, suitable for wide-angles taking lenses, an extreme Fresnel lens in terms of f/number, not so easy to find!

And thanks to Doremus for his concise explanation!
Explaining the principles of a Fresnel lens in "Twitter-mode" with not more than 140 sign is the next challenge for you, Doremus