Anybody ever tried a homemade fresnel lens?

Gavjenks

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I am considering either trying a plastic DSLR fresnel lens like in the back of rear projection TVs, or one of the ones they use for studio lighting, that doesn't have the stipling on the back of it to make it soft.

I'm well aware that this will have all kinds of terrible aberrations. What I'm wondering is just if anybody has tried this at all compared to homemade normal lenses, using for example lenses from surplusshed.com ?

The goal is to attempt to make a hilarious ~1000mm, f/0.8 lens that can maybe resolve enough to vaguely actually identify things in photos (i.e. "there mayyyy be a building in this photo" levels of resolution, if lucky). Or if not that, then just regular types of lenses.

However, if somebody has tried this and fresnel lenses are vastly worse than uncoated surplus crown glass lenses of equivalent power (to the point of not being able to make out anything, even with a reasonable FL at f/8 or something), then I may not bother.
 
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Gavjenks

Gavjenks

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Hey, so I went ahead and actually did this!

I got some $2-3 each giant plastic fresnel lenses about 300mm focal length, made for reading books more easily. Chopped a circle out of the middle of one, and put it on a PVC tube, lined with painter's tape for a little bit of contrast help instead of a white interior. I already had a universal PVC 2" Tube -> Canon EOS adapter and push/pull focuser I made a long time ago.

Total build time like 10-15 minutes. Product was this:
Amazon.com: 5 Fresnel Lens -- Solar Oven + DIY Projection TV: Office Products

Lens:
$lens.jpg

Sample photos (LOTS of RAW editing to make these not hazy pieces of junk, but for a $3 plastic lens weighing half an ounce, these are pretty great I think!) A little chromatic aberration, almost no coma or astigmatism, decent amount of spherical aberration:

$IMG_5754S.jpg$IMG_5763S.jpg$IMG_5771S.jpg$IMG_5774S.jpg$IMG_5775S.jpg
 

amolitor

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That's awesome, dude! It's like a telephoto lensbaby. I dig what it does to specular highlights.

I wonder what portraits would look like?

You could *totally* rock a Pictorialist gum bichromate look with this thing.
 

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Did you think about the interior of the lens being black instead? Wondering if that would make much difference, anything I have, even pinhole cameras, have a black interior.

Somewhat similar looking effects to what people get with some plastic or homemade or pinhole cameras. I think it can be interesting depending on the subject, I like this best with the floral shots I think, although I sort of like it with the motorcycle, gives almost a bit of an illusion of movement.

Now you need to put some black stripes on that lens so it goes with your camera body.

edit - I can't say though that it has the look of photos I've taken with one of my Lensbabies, doesn't have the look you get when you turn and angle the lens.
 
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amolitor

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Yeah, the lensbaby definitely modulates its effects across the frame, which this thing does not. I find this to be pretty much the single most irritating thing about the lensbaby, though. If a lens is soft, even extremely soft, I can see how to work with it. A lens with multiple personality disorder just does not produce results that make any visual sense to me.

I probably should have said "lomographic" rather than "lensbaby" or something.
 
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Gavjenks

Gavjenks

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Yeah it's actually fairly different from a lensbaby. Lensbabies work by having a huge amount of uncontrolled astigmatism and coma, both of which create more streaking and directional blur away from the center of the plan of focus. As you tilt, then, the center moves by Sheimpflug principle, moving the "sweet spot."

In fact, this lens is almost exactly the opposite of a lensbaby. It has coma and astigmatism seemingly very well under control, but spherical aberration and polish (general softness issues) are both poor, whereas lensbabies make a priority of controlling for those so that the middle is nice and sharp.

If you were to tilt this lens, it would actually look much more like a traditional tilt shift lens, with a fairly uniform line of focus from a nice evenly moved plane, and less of the circular effect.

black inside tube
Yes, that absolutely makes a difference. I've made multiple homemade lenses before, and typically what I do is to flock the inside of the tube with thin, fine black felt, which just sucks up light like it's nobody's business. As good or better than the interior of Canon lenses, probably.

The painter's tape is because I'm a lazy bastard and made this in 10 minutes.

It also has no lens hood, which makes a huge difference. A few dollars on a 2" to 3" PVC coupler and a foot or so of 3 inch pipe also lined with felt would massively help contrast as well.

However, even with both of those improvements, which normally would make even a terrible glass lens super contrasty, it still wouldn't help entirely with the fresnel lens. Part of the fogginess is due to light hitting the flat sides of each fresnel lens, which can't be helped fully ever, no matter what. If I could choose my material and use coated glass with very little reflection but lots of refraction, then it would help (like Canon does in their diffractive optics fresnel lenses), because than light hitting the straight parts would just go straight through and bounce off of felt 10 times and not be an issue. The relfecting off of the straight parts on my uncoated plastic, though, kills contrast like it's its job.

specular highlights
This is partially due to the lens, but also due to the fogginess / very low contrast, which is making me get like 3 stops of dynamic range in each shot. RAW helps tremendously by giving me the data to stretch that back out in converting to a jpeg. The extra 4 bits of data mean I essentially have 4 additional stops of dynamic range on top of that, which makes it workable.
 

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It rivals many of Canon's telephoto zoom lenses! Nice work man! I'd consider painting the barrel solid black, then painting a nice, thin red stripe around it.
 

Derrel

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It rivals many of Canon's telephoto zoom lenses! Nice work man!

Really? I thought it looked more like Sony, myself!


Ehhhh, not quite, and I say that because Sony lenses usually have lower levels of contrast than this eccentric beauty Gavjenks hath wrought, but I get whatcha' mean Charlie---it does have something of a Sony-esque look to its drawing style...
 

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Nice one mate! This kind of stuff is right up my street, I love making stuff like this. Plus you can totally get that authentic 60's oof look should you wish, sweet!
 
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Gavjenks

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Note that this is not actually a telephoto lens, as that implies a telephoto group that makes the lens physically shorter than its FL by having a positive element then a negative element. It's just a long lens.

However, I did also buy a plano-CONCAVE fresnel lens online, which is marketed to put on the back of your car's rear windshield to see the area around better and not run into kids and trees and stuff. It is more expensive though, so I'm biding my time on deciding what to do with it. A telephoto (or retrofocus) lens is possible. Or maybe some other things.

Also, the original fresnel positive sheets are like... the size of an A4 piece of paper, they're HUGE. They cast a HUGE image rectangle, enough to maybe even cover an 8x10 large format piece of film. Certainly 4x5, at like potentially f/1.4. I only cut out a 2" piece here, and it's at f/6.

The softness is only going to get more out of control at larger apertures, but I still feel like it is my civic duty to go get a piece of like 6" pipe and step it down to make an f/2 lens or so out of one of these, and possibly a wacky large format cardboard camera out of another one.



Another fun thing to do with a positive fresnel sheet: Draw a little area of black on a piece of computer paper with a sharpie (to absorb more light), and line up one of these with it in a beam of sunlight, and it will burst into flames in approximately 1 second. =P Amusingly, they do not warn you about that on the packaging.

One possible thing to do with the negative fresnel element of roughly the same but opposite FL is to make a collimated beam of high intensity light by placing it near the focal point but not quite at it. Assuming it doesn't melt my plastic lens, that would = a no-focus-required, ranged death ray
 
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Gavjenks

Gavjenks

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Actually, come to think of it, if I put 3 in a row in a 6" piece of pipe, I could make an f/0.65, 100mm lens!

And then never point it at the sun... because it might actually be able to burn my shutter leaves.

Lol.
 

Tony S

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Your shutter has leaves on it? Do they blow around in the wind? :lol:

The aperture blades are in the lens usually, you might burn the sensor when the mirror flips up though. Or melt the back side of your viewfinder.
 
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Gavjenks

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Your shutter has leaves on it? Do they blow around in the wind? :lol:

The aperture blades are in the lens usually, you might burn the sensor when the mirror flips up though. Or melt the back side of your viewfinder.
I said shutter, not aperture diaphragm. The thing that is right in front of your sensor that stops it from being exposed and that is being referred to by "shutter speed."

It consists of two curtains, each of which is made out of a bunch of tiny thin little overlapping leaf/blade thingies, almost like venetian blinds. Video:

This is the thing that is most exposed to the sun's image at the focal point of the lens while not exposing. The sensor is very unlikely to receive damage even if you took a picture, because with that much light, it would always be at 1/4000th of a second which isn't enough to cause damage, unless I set it to manual and forced it higher or whatever like a chump.

I think they're made out of metal, though, so they probably wouldn't actually be damaged.





Also, after more research, I discovered that the size of the mount versus the flange to focal distance makes it only possible to go down to like f/0.8 or something on Canon. So I may have to settle for that, or more like f/1.0 ish.
 
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