Anybody ever tried a homemade fresnel lens?

[Derrel]

Not sure what you mean the shutter is being subjected to light when not exposing...the reflex mirror is sending the light up through the pentaprism, and right out the back of the camera...the shutter is exposed to light for a few milliseconds before it begins the exposure....and then the mirror comes back down...

Burning a hole in a focal plane shutter dates from the rangefinder era with rubberized silk shutters, not modern, titanium-alloy or high-tech composite shutters with a honking reflex mirror in front of them...

 

[Gavjenks]

[Accidental double post]

 

[Gavjenks]

Well right, I guess a given section of the shutter is exposed for approximately 1/25th of a second before it starts sliding and exposing a new spot.

And the ground glass isn't in any danger. Your eyes might be though.... it's not focusing on them, but just looking at something that bright even if it's diffusing for a short distance, etc. might still really hurt.


Dunno. f/0.9 lenses and such are commercially available, but they're never 100mm+ which could make a big difference. At that FL it could be I think 4x more overall light on the ground glass and thus flooding into your retina than a 0.9 50mm lens? Not all in one spot though, which may be the saving grace.

 

[Gavjenks]

New lens I made just now. It's that kind of day =P Was a tad bit more careful with this one.

It's an approx. 180mm single achromat (two elements cemented together to cut down on spherical and chromatic aberration) from surplusshack for like $4 or something. This one was slightly smaller than my 2" PVC tube standard, so I wrapped a piece of electrical wire around the inside diameter, duct taped it in place (which doubles as reflection prevention, black tape, still glossy though), and then seated the lens.

Being a proper glass lens AND an achromat, the results are substantially better. This lens has a fixed aperture of f/2.2. It is designed as a portrait lens, though I don't have any portrait subjects and don't feel like lining myself up right at the moment.

Instead, here is a typewriter and a snail lady sculpture in my hallway:


Compare to a hasty snapshot that i zoomed to make the composition the same on my Canon 70-300 IS USM (it's a tiny bit motion blurred, but I think that's only fair as a showcase of the homemade lens' 2 1/3 faster stop aperture hehe). The Canon was at its widest aperture of 5.0:



The lens looks exactly like the other one, except shorter, from the outside. Not gonna bother juggling smartphone photos.

The Canon is obviously better in technical control of aberrations, however not by much. It's a little difficult to directly compare, due to the inability to test both at the same aperture (without building a diaphragm for my lens) And regardless, I think the homemade one is actually much more attractive anyway, and especially would be for portraits (I'll update with one as soon as I have it). It has a very intentional-seeming "this is slightly imperfect and dreamy, but by absolutely no means a fuzzy wreck" look to it.

Also, the PVC lens is almost exactly the same size as the Canon fully retracted, almost 2 inches SHORTER than the Canon at equal focal lengths, and about 1/5th the weight of the Canon.



When was the last time you saw a 6 ounce, 180mm lens that was perfectly useably sharp at f/2.2 for $5 ?

 

[Gavjenks]

Okay, got bored and did a portrait of myself with the achromat lens. DOF is like an inch on this thing (maybe it's actually wider than 2.2...? Or it's slightly soft already, making DOF narrower in general), making it virtually impossible to take self portraits that show off the quality of the lens optimally, but whatever.

I took like 30 shots trying to get my eye in focus and gave up. You can see how sharp it CAN be though right at the point of focus, by looking at the stubble on my chin and the chevron patterns on my left collar. Plenty able to resolve impressively in a narrow narrow range. But it's almost buttery smooth blur on the cloth right behind that same collar! Craziness. It's acting like a f/1.2 or something, but metering at 2.2.

This particular shot is orthochromatic (I removed the red channel) to increase sharpness a bit, since red was actually aberrating noticeably softer than the other channels.

Anyway, totally a successful lens for this! Could take some awesome portraits if I could see what was happening before hitting the button.

 

[amolitor]

That's really nice. To my inexpert eye, it has some of the same qualities as an olde skoole portrait lens. Possibly it's because we're seeing (I think..) multiple images cast by what are really multiple partial lenses, with varying degrees of sharpness and defects.

Compare with the Cooke PS945 which is.. more expensive.

 

[grafxman]

You can stop the lens down and achieve some depth of field by using a disk of black construction paper over the front of the tube. You can cut disks with different size holes in them to change aperture size. I've done that with a couple of lenses. One is a Vivitar 800mm mirror lens and the other one is a Bower 650-1300mm zoom. A lot of light will be needed though.

 

[Gavjenks]

Yeah I did try stopping it down, but then it loses all its character and look like a boring normal lens, so I might as well use a modern autofocus, etc., you know?

 

[cynicaster]

View attachment 50197

It's Dexter Morgan!

Cool DIY.

 

[grafxman]

View attachment 50197

It's Dexter Morgan!

Cool DIY.

:lmao::lmao::lmao::lmao::lmao::lmao::lmao::lmao::lmao::lmao::lmao::lmao::lmao:

 

[Gavjenks]

I had the idea of putting the lens onto a macro bellows today, and using that to focus instead of pushing pieces of plastic together clumsily. Makes a huge difference, and this is now much more of a joy to use.


Also the huge range of the bellows means basically any lens I put on it is a true macro lens. The 180mm goes from infinity to 1:1, without me even having tried to design it like that! (You do have to extend the module forward a bit to get to 1:1. it's about 0.8x with bellows movement only) A 50mm for instance would go from infinity to extreeeme macro, 3:1 or so.

Snapshot from today with the above setup


next steps are:
1) A 2x 4" fresnel module with 150mm or higher focal length and about f/1.4
2) Aperture added to modular system
3) Eventually tilt movements added to modular system, maybe shift.

 

[Tony S]

All I can say is I had a brain fart..... lol

 

[grafxman]

Say Gavjenks, since you're clearly a bright, inventive guy how about inventing an antigravity device. Consider this. We control all the natural forces such as light, RF energy, electricity, etc. but no one knows how to control gravity. So get with it Gavjenks. If you invent a way to control gravity, anti-gravity shouldn't be too hard. Then we can cruise around the solar system with just solar power and storage batteries.