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UltravioletPhotography

Possibilities for Alternative Optics?


OlDoinyo

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We all know about the use of conventional refractive and reflective objectives for ultraviolet photography, and many of us have heard of pinholes used for this purpose, as well. But what about Fresnel lenses, passthrough Fresnel mirrors, or zone plates? These would obviously not produce images of the highest quality, but for someone of a low-fi/Lomo bent who is interested in getting into the field without spending a lot of money, would these be reasonable alternatives to conventional lenses? Fresnel lenses are quite thin compared to their conventional counterparts, and the other two options involve nothing but air in the light path, and might be usable even at wavelengths where conventional optics are useless.
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Here are some Fresnel shots with Fresnel lenses from flash units. Pictures talken with leses in daylight did look like London in the fog. Nothing was visible!

 

To avoid some lens errors I used the green (almost) monochromatic light from a power led.

 

The result is here lens fully open (Nivea tin):

 

 

http://up.picr.de/20183026er.jpg

 

Then I put some apertures (9mm, 12mm, 25mm) to the lenses and changed the scene:

 

http://up.picr.de/20183027xt.jpg

 

http://up.picr.de/20183028kh.jpg

 

Here the results:

 

http://up.picr.de/20183029du.jpg

 

http://up.picr.de/20183030kr.jpg

 

http://up.picr.de/20183031vn.jpg

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Welcome, but I think my fresnel lenses are not suitable to make pictures...

 

Using a narrow part of spectrum enables new ways. For example using uv led light sources.

 

It might be possible to use a single converging lens or a simple group instead of a highly corrected lens consisting of different lenses.

 

Sounds like nonsense. My problem is that am used to the german words "Objektiv" as name for a unit and "Linse" for the elements it is consisting of. In english both are named "lens". (???)

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Slightly off topic - several years ago I found that Better Beamer flash extenders (Fresnel) pass UV quite nicely. I recorded a transmission spectrum which is around here (in the bit vault) somewhere.
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Are the Fresnel elements on flash units even intended to be imaging? I know that there are non-imaging and even divergent Fresnel lenses, which obviously do not bring light to any focus. The ones sold as "solar fire starters", on the other hand, definitely do form images.

 

N.B. Yes, the English usage is confusing. There is also the cognate word "objective," which refers to an entire optical assembly (reflecting or refracting.) However, English speakers don't like to use it very much, as it sounds a bit pedantic.

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No, the are not intended to shoot photos. But using a led and projecting to a wall looked fine. They also work in magnifying things.

 

Would you please show some images taken with your solar fire starter?

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Sounds like nonsense. My problem is that am used to the german words "Objektiv" as name for a unit and "Linse" for the elements it is consisting of. In english both are named "lens". (???)

 

Yes it is confusing, especially when the context is not clear.

 

For your own sanity, try using 'lens element' for Linse and 'lens' for Objektiv. :)

And always ask the writer for clarification, if needed.

We are all happy here to clarify what we write because we know English idiomatic usage can be confusing sometimes.

 

We do use the word 'objective' in English - usually in the context of discussing microscope or telescope gear.

But we use 'objective' less often when discussing photographic gear.

I don't know why. It seems more sensible to use the word 'objective'.

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Now I try again:

 

If one uses a single lens element or a simple group then it might be possible to shoot sharp pictures if the illumination is monochromatic.

 

That should be possible with uv and Led illumination like some of us use it as it is nearly monochromatic.

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  • 4 weeks later...

Here is the Itoya Pocketlens in its PVC sleeve, standard credit-card sized, borrowed from my wife:

 

post-66-0-61865500-1419096054.jpg

 

This (about $9 from amazon.com) proved to be an imaging Fresnel lens with a focal length around 120mm, according to quick measurements. The first step was to check if the bandpass characteristics were reasonable. To this end, the lens was stood upright in a business card holder and propped with its own sleeve rolled up. The lens was photographed in front of an illuminated background:

 

Sony A900, Goema/Makowski 500mm mirror optic, extension tubes, Baader U2 filter, 15 seconds@f/8 and ISO 800; display intent BGR.

post-66-0-45209100-1419095995.jpg

 

The lens appears not only clear, but untinted, which is an excellent sign. Compare this with the deep orange of the PVC sleeve, which indicates UV transmission only at the longest wavelengths. Had the lens itself shown such color, it would have been deemed unsuitable for further investigation.

 

Using tape and cardboard, a sleeve was improvised with a crude lensboard on one end. A hole was cut in the lensboard and a 55-->49mm stepper ring was taped to the outside of the lensboard, allowing the filter to be screwed on. The Fresnel lens was then held to the outside of the filter with additional tape. The sleeve was slid onto the extension tubes mounted on the camera. The finished apparatus appeared thus:

 

post-66-0-66116700-1419096094.jpg

 

Preliminary tests without the filter verified that a recognizable image existed at the focal plane and also indicated that sliding the sleeve in and out on the extension tubes allowed focusing from infinity to about 3 meters.

 

The apparatus was taken outdoors and two targets were photographed: a shed and the eaves of a cottage:

 

Sony A900, Itoya 120mm Fresnel optic, Baader U2 filter; 1/2 second@ f/2.8 and ISO 400; display intent RGB (upper images) and BGR (lower images)

 

post-66-0-15779200-1419096130.jpgpost-66-0-05512500-1419096162.jpg

 

The images are rather veiled and overexposed and the "red" channel is badly blown out, especially in the sky areas; work-up allowed the images to be brought to a state where evaluation was reasonably possible. Focusing was very tricky and basically a matter of trial and error because it could not be done by eye; it took a number of tries to get these images. Some of the veiling could be the result of light bouncing between the filter and the lens-card.

 

The images exhibit some characteristics typical of UV images, such as dark foliage and bright skies. The reddish tinge on the sides of the buildings is due to the absorption of shorter-wave UV by titanium dioxide paint. The bluish appearance of the window frames of the shed could be an absorption quirk of the maroon paint, or it may merely be an artifact of overexposure.

 

It would clearly be a waste of time to compare these images with anything obtained by precision optics; I will therefore confine myself to comparing them to what might be obtained using that other low-budget device, the pinhole.

 

PRO:

 

-Much faster (12 stop advantage over a standard f/180 pinhole, in this case.)

-Modestly better relative angular resolution than an f/180 pinhole; clearly better in absolute terms due to longer focal length. Higher number pinholes could erase this advantage only at a prohibitive cost in speed.

-Possibility of selective near focus

 

CON:

 

-Tricky focusing (pinholes do not need focusing.)

-Shallow DoF

-Image veiling (lesser input dynamic range.)

-Possible focus shift issues (unverified.)

 

I was unable to find any indication that the bandpass characteristics of the lens card (within the filter transmission band) are much different from those of the pinhole.

 

As depicted, this exercise is not quite "Instructables" material, as it employs three pieces of specialized gear: the converted camera, the extension tubes, and the filter. One may speculate that the camera could be replaced by an inexpensive film camera loaded with monochrome film, whereas the extension tubes might be replaced by something contrived from more cardboard and tape. The filter represents the greatest challenge; the classical strategy is to break an incandescent black-light bulb and tape a fragment thereof in the optical path. If one could get access to the plastic applied to such bulbs, it might be possible to coat the lens-card with it. There might also be sources of inexpensive Wood's glass.

 

Images obtained by this method, if they have any merit at all beyond science demonstrations, will most likely appeal to those who are into the Holga/Lomo/Diana low-fi photography movement.

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Bjørn might have seen that. During the Photokina this year, Canon showed a new tele lens with a diffraction optic. One of the lens elements was a lens made of two lenses, somehow glued together to one piece, and the "trick" is the interface between the two elements, which looks like a lot of small steps (the lens element was shown cut in half in cross section). My first thought was, this interface is a fresnel lens element.

 

Werner

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Bjørn might have seen that. During the Photokina this year, Canon showed a new tele lens with a diffraction optic. One of the lens elements was a lens made of two lenses, somehow glued together to one piece, and the "trick" is the interface between the two elements, which looks like a lot of small steps (the lens element was shown cut in half in cross section). My first thought was, this interface is a fresnel lens element.

 

Werner

 

Canon had the 400mm F/4 DO (diffraction optics) lens for many years, they intoruduced it in 2000 and just upgraded it in 2014. It is an interesting concept but there are probably some important reasons why it is not common.

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Very prone to flare plus an 'interesting' bokeh.

 

Its optical design shortens the physical build significantly so the lens can be made smaller (and hence lighter), but at a price.

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