Single element lens

[david_carlisle1]

Now that it seems possible to have lenses with aspheric surfaces produced at reasonable cost, what is the

possibility of having a camera lens consisting of only ONE element, with both surfaces aspheric?

[winfried_buechsenschuetz1]

First, you could not use a single-element lens for photography, whether it is asperic or not. Such a lens would not be

color-corrected and would have significant chromatic aberrations. This would result in a "soft focus", even if other

aberrations (mainly spherical aberrations) are corrected to a better degree using aspheric surfaces.

 

Second, the only advantage of low lens-element count is reduction of glass-to-air surfaces. But with coated and multi-

coated surfaces this is not a big problem today. Using several lens elements (or, to be precise, a system with

several surfaces) allows for better correction of aberrations.

 

BTW Edmund Optics offers achromatic lenses with one aspheric surface, they consist of a normal (spheric) glass

lens and a layer of aspherically moulded plastic. The refraction index of the materials used yields achromatic

correction. They are much better corrected for spherical aberrations than their standard achromats.

[aplumpton]

The lowest number of elements I have seen successfully used in a lens were the Leica 400 (or 560?) to 800mm lenses,

made for the reflex housing adapted M cameras and their SLR. They used only two non-aspheric elements. One of my old

books shows that they were beautifully used by Duncan in his 1968 detailed head shots of delegates at the USA Democratic convention

in Chicago.

 

Of course, by restricting maximum aperture and using long focus lenses (small angle of field), you can get away with this lesser

number of elements.

[tommyinca]

One element is not so good with color print as post above due to the lack of color corrections. Two elements in one

group would be fine for color as long as once care about on axis performance only. From that, Pentax did made a

85mm f2.2 portrait/soft focus lens. Aspheric surface can help off-axis performance but then the marketing

department has to explain why plastic is better then glass or molded is better than percision grind and one is better

than six. All up hill battles.

[winfried_buechsenschuetz1]

"One element is not so good with color print as post above due to the lack of color corrections. "

 

A non-color corrected lens will not work fine for b/w pics, too. The "color fringe" you will see on color pics will show up as soft contours on b/w prints. Some of the older "soft focus" or "portrait" lenses were not fully color corrected on purpose (of course such a lens would be completely useless for color pics).

[Brian1664876441]

You end up with a Holga. Of course, I know two people using RF coupled "monocle" lenses on their M8's now.

 

An I61 mount works great for 50mm single element lenses.

[john_shriver]

A lot of Vest Pocket Kodak cameras with the slow meniscus lens have had the lens adapted to SLR mounts in Japan. They also un-mask the lens, increasing the maximum aperture, to make it even softer.

[zane1664879013]

> You end up with a Holga

 

Or a box camera (meniscus) lens. They both can have a certain appeal, but won't be a mass-market item.

[tommyinca]

"A non-color corrected lens will not work fine for b/w pics"

 

You are assuming the world has no color filters and film is equally sensitive to all colors.

[winfried_buechsenschuetz1]

""A non-color corrected lens will not work fine for b/w pics"

 

You are assuming the world has no color filters and film is equally sensitive to all colors."

 

Of course using a filter might help to make a single lens work somewhat better for b/w.

 

Today's panchromatic film is more or less sensitive for ALL colours. Even if it is less sensitive in some parts of the spectrum, chromatic aberrations will show up.

 

And there is another problem: focussing. The maximum sensitivity of the human eye is in the green range, so the green part of the light on a focussing screen will contribute most to the impression of sharpness. B/w film (especially non-panchromatic) is most sensitive in the blue range. If the lens is not color corrected, the focal length will be different in the green and blue range, and focussing with the human eye will not yield correct results. Some older lenses had marks for the "chemical focus", i.e. for the displacement needed from "eye focussing" for correct "film focussing".

[andy_piper2]

The Leica Telyt 800 had a THREE-element cemented group, the 400 and 560 two cemented elements as mentioned.

 

And yes, two has been the minimum to correct color, because you need two different types of glass, with different refractive

indices, to get the color back in line. ASPHs alone couldn't solve that by themselves.

 

The long Telyts handled color very well, but the field curved - i.e if the center was sharply focused on a subject at 200 feet, the

edges would be focused at about 185 feet. Made for interesting apparent DOF effects.

 

It is probably possible to make a corrected single-element lens by employing a bunch of tricks together: Fresnelling, ASPHs,

blending two (or more) glass types into one solid piece, or impregnating one side of the glass with minerals or some such to

modify the dispersion on that side.

 

Not unlike those Edmund lenses mentioned above, using plastic-on-glass (although that sounds pretty close to a traditional

cemented doublet anyway)

 

Question is, would it be cost-effective compared to just using traditional cementing of single-glass elements?

[winfried_buechsenschuetz1]

Cost should always be taken into consideration when thinking of aspherical elements. Conventional lens grinding machines are suitable for spherical surfaces only. In the early days of aspherical elements, the aspheric surfaces sometimes were ground manually (i.e. using machines but no automatic process).

 

Today things have changed a bit. Now we have plastics which can be moulded to high precision and the properties of which meet optical requirements. Also today it is much easier to pre-mold glass elements, so the aspheric surface just needs to be polished.

 

We all remember that many years ago zoom lenses used more than a dozen elements. Topcon (which used to be active in the field of photography but today just makes industrial and scientific optics as well as optical research) has published a design for a simple zoom lens for low-end digital cameras using just three elements with several aspheric surfaces.

[pc_b]

Winfried: ..."you could not use a single-element lens for photography"

 

Winfried: ..."older "soft focus" or "portrait" lenses were not fully color corrected on purpose (of course such a

lens would

be completely useless for color pics)"

 

Ahm...since when has photography been about sharpness/CA/etc. only (except for technical applications and in the

heads of nerds)?

Aren't there plenty of single-lens-cameras, and single-lens-optics for cameras out there to prove demand and

therefor motivation for such 'useless' photographic use? Weren't the results worthwhile?

 

The technical quality of single lens photos is often much higher than most people expect it to be, esp. on this

forum (actually,

it's the wrong forum for this topic/thread, .. wait, I take this back: it is absolutely right here!). It is miles

ahead of pinhole photographs.

 

Intermezzo story: I scanned the center 4x4cm of some Holga-Negs for a website and they came out so sharp that the

guy wouldn't believe I had actually used a Holga as demanded. I had to send the scans of the contact sheet as

proof!

 

I think there is only _sufficient/non-sufficient_ sharpness/contrast/color fidelity/veil suppression/etc. for a

specific

photographic task (incl. art photography). History of optical lens engineering and manufacture and history of

photography have shown this times and again.

Let's look more around (photographically speaking): there is much more to photography than optical engineering and

optimization/maximization.

 

Cheers, Pete

 

P.S: Winfried, I agree with you on all technical points you made except the two (not really technical ones) I

cited above. Zero doubt, you do know your way around in the field of optical engineering! And I am thankful to

you for sharing your knowledge with the feisty bunch of guys on this forum!

[winfried_buechsenschuetz1]

You are completely right that the term "sharpness" needs to be defined to the needs of the user. Of course a single-element lens will be sufficient for many purposes. And I am the owner of a box camera (my father's first own camera) and enjoyed shooting some rolls with it. No worries about focussing and shutter setting - simply use f/16 for sunny weather, f/11 for cloudy weather and do not use this box if weather is even worse...

 

And of course sharpness is not paramount for each and every application. I know that the Zeiss Trioplan lens on the old Rolleicords was preferred by some photographers for portrait shots - although this lens is everything but sharp from corner to corner wide-open. I think that the drop-off of sharpness off-center of "bad" lenses rather meets the properties of the human eyes the resolution of which also gets much lower off-center.

 

But you are completely right that my statement that a non-achromatic lens would be useless for photography is not correct. I know a semi-professional photographer who made a series of soft-focus shots - with hacked zoom lenses. (BTW reversing the center element on old Zeiss Novar lenses produces nice effects, too.)

 

BTW I am not an optical engineer (I have a degree in precision engineering and I design electronic circuitry for laboratory systems) but I always had an interest in optics ... and an excellent lecturer on applied optics at college. I attended all his lessons whether mandatory or not and I still keep on running through german, english and french books about photographic optics.

[MTC Photography]

Many box cameras were of single lens variety, with a crescent moon shape single element glass lens.

[winfried_buechsenschuetz1]

Yes, in most cases the single element lenses of box cameras had a meniscus-shape cross section. This is used to reduce spherical aberration. Unfortunately shaping a lens element like that increases astigmatism. So even if we do not talk about chromatic aberrations a single element lens is always a compromise between these two major aberrations.