Orientation of the meniscus lens in a simple box camera

[Todd Barlow]

A colleague of mine has a Agfa Ansco 2A that belonged to his grandfather. I made two modified spools so he could use 120 roll film and try it out. The single meniscus lens was held in place between two pieces of wood.
Unfortunately the one piece gave way and the lens fell out when he was trying to clean it.

The questions is which way would it have left the factory in:
Convex facing the subject or Convex facing the film?

I think it should be convex facing the subject but want to give the proper advice.

Thanks

Todd

 

[AgX]

The proper orientation for a meniscus lens in such case is the hollow side facing the film.

EDIT:
I stand corrected, it is the other way round in this case.

 

[Andrew O'Neill]

...and if you want to have soft, out of focus edges, orient the lens concave side facing the subject.

 

[NedL]

I don't know about the Agfa Ansco 2A specifically, and I could easily be wrong, but I think many early cameras used a meniscus lens with the concave side facing the subject. That's the opposite of what Andrew and Agx wrote ( By "hollow", I think Agx means "concave" ).

Meniscus lens have an interesting asymmetry: the focal length is different in the two orientations. It's longer when the concave side faces the film and shorter when the concave side faces the subject.

It has become a fad to take advantage of this and "flip" the lens in a Kodak Brownie Hawkeye, which is a fixed focal length camera. When the lens is flipped so that the concave side faces the film, the focal length of the lens is made shorter, so the camera is then focused on a point 1 or 2 meters in front of the camera and distant objects will be out of focus.

 

[removed account4]

all my box cameras have the rounded part facing the film
the flat part outside the camera .. they are like reinhold's woillaston meniscus configuration...

 

[NedL]

Yep, I think the rounded ( convex ) side would usually be facing the film in these old cameras. That's also how the first "landscape" lenses were made.

 

[AgX]

all my box cameras have the rounded part facing the film
the flat part outside the camera .. they are like reinhold's woillaston meniscus configuration...

A lens with a flat plane is no meniscus lens.

 

[removed account4]

hello agx

planoconvex lens isn't a meniscus lens ?

my mistake ..

 

[AgX]

Meniscus lens have an interesting asymmetry: the focal length is different in the two orientations. It's longer when the concave side faces the film and shorter when the concave side faces the subject.

The focal lenght of a meniscus lens does not differ at both sides.


By reversing a collecting meniscus one moves the cardinal plane and thus the focal point.
The worse orientation of a collecting meniscus has a larger caustic.

 

[AgX]

planoconvex lens isn't a meniscus lens ?

A meniscus lens is a lens with both radii having their center at the same side.
There are converging as well as collecting meniscus lenses.
In this thread we are speaking of collecting meniscus lenses.

Without going into detail, a meniscus collecting lens has an optical advantage over its respective double-convex counterpart.
But also disadvantages, these vary with its orientation.

 

[bvy]

It has become a fad to take advantage of this and "flip" the lens in a Kodak Brownie Hawkeye, which is a fixed focal length camera. When the lens is flipped so that the concave side faces the film, the focal length of the lens is made shorter, so the camera is then focused on a point 1 or 2 meters in front of the camera and distant objects will be out of focus.

Blurred edges too...

 

[Gerald C Koch]

Example givenFig 2.2

http://nightlase.com.au/education/optics/lenses.htm

 

[NedL]

The focal lenght of a meniscus lens does not differ at both sides.


By reversing a collecting meniscus one moves the cardinal plane and thus the focal point.
The worse orientation of a collecting meniscus has a larger caustic.

Yes, thanks! I think I should have written the "back focus" or distance from film to lens when focused at infinity. It is longer when the convex side faces the film and shorter when the convex side faces the subject. I don't know about the ansco 2A, but I think at least some early cameras were made with the convex side facing the film and the aperture stop further from the film.

 

[removed account4]

Example givenFig 2.2

http://nightlase.com.au/education/optics/lenses.htm

thanks for the diagrams gerald !

i inadvertently, and wrongly refer to
a planoconvex lens as a menisicus lens
when it is different a meniscus lens is scooped
on one side and round on the other, and a planoconvex
is curved out on one side and flat on the other. i have been under
the impression ( wrong i understand now ! ) that some of the lenses i have
been using might be different than i thought. i have been under
the impression that lenses on my box camera are flat outside and curved inside
(towards the film ) i didn't realize that they might have a curve on the outside too!
thanks to you and AgX i probably have to check what i am actually using !
( so i can describe them accurately ! )

john

 

[Nodda Duma]

A Landscape lens design *always* has concave towards the aperture stop.

In the Agfa the lens is behind the aprture stop, so concave forward.

 

[Todd Barlow]

Thank you for the responses,
The camera has the shutter and aperture in front of the lens.
So concave toward the subject.

 

[Gerald C Koch]

Described in the article below box Brownies had a meniscus lens with the concave side facing the subject and the aperture in front of the lens. This agrees with the two different models that I have.

http://www.brownie-camera.com/articles/petelutz/article.shtml

 

[AgX]

The camera in question is not a Brownie.

 

[Gerald C Koch]

The camera in question is not a Brownie.

Isn't the term Brownie generic for all simple box cameras. It is a case where a trade name has become generic like Victrola and Xerox. I mention the Victrola because IIPC there was a landmark case where the court decided that the name had become vernacular for phonograph and was no longer protected by trademark by the Victor Talking Machinecompany.

 

[AgX]

Maybe it is a generic term in the US. Here in Germany I practically never ever saw any model designated as Brownie.

 

[desertrat]

In his History of The Photographic Lens, Kingslake describes both orientations for a meniscus lens. The early box cameras had the concave side facing the subject, and the lens was placed behind the stop and shutter. Later, many cheap compact box cameras were manufactured with the convex side of the lens facing the subject, and the lens was mounted in front of the stop and shutter. This later arrangement used the lens to protect the shutter and stop, and also made the camera more compact for a given focal length. Kingslake noted spherical aberration and distortion were worse with lens mounted in front, but it was popular anyway.

 

[Nodda Duma]

Yes: concave side towards the stop.



To add some more detail regarding the Landscape Lens design: The lens shape corrects spherical aberration. In fact, it is the ratio of curvatures on front and rear surfaces that minimize spherical, while the actual curvature values set the focal length. Astigmatism is entirely uncorrected: The distance from the stop balances field curvature for the most pleasing off-axis image. If you've ever tinkered with this layout, you'll quickly learn that the distance is set to flatten sagittal, since this makes for a better looking image (the "swirly" effect at the edges of a Brownie photo are due to uncorrected tangential).

Distortion and coma are minimized by the combination of lens position, balancing against field curvature, and reducing the aperture diameter (the reason these cameras are so very slow). Distortion is also helped by keeping with "normal" focal lengths.

Kingslake traces the evolutionary design path to the Double Gauss, but that interesting topic is beyond the scope of this thread.

edit: now that I'm home I looked back through my notes and noted that the lenses in box cameras corrected for *tangential* field curvature (not sagittal...my memory is not as good as it used to be). In addition, spherical aberration is slightly better corrected with a lens behind the stop.

 

[1L6E6VHF]

One other method (If the camera should have a "B" setting):

Leave the shutter open, then stretch a piece of wax paper across the film plane. What you see is what you will get.

I don't know how much of my life I've wasted doing this, LOL.

 

[james orme]

Does anybody know if there is a manufacturer who makes new meniscus lenses? Many thanks (I have more questions...)

 

[Denverdad]

I see that Edmund Optics has some inexpensive "experimental grade" meniscus lenses for cheap right now.