Single Element Meniscus Lens - Aperture Location?

[JoeV]

Folks;

I putting together an 8"x10" nested box camera that uses a single-element meniscus lens, salvaged from an industrial photo-lithography machine. The lens is convex on one side and slightly concave on the other, about 4" in diameter, with a focal length around 10.75".

I've initially mounted it in the camera with the concave side facing the subject, and have initially used a 3mm aperture, located about 1" in front of the lens, in order to get a wide DOF, as a kind of "hyperfocal" setup, requiring minimal focusing for landscape images.

http://farm5.static.flickr.com/4051/4423318639_0275fa4425_o.jpg
http://farm5.static.flickr.com/4060/4423318675_54ee72c590_o.jpg

The initial test photos, subject matter not exactly thrilling (of my desolate winter-time backyard) indicate that the lens is pretty sharp, corner-to-corner, at F90, but I'd like to improve its performance if I could.

http://farm5.static.flickr.com/4040/4423778886_e0be317221_o.jpg

Is there an optimal location for the front aperture? Like the front nodal point? And how would I, in actual practice, go about testing for this point?

I am also planning on using this lens wider open, for fuzzy-edged portrait kinds of images (notice how I avoided the use of the term "bokeh" ... oops).

Thanks for your help.

~Joe

PS: The test image was exposed onto a preflashed grade 2 paper negative, hence the washed out sky.

[GPS]

What does the "convex on one side and slightly concave on the other" lens show hanging on the tree? A series of bird feeders??

[JoeV]

Several wind chimes. Exposure time, on this cloudy afternoon, was about 30 seconds. You can see the pendant of the chime is blurred by motion.

~Joe

[GPS]

I see... thanks! Not exactly fuzzy edged though... ;-)

[Steven Tribe]

The old landscape/meniscus lenses were apertured by waterhouse or washer stops.
Looking at those I have, the postioning is between 55mm for a 12 inch lens (Swift), 65mm for an 18" (Perken Rayment) and 75mm for a 24" (Port-Land). I would have thought washer stops were easier to make. So around 45-50mm perhaps for 10.75inches?
As I am in middle of garden pruning, I couldn't help but notice that the lower angled branch needs to be sawn through as you will soon have a forked tree!

[JoeV]

Thanks for the feedback. Since I have several handmade waterhouse stops already made, I'll have to experiment with various positions. I'll try the recommendation of 45-50mm in front of the lens. IIRC, at the correct position it should reduce spherical aberration; but it would need multiple elements to correct for other effects like coma, etc. I'm not looking for perfection, just to see how good of an image I can make from a single element lens.

Regarding the tree, we've let it grow way too big before deciding to prune the branches; last summer it caught some kind of blight, an arborist was called in, who injected some minerals and whatnot** around the trunk. We're hoping it recovers this spring.

~Joe

**"whatnot": another term for money; at the price he charged, he must have directly injected our cash, straight away into the roots.

[Steven Tribe]

You have read the discussions on the use of wide open meniscus lens for "soft" effects here? Apart from my "regular" landscapes I have a couple of project lens underway with simarly mounted landscape.

I don't want to depress you about the the tree - this kind of 30 degree fork is a real opportunity for fungus development (entry of rainwater and spores). Foliage discolouration and loss usually means fungal damage to the water capilliary layer just under the bark.

[russyoung]

. I'll try the recommendation of 45-50mm in front of the lens. IIRC, at the correct position it should reduce spherical aberration; but it would need multiple elements to correct for other effects like coma, etc. I'm not looking for perfection, just to see how good of an image I can make from a single element lens.

My notes are not at hand (packed from a move) but my sometimes foggy memory recalls:
1. stop position is a trade-off of field flattening and distortion (pincushion vs barrel) minimization
2. theoretical best placement for distortion is at the nodal point
3. Thomas Dallmeyer wrote that a rule of thumb was to place the aperture 1/5 of the focal length in front of the lens
4. position of the stop has no effect on spherical aberration; only the f/stop number. You should find that about f/8 will bring SA under decent control, f/16 it is almost nonexistent.

My experiments with single meniscus lenses of 250-320 mm seemed to indicate they could perform quite well over a 35 degree field at f/8-f/16 with high contrast even if uncoated. Cheap achromatic menisci are remarkable performers by f/11.

Russ Young