Meniscus lens iris "sweet spot"???

[Mark Sawyer]

Just a note that aperture position is one of the "design freedoms". There's some choice involved because different positions are the "sweet spot" for minimizing different aberrations. The sweet spot for minimizing coma may not be the sweet spot for flattening the field curvature, and neither may be right for minimizing spherical aberration, (although users of meniscus lenses usually want that aberration). And the farther out you move the aperture, the less coverage you have, which may also affect the decision if you're into wider views.

Anyways, your sweet spot may not be someone else's sweet spot.

[LabRat]

I'm the guy that wrote the web page on the Wollaston Landscape Lens someone gave a link to above. https://sites.google.com/view/wollas...cape-lens/home

Would you tell me a little more about the lens you are using and where you got it? Diameter, focal length, shape, image size, etc. Where did you end up putting the stop? Thanks.

Thanks Michael, great article!!!

I will PM or email you soon to help clarify a couple of points I'm missing, but very useful!!!

My new lenses are assembled, but need lensboards, aperture markings/pointers, and testing... Quick tests have what I wanted; soft wide open, turns sharper mid aperture, and sharp stopped down... My first attempt used a 1.125" dia (-2) and it tended to be sharpish, but newer ones are 2 " dia (-4), and soften wide open nicely... (all lenses are surplus as - found scrap optics)... I have some effects in mind, and I'll see if I can post or link...

Watch your mailbox!!!

Thanks,

Steve K ( LabRat )

[plywood]

Fascinating discussion, and very interesting link about using lenses from old Brownie cameras. As it happens, the pandemic stay at home time has had me mounting oddball lenses on homebuilt cardboard and plywood 4X5 cameras just to see what they will cover. I'm also using photo paper as negatives. So far this assortment has included a plastic triplet from a Polaroid Color Pack, about 100mm in focal length, a couple of 130mm f7.7 Kodak lenses from broken folders. A 110mm plastic meniscus lens from a failed Kickstarter. (Lost $50 on that piece of junk, lesson learned.) And a 100mm f8.8 triplet from a Kodak Tourist. This last will cover a diagonal of 135mm at f32.
Except for the lens recovered from the KS project the others were all donations, gifted by friends who know I like to mess around with old cameras.
I've only tested the lens from the Tourist 6X9 but by the end of next week hope to have tried out the other combos.
Cheap fun.

[Nodda Duma]

Just a note that aperture position is one of the "design freedoms". There's some choice involved because different positions are the "sweet spot" for minimizing different aberrations. The sweet spot for minimizing coma may not be the sweet spot for flattening the field curvature, and neither may be right for minimizing spherical aberration, (although users of meniscus lenses usually want that aberration). And the farther out you move the aperture, the less coverage you have, which may also affect the decision if you're into wider views.

Anyways, your sweet spot may not be someone else's sweet spot.

Distance to the stop doesn't correct field curvature, only coma.

(Field curvature / astigmatism doesn't change with distance to stop.. what changes is where your flat image plane lands relative to the tangential and sagittal fields. Hence my comment above about "balancing").

[Mark Sawyer]

Distance to the stop doesn't correct field curvature, only coma.

(Field curvature / astigmatism doesn't change with distance to stop.. what changes is where your flat image plane lands relative to the tangential and sagittal fields. Hence my comment above about "balancing").

As you change the position of the stop, you change where rays coming from off the optical axis though the aperture strike the lens. In a lens with significant spherical aberration (like a meniscus), that different area of the lens will have a different focal length, hence field curvature. The smaller aperture may "sharpen" the spherical aberration, but it does so on a different focal plane.

[Nodda Duma]

As you change the position of the stop, you change where rays coming from off the optical axis though the aperture strike the lens. In a lens with significant spherical aberration (like a meniscus), that different area of the lens will have a different focal length, hence field curvature. The smaller aperture may "sharpen" the spherical aberration, but it does so on a different focal plane.

You're talking about Coma, an off-axis aberration. The aberration illustrated in that link is coma as well.

Spherical aberration is an on-axis aberration, due to the lens not being the ideal (aspheric) shape to bend rays at different pupil heights to the same on-axis distance. It is independent of coma, which arises from a variation in the magnification over the entrance pupil (what you are describing).

You eliminate coma by adjusting distance between lens and stop. You reduce spherical aberration by adjusting the lens shape.... the ideal shape for minimizing spherical aberration in a simple lens is a meniscus. You can also eliminate spherical aberration by using an aspheric lens.

Field curvature arises from the fact that a simple lens has the *same* focal length at different ray angles. Modern lenses correct field curvature by varying the focal length vs. ray angle. The landscape lens reduces field curvature (and lateral color) by reducing the aperture stop. Glass-type and thickness are used to adjust field curvature so that the sagittall field lies flat (as implemented in the kodak brownie) while allowing the lens shape to reduce spherical aberration.

Chromatic aberration is not corrected at all in the landscape lens, except via reducing the aperture stop diameter.

I actually wrote a white paper on the landscape lens quite some time ago, as part of some coursework I went back to school for several years into a multi-decade career in lens design. The course work was offered through a school in your town.

[Mark Sawyer]

You're talking about Coma, an off-axis aberration. The aberration illustrated in that link is coma as well.

If I was talking about coma, I would have said coma, as I did when talking about coma earlier. I didn't post a link.

the ideal shape for minimizing spherical aberration in a simple lens is a meniscus.

Yes, and you can also increase spherical aberration by altering the meniscus' lens shape, which is done in the vast majority of soft focus lenses. The main attraction to soft focus meniscus lenses is their pronounced spherical aberration, which is how most soft focus lenses achieve their effect. Spherical aberration is the whole point behind meniscus landscape lenses like the Imagon, Kodak Portrait Lens, Cook RVP, Spencer Port-Land, converted Verito, etc. etc.

Chromatic aberration is not corrected at all in the landscape lens, except via reducing the aperture stop diameter.

Chromatic aberration is corrected to a pretty high degree in meniscus achromatic doublet landscape lenses. That's why they're call "achromatic" doublets. Like the Gundlach Achromatic Meniscus Portrait Lens and the Cooke Achromatic Portrait Lens, and all the landscape lenses I mentioned above. Single landscape lenses like the Wollaston are not corrected for chromatic aberration, but that's a whole different issue with no bearing here.

Anyways, with spherical aberration being pronounced in a Wollaston lens, the outside of a 10-inch Wollaston lens focuses the image at a closer distance (let's say 9 inches) than the center, (10 inches). Moving the aperture farther forward moves the image-forming light coming from the side farther to the side of the lens, shortening that focal distance, thus curving the field even more. Putting the aperture close to the lens (imagine it being in contact with the front of the lens) means light from both the side and center of the field of view pass only through the center of the lens. Still a curved field with the light being at a 10 inch radius, but not as curved as a parabola with the center focused at 10 inches and the periphery at 9 inches.

[Gasperi]

Due to the reactions above, thought I should add a little more analysis to the web page. I think this link will take you to just the new part: https://sites.google.com/view/wollas...h.khu6tew41obf . Otherwise it is the "Where does this equation come from?" section.

At least the way I'm doing it, the position of the stop is ONLY driven by trying to flatten the field. If you happen to know the radius of the convex side of the lens (R2) then the stop is simply R2/n. The distance is measured from the surface of the convex side NOT the edge like you might expect. My equation tries to work from lens values you have and account for the lens thickness so it is measured from the edge.