Holga lens formula/diagram?
I'm looking for some information on the actual lens formula of the plastic lens that comes on a Holga. If anyone knows of a diagram of this, it would be extremely helpful. I'm adapting the lens from a Holga to a Mamiya 645 and need to add an element to essentially make the lens retrofocus so I can clear the mirror box. This would be much easier if I had some info on the formula of the lens to start with. Thanks!
I know nothing about the material, but there's a cross section of the lens here at holgamods.
That's actually all I needed. Awesome - thank you!
I'd suggest experimenting because with a meniscus the critical optical 'position' (node) is going to be hard to determine - it's typically not at the lens, somewhere behind the concave surface.
If I didn't know better (I don't, but it's just an opinion), that funky shape around the perimeter contributes to the cross section a more 'exotic' appearance than a standard meniscus...like an aplanatic meniscus or aspherical...my assumption of 'knowing better' is that both of these would probably be overkill for a lens and camera that is beloved for its lack of quality...or quality control.
That thickened area around the perimeter might not be for optical qualities but to stiffen the lens or provide a predictable mounting surface. If they mask (mechanical vignette) the edge anyway with a smaller aperture than the lens diameter, the central part of the lens is doing the lion's share of the work of producing the image.
A true retrofocus lens has even screwier nodal properties.
As far as experimental direction, adding a negative lens will lengthen the focal length. If I understand your requirements, this will require the lens to be further away from the body so the mirror can operate without hitting the lens.
If you were to add a positive lens, it would shorten the net focal length, placing the lens closer to the mirror's trajectory.
Adding another lens element might alter the existing aberrations and look, but isn't each Holga somewhat different from the next, anyway?