Opposite of close-up filter?

[Robin Harrison]

Close-up filters are just +diopter lenses, right?

Are the opposite readily available?

I have a c-mount 50mm f0.95 lens that I cannot mount close enough on my G1 to achieve infinity focus. Could I in theory just mount a focus altering filter to correct that? It wouldn't have to be accuracte given the EVF manual focus of the G1.

 

[Seele]

I often have to say that "close-up filters" are not filters at all, but supplementary lenses which are positive in power. When attached to a lens, a close-up supplementary lens shortens the effective focal length, but as the lens extension (distance between lens and film plane) remains the same, it is like having a lens of shorter focal length being held at a greaterthan normal extension, thus decreasing the focussing distance.

Negative supplementary lenses were indeed made: for instance, the positive ones made by Carl Zeiss were called "Proxars" and negative ones, obviously, "Distars". If you attempt this, it would need a bit of calibration to get the positioning of the lens just right, of course.

 

[Sparrow]

Or would reversing the glass in an ordianry close-up “filter” work?

 

[Robin Harrison]

I often have to say that "close-up filters" are not filters at all, but supplementary lenses which are positive in power. When attached to a lens, a close-up supplementary lens shortens the effective focal length, but as the lens extension (distance between lens and film plane) remains the same, it is like having a lens of shorter focal length being held at a greaterthan normal extension, thus decreasing the focussing distance.

Negative supplementary lenses were indeed made: for instance, the positive ones made by Carl Zeiss were called "Proxars" and negative ones, obviously, "Distars". If you attempt this, it would need a bit of calibration to get the positioning of the lens just right, of course.

Interesting. Thanks for the insight.

I presume that these Distars are quite rare? I think the filter thread is around 62mm from memory.

In terms of calibration and positioning, presumably this can be calculated? Isn't there a relationship between the strength of the positive suplementary lens and the distance at which a lens will focus at the infinity setting? Presumably the converse exists for negative power lenses? If I find that at the infinity setting the lens is focussing at 1.5m, could I work out the minimum power of lens to enable infinity focus?

 

[Robin Harrison]

Or would reversing the glass in an ordianry close-up “filter” work?

I presume not. Are they not just symetrical convex lenses? Would I need a convex lens to create a negative power element (like the difference between eye glasses for long- and short-sighted folk?)

 

[sevo]

I presume that these Distars are quite rare?

Not really. They weren't as frequent as the Proxars back then, but they usually saw much less use, so a fairly high percentage of them survived. They turn up on ebay frequently and tend to attract few to no bidders, as anybody using cameras with free bellows adjustment nowadays does so for maximum quality, where diopters would usually be detrimental.

As to the original question: A 0.95 lens is a optically rather vulnerable construction - the optical degradation due to any extra element at fully open aperture will be quite radical. Besides, you'd convert it to longer focal length (and in relation to MFT, 50mm is already in the upper range of desirable lengths for portraits) and will correspondingly lose aperture.

Sevo

 

[Seele]

Sparrow,

A positive lens is a positive lens; a magnifying glass (a la Sherlock Holmes) would not make things look smaller if you flip it over.

Robin,

Originally, Distars were made for use with large format cameras where the extension can be varied to a great degree. Add a Distar to the lens, it gives a lengthening of focal length, so by increasing extension you get larger image size. Obviously, with smaller cameras with limited extension becoming mainstream, Distars became quite useless, while Proxars remained (and still remain) useful. While large format lenses at the time tend to be quite small in diameter, the Distars were not made in such large sizes; I have one in my bottom drawer, its push-on mount fits a lens front external diameter of 28.5mm. I am sure bigger ones were made but am very doubtful if one was made to be over 62mm.

The increased extension and strength of the Distar of course are intimately related. If you have a given extension greater than the proper one required by the lens to reach infinity, then you have to get a Distar of exactly the right strength to match; by the sound of it, it might be a very tricky scenario. Even if you can figure out the exact negative power, it might not be available off the shelf.

The design of the Proxars and Distars are not simple bi-convex and bi-concave lenses, but are positive and negative meniscii respectively, something like eye glasses. But the curvatures in both scenarios have to be optimised for a flat image plane: sure enough there're people using eye glasses lenses as Proxars for decades but the image would not be sufficiently satisfactory: eye glasses lenses are made to give a curved image plane in conjunction with the lens in the eye, as the retina at the back of the eyeball is curved, while the film plane is flat.

Sevo,

Judging by Robin's description, the required Distar would indeed be extremely weak in power, and even if an optician is prepared to make such a lens, it would be very expensive and fragile, along with the possible image degradation, I honestly do not believe it is a productive route to pursue.