Rectangular stops in reproduction lenses? purpose?

[walter23]

I've seen a couple of lenses on the auction site that have rectangular stops kind of like waterhouse stops in addition to a circular iris. What is the purpose of these stops? Boosting constrast by blocking internal flare or something?

Most recently, this Industar lens:
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=7558688868

[Jim Jones]

A non-circular aperture can give a more uniform distribution of tones when generating half-tone negatives with some half-tone screens from photographs. The effect of non-circular apertures on bokeh might be an interesting project for some energetic young photographer to research.

[walter23]

Jim, that latter suggestion has been done:
http://www.lensbabies.com/index.php?page=lcak/galleries

The effect is to change the shape of the out of focus highlights, in addition to other more subtle things I suppose.

[KOG]

I believe that photographer Karl Struss applied for a U.S. Patent for square apetures back in the early 1900's. He was a pictorialist, so I think his goal was to increase some sort of pictorialist effect.

Kevin.

[Alan Davenport]

That might be trick for night photography. The square aperture would produce perfect four-pointed stars on bright lights...

[Ash]

That might be trick for night photography. The square aperture would produce perfect four-pointed stars on bright lights...

That kinda thing has been done for a long time via the various Cokin filters available.

[David A. Goldfarb]

In this case, with a process lens, the halftone screen answer is the right one, though. I've seen them with diamond shaped apertures for this purpose

Maybe someone in the printing industry can fill in the details.

[Mick Fagan]

Moiré effect often happens when you are photo copying a halftone picture at a different screen ruling. Mostly this would happen when you copied a 150 LPI four colour halftone, through a 62 LPI halftone screen for a newspaper type of reproduction.

The different colours in the original halftone picture, say out of a glossy magazine, are all running at different angles, one colour will be at 45° whilst another is at 90° and so on. A moiré pattern will often emerge when this type of photo copying is carried out for a one colour reproduction, with a coarser screen ruling.

One of the official ways a process camera operator was supposed to reduce or eliminate this effect was to use square stops. Well the unofficial way to do this was to wave a piece of glass in front of the camera lens for about 90% of the exposure, which was often something like 15 to 40 seconds. Every camera operator I knew of used the unofficial way.

There were some other reasons for the square stops but I'm unaware of them. I have a complete set of square stops which run in ½ stops from f22 through to f256 for a 460mm f16 process lens.

Mick.