Perhaps an 'illegal' question from someone who has been in the darkroom since 1964:

MattKing said:

Finally, with some enlargers and lenses and focus aids and magnifications, when the lens is wide open, the image can be very bright. It may be more comfortable to work with less brightness.

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Thanks a good point, I have to focus my enlarger when the lens is wide open with the room lights on or it will totally blind me lol

Lachlan Young said:

@Jens Hallfeldt do you happen to know what apertures they were fixed at?

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Hi Lachlan,

sorry,
I've searched twice, but only found cataloges from the late 1990s where Salthill was no longer listed at german MonoC...

Best wishes
Jens

DREW WILEY said:

There was one model of the Apo Nikkor 369/9 which had both an adjustable square aperture, plus a multi-bladed circular aperture, while also having a slot potentially accepting Waterhouse apertures.

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The square apertures in process lenses, or even stranger shapes, may be used to correct fall-off of the lens in the corners to some extent. It would not work if we were to print a circular format, but it helps for squared/rectangular formats.

The square also promotes "Joining the Dots" in half tone process, this is the main purpose, which may be specially interesting for oriental characters (Hanzi, Kanji...)

A nice year 1923 reference:

http://www.survivorlibrary.com/libr..._practical_manual_of_photo-engraving_1923.pdf



The Penrose system allows to rotate the square to provide an squared blur in the desired direction, to match the rulling of the screen, without having to rotate the lens. This "iris" has two blades... In that way a squared blur can be generated. The effect is that a screen made of dots (geometrically arranged ) provides a nicer feel in the result.




This is my LOMO O-2 600, it has a very circular iris, and also a set of different square apertures and a set of equal circular apertures. Perhaps this is also intended to glue on them gelatin filters (or other kind) to cancel CA or to make separations, if not going to place filters on the circular apertures then the repeated circular apertures would not make sense, while also having an iris, a very circular one with a lot of blades.



(Under the LOMO an ATMEL ICE is seen, this is for example to debug the Marlin firmware controlling 3D printers. The ICE survived )

The various other aperture shapes supplied as Waterhouse stops with the lenses intended for halftone/ halftone separation work have nothing to do with fall-off correction. Instead they are there to produce specific dot shapes demanded by various printing processes. The slot in the lens has screen angles marked so you can shoot the correct angles for making CMYK separations using traditional halftone ('AM screening') - as opposed to stochastic ('FM screening') screens. See pg. 12 in this Getty document for a clear illustration.

The narrow angle of view of many of the traditional dialyte 'Apo' lenses is not just demarcating their field of apochromatic correction, but also their area of flattest possible field and zero (or near zero) fall-off and distortion. Any distortion, field curvature or fall-off are very bad news when you are trying to make an absolutely even halftone dot from edge to edge on your film - and in the bigger picture of pre-press, absolute sharpness mattered less than freedom from those aberrations.

Jens Hallfeldt said:

Hi Lachlan,

sorry,
I've searched twice, but only found cataloges from the late 1990s where Salthill was no longer listed at german MonoC...

Best wishes
Jens

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Cheers for looking - my own suspicion is that they were set to f/8.0, but I'd like to see if someone has official confirmation.

Regarding widest aperture open enlarging lens focussing, you simply cannot make blanket generic claims. Most modern enlarging lenses are free of focus shift, but not all. You have to check for this, especially with cheaper lenses having relatively wide max apertures. Focussing wide open allows brighter, more accurate viewing when fine tuning the focus. But that's all contingent on your neg being perfect flat and parallel to begin with. If the light is simply too bright for comfortable wide-open focus, simply dial in equal amounts of CMY on the head temporarily to provide neutral density, or simply use an ND filter if you've got one of those ancient beehive enlarger heads complete with bees flying in and out of it. My brother used on of those, presumably stolen from a hair salon.

Now back to that Waterhouse stop process lens etc topic. Such lenses were engineered for a specific angle of view determined by both recommended magnification range during copy work, and with respect to recommended aperture when doing this same task (generally f/22). But neither spec necessarily applies to different kinds of applications these lenses might also prove useful for, including enlarging and large format photography. Your angle of view will substantially increase at those smaller apertures likely to be used for 8x10 and larger film formats, while on an enlarger, apertures significantly wider than f/22 might prove optimal. You have to take it on a case by case basis.