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

David Lyga

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I will ask this because, at least in the past, manufacturers have spent good money in getting this done:

Is there ANY advantage (aesthetic or otherwise) with making an enlarging lens with a stopped down aperture which is nearly circular? Bokeh is of no concern here for obvious reasons. More blades = more manufacturing expense. - David Lyga

EDIT: My question is NOT asking how far one should stop down, but, rather, it is this: Is the SHAPE of the stopped down aperture (circular or otherwise) a factor in print quality?
 
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Printing with your lens wide open isn't making most of the sharpness of your enlarger lens. Also stopping all the way down will cause diffraction. There's no bokeh because you're just projecting a 2 dimensional object (your negative).
 
Concerning control of lens abberations one could emoploy a square or star-shaped opening. Here the adding of "worse"rays from toutter part of the the lens to some extent woud be counterd by adding "better" rays of the inner part.

All my enlarger lenses have rather few blades. Which of course might be an issue when using such lenses as taking lenses of 3-D objects.
 
David,
Are you asking if an enlarging lens with a circular aperture will perform better than an enlarger lens with a more irregularly shaped aperture?
I don't know, but it might.
Particularly in a system that uses diffused light sources.
Apertures affect flare, and the flare from a system that employs non-circular apertures is likely to be different than from a system that employs circular apertures.
As to the magnitude of that difference, I have no idea.
 
How much of an impact could you expect diffraction spikes to have on an enlarger?

In the grand scheme of things, unless you're down at the rather low end of a lens market, I can't really see aperture blades playing a huge role in over all costs between having a few and having a bunch, so it wouldn't surprise me at all to find some designs throwing 'a few extras' in the mix simply as a placebo to convince buyers that an otherwise similar lens is 'totally far better'.
 
Mainecoonmaniac said:
Printing with your lens wide open isn't making most of the sharpness of your enlarger lens. Also stopping all the way down will cause diffraction. There's no bokeh because you're just projecting a 2 dimensional object (your negative).
Click to expand...
No, my question was a bit misinterpreted. I am not asking how far to stop down. I already know the answer to that. I am talking about the construction of the aperture. Is an aperture with MANY blades, enough to give a near circle as it is stopped down, of any benefit? As Agx rightly says, today's enlarging lenses have only a few blades.

Matt, yes, this is what I am asking. I guess that if this is a 'flare' issue, is anyone able to quantify that effect? - David Lyga
 
Don't forget that enlarging lenses shared manufacturing lines with lenses for process cameras and automated colour printers and other commercial and technical lenses. If circular apertures benefited them, it would probably have been cheaper to just use the high end aperture mechanisms in the cheaper lenses than it would be to design and manufacture different apertures for cheaper lenses.
 
I had never thought of that placebo effect. You just might be correct here. Let us see what others have to say. - David Lyga
 
Another important factor which I failed to take account of. Yes, macro work could well involve bokeh. - David Lyga
 
Should it have more or less than the lens that was used to get the image on the negative?
 
Modified for Monochrome? And why? Seemingly as "placebo" as indicated above.
 
Vaughn said:
Should it have more or less than the lens that was used to get the image on the negative?
Click to expand...
Well, if used strictly for enlarging, I don't see why it has to have the 'same' as for taking the original picture. - David Lyga
 
Given that your neg and paper should be better than reasonably flat field for enlargements, if everything is aligned correctly and you are using a flat-field enlarging lens there should be no out of focus areas, therefore the aperture shape should be utterly immaterial. What does matter is the evenness of exposure between the marked aperture stops.

Salthill apparently had a set of Schneider Apo Componon HM lenses made for their enlarger with fixed apertures for each lens' optimal f-stop.
 
This is what I had thought ... but only as pertains to enlarging and flat fields. Others have inferred macro work and other uses which do NOT involve wholly flat fields. This topic is getting more attention than I would have imagined. - David Lyga
 
Lachlan Young said:
Salthill apparently had a set of Schneider Apo Componon HM lenses made for their enlarger with fixed apertures for each lens' optimal f-stop.
Click to expand...

Not practical at all for general enlarging work. A more pronounced click-stop at that very aperture would be practical instead
 
Aperture shape can play a big role in screen printing. The aperture shape affects the shape of the dots in the screen. If you want round dots, you need a round aperture.

I think the above is the reason some older enlarging lenses wound up in barrels with round apertures. That would make the lens suitable for creating screens for screen printing.
 
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David Lyga said:
This is what I had thought ... but only as pertains to enlarging and flat fields. Others have inferred macro work and other uses which do NOT involve wholly flat fields. This topic is getting more attention than I would have imagined. - David Lyga
Click to expand...

It's because people insist on using optics outside their intended purpose & then complaining that the results are weird!

AgX said:
Not practical at all for general enlarging work. A more pronounced click-stop at that very aperture would be practical instead
Click to expand...

It makes considerable sense, if you have several stops of ND available & are printing colour/ want consistent sharpness & exposure times across a range of print sizes.
 
Lachlan, I don't get your point.
At first you had it about the optimum aperture for a lens. That is ONE aperture. Here my clickstop marking of this one apeture is more versatile than substituting an iris for a one-stop aperture.

Concerning optimum apertures for different image scales is something different. But whether exchangable stops are more practical than an iris, I doubt.
There were repro cameras that had a lever attached to their Rodenstock lenses. These lever offered at a scale much more precise readings and thus settings. This would be an alternative to using several one-stop apertures.
 
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Lachlan Young said:
...
Salthill apparently had a set of Schneider Apo Componon HM lenses made for their enlarger with fixed apertures for each lens' optimal f-stop.
Click to expand...

Ahh, those should be the ones sold by Monochrom. They were a Salthill dealer that time.

Thanks, Lachlan.

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

@AgX Imagine your enlarging lens is as perfectly sharp as possible at f5.6 across its enlargement range, as you stop down further, diffraction starts to kick in - so you want to be able to use this absolutely optimal aperture across as wide a range of print sizes as possible, without needing to adjust the aperture - that's where a fixed aperture & ND comes in.
 
Thank you, I completely misunderstood your idea.
 
David Lyga said:
Well, if used strictly for enlarging, I don't see why it has to have the 'same' as for taking the original picture. - David Lyga
Click to expand...
Just being a bother...but I would think that if a perfectly round aperture was critical for sharpness, high quality lenses would probably still come with a slot for waterhouse stops.
 
And, might I ask, where does focusing the image enter into the equation? At that difficult, less accurate stopped down level? - David Lyga
 
Vaughn said:
Just being a bother...but I would think that if a perfectly round aperture was critical for sharpness, high quality lenses would probably still come with a slot for waterhouse stops.
Click to expand...
THAT is the whole purpose of this thread. The fact that they DO NOT speaks volumes. - David Lyga
 
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