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five bladed apertures?
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PostPosted: Mon Jun 18, 2018 2:52 am    Post subject: five bladed apertures? Reply with quote
At some point in the dim past I remember reading or being told that pentagonal apertures produced sharper or higher contrast images especially in wide angle lenses, when compared to iris assemblies with more blades. Has anyone else ever heard this bit of (possibly apocryphal) lore and if so can you recall the source?

All I can find on the subject by Googling is the oft repeated conventional wisdom that bokeh is better with round apertures, so pentagonal apertures are the Great Satan of the anti-bokeh Apocalypse Wink (if that was true then all of my Bronica ETR lenses would have crap bokeh, which IMO, couldn't be farther from the truth).


PostPosted: Mon Jun 18, 2018 4:23 am    Post subject: Reply with quote
Face it, there are a lot of things that determine bokeh -- good, bad, or indifferent -- and the iris blades are just one aspect of the whole.

Regarding the five-bladed hypothesis, I've never heard that before. I've always thought of lenses with five blades as being on the cheap side. One thing is true about a five-bladed iris, however -- point light sources will have a five-pointed star pattern. Perhaps that characteristic is what people liked who spread this bit of unproven wisdom.


PostPosted: Mon Jun 18, 2018 7:38 am    Post subject: Reply with quote
Bokeh is dependant on a lot of things, the aperture shape, f stop, FL, focus distance, distance to background, how busy the background is.....
If the lens has straight blades and the subject has straight lines, it can enhance the perception of sharpness, it's just that the blades have to align with the subject for that to happen, for example, there are a few lenses with square apertures, if you align the sides of the aperture with horizontal and vertical lines, they seem enhanced, this is essentially noticeable in the bokeh with H & V lines.


PostPosted: Mon Jun 18, 2018 7:59 am    Post subject: Re: five bladed apertures? Reply with quote
awa54 wrote:
At some point in the dim past I remember reading or being told that pentagonal apertures produced sharper or higher contrast images especially in wide angle lenses, when compared to iris assemblies with more blades. Has anyone else ever heard this bit of (possibly apocryphal) lore and if so can you recall the source?


The only vaguely appropriate reference to 5-bladed apertures that I can recall is that with less mechanism in the aperture there's less chance of the aperture blades not having closed properly when the shutter fires, therefore less chance of mechanically induced vignetting.

I suppose there's chance of diffraction issues at small aperture openings, but without two optically identical lenses with significantly different aperture mechanisms to test, it'd be difficult to prove.

Historically, some "old favourites", the Tessar or Helios-44, for example, have been available with a different number of aperture blades. Unfortunately, over the years, the coatings, and possibly the exact nature of the optical glass, will probably have changed, so a side-by side comparison might not be valid Smile


PostPosted: Mon Jun 18, 2018 8:49 am    Post subject: Re: five bladed apertures? Reply with quote
awa54 wrote:
At some point in the dim past I remember reading or being told that pentagonal apertures produced sharper or higher contrast images especially in wide angle lenses, when compared to iris assemblies with more blades. Has anyone else ever heard this bit of (possibly apocryphal) lore and if so can you recall the source?

All I can find on the subject by Googling is the oft repeated conventional wisdom that bokeh is better with round apertures, so pentagonal apertures are the Great Satan of the anti-bokeh Apocalypse Wink (if that was true then all of my Bronica ETR lenses would have crap bokeh, which IMO, couldn't be farther from the truth).


Repro lenses had square apertures at times, to produce sharper edges used only for the reporduction of printed documents and technical drawings, I was told by lens makers many years ago. Those still can be found at times. I have never heard such for wide angle lenses, as teh produced bokeh is rather harsh...which was the intended purpose to get that edge enhancing effect.

HERE is some scientific work done for that:
SQUARE Aperture: http://www.iue.tuwien.ac.at/phd/minixhofer/node60.html
ROUND Aperture: http://www.iue.tuwien.ac.at/phd/minixhofer/node59.html

And here is some interesting read also, including lens flkare computation:
https://gamedev.autodesk.com/blogs/1/post/8316683159472521589


PostPosted: Mon Jun 18, 2018 3:33 pm    Post subject: Re: five bladed apertures? Reply with quote
kds315* wrote:

Repro lenses had square apertures at times, to produce sharper edges used only for the reporduction of printed documents and technical drawings, I was told by lens makers many years ago. Those still can be found at times. I have never heard such for wide angle lenses, as teh produced bokeh is rather harsh...which was the intended purpose to get that edge enhancing effect.

HERE is some scientific work done for that:
SQUARE Aperture: http://www.iue.tuwien.ac.at/phd/minixhofer/node60.html
ROUND Aperture: http://www.iue.tuwien.ac.at/phd/minixhofer/node59.html

And here is some interesting read also, including lens flkare computation:
https://gamedev.autodesk.com/blogs/1/post/8316683159472521589


I won't pretend to understand the maths involved in either of these articles, but they certainly seem to touch on the subject and are fascinating in their own right. I wonder if the square aperture might prove useful if accurately aligned with a digital sensor (and its gridded pixel layout), by encouraging more defined contrast divisions along strictly horizontal and vertical lines? I can also envision that combination going horribly wrong...

Perhaps the gist of the (mis?)information I had heard was that by having fewer and larger lengths of straight aperture blade in the light path, lines that are in sympathetic alignment to those might receive a sort of sharpening from OOF artifacts that echo the influence of that blade edge?


PostPosted: Thu Jun 21, 2018 7:59 pm    Post subject: Reply with quote
cooltouch wrote:
One thing is true about a five-bladed iris, however -- point light sources will have a five-pointed star pattern. Perhaps that characteristic is what people liked who spread this bit of unproven wisdom.


No. It will produce a 10 pointed star.
Every blade produces light beams going to the left and right of the center. Therefore an uneven number of them will produce double the amount of rays/beams, while in a lens with even number of blades the number of beams is identical with the number of the blades as opposite situated blades erase half the beam produced by the blade sitting face to it.


PostPosted: Thu Jun 21, 2018 8:37 pm    Post subject: Re: five bladed apertures? Reply with quote
kds315* wrote:
awa54 wrote:
At some point in the dim past I remember reading or being told that pentagonal apertures produced sharper or higher contrast images especially in wide angle lenses, when compared to iris assemblies with more blades. Has anyone else ever heard this bit of (possibly apocryphal) lore and if so can you recall the source?

All I can find on the subject by Googling is the oft repeated conventional wisdom that bokeh is better with round apertures, so pentagonal apertures are the Great Satan of the anti-bokeh Apocalypse Wink (if that was true then all of my Bronica ETR lenses would have crap bokeh, which IMO, couldn't be farther from the truth).


Repro lenses had square apertures at times, to produce sharper edges used only for the reporduction of printed documents and technical drawings, I was told by lens makers many years ago. Those still can be found at times. I have never heard such for wide angle lenses, as teh produced bokeh is rather harsh...which was the intended purpose to get that edge enhancing effect.

HERE is some scientific work done for that:
SQUARE Aperture: http://www.iue.tuwien.ac.at/phd/minixhofer/node60.html
ROUND Aperture: http://www.iue.tuwien.ac.at/phd/minixhofer/node59.html

And here is some interesting read also, including lens flkare computation:
https://gamedev.autodesk.com/blogs/1/post/8316683159472521589


Perhaps there is some validity to it then.
A 5 blade aperture would have 5 different directions covered by the blades & creating this pseudo sharpening effect. While a four blade aperture has 2, a six blade has 3 & an eight blade has 4.
Of course 7, or 9 blades would be better still, but with these there may be the effects of friction starting to show - which is I beleive why the number of aperture blades tended to reduce with the advent of auto-shutdown SLRs.


PostPosted: Fri Jun 22, 2018 12:07 am    Post subject: Re: five bladed apertures? Reply with quote
[quote="DConvert"]
kds315* wrote:

Of course 7, or 9 blades would be better still, but with these there may be the effects of friction starting to show - which is I beleive why the number of aperture blades tended to reduce with the advent of auto-shutdown SLRs.


You may be right about this, where a 9-bladed iris might be on the ragged edge of practicability with an auto-opening (and closing) lens diaphragm. But I would like to point out one well known lens that does have 9 blades and appears to be no worse the wear for them: the Canon FD 85mm f/1.2 SSC Aspherical. This lens is the granddaddy of Canon's 85/1.2 line -- later lenses were the "L" models. Now, I dunno if the Ls have 8 or 9 blades, but my 85/1.2 Asph definitely has 9, and it's experienced no jam-ups in the seven or eight years I've owned it.

Also, I'd like to correct a statement I made in my earlier post here. I stated that a five-bladed iris will yield a 5-pointed star pattern. when photographing a point light source. Apparently this is incorrect. These -- I dunno what you call them -- "diffraction points," maybe? -- that you see when a point light source is displayed are double the number of iris blades. I'm not sure exactly why this is, but if I had to hazard a guess, the doubling has to do with the angle where the blades come together as one point source, and then the flat of another blade 180 degrees from and perpendicular to this angle, thus creating a reflection of this point source. I dunno, whatever.

Last edited by cooltouch on Fri Jun 22, 2018 5:18 am; edited 1 time in total


PostPosted: Fri Jun 22, 2018 3:23 am    Post subject: Reply with quote
Diffraction spikes, some call them diffraction stars.
https://en.wikipedia.org/wiki/Diffraction_spike


PostPosted: Thu Jun 28, 2018 1:30 pm    Post subject: Re: five bladed apertures? Reply with quote
kds315* wrote:
awa54 wrote:
At some point in the dim past I remember reading or being told that pentagonal apertures produced sharper or higher contrast images especially in wide angle lenses, when compared to iris assemblies with more blades. Has anyone else ever heard this bit of (possibly apocryphal) lore and if so can you recall the source?

All I can find on the subject by Googling is the oft repeated conventional wisdom that bokeh is better with round apertures, so pentagonal apertures are the Great Satan of the anti-bokeh Apocalypse Wink (if that was true then all of my Bronica ETR lenses would have crap bokeh, which IMO, couldn't be farther from the truth).


Repro lenses had square apertures at times, to produce sharper edges used only for the reporduction of printed documents and technical drawings, I was told by lens makers many years ago. Those still can be found at times. I have never heard such for wide angle lenses, as teh produced bokeh is rather harsh...which was the intended purpose to get that edge enhancing effect.

HERE is some scientific work done for that:
SQUARE Aperture: http://www.iue.tuwien.ac.at/phd/minixhofer/node60.html
ROUND Aperture: http://www.iue.tuwien.ac.at/phd/minixhofer/node59.html

And here is some interesting read also, including lens flkare computation:
https://gamedev.autodesk.com/blogs/1/post/8316683159472521589


In the graphic industry square apertures and more odd shaped apertures were used in combination with lined, cross lined and rhomboid glass screens to create halftone films for further steps in reproduction printing of paintings, drawings, illustrations, photo's. The main aim of these aperture shapes was to control the tone values. Often two aperture shapes/sizes were used after another for a single film. To quote the start of a chapter on this subject in the "Ilford manual of process work" 1926 ; "Special Diaphragms - A considerable number of diaphragm shapes have been successively proposed for use with the cross-lined screen , some of them unnecessarily complicated."
There was quite a complicated set up for the glass screen at some distance from the film to be exposed, the magnification factor and the choice of aperture opening + the exposure time. Tests have shown that with ideal settings a single aperture + aperture could achieve best results but in practice two exposures were more often done and special diaphragms seen as necessary.

This all disappeared with the introduction of the contact screens, Policrome etc, that replaced the complex optical dot formation described above by a pattern of soft dots on a film regulating the dot formation in the film by filtering. The sheets themselves probably made with old process step. Used either negative or positive depending on the printing technology afterwards.

That process gone when image setters and appropriate software replaced the analogue reprography.

Ernst Dinkla


PostPosted: Thu Jun 28, 2018 2:25 pm    Post subject: Reply with quote
That's very interesting, I was trained in photo offset printing and production darkroom processes back in the 1980s... we didn't learn any history of what came before the darkroom techniques we used (although there was a bit of education about cast and moveable type), so all of that information is new to me.

Our class/lab darkroom was quite basic, so the finer points of color separation weren't covered except in our reading material, though we did print the occasional "newspaper" quality photo by sending originals out to another darkroom that could do four color separations.


PostPosted: Fri Jun 29, 2018 6:16 am    Post subject: Reply with quote
Google scanned that book, you might be able to find the specific Pages. Roughly near page 113.
I have an original one.
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