Originally Posted by photobulley
coma is NOT the main contributing factor.
coma is NOT the main contributing factor.
Not too far. I'm over in Kalamazoo, Michigan but I head to the western UP every couple summers. I may yet make it there this year. There's also a get-together of midwest large-format folks the weekend of August 17 in Munising that I may rendezvous with. If I do I'll have the wetplate stuff with me....
Joe, my next question is do you live anywhere near Saint Paul, MN so you could teach me the fine art of collodian?
There were also a couple folks from Minnesota at Bill Schwab's get-together in June in Cross Village near the straits. I had the collodion rig there, did a demo then,and plan to do it again next year.
Joe
Ole, we are going to disagree here -- so tell us what you believe causes the swirlies .coma is NOT the main contributing factor.
FTR, I say the elongation of circular highlights at the outer edges of the IC forming around the center of the lens axis IS the main contributing effect of the SWIRLIES. Coma is derived from the word Comet as it describes the shaping effect on point source and wider oof point source highlights. It's effects increases as you get further from the center of the IC and forms perpendicular to the central axis. I did not say it was the main factor of bokeh, but stand behind my statement that coma is what generates the swirlie effect in oof highlights and speculars.
Furthermore, I say it is elusive because it requires oof specular highlights surrounding a significant portion of the lens' optical axis for the effects to be seen...
Jack, I know that petzvals and modified petzvals like the verito "suffer" from this condition but are there other lens designs known to exhibit the swirl to such a degree? Or were lens makers too keen to "correct" this "problem"? A problem I love to see.
Joe, I hope to catch a demonstration in the near future, I enjoy seeing your work and would like to see the process.
It is my understanding that most any fast aperture, wide-field lens will exhibit coma at their wider apertures towards the outer region of the IC. The presence is apparently even more pronounced as spherical aberrations are corrected, thus a balancing act is required in the design to keep both under control. Interestingly, both effects get subdued quickly as aperture decreases, so they are rarely of concern in slower and narrower field lenses. So I'd look to the faster and simpler designs to find more of it.
Cheers,
As I said in an earlier post, there are at least two causes of swirlies.
From your own description, coma forms "light trails" away from the image center, not around it.
The main causes of swirlies seem to me to be:
1: Astigmatism. A large difference between tangential (around) and sagittal (away from the center) will cause highlights outside the intended image circle to be smeared out either around the center or radially away from it; depending on which one is worse.
2: Vignetting. when you tilt a lens you will see that there is some point at which the aperture starts to look like a lozenge (or american football) and not an ellipse. Out-of-focus highlights will reflect this change in shape, becoming narrower with increasing distance from the center. I believe this is the main cause in lenses like the Leitz Elmar and many other "famous" 35mm lenses.
My internet connection has been on the blink all day, but I have a picture with a classic example of coma ready for uploading as soon as my connection speed crawls above single-digit baudrate.
Ole:
First off, the tails on the coma are often barely visible, and depending on the spherical correction can point towards or away from the center axis. HOWEVER, the broadening or ovalation of the round oof highlights remains very visible. Since it also occurs at 90 degrees to the tail, the ovals appear to swirl around the central axis, hence swirlies.
Astigmatism causes star shaped highlights more often than smearing, and the smearing is usually on a small scale, but I'll grant that the smearing may affect the appearance of the swirlies.
Vignetting is a cosine 4th falloff factor due to the aperture changing shape to a football and nothing more. The fact that oof highlights follow that shape is due to under-corrected coma. (FWIW, coma is also called comatic aberration, NOT to be confused with CHRomatic aberration, and the resulting comet-shaped highlights are actually called "comatic flare"). In a more perfect lens design, like one using aspheric elements, you don't get them even though you still get cosine 4th falloff.
Cheers,
Hi Joe,
Is the Munising get together a private party, or can anybody join?
Thanks, Steve
Jack, I've been looking through some old posts at photos that have swirls and here is a photo by William Linne http://www.largeformatphotography.in...1&d=1173825332 that has excellent swirl. Now I'm a little dense (like my negatives), if you could explain in laymens terms how coma creates curves? I've read a little and it seems coma is usually present and noticeable in highlights but this photo seems to be something different. I don't know if any tilts were involved but to my untrained eye this appears mostly like lens mojo.
Because coma affects ALL of the image formation at the outer edges of the IC -- it is usually just most visible with oof highlights in lenses where it has been partially corrected. The lens used in the link you gave was probably a simple design and had no correction for coma.
If you google "comatic flare," "spherical aberration" and "astigmatism," I'm sure you'll find several references and some should have diagrams and examples.
Cheers,
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