Ctein grain focusing "gotcha"

[DREW WILEY]

His name is pronounced Kuh-tine, with no first name. He's a real interesting and friendly guy and a friend of mine. A certain amt of info in Post-Exposure is indeed somewhat out of date, but he is still very active investigating certain related questions.

[MattKing]

Ctein is very approachable. When he made the pdf of his book available I decided I wanted to use a portion of it as a reference in a session on "sharpness" for my local photo club. I emailed Ctein to ask for permission, and he quickly responded with a very nice email (and the requested permission).

The Online Photographer has a little bit of info, complete with photo

http://theonlinephotographer.blogspo...eck-is_22.html

[pentaxuser]

http://theonlinephotographer.blogspo...eck-is_22.html

Thanks for the link. Ctein's description of himself is very funny and brilliantly accurate. A man with a dry sense of humour - just as his witticisms in his book suggest.

pentaxuser

[Mainecoonmaniac]

I remember his articles in various darkroom magazines over 20 years ago. Always interesting reading.

[Smudger]

Drew, since you know Ctein, do you think he might be interested in contributing
to this discussion?
On another tangent, I get final focus with a contrast filter in the light path,either under lens for 4x5, or under the condensers for other formats - in neither case am I focusing in "white light".

[DREW WILEY]

Right now Ctein is deeply buried digitally restoring a lot of old photos for the Ken Burns PBS documentaries.
But he'd probably reply to an e-mail. I haven't been across the bay to visit him for awhile. But he travels and lectures quite a bit too.

[Ctein]

Dear folks,

This thread was only recently brought to my attention. It appears there are questions that people would like answered, but no one bothered to e-mail me. Please, if you want answers rather than just guessing among yourselves, come to the source! I'm always happy to answer questions, as some of you know.

Okay, tackling the big one, first:

~~ Leigh B, Please download the **FREE** copy of my book and read the full article there instead of speculating on what I did or didn't research. In particular, pay close attention to pages 148-150. The very first thing I did to test my hypothesis was to run a “sanity check” to determine if there was enough violet-UV light getting through the system to even matter. The computations, based on the real spectral characteristics of the lamp, enlarger, enlarging lens, and print papers said yes. I confirmed this experimentally using a set of sharp-cutting UV filters. These results are included in the book.

With the exception of the justifiably-revered apo El-Nikkor, enlarging lenses are actually poorly-corrected for longitudinal chromatic aberration. It's by far the dominant aberration in almost all top-of-the-line enlarging lenses. It's especially bad when you push outside the normal red-green-blue wavelengths, into the violet and shorter. Most enlarging lenses aren't at all well-corrected for that range.

(As an aside, be aware that the designation "APO" attached to the name of enlarging lens is nothing but marketing bull. It does not indicate that the lens is a true apochromatic, in fact many of the "APO" lenses show no better correction for either longitudinal or lateral chromatic aberration than their non-"APO" counterparts.)


~~ Will any particular printer encounter this problem? No way for me to say. It will depend on the exact spectral characteristics of your enlarger, your enlarging lens, and the spectral sensitivity of the print paper you're using. Maybe it will turn out to be a problem, maybe not. That's why I came up with a simple way to quickly check to see if there was a problem.

In all likelihood, most people won't have to worry about this. There's a reason that the subtitle of the book reads “ADVANCED Techniques for the Photographic Printer.”


~~ Finally, someone asked if this mattered for color printing. The answer is no. If the color print paper is even sensitive to these ultra-short wavelengths, it only affects the yellow-dye image layer in the print. The human eye is terribly bad at seeing fine detail in yellow. A sharpness error there simply isn't visible. Regardless, white light focusing is your best bet because the paper is sensitive to the full spectrum of light. What looks sharpest to the eye is going to roughly correspond to what will look sharpest to the print paper.


pax \ Ctein
[ Please excuse any word-salad. MacSpeech in training! ]

[Leigh B]

Hi Ctein,

I think the discussion on page 147, and particularly the beginning of the last paragraph in the left column, identifies the systemic problem:

"Then I made a print with no filtration. Removing the magenta filtration from the enlarger eliminated the focusing discrepancy..."

Magenta is red plus blue. It's well known that the human eye cannot focus on red and blue simultaneously.

By using the magenta filter you introduced red into the focusing system.

Since the eye is more sensitive to red than to blue, you were focusing on red, which would indeed introduce an error.

As a point of comparison, I use the same Micromega focuser, with an Ilford Multigrade 500H additive head.
That head has two 300-watt lamps, one with a blue filter, the other with a green filter. There is no red anywhere in the system.

I find no focus shift whatsoever using that equipment.

Yes, I have a hard copy of your excellent book, purchased some time ago. Thanks for the freebie offer.

- Leigh

[Ctein]

Dear Leigh,

No, it's not a trick of the eye. I do not routinely focus by magenta light, and I did not do so in the course of this testing. Also, very early in the process, I did look to see if focusing with the color filtration in place vs white light focusing caused me to be focusing in a different plane. The difference was negligible.

I ran through pretty much every imaginable "user-error" scenario before I got to the wavelength business. I don't include them all in the book for reasons of brevity and clarity. Frankly, a measurable percentage of readers got lost trying to follow the scientific logic of what I did report. A "too much information" problem. Ive learned to NOT tell people everything.

In the same vein, as I mention in the book, I see very little "Gainer effect." Not an unmeasurable amount, but not enough to matter.

It's a short-wavelength sensitivity thing. Really, I pretty solidly proved that.

I have no idea how much, if any short-wavelength light makes it through your system. I suspect very little, if the blue and green dichroics are designed well.

pax / Ctein

[Leigh B]

Hi Ctein,

First off, I agree that an enlarger lens would focus UV at a different distance than blue or green light. That's not the issue.

My problem is with your suggestion that UV constitutes the predominant energy reaching the paper, significantly more so than blue. The only explanation for a 1/2" focus shift between blue and what the paper sees would be if the image was formed almost exclusively by UV light.

I can't find any explanation for the source of UV at any significant energy level when using a thermal light source.
Zeiss claims that UV represents less than 1% of the total energy output of a halogen lamp. The level would be lower for plain tungsten.

Also, very early in the process, I did look to see if focusing with the color filtration in place vs white light focusing caused me to be focusing in a different plane. The difference was negligible.

This seems to contradict the statement from the book that I quoted in #68 above.


- Leigh