Recent content by DonF

Here is the link to the full document: Rubylith curve plot document I still have 4 large sheets of the stuff. It's available from a few places, mostly for astronomy night sight preservation screen filters over iPhones and iPads, it seems. Here is my recent modification of an old Arkay OC safe...

I have been looking for a transmission curve for Ulano's Rubylith masking film for some time. Ulano has not made one available. I use the stuff to provide additional filtering for a Superbrightleds.com red LED 7.5 watt bulb, popular for use as a safe light (E27-R8-G). Empirically, the Rubylith...

As referred to in an earlier comment, another way to render a positive image from a negative is by reversal processing. The basic process is to develop an over-exposed negative image to completion, bleach the image in a non-halogenating bleach to whiten the dark areas of the negative and render...

Chrome alum (purple crystals) is used as a hardener, not for subbing, but has the same effect of improving adhesion and reducing reticulation or wrinkling of the coating. I tried albumin and Knox gelatin for subbing. Both worked, but not particularly well. Proper photographer’s gelatin would be...

The recipe is buried in this thread back several years, in bold type. I looked up the unlabeled ingredients in their bulk kit in the MDS, then calculated the formula from their own mixing instructions in the kit. It worked just like their stuff.

Even a normal film negative may be viewed as a positive if placed emulsion side up in a bright light against a black background and tilted so the light reflects against the emulsion. The Rockland kit uses high silver content AG-Plus emulsion, which is necessary for good results. The developer is...

I have one of these tanks in all-black. It came with an insert for 12 4x5 film sheets. The hanging plate holders could also be used.

The "old" (1968) Kodak Polycontrast enlarger filter exposure adjustment calculator comes with each filter set. Each filter in the set requires an adjustment to exposure times. The circular slide rule shows exposure adjustment in seconds. I wanted to see the f/stop change going between any two...

2 plus years since this post and no noticeable reduction in light output. No LED or individual LED chip failures that I can see.Exposure times have been consistent over time. The lamp has dropped in price to 28.00 from 38.00 as well. Works great for "sunning" silver nitrate solution to remove...

Duh. You're right of course. The cyan filter should only increase or decrease the amout of red light hitting the paper, regardless of setting. The paper is insensitive to red, so it is effectively clear, regardless of setting. Need more coffee. Regards, Don

Although the cyan dichro filter does not affect the contrast, it certainly attenuates light in varying degrees. That attenuation would certainly be "seen" by the paper. It is not clear unless set to 0cc. I get the step wedge part, though. Best, Don

Aren't these two statements saying opposite things? Just trying to grasp the advantage of using red-only to adjust exposure.

I don't understand why you would only use only red (equal amounts of Y-M) for neutral density? Using equal amounts Y-M-C provides true neutral density with more available attenuation. Also, as you pointed out, the 1 stop per 30cc change of all three filters only holds true if all three filters...

Hmm...I think I grok your terminology about using "red" as a neutral density color. In the context of a dichro head using subtractive Y-M-C controls, "adding red" means dialing in equal amounts of Y-M while leaving C out of the path completely, correct? Therefore, one could add red "neutral...

Thanks! I'm not so interested in absolute calibration of the color scales as to how to achieve an equivalent exposure time while varying the contrast mix. Your example seems to indicate my proposed adjustment would work: "So, for example, 80cc of Yellow gives ISO R (contrast) of about 120...