J Lane Dry Plates exposure question

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I have some J Lane Dry Plates (glass with emulsion on them). They are rated at ISO 25 - this varies a bit depending on the colour of the light as they are mostly blue sensitive. If I put them in a plate camera and want to expose then I can get a good approximation with a light meter - fine.

What I want to do though, is expose them with a contact positive or negative using a process similar to some alternative processes such as palladuim/platinum. I can put my light meter in the exposure cabinet - fine - but what do I use for aperture? If I have a lens, I can say find the exposure for f8/ISO 25 and it will be say 1 second. If I measure the light in the light box, what "aperture" do I use when there is no lens in the equation?

I can obviously do a test with a step wedge or whatever - but this could tunr out quite expensive, as I can't just cut a strip off the plate. Where do I start with my exposure?

Thanks for your thoughts.
 
Off the top of my head - and this is something I'll investigate as it interests me - film speeds are set according to the exposure required to give a density of .1 over base plus fog. This would imply that the standard illumination is defined; and if you have a measure of the density of the plate for a mid tone, you should be able to apply that to the "standard exposure" to give a value that would result in base + fog + .1. A check of the characteristic curve of the plate (or a good guess at what it is). This should give a good approximation.

Another line of attack is to use the fact that f numbers measure how much light the lens holds back, and the theoretical maximum f number is f/0.5 and use that.

Or ask Jason...

I'll see what others say.

At the moment, I have a cat on my lap and have to tap on a tablet, which is laborious.
 
Thanks - I saw a thing talking about F/ 0.5 being the practical limit - but with diamond or other materials its theoretically be possible to make one with a smaller F number - obviously practically useless and super expensive though.

I wonder if no aperture and no glass is effectively F/0?

Google Answers: Optics

answers.google.com
 
It's a matter of illumination. At f/0.5 the image is as bright as the subject. It is possible to boost the illumination slightly by using aberrations, but in the real world 0.5 is effectively no lens involved.

I'd probably either use printing paper for test strips, or use an incident meter to measure the exposure at the level of the printing frame and use that directly, or possibly add a small amount to compensate for the base plus fog of the negative. This would be about half a stop on average, but will depend on the material.
 
Having had a quick glace at your Google result, it's probably the same reasoning as applies in a parabolic reflector; by reflecting (or diverting) more of the light than actually goes in a straight line towards the observer, you can get (apparently) something for nothing, by robbing Peter to pay Paul.

As you're concerned with real life practical illumination, I'll stick with my original value.

From memory, the issue is covered in Sidney Ray's Applied Photographic Optics. If you'd like a page reference, the book is actually in a book frame next to the chair I'm in (and I have the same cat back on lap :))
 
Thanks. I hadn't realised that F/0.5 is full illumination. Will try that.

The issue with printing paper for test strips, is that I don't know the ISO of the paper - I make my own, either Cyanotype or Platinum/Palladium, but they are much less sensitive than ISO 25 - I also have POP paper, but again don't know the rating. I don't have any commercial paper with a rating - but isn't the ISO of paper different to the ISO of film or sensors anyway?
 
Thanks for the offer of the page ref - its £165 on Amazon so am unlikely to get it I'm afraid. That's a lot of cat food.
 
As HM Government says in their financial warnings - prices can go up as well as increase :). My copy only cost £90, although I did some years before see the first edition on one of the stands at Focus on Imaging (yes, not The Photography Show) at £6. I already had the second edition at that point, but I noticed it sold on the same day. It is well worth having if you like some relatively hard to find pieces of technical data.

Standard printing paper has been used by many on here in pinhole cameras, and an estimated speed is usually given - somewhere in the 3 to 6 ISO region.
 
I just took a look at the Internet Archive archive.org and found the book us available as a free borrow. And that it's the third edition I have, same as the loan book, so a page reference could be used.
 
I'll spend a little time to ensure I can give a complete reference, just in case there's more than one place where it's discussed. As I have the print version, I have access to the index etc. Given that part of the information depends on (I think) correction of spherical aberration, I may check a few other optics and lens design books as well just in case. Unless you really don't want to delve deep. In case you haven't noticed, I can be rather prolix.
 
You probably need to read pages 129-131 and follow the mathematics, before reading the summary on p 288. If reading this whets your appetite, you probably would enjoy the whole book. But I can say this as I already have a copy :); I appreciate that the price is now rather steep, although Amazon for me shows a lower price of £115 new or £135 second hand (!).

I said elsewhere that modern books on optics (meaning, ones without "lens" or "photography" in the title) are much less relevant to photographers than the then current books were when I was a student. Research in optics has moved into different fields, and the standard texts reflect that. If there is a "general" book on optics better (from a photographer's viewpoint) than Longhurst's Geometrical and Physical Optics I'm not familiar with it. I have Ditchburn's Light sitting on the shelf just behind the monitor I'm using, and this is a book also from the 1960s. The treatment is broader (Longhurst's title shows the main thrust) but less photographically relevant.

I should perhaps also remind people that I'm NOT a physics graduate - my subject was chemistry, even if it did involve a lecture series on optics! I was just always interested in optics..

Edit to add. I was thinking primarily about discussions relevant to lenses in my remarks above, but the older books also treated photometry - the measurement of light - which is relevant to photographers. This appears to be a topic omitted to be able to insert more "modern" material in new books. Not really unreasonable, if access to older works is easy via a library, and there is nothing new to add to them. On older books, I always have a wry smile when I remember one of our lecturers recommending Pauling and Wilson's Introduction to Quantum Mechanics and explicitly telling us not to be put off by the date - at 1935, it was about 32 years old at the time. My copy is by coincidence sitting next to Ditchburn on the shelf.

Second edit to add: I just checked and both Longhurst and Ditchburn are available to borrow on the Internet Archive.
 
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Thanks again - both for the link and page references. I'll have a look over the next couple of days and then post here again.
 
Not read the book yet - will do that on Weds. I have put my light meter in my UV box though, and it would measure at F/0.5 - at ISO 25, it gave a speed of 1/250s. I can't turn it on for that short a period, so I'll need to use an ND filter - I have some 1 stop gel, but a stack of that would be optically bad (Newton's Rings etc and dust being the major problems). I also have some Baader astrofilm (about 16 ish stops) - my calcualtions (to be checked!) show this to be about equivalent of F/128 - which means an exposure of about 4mins 16s. The issue now is reciprocity failure of the emulsion - but at least it gives me a time to experiment with.
 
Just a thought, but is whatever meter you are using accurate with UV light? Meters do have a spectral response curve, and (like films ) do not respond equally to all wavelengths.
 
Have had a read - good stuff. Not sure I am ready to buy the book, bt I can read it when I need to on archive.org.

Yes - I have the same feeling about the exposure meter - its a good one - but I was thinking this: On the plates, it says to put a blue filter over the sensor to exclude the red end of the spectrum, as the plates are only/mainly sensitive to blue light - however, my UV box gives out almost completely blue light, be it visible or UV - so I was thinking that the light measured would be missing the red end anyway, so shouldn't be far off. Not sure if that reasoning is flawed or not, but I will give it a go. I also managed to get some slower plates yesterday, ISO 2 instead of ISO 25, so I have a few stops to play and should be able to time to exposue without much of an ND filter.

I'll give it a go and post here with results - might take a few days, so bear with me! Likely to be at least a couple of weeks before I get any meaningful results. In the meantime, I am grateful for all your help.
 
On meters:

When CdS meters first came out, they were hailed as a big advance on the older selenium cells as they were much more sensitive. Selenium meters generate a current, so big cells were needed if sensitivity were to be boosted. CdS cells varied resistance depending on the light, so required a battery but were more sensitive.

When silicon blue meters came out, the deficiencies of CdS was then made a marketing feature - excessive sensitivity to red, and slower to react to changes in lighting levels. Hence (I suspect) the warning re blue filters. There also used to be warnings about attempting to correct exposure through filters by placing them in front of the meter - OK except for red.

On exposure:

I suspect that some reading up on alternative processes and ranges of exposure times might give you an estimate; particularly if your UV source has a rated output. I assume someone somewhere gives an approxiamte ISO rating for the various alternative processes, and this might be a better starting point.
 
The issue is that I made my UV box - so no idea of how much it gives out, except for trying my process and experimenting. Printing out paper takes about 4 mins, similar for cyanotype. They are less sensitive that my ISO25 plates. I also print platinum, although I am at an early stage with that and similary want to reduce the amount of test pieces I do due to the expense.

I have a Stouffer step wedge and some I made myself, so what I think I'll do it cut the plates and use the wedge and then get a ballpark exposure - but using my light meter etc at least gives me a good starting point. If my calcs are rigt above (dubious!), then trying 30s or so will hugely overexpose, so getting a good idea first will save me tons of cash.
 
Success!

Printed my first dry plates today. I got hold of some ASA 2 plates as well as the ASA 25 plates I mentioned above. With a piece of Baader astrofilm, the ASA 2 plates have an 8 second exposure and came out pretty perfect stright away - some tweaking necessary to get it spot on, but I was really surpised at how accurate the exposure was.

The ASA 25 plates didn't go so well - the 1-2 s required was too much - a quick "flash" of the UV unit worked on a test piece, but I've not printed a whole plate yet - I have some ND filter and I think using some of that with the Baader film would make my exposures more manageable - I'll get to that soon.

However, using F/0.5 and the ASA/ISO of the plate, with the film over the sensor, in my UV box sorted out the exposure almost perfectly. Thanks so much for your help.
 
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