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  #1  
Old 30th December 2009, 11:33 PM
Mike O'Pray Mike O'Pray is offline
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Default Calculating New Exposure Time for An Enlargement

I have a Philips PDT2020 enlarger timer which consists of a probe that is placed on the easel to take readings from the neg projection. It has always seemed to work very well and was, I thought, very useful if I altered the height of the enlarger to make a bigger print. I simply left the probe on the same piece of the neg projection, raised the column and then read the new time.

The new larger prints generally seemed to look the same as the old smaller prints or so I thought.

Tonight as an experiment I used the probe on a neg projection, noted the time(8 secs) then raised the column height from 33cms to 44 cms and noted the new time of 15.8 secs.

I then used a website which contains an "excel" programme which calculates the new time for you and it was 14.2 secs. I also measured the longest length of the two projections as a further check and using the formula in Tim Rudman's book arrived at the same time as the calculation. I add this so you may know that it isn't the website that is wrong( or if it is then Tim's formula is as well)

So my probe leads to about a 10% error or 1.5 secs too much in what should have been a 14.2 secs exposure.

The probe may not be accurate of course but I'd have thought that any inaccuracy would have remained consistent. It is simply responding to a lower light level and should have delivered a proportionally bigger reading. The analogy I am drawing is with that of an uncalibrated thermometer. It says 20C when it should say 22C but if it is put into water which is 10 degrees warmer than this then it should read 30 even if the water is actually 32C.

Is there a flaw in this analogy and can anyone comment as to why the probe shouldn't be at least consistently wrong?

I'd be interested in hearing from anyone who uses such a similar timer and who knows more about such instruments than I do.

I suspect that Richard Ross may well have something meaningful to say, even if it is only " Buy my machine which is accurate"

So all comments welcome but especially cue Richard Ross as they would say if it was a script for a play

Thanks

Mike
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Old 31st December 2009, 07:52 AM
Dave miller Dave miller is offline
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Unless you are measuring from a uniformly plain area of the negative the light measured will change as you move the head because you are effectively measuring from a different part of the projected image.

If the calculation is made using the inverse square law then the result should be correct, some published formulae seem to add unnecessary complicating factors, the need for which I fail to see, or worse measure the wrong parameters.

The formula I use is based on the paper to lens distance, and is the new distance squared, divided by the old distance squared, multiplied by the original exposure time. This provides me with the new exposure time when all other parameters are maintained.
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Old 31st December 2009, 08:25 AM
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Another possibility that occurs is light from your safelight effecting the readings. Obviously, if your safelight is off during the reading then ignore me, but check for other light leakage from the enlarger itself (tho this is unlikely to be much in practice: it usually looks a lot worse than it is).

Last edited by Bob; 31st December 2009 at 08:31 AM.
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Old 31st December 2009, 08:57 AM
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Mike - which of the large prints is closer to the tonal values of the small print? The calculated one or the measured one? After all, that's the result you want to achieve.

Dave's right about the measurement area - unless the negative density is absolutely uniform over the area of measurement then errors can creep in.

As for formulae, there have been endless arguments as to the accuracy or otherwise of these. As I understand it (I'm not an optical engineer) it's not a simple inverse square law as other factors are involved, so the formula you used may not be absolutely accurate.

Don't forget paper reciprocity failure either, this can lead to prints made with longer exposures looking lighter than expected. Maybe like ours your Philips meter has compensation for this effect, whereas maybe the formula doesn't.
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Old 31st December 2009, 10:49 AM
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The formula in Tim Rudmans book is a ball park approximation.
Fact is when you change enlargment size you should do test strip because as you change size you often need a small contrast change too to give the same look.

An accurate formula using a digital spot meter is:

T2 = T1 * 2^Fstop difference (when going bigger)

or

T2 = T1 * 2^ (-1 * Fstop difference) (when going smaller)

meter something light in the center of print. Then change height and meter same point again and use difference in ftsop reading in above formula.

The formula in Tims book would be more accurate if you measured the full width of the projected film area and not the length of the easel framing which is not the same thing unless you are printing full frame.

ps you might get more accurate results doing the metering tests with white light and it could be done with a digital incident meter on baseboard too. Or if your enlarger has a diffuser filter then swing it under the lens. That way you should get accurate measure of brightness difference from one height to the next.
Either way, make sure focus is done before metering.
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Last edited by Argentum; 31st December 2009 at 11:09 AM.
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Old 31st December 2009, 06:09 PM
Mike O'Pray Mike O'Pray is offline
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Thanks all. You have hit the nail on the head. It was after I had switched off the computer and was sitting with a glass of wine that the very point that has been made on the measuring spot changing suddenly struck me. I was sober when I measured and wrote the tread - honest Guv. Just not thinking straight

I have just re-done the measurement using the exact same spot on the neg and the probe and calculation agree 100%.

I had previously done the calc on the website using the predetermined exposure and heights and deciding to stick with this led me to leave the probe in the same place on the projection where I had obtained the said 8 secs reading which of course isn't the same place once the projection is enlarged.

To quote a once famous politician and enthusiastic photographer " what a silly billy I was"

If you remember who he was and the quote used by Mike Yarwood then you are seriously too old like me!

Anyway I'll re-read your posts carefully for the additional stuff and thanks RobC for the suggestion about the diffuser filter. I do have one on the same swing rod as the red safety filter on the Durst.

Mike
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Old 31st December 2009, 06:27 PM
Tony Marlow Tony Marlow is offline
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Quote:
To quote a once famous politician and enthusiastic photographer " what a silly billy I was
I suspect said politician will deny ever saying this, it was just the creation of the likes of Rory Bremner.
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Old 31st December 2009, 06:47 PM
Mike O'Pray Mike O'Pray is offline
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No it was a real quote and he doesn't deny using it. Yarwood of course overused it in any impersonation he did of the said politician.

It was still in the days when politicians were generally respected and impersonation was gentle comedy and not withering satire

Mike
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Old 31st December 2009, 07:00 PM
Dave miller Dave miller is offline
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Quote:
Originally Posted by Mike O'Pray View Post
No it was a real quote and he doesn't deny using it. Yarwood of course overused it in any impersonation he did of the said politician.

It was still in the days when politicians were generally respected and impersonation was gentle comedy and not withering satire

Mike
It was a real quote. Mike Yarwood invented it, and Denis Healey subsequently used it.
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Old 31st December 2009, 09:00 PM
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To quote Wikipedia (so it must be true): "The impressionist Mike Yarwood coined for him the catchphrase "Silly Billy", which Healey had never actually said until that point, but he adopted it and used it frequently." Denis Healey is a keen photographer too.
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