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• 12-14-2008, 08:13 AM
Keith Tapscott.
Quote:

Originally Posted by Mike Crawford
That's really interesting and something I had never realised. However, trust us pesky Europeans to come along with our 12 x 16 paper and ruin the sequence!!

9.5 x 12 inches and A4 also to be included.:confused::D
• 12-14-2008, 11:52 AM
ic-racer
Quote:

Originally Posted by Nicholas Lindan
Comparison of methods for determining the exposure correction required for magnification changes using a 50mm lens, Beseler 45 enlarger and condenser lamphousing...

Fantastic!
• 12-14-2008, 12:49 PM
Nicholas Lindan
The flies over this well flogged dead horse get thicker and thicker and the horse keeps twitching.

For making moderately cropped 4x5, 5x7, 8x10 and 11x14 prints from 35mm and 4x5 negatives, the exposure correction factors for changes in magnification follow no discernible pattern. The geometric methods [either the DA correction ruler or the (M+1)^2/(m+1)^2 method] can result in answers that range from exact to 0.2 stops off from the metered values. The previous tests show the metered values are the correct values. All the meter readings and correction values are in stops.

Also included are test results with an opal diffuser over the negative. The results with the diffuser are close to identical to those with the normal Beseler condenser lamp housing. The conclusion is that the same behavior will be exhibited by diffusion, cold-light and dichroic enlargers.

http://www.darkroomautomation.com/su...xpmagtest4.jpg

The cropping is for the central 2/3's of the negative to be enlarged to the full size of the print - the central 1" for 35mm, the central 3" for 4x5.
• 12-15-2008, 11:37 PM
dancqu
Quote:

Originally Posted by ic-racer
The equation I use is:
new_time = old_time x (new_M +1)^2 / (old_M+1)^2
where M = new magnification (print/neg) and m = old
magnification (print/neg)

The exposure time factor would be:
Factor = (M + 1)^2 / (m + 1)

My quick study of this matter a few years ago had
me turn my attention to the purchase of an OM-10.
A more thorough study this pass confirms the
equation you use to be THE one to use. Print
size divided by negative size is specified
to be the method for deriving M&m.

Although a + 1 in each term appears to compensate
for aperture changes and the equation is described
as being accurate, I've not encountered any claims
of it being exact. So I wonder if the equation is
the last word?

Without the + 1s results will equal those of the
equation Nt = Ot x Ne^2/Oe^2; where the terms
are Old, New, time, and edge.

of other exposure related variables search Google
for, exposure enlargement calculations . Dan
• 12-16-2008, 06:08 AM
dancqu
Pardon me. That should be EM-10. I think Darkroom
Automation's combination Enlarging Meter/Densitometer
a good value. I'd have one except for it's appearance of
being bulky and lacking a narrow up front sensor; good
for metering small prints and any size print's corners.

Any chance of a redesign Mr. Lindan? Dan
• 12-16-2008, 10:00 AM
Nicholas Lindan
Quote:

Originally Posted by dancqu
I think Darkroom Automation's combination Enlarging Meter/Densitometer a good value. I'd have one except for it's appearance of being bulky

It is 1" shorter and a 1/2" wider than an EM-10, with the same thickness. It is the size of a slightly long pack of 100's cigarettes (an obsolete unit of measure, I know).

Its silicon-blue photodiode sensor is 0.008 sq in, a bit less than 3/32" on a side, comparable in size to the EM-10's CdS cell. Spots much smaller than that probably aren't important metering tones and a very small sensor is noisy because it starts to pick up grain and very fine image detail.

It comes with a 30 day money back guarantee, if it fails to satisfy then send it back.
• 12-16-2008, 05:52 PM
Nicholas Lindan
Sanity and grace ...

After a somewhat sleepless night, and a distracted day, I went over the magnification exposure correction calculations and measurements and found a pair of dumb errors that caused the results to get skewed.

The (M+1)^2/(m+1)^2 formula and the metered values all agree within a few hundredths of a stop -- which is what they are supposed to do.

http://www.darkroomautomation.com/su...xpmagtest5.jpg

The 'square law', beloved of all 7th grade science students, does still work. The law doesn't work on magnification but on distance from the light source to the screen. In this case the light source is the illuminated aperture of the lens and the screen is a point on the photographic paper (or you can turn it around, projecting a point on the paper onto the area of the aperture). The DA magnification exposure correction ruler works on this principle. An error in it's use can arise if the position of the lens' apparent aperture isn't located properly.

The geometric forms - the ruler and (M+1)... formula - are based on thin lens formulations. The actual light fall off will deviate slightly as the size of the front entrance pupil - the effective aperture - changes a bit with lens to paper distance. You can see this by looking into a wide-open lens at several distances. In practical terms this slight error can be ignored.

The meter has one great advantage - no math and transcription errors.

The horse is finally dead, the flies have gone.
• 12-17-2008, 09:24 PM
ic-racer
Thanks for spending the time on this!
• 09-05-2009, 07:27 PM
Jimpix
Hi boys & girls.
I'm new here & checked out the original question about print exposures & formulas etc....
then came to this last page to see how long it went on for. mmmmm?
So, just in case no-one has mentioned this method. Read on.

I haven't done any printing for a couple of years now. But I have always used the guide told to me by my darkroom manager when I was a young lad starting in BW photo-printing.
Based on knowing the correct exposure for an SS print (e.g. 5x4 neg to 5x4 print).
If you measure the length of the print for a 10x8 - 10 inches (twice as long as 5 inches).
You just open up the lens 1 f-stop. Giving twice the light strength.
Then with the same neg do a 20x16 print (20 inches long = 4 x the original length) you open up 3 stops from original exposure.
For example: SS exposure of 5x4 neg to 5x4 print is, say 10 sec @ f32,
So 5x4 to 10x8 = 10 sec @ f22, & @ 15 inches long = F16 & 20 inches long = f11 @ 10 sec.
When you run out of f-stops, you then increase the time comparable to f-stops to suit the magnification.(open up 1 stop for every 5 inches) (that's what she said?).
This method flies in the face of the Inverse Square Law. However, it does work. I have used it since 1976 & it is usually on the button or within a 1/4 of a stop on massive blowups. But it is a very easy quick guide to save time. It also takes the 'Brain Damage' out of the job.
BTW. 5x4 to 20x16 I open up 3 stops (equal to 8 times the illumination. Whereas Inverse Square Law would say the area is 16 times as big as the original 5x4. Thus indicating a 4 stop difference. So anyone who would like to try that little experiment & see how it pans out, please do. sometimes it can be that simple. Honest.
Like a 10x8 neg to 15x12 print in this instance would be either 10 sec @ f22.1/2 or 15 sec @ f32 (same thing).
And so on throughout all the percentage ranges.
So a 10x8 neg printing to 5x4 (50% print size) would need to shut down 1 stop to f45 ( or 5 sec @ f32).
Remember though. Start with a really good neg to form your basis. Then you can up or down the corrective exposures for heavy or thin negs accordingly.
I think i've been quite thorough there.

Happy printing.

Jim
• 12-06-2009, 03:15 AM
hoffy
Quote:

Originally Posted by Bruce Osgood
You're dealing with the Inverse Square Law. Moving a light source farther from the subject it diminishes proportionately to the distance.

I've worked out a simple Excel program that I simply plug in three variables and come up with New Elevated Exposure

New elevated exposure equals:

((New elevation / Old Elevation) ^2) X Original Exposure

Supposing your 8x10 elevation from lens to paper was 24 inches @ 16 seconds and your New elevation is 30
inches, then: ((24/30)^2) = 2.44
2.44 X Original Exposure (16 seconds) = 39.4 New Elevated Exposure.

I read this thread last night and decided to put this theory in practice.

While I don't agree with your example(!?), I found that the formula worked fine. Well, fine enough to be a pretty good starting point (I always test strip regardless).

I also found this one elsewhere on the net. Same formula in essence:

(ot/oh^2)*nh^2=nt, (old time divided by square of old height) times square of new height equals new time.

So, thanks to this thread, I am now saving a lot more time in the darkroom!!

Cheers
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