alfred_robinson

I have a different approach to the same problem. I built a mask to

put over the dark slide of a sheet film holder. The mask has 15

holes, each 0.5 inches square. I have a means to cover each hole

independently. With the mask taped to the film holder, and the camera

aimed at a fairly large evenly-illuminated surface, take the cover

off of one hole, put the filmholder in the camera, pull the dark

slide and make an exposure. Then replace the dark slide, take the

filmholder out of the camera. Cover the first hole, uncover a second,

etc. This is described in more detail on my website

www.vsta.com/~alrob, Appendix E.

 

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Al Robinson

I recently ran across a problem with pre-wet which has not been

reported previously. I was measuring some characteristic curves for

Agfa APX 100 developed in POTA. I found that both 5-minute and 1-

minute pre-wets basically destroyed the negatives. The density was

increased markedly in the higher density region, so that the overall

contrast was substantially increased. The characteristic curves with

and without pre-wet are shown in www.vsta.com/~alrob/prewet.html.

 

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However, when I developed the same film in HC-110, there was

absolutely no change in the characteristic curve with the prewet. I

did some asking around, and I think that for most film/developer

combinations, the pre-wet makes little or no change in the

characteristic curve, and it probably does improve temperature

control and even-ness of agitation.

 

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I would suggest caution, however, when phenidone developers like POTA

are used. Technidol is a prime example. Xtol has (apparently) a small

amount of phenidone and should be checked. Jobo does not recommend a

pre-wet with Xtol, and perhaps this is the reason.

If you are interested in the zone system, you might take a look at

the new version on www.vsta.com/~alrob.

 

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Al Robinson

If you are getting into sensitometry and the zone system, might as

well look at the latest and greatest. www.vsta.com/~alrob.

 

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Al Robinson

The only combination I have tested for prewet effects is not the one

you are asking about, but it might provide a cautionary note. I

recently decided to switch to Jobo rotary processing for LF and MF

film. The first combination I checked throroughly was Agfa APX 100 in

POTA. I measured the characteristic curve with and without the

prerwet. The effect of prewet was disastrous. It did not affect the

lower densities much, but raised the upper densities by a large

amount, and produced a very humped characteristic curve. With no

prewet, The curve was nice and straight as you please.

Before doing any important work, I would suggest you measure the

characteristic curves with and without. I am sure that different

film/developer combinations will not all produce the same effect.

 

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Al Robinson

There is one idea I have not tried, but which might work. There is a

method for measuring density which involved making prints of test

negatives and matchin gray areas. Since it works with actual prints,

it should work with pyro or other staining developers.

The method was described in a recent issue of one of the photo

magazines. I do not have the reference in front of me, but there is a

link to it in my website www.vsta.com/~alrob.

 

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Al Robinson

I would suggest that a better approach is to use a low-contrast

developer, Kodak Technidol or POTA. I have read that POTA has a

capability of capturing 20 stops on film, but I have not gotten that

far with it yet. I have done 13 stops, and am still working.

When using such developers, there is a problem in determining

exposure and development. I treat these problems in my website

www.vsta.com/~alrob.

 

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Al Robinson

I recently got a rather good deal on a used densitometer on eBay, but

of course there are always risks with that route. It turned out well

for me.

You might also consider avoiding the purchase altogether, and still

being able to measure densities. There is a recent article on how to

measure densities using only your enlarger, and some printing paper.

I do not have the reference with me now, but there is a link on my

website www.vsta.com/~alrob which will give you the information.

 

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Al Robinson

If you want to lower contrast, the real way to do it is use a low-

contrast developer. I know of two, Kodak Technidol and POTA. Both use

phenidone at the developer. With these developers, you can process in

the Jobo and get down to N-6 and possibly further. This is explained

in further detail in my website www.vsta.com/~alrob.

 

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Al Robinson

I recently decided to switch to rotary processing for my negatives. I

decided to use the Jobo 2500 series tank for 120 and 4x5 negs. I was

having nothing but trouble until I discovered that the 5 minute

prewet suggested by Jobo was the problem. I just completed a test of

Agfa APX 100 in 120 size, developed in POTA. I found that the prewet

seiously distorts the characteristic curve, causing an increase in

density at the higher densities, thus increasing the overall contrast

by quite a bit. The curves with and without prewet fall on top of

each other up to a density of about 0.4. After that, the prewet

causes a growth of density which reaches 0.3 or so, and has the

effect of producing a very peculiar curve shape. Without the prewet,

the curve runs smooth and straight as can be.

Of course, I have not tested every film and developer, but my belief

is that the 5-minute prewet should be viewed with deep suspicion. I

will be keeping you posted on my website www.vsta.com/~alrob on

further research in this area.

 

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Al Robinson

If you are going to learn the zone system, let me humbly suggest you learn the new and improved one. It is much more powerful, easier to understand (somewhat) and is available free at www.vsta.com/~alrob.

I think the tie-in that you are looking for is to plot the densities against the Exposure Index values which were used in making the exposures for which you are measuring the density. The EI is a direct measure of how much light is reaching the film. It should be in logarithmic units, like the old DIN film speed ratings. This is explained in more detail in my website www.vsta.com/~alrob