Exposure/Contrast Changes

[David Ruby]

This is going to show my lack of knowledge right off the bat, but could someone explain, or point me to a good book about how to deal with changing exposure times when changing filters with VC paper?

I seem to remember reading about a chart possibly that you could use after you've determined the proper base exposer and then wanted to change the contrast with filters. Do different filters have a factor that you add or subtract to maintain the proper exposure?

Would anyone recommend the Kodak darkroom dataguide? If so, anyone know a good place to look for one at a decent price?

Thanks.

 

[Robert]

My understanding is that until you get up to the higher grades exposure stays constant. This assumes you first figured out exposure with the #2 filter in place. Which paper are you using? I know the Agfa datasheet claims that up to grade 4 it's constant. 4 and above it looks like you lose a stop. From looking at it I'm guessing in reality what they've done is put some ND into the lower filters so they print the same across the range. Grade 2 without a filter is much faster then grade 2 with a filter.

 

[ann]

Some papers (not Kodak) will have a data sheet with the paper that will give
you that information. Generally speaking the rule of thumb is that from 00 to 3.5 will be the same time ; with 4 up needing a 100% increase as the paper speed decreases in half.
The small print also says "must test for personal results". Or something along those lines. Take that to heart.
With some, the times will vary from grade to grade before reaching 3.5
The paper defaults to grade 2 without a filter, however, make a print a grade 2 filter and then one without and they do not look alike.
In our lab we recommend that students increase exposure about 50% when moving up from grade 3.5 to 4 and above as that works better with the paper, developer and water content in our lab.

 

[David Ruby]

thank you. i'm using Kodak polycontrast now, that's probably why I didn't see anything in the box. I'll experiment a bit.

 

[Black Dog]

Try Steve Anchell's Variable Contrast Printing Manual. Also useful info in mags like View Camera, Photo Techniques etc.

 

[Lex Jenkins]

If you're using a dichro head the usual rules of thumb for VC heads don't quite match. With my Durst M605 color head I find it necessary to make exposure adjustments with as little a change as from neutral or Grade 2 to either Grade 2.5-3 or 1-1.5.

This could be due to the condition of my filters or personal preferences (I tend to print dark) but in my experience with this enlarger and Ilford and Agfa variable contrast, multigrade, whatever, papers, the transition from neutral to either direction via magenta/yellow filters is not linear.

Being a lazy bum I haven't bothered implementing the various techniques published on the web to accomodate these differences. I simply make new test strips in quarter stops when changing a full grade or more and go from there.

 

[dr bob]

There are all kinds of methods of dealing with the exposure variations due to filtration, many dependent on the producer of the products involved. I have not seen any universal method of compensation that work well yet. I always have had to make strip tests.

There is another issue here. As the contrast changes, the overall exposure usually has to be “corrected” because every “zone” will be rendered differently. This may be the primary reason no one has been able to produce the
”universal correction” chart or formulae (equations). I love this stuff ;-)

dr bob
"I love the smell of fixer in the morning. It smells like - creativity!"

 

[Jeremy]

it is supposedly possible to adjust the contrast with a color head without changing the time by using more than one color so that the overall density of the filters does not change, just the color. But I've personally never tried this as it also increases the exposure time.

 

[Ole]

I've been through this, and given up. I now use an Ilford EM10 enlarging meter: Regardless of contrast and filtering, putting nearly-white on 80 on the meter gives an exposure time of 10 seconds on Ilford Multigrade. I adjust aperture and filtering until the reading is right, not the other way round.

 

[Aggie]

..

 

[Donald Miller]

000

 

[Aggie]

..

 

[Jeremy]

http://www.ilford.com/html/us_english/pdf/Cont.pdf

If you go to page 3 of this PDF (and since it's a PDF and I couldn't just copy and paste) you'll see a section titled "Dual Colour Filter Settings." I'm not touting this as true (and I note now that it says "...but should need LESS adjustment to exposure times when changing contrast." not "no") just passing the information along I had read.

 

[mvjim]

Not for nothing - but - "......Adding magenta to a yellow, only cancels part of one out. say you have a 65 yellow and want to add 20 magenta. What you are effectively getting is a 45 yellow. It will not effect the density at all. just dial in the 45 yellow......."
is not completely true. Most multi-garde papers are now made up of two emulsion layers, and as stated, each activated by one of the above filters. Mixing these will produce a contrast (if not density) different from using only one or the other. Another example is the design behind most VC cold light heads where two bulbs change intensity of mixed color as you shift contrast.
With that said- David - Why not teach yourself the difference with the; paper, enlarger, etc. you are using by running comparitive test strips. Seeing the difference will help you understand the materials response to these changes.

 

[Donald Miller]

000

 

[MikeK]

This discussion had me curious so I went and fiddled with the contrast controls of my Aristo VC Head. Cool little devil I can dial in contrast settings in 1/10th grade steps.

At the lowest contrast setting the light is blue, but moving through the range you can see the colour of the light change from blue, blue-ish green to green. So there is a mixing of light occuring from the two tubes. Never really paid attention to this as it just magically works.

- Mike

 

[lee]

Mike,
I think it is the other way around isn't it? Full on Blue should be the highest contrast and green full on the lowest contrast.

lee\c

 

[MikeK]

Lee:

Of course you are correct. Just went into my darkroom and turned the beast on and checked again.

That's my second senior (dumb) thing I have done today. It's either lack of sleep or dementure setting in. Of course could be both

- Mike