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Bruno Di Nunzio
18-Aug-2004, 10:33
I would like to do some film tests using a step tablet but I haven’t understood one thing. It’s clear that if I increase developing time I will get more bars but how bars # 21 and 20 should look like? Before putting all the data in a PC how can I be sure that reference speed I have used is “right”.

Many thanks.

Bruno

Kirk Keyes
18-Aug-2004, 12:15
Bruno,

This question really deserves an answer that is longer than what could be given in a forum.

I suggest that you find a copy of "Beyond The Zone System" by Phil Davis. It has a lot of detail about your questions and it should answer your subsequent questions as well.

Kirk

Tim Curry
18-Aug-2004, 12:59
Bruno, I'm at lunch so I'll keep it short. You just want to match the film's response to light to the paper's ability to see a scene with full detail.

Use the step wedge to expose a sheet of film. Get close to a white wall. Use infinity focus (no bellows factor or film reciprocity please). Meter the wall and you will have a zone V exposure, so add 5 stops to your exposure so you are exposing the wedge and film at zone X. This will give you a normal exposure with zones 0-10 in evenly graduated increments on a sheet of film (if your film speed and developmenbt are correct). If you are correct, step 19 should just begin to show a faint lightening and step 5 should zone VIII WHEN YOU PRINT the test film on paper.

I don't have a densitometer, so I use printing on the actual paper to read my test results. If, for example you have step 18 as your first visible step, add exposure (1/2 stop, or .15 density). If your step 5 (zone VIII) shows at step 7, you have overdeveloped the film by a full stop (N+1). Cut development ime and try again.

If I have made any errors in this approach, I'm sure sufficient band-width will be used to correct my hasty description.

Bruno, please contact me off list to ask any further questions. I'll answer if I can. tim

Bruno Di Nunzio
19-Aug-2004, 03:33
Thanks, I would like to give you some comments:

Kirk, I have bought Davis' book, maybe its me but I haven't found clear details on this aspects... but I will give it another look at it again.

Tim, I have a transmission step wedge. My idea is to put it on a light table and take the shots. I have to fix exposure and verify it. Then I will take all the five shots, develop them for different times and so on.

If it clear to me, in Minolta Spotmeter F I should select my referece speed, check uniformity, take the measure and increase the exposure of five stops. Take the shot and then develop (for example 10-12 min, I am using HP combiplan T tank).

Then: Step 21 is zone 0 Spet 19 is zone I .... Step 11 is zone V Step 5 is zone VIII Step 1 is zone X

If my first density is for example at Step 17 I should increase exposure of one stop. Is that right?

It is better to take the measure on the light table and increase it of five stop or directly on Step 11 and live it the way it is? In principle it should be the same.

Kirk Keyes
19-Aug-2004, 09:17
Bruno,

I understand what you mean about Davis' book, and actually this entire subject - it can be confusing. But at least you have the book so you can go and get more detail than what we will give here.

I take it you are using sheet film - since you are on the large format forum. Do you have an enlarger? It may be easier to make your test film exposure by contact printing the step tablet to the film under the enlarger. If you do use a light table, you will want to mask-off the table around your step wedge to cut out any extraneous light from entering your lens. The other problem is your will need to use a lot of bellows extension to get the image of the step wedge large enough to measure on the test film. Don't forget to take the extension into account when making the exposure and calculating your results.

"It is better to take the measure on the light table and increase it of five stop or directly on Step 11 and live it the way it is? In principle it should be the same."

Yeah, this is close enough for what you are doing.

Do you have a densitometer? If not, you can use your Minolta Spotmeter F - get yourself some cluse-up filters and look in the back fo the BTZS book for more info.

Bruno Di Nunzio
19-Aug-2004, 11:55
Kirk I will give you more details:

1) I use sheet film 2) I don't have an enlarger, but my light table is 10x15 inches, that is I am able to musk it without particular problems 3) I can use a 300, 150 or 75mm to crop the image. If necessary I can take into account bellows extension 4) I can use a densitometer of a lab 5) at page 72 of the book, fourth edition, under the chapter "Determining the Film Test Exposure" he says ".. if you run the test series with the wrong exposure, you'll waste your time as well as several sheets film" The problem is that he apparently makes film speed fine tuning for a developing time that has been fixed, but according to which criteria. Developing time and speed are changing accordinly so it should be different if I have fine tuned exposure for what I will later discover being a N-2 development then fine tunig for N+2, for example. This is the part it is really not clear to me.

Hoping it helps

D. Poinsett
20-Aug-2004, 09:44
To answer the first question, developed film that was exposed from tablet steps 20 and 21 should be clear. These must be clear because they are the reference density for all other measurements of that film. If many more steps are clear, the exposure is too short.

I have never tried the method you are going to use: photographing a step tablet that is back-lit by a light table. Here are some things to keep in mind.

1) As Mr. Keyes pointed out, you must mask the areas around the step tablet to prevent flare. Even so, you will still have some flare which will affect the results of your film test. This may be a good thing or a bad thing depending on your point of view but your test will not yield results due to film and development alone. The effect may also be too small to matter.

2) The spectrum from the light table will have an effect on the resulting film curve. It may be difficult to determine if it will be a problem. For outdoor photography, Mr. Curry's exposure method comes closer to duplicating lighting conditions that are likely to be encountered. (I presume he meant to sandwich the step tablet on top of the film in the holder.)

3) Depending on film, exposures longer than, say, 1/4 second may begin to show reciprocity effects. Again, that may or may not be a problem depending on the purpose of your test. Some light tables have a high frequency flicker that may affect metering and fast exposures. Light table output is often temperature sensitive.

Regarding your last post, the N, N-1, N+1, etc. development time and corresponding film speeds are derived from the film test and are usually interpolated from the results. As you have reasoned, you cannot know how to expose and develop for N, N+1, etc. until you get the results. Actual results might show N-0.3, N+0.6, etc. along with calculated film speeds. But knowing this allows you to interpolate the standard N, N+1, etc. development times and associated film speeds.