Film tests

[Yves Gauvreau]

I want to do some testing where I need to expose by contact 4x5 negatives with a step tablet.

I have no enlarger and no timer because I intend to print digital. I would need a way to expose the negative to uniform light otherwise the density reading will be useless. Also the exposure time as to be such as to avoid reciprocity failure but the film I'll be using for this is suppose to be ok between 1/10 and 1000 secondes but I would like the method to do this applicable to less tolerant films.

I'm willing to spend a minimal amount of money for this, say less then a $100 much less if possible. If you have any suggestion I would appreciate greatly. Thanks

[Scott Rosenberg]

yves...

why don't you see if a local lab with a traditional darkroom can do the contact printing for you. alternatively, you may want to consider finding a 'darkroom-for-hire' facility whereby you pay to use the room on an hourly rate. thirdly, the local community college or high school may have a darkroom, though gaining access will be the most difficult.

good luck,
scott

[Jeff Moore]

Here is another option:

The View Camera Store will do the complete tests for you and supply a family of film curves with all the relevant information you need for $44.95.

The steps are as follows: 1) You tell them which film you wish to test; 2) They expose five sheets of the film on the step tablet with a calibrated light source; 3) They then send the film to you for you to process using your specific development method; 4) You return the film to them and they read the negatives and send you a complete set of curves. The packet of materials containing the test results is really substantial, including everything you need to know about your film test.

This is a terrific source for those without access to a densitometer or a darkroom. If you have any other questions, Fred Newman at The View Camera Store is very helpful and friendly.

Here is the link: http://www.viewcamerastore.com/product_info.php?cPath=35&products_id=82 (http://www.viewcamerastore.com/product_info.php?cPath=35&products_id=82)

[Yves Gauvreau]

Thanks Jeffrey, this is probably the most simple solution for standard films but there are 2 problems for me with this approach. The film I want to test is not standard, I'm sure they don't have it for one and the other and most important one is the cross border problem (X-Ray 2 times).

I think I'll have to buy the cheapest enlarger I can find and do it myself and because I also want to test other films as well in the future.

[Joe Smigiel]

Yves,

Can you describe in more detail what you are trying to test? Are you testing to find proper development times for in-camera negatives? If so, and assuming you have a 4x5 camera system, why not just tape the step wedge to a sheet of film loaded into a film holder and expose it in camera to Zone X using an appropriate evenly-toned light surface? Schaeffer goes over such a film testing procedure in his Ansel Adams' Guide to Photography (Book #1 I believe).

[Yves Gauvreau]

Thanks Joe, this is exactly what I've done so far but it's very difficult to have uniform lighting. I have a fiber optic probe (Luna Pro) and the intensity varies by as much as 2EV on the ground glass. When I devellop such expose negative using a 4x5 step tablet on which there are 2 step 11, the negative density is significantly different for this step. This suggest to me that light isn't uniform and thus I can't build my charts for proper development times.





Ideally the proper mean is to expose the film is by contact under uniform light making sure exposure time wont cause problems (reciprocity). You expose identically a few negative and develop for different times from about 40% normal to 250% normal then you measure the density of each step on each film. From all this data you can extract enough knowledge about your film for practically any subject luminence range and develop the film accordingly.

[Paul Fitzgerald]

Hi there,

You might want to try the same set up with the lens cells removed and max. bellows extention.

The de focused light may have less falloff to the corners.

Just a thought.

[Michael S. Briggs]

What film has near perfect reciprocity behavior out to 1000 seconds??

You don't need an enlarger to exposure your film. Place your step tablet in contact with the film and then expose with a bare incandescent light bulb. If you place the bulb far enough away, the light will be nearly uniform -- you can calculate from the inverse square law and the cosine for the slanted rays, or measure with a light meter. A quick calculation gives 1% uniformity if the bulb is 1 meter from the 4x5 film. At least in the US, I think you could find a used enlarger timer for well under $100 -- you can use this to control the light bulb. You can adjust the wattage, time and distance to get the correct exposure, subject to needing sufficient distance for uniformity and an exposure time that avoid reciprocity failure. If your film really has no reciprocity failure to 1000 s, you could even control the exposure with a watch -- a 0.5 s error on a 100 s exposure would be a 0.5% error.

An alternative would be to expose with a electronic flash placed at a sufficient distance, but this might have reciprocity failure from a short exposure.

[Yves Gauvreau]

Thanks Paul , It's funny you say that, I tried to extend the belows this afternoon and I noticed the light falloff was a bit less. I'll be sure to try your suggestion.

Thanks also to Michael, the film I use is an old aerial Panatomic X #3214 and the figure I gave comes from Kodak themselves. Supposing this is true, long exposure with this film should not matter so much but I still want to find a way to expose other films as well.

I know about the inverse square law but what's the cosine "law" I presume? I studied in french maybe we call it something else or I have a memory lapse.

Would you suggest using a bulb with a known CRI and temp (K) to simulate daylight as close as possible or do you think any bulb would do (film spectral response)?

[Michael S. Briggs]

Yves, have you transposed the digits in the name? I found Panatomic-X Aerocon II Film 3412 on Kodak's website. Or is 3214 an older version?

What the datasheet for 3412 states for "Reciprocity Characteristics" is "No exposure or development time adjustments are required for exposure times from 1,000 second to 1/10 second. At 1/10,000 second, increase development time by 20 percent." I strongly suspect a typographical error here -- I think it was supposed to be "from 1/1,000 second to 1/10 second". First, the English way of stating the other meaning would be "1/10 second to 1,000 seconds". Secondly, not needing reciprocity corrections past about 10 seconds is not technically plausible. Thirdly, an exposure of many seconds is not plausible in the intended use of the film, aerial photography.

The "cosine law" is not an official name. I am referring to the fact that the light rays will reach the edges of the film at an angle and therefore be spread out over a larger area by the factor cosine theta. This is the same effect that causes the seasons on the earth. Simple calculations won't be exact anyway because a bare bulb isn't a point source until it is far enough away from your film. The calculation was meant to illustrate how even the illumination could be. You can use a meter to check your setup.

The film tests that you are planning are most useful for determining development times and not quite as accurate for determining your exposure index. For example, you are making exposures by contacting printing rather than in a camera with a lens imaging a scence, and these tests probably won't exactly duplicate your metering procedures in the field. So my suggestion is not to worry too much about the color temperature of the light and to adjust your exposure index based on the results of your actual photographs. If you want to get the color temperature right, then flash would be a good choice, or you could use a filter with an incadescent bulb.

[Yves Gauvreau]

Michael, your are right about the number it's 3412.

I was skeptic about this 1000 sec and it's the reason why I wanted to expose at times that should not cause problem with reciprocity.

I did a kick search on Google and found the Lambert's cosine law and a good definition. I know this doesn't matter for practical purpose. The cosine law is for emitted radiation from a surface which is not exactly the case here but anyway I've done calculations with both laws and I've made a couple of test with the meter as well. I choose 0.5% uniformity and with the cosine law this gives us about 25", with these number I check the inverse square law and it give a bit less, about 1% uniformity. For the same uniformity (0.5%) with the inverse law I get about 36". Using the meter and trying to get no perceptible mouvement of the needle, I found the inverse square law is probably sufficient by itself.

As for the test, I agree with you, determining the film exposure index using this approach is only part of the solution but I think you'll agree with me that if we find the caracteristics of the film itself without any influence of a lens, a camera, flare or whatever. the result we get can be adjusted or corrected if you prefer, for a specific camera lens system. Otherwise we'll always have to "subtract" the effect of a particular system before applying it to another one.

I think the best light to use would be daylight or equivalent, this basically for the same reason as above. Each film as a specific spectral response, using a different light source then daylight means introducing another variable which will have to be compensated for later. I'll admit I have no idea how much correction would be needed if any in practical terms but I still think minimizing the number of variables or treating them one at a time is a sound approach.