Exposure time for step wedge?

[Kirk Keyes]

Just use the light that's in the enlarger, don't mess with using a flash.

 

[ic-racer]

Just use the light that's in the enlarger, don't mess with using a flash.

Like Kirk points out, if you have a fully functional enlarger, use its own light. If you are building something from enlarger parts, then you could consider a flash.

My Edgerton xenon sensitometer has a little chamber below the step wedge that presumably provides for even exposure from the bare flash tube. Its a pretty simple setup, might be easy to copy.

 

[Stephen Benskin]

Like Kirk points out, if you have a fully functional enlarger, use its own light. If you are building something from enlarger parts, then you could consider a flash.

My Edgerton xenon sensitometer has a little chamber below the step wedge that presumably provides for even exposure from the bare flash tube. Its a pretty simple setup, might be easy to copy.

The EG&G Mark series is theoretically simple. The difficulty with any scientific testing equipment is with accuracy and repeatability. What are the variables that can influence the outcome? The ISO speed standard requires only a NON-intermittent sensitometer which eliminates the EG&G flash type. Personally, I have one and it works well enough for me. You need to ask yourself what you want out a procedure and then decide if it's cost and time effective.

 

[kenkuro]

Excuse me for beeing picky.

Stephen Benskin wrote:

Example,

Step Tablet aim Density - 4 steps down from 3.0 = 2.6
2.6 is around where I want the speed point exposure for fall.
Opacity = Antilog(2.6) = 10^2.6 = 398.107
Transmittance = 1 / 398.107 = 0.0025
Finding aim transmitted light for 125 speed film = .8 / 125 = 0.0064 mcs
Necessary incident light is Transmitted light / Transmittance = .0064 / .0025 = 2.56 mcs

That would equal 27.5 footcandles for one second - 2.56 * 10.76 = 27.5

So, meter the enlarger for 27.5 footcandles, set the exposure to 1 second and you should get the desired results.



The number does not agree with my ovservations. I have been using Timo-O-Lite contact printer adjusted to 1fc and giving 1s of exposure with Kodak #2 step tablet. With Kodak 5231 Plus-X motion picture film, which has a published speed of ASA 80. I am getting 0.05 over b+f at the step density of 3.05. And I think I found the problem.

Since 1 fc = 10.764 lux, one has to divide 2.56 by 10.764 instead of multiplying.

2.56 / 10.764 = 0.238 [fc]

It is about a 1/4 or 4 steps less than what I usualy use. Now we are in the same ball park.

 

[Stephen Benskin]

Thanks for the correction. Told you it's been awhile.

 

[kenkuro]

Thanks for the correction. Told you it's been awhile.

your wellcome.

By the way does anybody know if the method used to determine a motion picture film speed. Is it same as still films?

 

[Stephen Benskin]

your wellcome.

By the way does anybody know if the method used to determine a motion picture film speed. Is it same as still films?

As far as I know, but you could always check with the ANSI site and see if there's a separate standard. Considering development specifications (motion picture film has to have roller transport) and the film backing, it probably does. Still, the emulsions are the same so the sensitometry should also be the same (except for the release print).

Update - I was curious and checked ANSI and ISO and found nothing. I did a Google search and found this from a Kodak site.

Exposure Indexes (EI) Used to Measure Motion Picture Film Speed

Unlike still photography film, there are no published standards to determine the speed of motion picture film. Motion picture film speed and the suggested filtrations are determined by practical picture tests. Suitable safety factors are included to allow for camera differences, lighting variation, etc. Exposure Index (EI) values do not express the absolute film speed or sensitivity, and should not be regarded as fixed values. Because EI values are not fixed, they can be changed if repeated testing indicates the need for a change.

Got to tell you, I'm a little surprised.

http://www.kodak.com/global/en/service/tib/tib5209.shtml

 

[Usagi]

Like Kirk points out, if you have a fully functional enlarger, use its own light. If you are building something from enlarger parts, then you could consider a flash.

My Edgerton xenon sensitometer has a little chamber below the step wedge that presumably provides for even exposure from the bare flash tube. Its a pretty simple setup, might be easy to copy.

Ok. I will use enlarger only. The reason why I was thought of flash was exposure time. How accurate are short exposure times of my darkroom timer?
One second is probably quite accurate, but there reciprocity may raise it's ugly head.

Sensitometer style approach would be best. There are lot of used sensitometers in the eBay. Unfortunately none from europe, which leads problem with current. Need to buy transformer also.


I think that I am cutting hairs here. How exact measurement are even practical? When using sensitometry for calibrating process, eg. zone system, there is always a quite big tolerance in the values. For example, the VIII for diffuser is between 1.25 - 1.35, so it does not need very precise exposing device. How much will camera introducted flare etc. affect the results? After all same camera and lens will be used for real photographs.

Ofcourse it's different if you have tested which density gives best VIII tonality with your enlarger and your favorite paper.
I know only that from stained negatives, I get perfect VIII tone when densitometer with blue light measurement gives 1.35 above BF.

 

[ic-racer]

The main things I get from sensitometry are the film curve so I can examine the shape and slope to aid in correlating development time to ease and quality of printing. For example in my system a slope around 0.65 is great, but thats a personal value. If your light source is constant you can do RELATIVE speed tests (compare one film to another). But I do my EI test in-camera, not with the semstometer (Zone I frame = 0.1 log). In a RELATIVE speed test you will be able to say comfortably that under your test conditions, film A, for example, has a speed point 0.3 log (or whatever) faster, so its one-stop faster than B.

If you are going to the trouble to expose the whole step wedge, it might be better to get a slope or contrast index, rather than relying on a single zone (VIII) for development times.

If you use just Zone VIII for development times will find that you will probably NOT be able to get RELATIVE speeds with the same enlarger settings for all films. Thats because your Zone VIII is going to be sixteen steps up from your SPEED POINT (Zone I or 0.1 density on your negative). So thats just 5 steps to spare (out of 21). When testing a second slower or faster film, Zone I or VIII will likely fall off one end or the other of step wedge. Once you alter your exposure to get the second film's sixteen values all on the step wedge you can introduce a lot of error into your relative speed measure (for example are the F numbers on your enlarging lens exact, etc)

If you measure the slope of the curves, you can get a good representation in the first 10 to 12 steps above the 0.1 slice, so its OK if zone VIII falls off the end of wedge exposure.

And I think its clear to everyone that unless your light source is calibrated to an ANSI standard, you can't do an ISO test, but who cares, everyone uses (or should use) exposure index anyway.
Just a few things to consider.

 

[Chuck_P]

Usagi,

Pardon the interruption of this discussion, but why are you using an enlarger to determine EI of your film and "normal" development time? I've been trying to follow this discussion, perhaps I've missed something. I realize it is stating the obvious, but since the camera takes the picture, why not use the camera? Like I said, maybe I missed something.

 

[Stephen Benskin]

There's actually a common, or even universal, misunderstanding about film speed. The testing procedures defined in The Negative are partly to blame, but there's also a misreading of the ISO/ANSI standards. Simply put, film speed isn't about density.

Since that isn't the topic of this thread, I direct you to the the last two pages in the Understanding ISO thread where I've outlined the argument and referenced some additional information sources.

BTW, my earlier reference to sensitometer calibration and how non-intermittent sensitometers aren't accepted by the standards (mostly because of short exposure reciprocity failure which is part of high illuminance reciprocity) was only to give an example of the various levels of accuracy and acceptability. Just like when talking about speed, there's the functional and pragmatic ways to produce a good exposure and then there's the definition of what speed is.

The thing I like about film curves is not only the obvious ability to compare the characteristic of different films, but help in understanding the interrelationship of the photographic process as a whole. The film curve takes on so much more meaning when you relate it to the whole. How it relates with the paper curve and how both relate to the subject/camera image and then how everything interacts to form the reproduction.

 

[Stephen Benskin]

Usagi,

Pardon the interruption of this discussion, but why are you using an enlarger to determine EI of your film and "normal" development time? I've been trying to follow this discussion, perhaps I've missed something. I realize it is stating the obvious, but since the camera takes the picture, why not use the camera? Like I said, maybe I missed something.

Scientific testing is done to understand and control all the elements and eliminate the variables. If you are talking about speed, why would you want all unknowns of the lens, shutter, light meter, and color temperature of light source just to name a few? Never discussed enough is flare which plays a hugh roll. Contacting a step tablet eliminates that unknown. Then a statistical average flare value is calculated back in.

Of course, all the lab work has to relate to real world use.

 

[Kirk Keyes]

timer - you'll want to use an electronic one. They are usually accurate to within 1/60th of a second. Don't bother trying to use a dial type like an old Gralab.

 

[Stephen Benskin]

Also, as Kirk states, controlled testing increases accuracy and repeatability.

 

[Photo Engineer]

Having read most of this thread, and having done speed determinations (both ISO and in-camera), I can say that it is quite simple. It is a matter of producing a sensitometric curve of the film at your exposure and process conditions and then plotting the step scale (the image should be overexposed).

Then place your picture on the straight line portion.

The speed displacement of this "picture" from the curved portion of the toe is the actual ISO or ASA rating for what you are trying to do.

PE

 

[Usagi]

Usagi,

Pardon the interruption of this discussion, but why are you using an enlarger to determine EI of your film and "normal" development time? I've been trying to follow this discussion, perhaps I've missed something. I realize it is stating the obvious, but since the camera takes the picture, why not use the camera? Like I said, maybe I missed something.

No problem

I don't use enlarger to determine EI. Currently I have used camera for exposing step wedge and from the densities on the developed negative I can found EI quite precisely.

However in my method there are some problems. Or shall I say difficulties, because I have attach step wedge to the 4x5 film holder as an sandwich with film. And it is not so easy as it sounds... Thus I am looking for alternativies.

One is to expose wedge using camera to photograph wedge from some kind of light table. Another is use of enlarger.

 

[Chuck_P]

No problem

I don't use enlarger to determine EI. Currently I have used camera for exposing step wedge and from the densities on the developed negative I can found EI quite precisely.

However in my method there are some problems. Or shall I say difficulties, because I have attach step wedge to the 4x5 film holder as an sandwich with film. And it is not so easy as it sounds... Thus I am looking for alternativies.

One is to expose wedge using camera to photograph wedge from some kind of light table. Another is use of enlarger.

I tape my wedge to the sheet and then expose the sheet in the camera to Zone X, or 5 stops over the meter reading of a white mat board on the shady side of my house, develop the film, then read the step densities and then plot the curve. It's very simple---I guess I don't understand the problem you are having.

 

[Usagi]

I tape my wedge to the sheet and then expose the sheet in the camera to Zone X, or 5 stops over the meter reading of a white mat board on the shady side of my house, develop the film, then read the step densities and then plot the curve. It's very simple---I guess I don't understand the problem you are having.

I do just like that, except I haven't taped the wedge. I guess that it's key to problems I am having.

My procedure has been following:
- Load sheet to the holder
- Put wedge above the film and 'secure' (actually very unsecure, I have found) it to the place by closing holder's 'flap' end.
- expose to zone X

That works fine sometimes, but sometimes wedge don't stay in contact with the film and it causes several problems.
The tape will cure that. Thanks!

 

[Chuck_P]

I do just like that, except I haven't taped the wedge. I guess that it's key to problems I am having.

My procedure has been following:
- Load sheet to the holder
- Put wedge above the film and 'secure' (actually very unsecure, I have found) it to the place by closing holder's 'flap' end.
- expose to zone X

That works fine sometimes, but sometimes wedge don't stay in contact with the film and it causes several problems.
The tape will cure that. Thanks!

Good, I hope you can get back on tract with it. I would recommend that you get John P. Schaefer's: The Ansel Adams Guide (Book 2), Basic Techniques of Photography. This procedure is very well described in this text and I think you would find it very helpful.

Chuck

 

[Chuck_P]

Usagi,

This is a negative scan of a wedge shown taped to the sheet of film---I used black electrical tape, because it's all I had at the time, but you can use masking tape or some other tape that is not too sticky. Be sure and practice attaching the wedge to an old sheet, it can be tedius, but it does not take long once you're comfortable with it; also, you should handle the wedge at all times only by the edges, try to keep your fingers off the steps of the wedge. You can also purchase a 4x5 21-step wedge that is the same size as the film, it is then slipped into the film holder directly on top of the film---you'll see this example in the book that I recommended to you.

Chuck

 

[ic-racer]

Nice. So if Step 19 is 0.1 log, that confirms your initial EI for the exposure. Is that what you got?
Thanks for sharing.

 

[Chuck_P]

Nice. So if Step 19 is 0.1 log, that confirms your initial EI for the exposure. Is that what you got?
Thanks for sharing.

The speed test is always done while rating the film at the ISO (box speed) and developing as per the manufacturer's time for whatever temperature you choose (I always use 68 deg F); this establishes a baseline time for the subsequent tests for "+" and "-" dev times, once you have established your EI.

Step 19 lands right at Zone I. But the question is how far up or down on Zone I it actually lands. Too far up ISO is too low, too far down, ISO is too high. All 21 steps are plotted, then connect the dots. Only then will you see where the curve actually crosses the 0.1 densilty line.

Of course, traditional ZS testing would have that curve cross the 0.1 line at a net negative density of 0.9 to 0.11. If the curve crosses the 0.1 density line at, say Zone II, then we know that the box speed with our process is one full stop underexposed (step 19 landed at Zone I, but with a net negative density too low)----we would then know to rate the film at one stop less. This would produce a curve that shows it crossing 0.1 density at Zone I, where we need it to cross to establish the EI.

This seems more complicated in discussion, but so simple when actually doing it.

 

[Stephen Benskin]

So, would you say step 19 equals around a density of 2.90?

 

[jgjbowen]

interesting thread

 

[Chuck_P]

So, would you say step 19 equals around a density of 2.90?

In the attachment you can see that the step density at step 19 is 2.71......so, when reading the density on the film at step 19, it translates to a negative density of .15 and after the fb+f density of .04 is subtracted, the net density is 0.11. So, the curve crosses the 0.1 density line at a Zone I net density of 0.11. Good enough for me and so for this test with TMX and d-76 1:1, the ISO speed of 100 became my EI of 100 because the curve crossed the 0.1 density line at Zone I (I don't have the zones well identified, sorry) or at least within the range of acceptable Zone I densities.