Sensitometry questions

[iandvaag]

I have a couple of questions about sensitometry. I have a very basic background (I have read the sensitometry chapter in Haist and gone through the Kodak sensitometry worksheet).

1. Is there a standard contrast measurement to compare reversal films? In PE’s photo system engineering thread, he mentioned that a positive film can have [presumably an average] gamma or contrast of 1.6 to 1.9. What is the method to measure this average contrast?

I can easily measure straight line gamma, but this is much higher, for example when I reversal process Delta 100 in D67 I get a straight line gamma of 2.7. Is there a standard measurement analogous to contrast index, but for positive film (ie. a standard method of measuring an average contrast which is meaningful for positive film)?

2. What is the difference between a “Transmission Projection” step wedge and just a regular transmission step wedge? Stouffer markets Trasmission Projection step wedges for use in determining speed and contrast of photographic paper. I have used the T2115 transmission step wedge in this way, contact printing it onto film and paper. Is the only difference the physical dimensions of the step wedge, or is there something fundamentally different about the two different step wedges? I will also be contacting Stouffer about this, I just thought I’d ask here first.

3. I’m a bit interested in measuring speed of materials using an H-D curve. Does someone have experience with the X-rite sensitometers? I presume you can adjust the exposure. Do they indicate the amount of exposure in some physically meaningful units (eg. lx*s or similar)?

If you were to do the exposure under the enlarger, what kind of light meter would be suitable for measuring the light in order to determine exposure?

Thanks.

 

[Rudeofus]

There is an ISO standard for determining film speed of reversal materials, but it is not freely available. Not sure, though, how such a document would help here. You have some means to adjust tonality of reversal processed film to your needs, use these to reach the look you want.

About the transmission projection step wedge: I have the impression, that the main difference between them and standard transmission step wedges is their form factor. The former fit well into standard enlarger masks, while the latter is expected to sit on top of a medium of given size and freely selectable length, such as film strips.

PS: Do not attempt to create an H&D curve of film by projecting such a transmission projection step wedge onto your test film, you absolutely have to use contact printing. Each and every enlarger lens has much more flare than the dynamic range of such a step wedge can accept. This flare completely messes up the measurement of lower exposures! To give you numbers I read somewhere: multicoated lens surfaces have reflectivity of about 0.5%, this corresponds to a density of about 2.3. Even the simplest enlarger lenses have multiple lenses, and therefore many internal surfaces, and all your measurements above D = 1.5 - 2 will be completely off. BTDT.

 

[Anon Ymous]

1. This web page has some information that you'll probably find useful:
http://jaz99.blogspot.gr/2016/01/velvia-50-iso-sensitivity.html

 

[ic-racer]

This may help:

 

[Photo Engineer]

Here are some answers, I hope.

1. Change in density / change in logE gives contrast measured as a straight line in the middle and straightest portion of the curve. This is easy with papers and negative films (which should have a long straight line portion) but more difficult with reversal films which are often quite bowed (see post above). The more bow in reversal, the lower the perceived contrast of the image, and so you can get quite good results with a variety of curves.

2. I'm not sure of the Stouffer product names. Sorry.

3. Speed can be measured in a variety of ways. The graph above shows the method for reversal which can be quite different for neg-pos systems. One true measure is the threshold speed of the emulsion which is the inflection point of the curve from Dmin in a negative. This is mainly invariant and only varies under special conditions not often observed. When one says that they have gained speed from a push, they have only increased contrast. A true speed increase might come by a sharper toe and normal contrast. In reversal, as we see above, the shoulder is quite bowed, but if you were to draw a straight line from Dmax where the inflection takes place, that would be the speed point. Ideally, higher reversal speed would take place at the same Dmax with either a sharper curvature to the shoulder, or a higher threshold speed.

All of this is spoken from the POV of a product builder, not a theorist.

I hope this helps a bit.

PE

 

[Bill Burk]

To find the exposure of a sensitometer, you can work backwards.
I expose fresh film, for example ISO 400 on a sensitometer and develop according to standards the best I can. When I find a test run which met the ASA parameters, I note the speed point and mark that down as -2.7 log mcs (the exposure at the speed point by definition for a 400 speed film).

 

[iandvaag]

Thanks very much to all! This was very informative, much appreciated.

 

[ic-racer]

I had a thread on using film to calibrate my sensitometer, and similar technique was explored in a lengthy thread of Bill's (post #6 above). One difference is I now ISO 100 film as my sensitometric standard, because if fogs less in storage and transport.

The concept that name-brand film has an 'exact' ISO to three digits is elucidated here in a post from our own Photo Engineer from 2010:

Internally, we had the ISO rating of every emulsion to 3 figures such as 351 or 433 or the like, but for the life of me I cannot remember any of these figures for those emulsions. Sorry.

However, the release aim of 400 means just that and it is adjusted at coating time by the use of a trimmer dye to get an exact speed. You see this dye and the acutance dyes washing out of the coating during processing. The 3 digit internal speeds are used to determine the amount of trimmer dye to be used.---Photo Engineer