Davis' BTZS is the closest method to good sensitometry, but it's still a method and while I've never found anything in Davis' book to be inaccurate, he has a tendency to short hand the explanations of some point especially theory. It's been awhile since I've read BTZS, but I believe he mentions how testing with an enlarge doesn't produce an accurate film speed. The color temperature of the lightsource, the lack of precision of most timers, and the difficulty in determining the illuminance of the light source are all contributing factors. The enlarger is a compromise. Isn't it better to know that than to think the results can be as accurate as with a sensitometer?
Originally Posted by AndreasT
The 1.5 you are thinking about is the log-H range used to determine the average gradient when doing fractional gradient testing. How it relates to how the manufacturer gets the ISO rating is what I'm talking about. By knowing how the details, parameters, and theory on a test, the photographer can better utilize the testing data. There will be a slight difference in the film's responce when testing under an overcast day and sunny day. the ISO speed standard uses daylight color temperature for exposure. When the speed standard changed the color temperature for the film exposure from sunlight to daylight they had to adjust the speed constant from 1.0 to 0.80. But if the photographer is testing using natural light, they are probably using a camera. All the variables associated with camera exposures will tend to obscure and speed change caused by different color temperatures.
It also relates to how the manufacturer gets this ISO rating and what it means. As I understand it and I may be wrong here but the rating correlates to a contrast of 1,5 am i wrong here? or me that is an overcast day in winter. Not a sunny day when many people take their cameras and run on the streets.
The value of 0.62 for contrast index that you said was standard in most books, I believe came from the contrast parameters defined in the ISO speed standard. The delta 1.30 log-H by delta 0.80 density has a gradient between 0.61 and 0.62; however, it can't be referred to as a contrast index because the use of that term implies a testing methodology. The 1.30 log-H range is intended to define the shadow portion of the curve and not the average gradient.
Now, a CI 0.62 is perfectly acceptable as a normal. It's just a little high to be considered the results from the statistical average conditions. Most books have aims more around 0.56 and 0.58 depending on the average flare factor - whether it's for a large format lens or medium and 35 mm lens.
My basic premise is that there is good testing and bad testing. The best possible decissions come from a position of knowledge.
Last edited by Stephen Benskin; 01-17-2013 at 06:51 PM. Click to view previous post history.
I see what you are getting at Stephen, or most of it. I use a enlarger to expose my film but I put in a lens with a shutter so that the times I would suppose is relativly accurate. While there is a strong talk about accuracy here I believe if one is pretty close the parameters should cancel each other out on average.
Though there may be problems here and there or even false results, by seeing the collected data one does get a knowledge or feeling how ones methods work. This will get us what we want.
I feel comfortable when I test my materials.
I think you don't have misconceptions.
When you expose a test strip, even if you set aside the whole concept of speed, you can always get a measure of the contrast.
Photographers could work from box speed, if they used appropriate metering technique (like incident metering). But people meter badly at times. I remember times when I got underexposed shots using the camera on automatic because I included too much sky or backlit situations.
All I know now is that I have become confused. I am going to have read up on all this. I feel like I am in a dark forest at night.
Andreas, I have no doubt about the quality of your results, or most people's for that matter. The manufacturers have strived to make the photographic process as fool proof as possible. Gone are the days of needing to be a chemist in order to make your own emulsion. When I talk about accuracy in testing, I'm not suggesting everyone has to adhere to the strictest testing standards. This is obviously not necessary for what most people require. What I do feel is important is that we should approach testing from a position of knowledge, to be aware of the strengths and weaknesses of a methodology, and have a reasonable understanding and expectation of the results. Knowledge is a good BS gauge.
Originally Posted by AndreasT
For an example of the opposite result, you need not go any further than the many posts claiming that a given in camera testing method proves the ISO standard is inaccurate, yet the poster has no idea what the standard contains. Another example is the conspiracy theory over the K-factor in exposure meters. What's worse, unquestioning confidence in knowledge that is wrong or no knowledge at all?
Last edited by Stephen Benskin; 01-18-2013 at 06:11 PM. Click to view previous post history.
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Yes I know what about the story of a grey card reflecting 18%, a lightmeter is actually calibrated for 12% reflection and if one measures a 50-50 surface of black and white we get a, I think it was 9%. (the last I do not remember) there are so many confusing things stated.
I feel secure about my own method, always learning something new.
It was certainly fun reading all this.
So many sources claim that exposure meters are calibrated to 18% (putting aside the fact that meters aren't calibrated to reflectance). Other sources say it's 12%. This discrepancy can make a difference with certain testing methodologies. Too many simply accept the calibration percentage their chosen method uses or adopt the percentage supported by an authority figure, but I think the smart idea is to do some research on their own?
Originally Posted by AndreasT
Last edited by Stephen Benskin; 01-18-2013 at 11:52 PM. Click to view previous post history.
To illustrate how you can gain confidence by acquiring knowledge, while at first feeling confused. Here's a thought that is confusing me lately.
You probably have no trouble thinking of filter factors as simple as 1 stop for yellow, 2 stops for green, 3 stops for red. But lately I've been reading a chapter from LP Clerc which explains the common sense that the factor you use depends on the effect you want.
If you want to make a gray card gray those factors may be correct.
But a green filter can be used to lighten greens and darken reds. Really brighten the greens while leaving the reds natural. Or really darkening the reds while leaving the greens natural. You would want to use three different settings for the three different effects.
I had to think through those last few sentences, because the idea's still sinking in. But it's pretty obvious.
In sensitometry there is plenty talk of making sure that you use the correct illumination. What if the light source of your enlarger is tungsten and you shoot daylight? Does it invalidate your tests? My opinion is that it matters, but not much. Suppose you develop your film to 0.6 Contrast Index (you think) but in reality you truly obtain 0.5 Contrast Index. You will still get great negatives. So long as you are consistent, you will make adjustments as needed.
Bill, this is not about how much one factor or another has on the results or determining what you can get away with. I'm simply advocating a little critical thinking to better understand the process. It's more about the philosophy of testing. I'm tired of reading "I've heard that", or "this famous photographer says", or seeing uncritical and uncommented on links. I brought up the question of the validity of the just black printing method in my first post. This technique is part of a testing method that was linked to in this thread and the OP was considering using. I believe evaluating the strengths and weakness of this method would make for a worthy discussion. Beginners tend not to be aware that there are variances, tolerances, and nuances involved, and few absolutes.
If one camp claims exposure meters "read" 18% and another 12%, shouldn't a person have the intellectual curiosity to investigate? How many of the people who do Zone System type testing ask themselves why their EIs are almost universally 1/2 to one stop slower than the ISO speed? This would be my second question after wondering if I did the test correctly. I guess it comes down to asking the right questions. Most ask "how to" and not "why."
Last edited by Stephen Benskin; 01-19-2013 at 08:26 AM. Click to view previous post history.
Yes Steven you are right. But...
Not everybody has the will or discipline to find the "why". Starting out one doesn't have a clue where to start so one has to rely on information coming from others. Some find it damn boring trying to find the why.
It takes time and money to find all this. I have little of both.
I do believe we may get blinded from all the information out there, while looking for that magic potion.
How often have I told people to read the Technical Pages published by manufacturers. Either they don't have the nerve or don't know they excist.
Yes to Bill with the filters, thank goodness I don't use filters that mush, lazy me. But when one has a sound idea how the film react I think not that much can go wrong. Besides when in doubt I just take two photos the second one with a half or full stop more and according to the first film change the processing of the second one as needed.
We still have multigrade to save us.