No matter what Kodak says

[Stephen Benskin]

A contrast index of 0.55 has always been "standard". It works for me under Rocky Mountain lighting. A CI of 0.65 works better under cloudy (typically European) lighting. If you read the tech sheets, Foma's reliably optimistic ASA ratings are for a CI of 0.6 to 0.7. A difference of 0.02, like between 0.56 and 0.58, is immaterial.
But what you really want to do is go out, shoot a bunch of pictures, print them in the darkroom, and use the ASA and development times that work for you. A CI of 0.5 to 0.55 works for me, except under cloudy skies when 0.6-0.65 works better. But I did those measurements after the fact, and arrived at the development times by experimenting.
The only exception - when I lived in Arcata, on those pea soup foggy summer days at the beach, a CI of 0.8 worked well. Under the redwoods, forget it. The contrast range there, between deep shadows and new green foliage, is much higher than it appears to the eye. I'm sure Vaughn has experienced that.

I believe Foma uses gamma (γ). A film with a γ = 0.67 - 0.71 range is approximately the same as a CI = 0.56 - 0.58 range. It's normal. Gamma uses a different method to read the curve.

 

[takilmaboxer]

Point taken. Thank you!

 

[Stephen Benskin]

It's easy to miss. Foma should present the information better.

 

[pentaxuser]

I believe Foma uses gamma (γ). A film with a γ = 0.67 - 0.71 range is approximately the same as a CI = 0.56 - 0.58 range. It's normal. Gamma uses a different method to read the curve.

View attachment 299041

Stephen where does Ilford's Gbar of 0.62 fit into this scheme of contrast measurement ? Thanks

pentaxuser

 

[Stephen Benskin]

Stephen where does Ilford's Gbar of 0.62 fit into this scheme of contrast measurement ? Thanks

pentaxuser

It's funny, I'm working on a post about average gradient. I don't remember Ilford having a target of 0.62 for normal but they adopted a run of 1.50 like Jones' fractional gradient method. I've often wonder why Ilford didn't update with the general use of coated lenses. Depending on curve shape, mostly long toed curves, G-bar will return a different number than Kodak's CI.

Just checked a couple of Ilford's datasheets and they don't list contrast aims or even a Time / contrast curve but they do seem to have a standardized statement of "These times will produce negatives of average contrast suitable for printing in all enlargers. The development times are intended as a guide and may be altered if a different result is needed." So it looks like they are protected from lawsuits.

 

[pentaxuser]

Thanks Stephen I was hoping that in the same way you translated Foma's Gamma into the CI used by Kodak, you could do the same with Ilford's Gbar of 0.62

It would be nice to know how close Ilford's Gbar of 0.62 comes to Kodak CI of, if I recall 0.58

Thanks

pentaxuser

 

[Stephen Benskin]

Thanks Stephen I was hoping that in the same way you translated Foma's Gamma into the CI used by Kodak, you could do the same with Ilford's Gbar of 0.62

It would be nice to know how close Ilford's Gbar of 0.62 comes to Kodak CI of, if I recall 0.58

Thanks

pentaxuser

Did another search. It does appear that they use a G-bar of 0.62 in some of their data sheets. Delta's curve looks average so it should about equal the same as CI. My first thought is they are keeping to the ISO speed standard for some reasoning. Problem is the ISO speed standard isn't about defining a normal contrast. Interesting.

 

[Stephen Benskin]

G-bar 0.62 is almost certainly an internal decision of Ilford's and not a product of the methodology. A possible reason why Ilford's aim G-bar for normal is 0.62 could be based on their aim LER of 1.10 for many of their grade 2 papers. Kodak uses 1.05 for their aim LER grade 2.

1.05 / 2.2 - 0.40 = 0.58
1.10 / 2.2 - 0.40 = 0.61
1.10 / 2.1 - 0.30 = 0.61

Comparing a couple of development times of Ilford films in Xtol as listed by Ilford and Kodak, the times are all within 30 seconds of each other and it isn't consistent which manufacturer has the longer times. HP5P and FP4P have Kodak's recommendation at 30 seconds longer and Ilford has Delta 100 as 30 seconds longer.

 

[Craig]

My first thought is they are keeping to the ISO speed standard for some reasoning. Problem is the ISO speed standard isn't about defining a normal contrast. Interesting.

For as long as I can remember Ilfords tech sheets have always said that film speed was based on a practical evaluation, not the ISO standard speed.

 

[Stephen Benskin]

For as long as I can remember Ilfords tech sheets have always said that film speed was based on a practical evaluation, not the ISO standard speed.

That is sort of true; however, I was referring to what Ilford considers the aim for normal with G-bar. The ISO speed standard has a contrast parameter that must be met before determining the film speed. Some people erroneously consider that to be the standard for normal contrast. The parameters equal a slope of ~ 0.62.

What you are referring to is where Ilford makes the statement "It should be noted that the exposure index (EI) range recommended for HP5 Plus is based on a practical evaluation of film speed and is not based on foot speed, as is the ISO standard." This is referring to where in the data sheet they are stating an "EI" or exposure index. It doesn't mean they didn't rate the film based on the ISO standard. If there is a speed number that has the prefix ISO before it, it means the film adhered to the ISO speed standard. EI can mean basically anything up to and including the ISO speed setting. In this case, Ilford means the camera rating or how the film was shot and not what the actual speed is.

"Best results are obtained at EI 400/27, but good image quality will also be obtained at meter settings from EI 400/27 to EI 3200/36."