Aparat I take it that CI is contrast index and if so, to get an index of 0.70 the development time is 5.75 mins The Ilford time is 6:30 but this is for a GBar of about 0.62 which make comparisons of time and contrast very difficult for non technical types such as I
So is there a conversion formula that turns times for a CI of X into a GBar and if so what would a CI of 0.70 become as GBar
If your CI is exactly the same as Kodak's CI and I imagine it is then can I presume that if Kodak gives times for a CI of about 0.58 it would recommend a much shorter time for D100 and Microphen than does Ilford recommend with its Gbar of 0.62?
Ilford gives 8 mins for TMax 100 which suggests a difference of 1 min 30 sec to achieve the same GBar of 0.62 but why this should be is something I cannot work out
As a comparison does Kodak give its own times for Tmax 100 or D100 in Microphen? This may help establish the relationship between GBar and CI
Thanks
pentaxuser
Very good questions. Let me see if I can answer them.
First of all, I must point out that my curves are for a home-brew ID-68, not for Microphen, so I cannot guarantee that Microphen would yield identical results. I have ordered a one-liter box of Microphen and will test it at some point to compare.
Contrast Index and
Ḡ are similar but different, and, unfortunately, there's no conversion formula. For most curves, they are going to track very close together, so much so, that I would not worry about them being different for the purposes of everyday photography. I would just pick one of them and stick with it. I typically use CI because it's very widespread, easy to calculate, and its definition is unambiguous.
To help figure out development times, contrast, etc., in my program, you can generate a curve of any
Ḡ or subject luminance range, and that should be helpful. For example, here, I generated a curve with
Ḡ = 0.62. It would need a development time of 5.17 minutes, according to my program.
delta100_id68Gbar_0.62Curve by
Nick Mazur, on Flickr
Having said that, the calculations included in the table (left upper corner of the plot in
post #68 above) do not account for lens flare. With average flare, to obtain the
Ḡ of 0.62, you would probably need to develop for around 6.5-6.75 minutes. You can look it up on the plot below. This is because lens flare reduces contrast. It can be handled by adjusting metering and exposure prior to taking the picture, or, afterwards, by calculating its effect on negative contrast and modifying development time, accordingly:
delta100_id68Gbar_gtime by
Nick Mazur, on Flickr
Unfortunately, I haven't tested TMAX 100 with ID-68, so I cannot help you there. However, I would not put too much stock into the differences between recommended development times by either manufacturer. We do not know exactly how they arrived at their recommendations. The data sheets are, in my experience, not always consistent. They do help very much, don't get me wrong, but I would only take their recommendations as starting point values, only.
And finally, the photographic film has a very non-linear response to exposure and development. This is why a certain interval, say 1.5 minutes, only makes sense, relative to a particular exposure/film/development combination. This is also partly why there is not fixed conversion formula for
Ḡ and CI. So If I tested T-MAX 100 and Delta 100 in one developer, the established differences would not always hold in another developer.