I like it when they keep this stuff a secret. The easier it is to do low-light photography, the less "wow" and "how did he do that" factor there is in Large Format low light photography. :)
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I like it when they keep this stuff a secret. The easier it is to do low-light photography, the less "wow" and "how did he do that" factor there is in Large Format low light photography. :)
Pere,
Ilford used a fixed density criterion (0.1 above Fb+fog) to determine these t-factors. As you know, the speed point under ISO conditions is the exposure H_{m} where density is 0.1 above Fb+fog. Using this as a reference point, they then plotted characteristic curves for successively longer exposures and measured the speed losses relative to H_{m}, by finding the exposure in each case that produced a density of 0.1 above Fb+fog.
Said another way, the equation gives you the adjusted time so that the density 3 ^{1}/_{3} stops below the metered exposure is always 0.1.
From a Zone System perspective (for those who use it), the equation can be said to give you a constant density at Zone I ^{2}/_{3}.
This is a little more conservative than Howard Bond's tests which targeted a constant density at Zone III.
Hope this helps.
Hello Michael,
The ilford technical notice https://www.ilfordphoto.com/wp/wp-co...mpensation.pdf speaks about "effective speed", not about ISO speed.
I understand what you say, giving a correction factor for the speed point it is not the same than for the meter point that has exactly x10 (3 1/3 stops) more light intensity...
IMHO a key factor in this discussion is if the new ilford correction is for the speed point or not. If it was for the speed point IMHO new factors would be higher than the old curve (I guess), but it's the counter.
Still, do you know for sure that the new values are based in the fixed density criterion (0.1 above FB+fog) ? Or factors are for "the effective speed" ? What is the "effective speed" ?
Regards
ISO speed point is the reference point, and the t-factors are based on the loss of effective speed relative to the ISO speed point.
Effective speed is based on finding the exposure that produces a density of 0.1 above Fb+fog under LIRF conditions. It is called effective speed because it is based on a fixed density criterion (ISO contrast conditions do not apply under LIRF conditions due to the inherent contrast increase).
Another way of saying this is that the correction is for the speed point (using your words).
This is the most logical way of doing it. It isn't very complicated, just time consuming to test without the right equipment.
The old curves were generic and also were not film-specific.
When you guys get done, will you please give a summation and succinct advice on actually using Ilford's new guideline.
Must be comprehensible to a 5th-grade level.
And very short.
Carry on, we can wait.:)
1) Look up the P factor for the film you are using in the Ilford table
2) Whatever your exposure time is (in seconds), put that number to the power of the P factor (the P factor is the exponent), and this is your new exposure time.
Example - you're using Delta 100 and your meter tells you to expose for 10 seconds:
1) Use a P factor of 1.26 from the Ilford table
2) Metered time^{(P factor)}=Adjusted time so 10^{1.26}=18.2 (round to 18 seconds)
Still would be more intuitive in graph form. Most of us carry a limited number of films at any given time, maybe only one. Alas, I'm down to my last three sheets of 8x10 Acros.