I assume you mean the table on Page 2 of F-4017; that's demonstrating what I mean in that adjusting by aperture is not the same as adjusting by time because adjusting by aperture improves the film's sensitivity (reduces the reciprocity failure). Taking the metered-100s example, there is approximately is 3.6 stops (12x) of reciprocity failure but if you applied that much correction to the aperture, it would be an over-exposure. This is because the film responds non-linearly to light at this point and opening the aperture means increasing the light flux and therefore increasing the film's sensitivity. Changing the aperture means changing the flux, which means you are now at a different point on the film's reciprocity curve.
Say you meter at f/16 100s. At that level of luminous flux on the film, the film has 1/12 of the normal sensitivity so you would need to extend the exposure by a factor of 12. You don't do any further compounding to that adjustment, you just apply it to the time. Single step, no compounding. Look up the table, apply the multiplier to the time.
If you instead open the aperture, you need to open it by less than 3.6 stops or you'll get additional exposure, hence the table tells you to use only 3 stops. Opening by 3 stops means you're now shooting f/5.6 for 100s (meter says 12.5s). That's actually pretty close to the metered situation on the previous line of the table (10s requires +2 stops aperture or +2.3 stops time), for which the table would recommend about f/5.6 70s. The 70s/100s difference is mostly down to the change in recommended development time. Arguably you would be better off just using the previous line of the table if you can reasonably open the aperture that far - in other words, treat your meter as saying f/5.6 12.5s, go to the second-last line in the table, adjust it to 70s and shoot.
Using the aperture adjustments (instead of time adjustments) in that table will result in lower-contrast negatives too. From the f/16 100s situation: if you start at the last line and aperture-adjust you will shoot f/5.6 100s at -30% development but if you start at the second-last line (from the same meter reading!) and time-adjust you will shoot f/5.6 70s at -20% development. Those negatives will both be fine but they will be different. The -30% contrast adjustment is intended for use shooting at f/16 (in this light level) so if you instead use it to shoot at f/5.6 (in the same light level), you will produce a contraction of about 1 stop. The higher flux at f/5.6 means that there is less contrast expansion (we're on a flatter part of the exponential reciprocity-failure curve), hence the contraction.
When shooting at night, I don't think it's ever really feasible to go around opening the aperture by 3+ stops because you either won't have enough DOF or your aperture just isn't that big. So it's much simpler to just get the time-adjustment table (which, we agree, is not the same as the aperture-adjustment table) and apply that to the time... once, not compounded in any way.
It's also worth noting the development adjustments in that table. 20-30% is a significant adjustment in contrast and it's required because severe reciprocity failure increases contrast. In effect, those large time increases in conjunction with the development reduction amount to a pull of the film in order to retain shadow detail and tame the contrast. In other words, the time-adjustment table provided by Kodak is a preserve-the-shadows adjustment, not a preserve-the-midtones adjustment.
(I'm pretty sure we are in complete agreement wrt mechanism and what ones does when shooting, but your use of the term "compounding effect" wrt time-adjustments muddies the situation. Yes, we agree that sensitivity loss and therefore exposure time is exponential).
Last edited by polyglot; 12-05-2011 at 12:51 AM. Click to view previous post history.