Different combinations of film and developer have different curve configurations. Therefore it is impossible to apply a blanket model to all of them. Some black and white films will actually deliver about twelve stops of straight-line separation without resorting to "minus" or compensating development, and at the other extreme, some might only deliver six stops of range. Eight is more common. Then you have to match that to the printing paper. And I'm more concerned about the shape of the toe than the overall range per se. This isn't a religion!!! There is no one set of rules or any
silver bullet. Rather, it's a simple tool kit for acquiring negatives which are reasonably within target range to print well. One has to customize all the variablse to their own specific needs. And as an anecdotal aside issue, I once measured a whole stack of gray card from various manufacturers on a high quality full range spectrophotometer, and not one of them was even close to 18% reflectance, nor did any of them match the peak sensitivity of my light meters. So one more reason you need your own standardized reference.
Mark, not really. The target points within the standard ZS are based on the ZS construct. The EI speed point is a good example. "0.1 above B+F", ie a target density, on its own doesn't contain much information about shadow tonality. It disregards the shape of toe, for example. It's just a rule of thumb, which is why ZS EIs change with variations in development. The fractional gradient speed, closely related to the first excellent print speed, is concerned with the gradient (ie contrast) at the speed point in relation to the total average gradient. This is a fundamental concept in the first excellent print and fractional gradient (0.3G) method - ie the speed is a means to getting sufficient toe contrast in relation to total contrast. This is different than targeting a fixed density for a speed point. It makes sense if you think about what determines local contrast and tonality in a print. A density on its own is meaningless without knowing the gradient at that point.
Why is Zone System EI often about half rated ISO/ASA?
I completely agree with Mr. Scudder above. For the extremely non-technical minded, half box speed for most films means overexposure of one stop. If we err on the side of overexposing in order to make sure we've placed generous shadow detail in Zone III (or even IV, as some would have it), then we've in effect overexposed by a stop, functionally the same as halving the box speed. Same end result.
That's the way my fragile brain sees it, anyway. :-)
I see target points above fbf as largely meaningless. It all depends on the actual curve shape and how you want the shadows to separate, based on your personal expectations and printing values. One reason that folks often rate film lower than box speed is to push the shadow futher up toward the straight line section of the curve. And in this respect, so-called staraight line films will dig way further down than something with a long toe. I really visualize the curve when I shoot, and only use the Zone System as a shorthand labeling tool to assign which batch of development I want a particular neg to go into (N verus N+1 etc). With some films I can easily get shadow separation way down to Zone 0. Only
with something like Pan F would I use Zone III as the metered shadow value. I work with a lot of different kinds of film and almost never botch an exposure unless the lighting very suddenly changes.
To elaborate, the first thing you do when researching a new film is go to the tech sheet and look at the
characteristic curve. That will tell you a lot. For instance, most Ilford films have a moderate toe to them, so that would inform me that in harsh lighting, I might in fact want to begin testing at half box
speed and work my way up (depending on my developer, of course). But with T-Max films, the toe is
much steeper, so I might start at actual advertised speed. With a true old-school 200 speed color separation films (most of which are discontinued), the toe retains contrast way down. Some films just
dont like overexposure of you'll push yourself off the cliff at the top, and blow out the highlights (that
is, if you want to maintain good midtone gradation). Then if you look at the curve for something like
Pan F, you'll see you don't have much wiggle end at either end of the curve, so maybe not the best
choice for harsh lighting at all. Once having made these generic distinction, you just have to test and
actually print until you understand a particular film's specific personality and pros and cons. Small camera users tend to have less choice because the priority is often upon fine grain, at potential sacrifice to tonality.
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Shadow gradient is crucial for the perception of quality. That's why the fractional gradient method, based off the results from the first excellent print test, uses a gradient of the shadow that is proportionate to the overall gradient. Film speed is arbitrary if it doesn't have a direct connection to the characteristics of the film. The ISO speed standard is linked to the fractional gradient method through the use of the Delta-X Criterion. When the contrast parameters of the ISO standard are followed, there is a known and consistent shadow gradient 0.29 log-H units from the 0.10 fixed density independent of curve shape. This is how we are able to make practical comparisons of speed between different film types and to assign film speeds with relevance. The ability for the film to reproduce luminance differences of the subject is what determines quality. A fixed point of density as Jones writes, "has no significance as an indication of the ability of the photographic material...except insofar as it may have some bearing on the exposure time required to make a print from the negative."
I just drew a set of curves for an APUG'r and the lowest contrast negs have beautiful smooth toes that I know I could just reach right into to print a negative shot at half box speed. If I took the 0.1 density to base EI, it would say quarter box speed. But half box speed is more appropriate.
Originally Posted by DREW WILEY
This is one of the factors I believe leads to lower Zone System speed ratings. When you develop N-1 etc, you may not achieve full speed. Especially if you rely on 0.1 density as the point where you take your speed reference.
And when I know that more of the toe is usable, I am reluctant to use 0.1 density-based speed.
Just depends on what look you want. Minus development sacrifices midtone expansion and microtonality
to some degree. I'd rather have full development and use and accessory unsharp mask to bring the
highlights into control, provided the original neg holds all the relevant information. The whole Zone
System mentality tends to be taught on a generic basis. But even when Ansel put the last version of
this in print, there were radically different popular films available. Something like Pan-X was engineered
for high-key studio portrait and fashion work, with a very long sweeping toe, while Super-XX had a
deadpan straightline from the basement to the moon. Tri-X was somewhere inbetween. Then Minor
White basically made a nutty religious cult out of the whole concept. It really isn't all that complicated.
Ooops, made a typo. Meant to say, Plus X pan had a long toe. The nearest equivalent today would be Delta film.
Bill, as you know that is essentially the logic underlying 0.3G (which is analogous to the first excellent print). Actually Stephen's summary paper on Delta-X does a good job illustrating how when developing to different gradients you still end up with virtually the same speed (as opposed to a fixed density criterion such as ZS).
Originally Posted by Bill Burk
My own caveat (not part of the theory) is you might have to assume a relatively normal luminance range. But I am not really there yet. Have a lot more reading to do first.
Last edited by Michael R 1974; 03-13-2013 at 02:28 PM. Click to view previous post history.