View Full Version : Understanding Toes and Shoulders.. not my curves!

brian steinberger
7-Dec-2005, 18:04
I've read numorous threads in here talking about different curves of different films. And I must admit that I'm still kind of confused about it. Right now I am trying to understand the differences between Tri-x 400, tri-x 320, and HP5. And I'm reading alot about how Tri-x has a very long toe, and this and that. Can someone explain what a long toe or shoulder means, in terms of "midtones are muted, or midtones are rather dark." I appreciate any help! Thanks


Ken Lee
7-Dec-2005, 19:18
A good stereo speaker plays all the tones equally: the low tones, middle tones, and high tones. What goes in, comes out, with no distortion. Also, it has a wide range of tones: from really low, to really high.

Usually, stereos have a big woofer to handle the bass notes, a small tweeter for the high tones, and mid-sized speaker for the mid-range. With good speakers, you can clearly distinguish all the tones from low to high, and it sounds like the original.

Unfortunately, we don't use 3 pieces of film: one piece has to do it all. Ideally, it should record the complete range of light, without distorting it, and without chopping off the bottom or top of the range.

When a film has a long toe, it chops off the low range: All the darks look pure black, instead of velvety. The curve in the low values (the toe) is flat: no matter how much light is given in that range, the result is the same empty black.

When a film has a long shoulder, it chops off the high values: all the high values are blasted out to pure white with no texture, instead of looking creamy. The curve in the high values (the shoulder) is flat: no matter how much light is given in that range, the result is the same empty white.

Witold Grabiec
7-Dec-2005, 19:46
The "toe" refers to low values (Zone 0 or 1) and "shoulder" to high values (Zone 8 or 9). The section of film curve between toe and shoulder is relatively straight. Your aim is to atain the longest straight line and shortest toe/shoulder for maximum range of tones and minimal tendency to loose detail at each end (lack of detail in shadows or blocked highlights), all of it will make processing less complicated.

Long toe or shoulder indicates film's limited ability to dealing with the extreme values, or (using your phrase) muted low or high values. With long toe/shoulder the curve in this section is at relatively low angle which translates to bigger exposure increase for smaller value changes. Long toe/shoulder automatically means shorter straight line inbetween (you can call it midtones, but the longer it is the more tones fall within it, so say you could have Zones 2 through 7 recording on a long straight line (or short toe/shoulder) while only Zones 4 through 6 would record on a short straight line (long toe/shoulder).

Your technique can and will affect the curve, but not necessarily its shape. Meaning manufacturer's curves are the good starting point for choosing a film that's right for your application (my statement above about wanting the shortest toe/shoulder assumes a need/desire for greatest depth of densities). So while development will affect mostly mid to high values, exposure will mostly affect low to lower mid values.

For more on this check out Ansel's "Negative" or "Basic Photographic Materials and Processes" (published by Focal Press). The latter is a great overall (and technical) reference on anything photographic process involves.

Oren Grad
7-Dec-2005, 20:52
A long toe or a long shoulder "chops off" the corresponding values only if you print that way. It's more precise to say that a long toe means reduced contrast in the darker areas of the picture, and similarly a long shoulder means reduced contrast in the brighter areas of the picture. If you print a negative that has both a long toe and a long shoulder with an overall contrast that allows all of the detail in the negative to be seen in the print, the result will be a picture with low-contrast shadows, low-contrast highlights, but midtones with exaggerated contrast.

With B&W negative films, the classic problem is preserving printable shadow detail. Films that combine a long toe with little or no shoulder are particularly difficult to print in a way that preserves detail in both the shadows and the highlights without extensive manipulation (e.g., dodging and burning, contrast masks, etc.). Thinking in terms of the films you mentioned, Tri-X 320 tends to be more difficult to expose and print in a way that preserves both shadows and highlights in a print than is Tri-X 400. On the other hand, Tri-X 320 will provide more brilliant highlight contrast (some say "sparkle") than will Tri-X 400. Some photographers like that and are willing to either sacrifice the shadow detail or put up with the more complex printing techniques required. HP5 Plus has traditionally been in between, though closer to Tri-X. There are some reports that the HP5 Plus curve may have "straightened" a bit in emulsion batches produced in recent years, though it is still a more forgiving emulsion than Tri-X 320.

Note that printing papers have shoulders and toes as well, so that the effects get compounded. In recent years, the characteristic curves of the papers available on the market have tended to change in a direction that makes it easier to print negatives made with films that have steep shoulders (e.g., Tri-X 320 in any developer, T-Max 400 in any developer, T-Max 100 in some developers). For example, Ilford MG IV FB tends to compress the highlight tones more than its predecessor, the original MG FB. This makes it easier to print negatives that have greater highlight contrast, but at the price of flattening even more the highlight constrast in more shouldered films like Tri-X 400.

Phil Davis used to show a set of prints made on different papers from the same negative, that provided a simple but revealing demonstration of the effect of different paper curves on the look of a print. He published a couple of articles about gradation in Photo Techniques a few years back that showed these comparison prints, as well as others that show the effect of different film curves. While the tonality of the examples suffered a bit in magazine reproduction, the articles were still instructive. If you're interested, I can track down the specific issue references.

If you want to learn more about what all this means for exposure and development in practice, Phil Davis' "Beyond the Zone System" materials are the most coherent, systematic and practical that I know of.

William Mortensen
7-Dec-2005, 22:59
Brian- I'm an "understand it but don't dwell on it" type, but if you really want to shove those toes and shoulders around, the stand- and semi-stand development methods that have become somewhat more popular in the last ten years or so are very effective. The use of a dilute developer for long times with reduced or no agitation allows the highlights (dark areas of the negative) to exhaust the developer and the shadows (lighter areas of the negative) to develop more. Combined with the old "expose for the shadows, develop for the highlights, beware the dynamic range" philosophy of the zone system and its relatives, it can give wonderful control and beautiful results.

There are a number of threads on this subject here and elsewhere.

8-Dec-2005, 00:35
I used to assume that the toe and shoulder represented flaws in the way film (and paper) depicted the world. I've grown to see them as just part of the innate look of the materials. My goal now isn't to eliminate the shoulder and toe, but have nice ones ... ones that print the way I like.

My first attempts at scanning and printing digitally demonstrated this to me. I was used to fortezo, which has a very long, sloping shoulder--this has the effect, among other things, of compressing the darkest shadow values and darkening the midtones. When I scanned, for the first time I had the "ideal" straight curve. No toe, no shoulder. And the pictures looked like crap! I found myself using photoshop to put that big shoulder back, so I could get the rich tones I was used to.

I know this doesn't add much clarity to the original question. A short answer is that toes and shoulders describe how the contrast changes in the shadows and in the highlights. they can be more or less prominent, and have different shapes, but are not inherently good or bad. They simply describe one aspect of the look a particular film or paper.

Ken Lee
8-Dec-2005, 06:45
Thanks for the corrections. Perhaps I am confusing certain things.

Here is a sample curve and the results, applied to a step wedge: before and after applying the curve.




Oren Grad
8-Dec-2005, 09:40
Ken, great idea to post the step wedges! It might make things really clear if you could post three step wedges with accompanying curves - one straight diagonal line, one with a big shoulder, and one with a big toe - might help Brian see at a glance what we're talking about.

Ken Lee
8-Dec-2005, 10:57
The first image is a step wedge with a straight diagonal line.

The second image contains both big toe and big shoulder.

The toe section is on the left, the shoulder section is on the right.

8-Dec-2005, 11:28
Ken, a problem with the curve you posted is that it's actually clipping both the highlights and the shadows ... so we're seeing shadows dropping straight to black and highlights dropping straight to white. Look at the points where the line intersects 0 and 100.

This can be tricky with the curves tool in PS sometimes, because the line likes to jump around. but ideally you should show a curved line that hits 0% and 100% in the corners ... same as the straight line. that will show the effect of the toe and shoulder, without also showing the effect of increased contrast.

Ken Lee
8-Dec-2005, 11:47
Here is another curve and set of wedges.


The top wedge is just a straight line. The lower wedge reflects the influence of the curves.


Is this the kind of curve you had in mind ? If not, please provide one.

Witold Grabiec
8-Dec-2005, 14:27

I'm not sure what horses have you been listening to, but what you said (or how you said it) makes no sense to me. Not to say your statement is entirely incorrect, but "desnities are pulled up" ... just because film used has a long toe?

Sensitometric curve shows relations between exposure values against tonal values, or how density changes with exposure. Long toe/shoulder indicates a large exposure change for a relatively small density change. If anything, this area would have a considerable exposure latitude, since small exposure inaccuracies would go practically undetected in density response. I suppose you can say it essentially improves on density resolution in this area (low values at toe's end) but would create problems in the remaining range. As I mentioned earlier, film needs to be chosen carefuly for a specific task and looking at the curve will help. But most films are pretty damn close in that department. Personally I believe in sticking to one or two (at most) film types and take care of business in the exposure/development part which is more controllable.

Oren Grad
8-Dec-2005, 15:02
Ken -

With the challenges of monitor calibration, it can be hard to keep the ends of the scale from blocking for every viewer. But I think that last pair nicely shows the redistribution of contrast on the scale as you introduce the toe and shoulder. Thanks for going to the trouble of setting it up and posting.

8-Dec-2005, 22:24
"Is this the kind of curve you had in mind ? If not, please provide one."

yeah, exactly ... you've kept the endpoints in the same place but changed the shape. now there's just one variable to compare, so it's easier to see.