Interpretation of a characteristic curve

[Anon Ymous]

Hi, I've been looking at data sheets for several films and I want to know if I got some things right. Please correct me if I'm wrong.

1) The density from which the graph starts is the base fog.

2) A steep curve means that the film has higher contrast.

3) A long curve at the top right part (shoulder?) means that this film handles highlights nicely. It doesn't overexpose easily

4) I saw somewhere that 0,3 units at the X axis (relative log exposure) equal to 1 stop. Is that correct?

From the above we can calculate the exposure latitude in stops, but what's the maximum useful density? The data sheets say that FP4+ has a Dmax of 1,8 and APX100 goes to 2,6. Tri-X seems to go even further and reaches 3, depending on the combination of developer and time.

 

[edtbjon]

You are correct at 1 and 4.
The angle of the curve (statement 2) is dependent on development time/temperature/etc...
As for statement 3, you're still looking at the result of the film/developer combination. Another developer gives another result. Also a long shoulder means that the highlight contrast is low, i.e. no "snap" in the highlights. The same is true for a long toe, i.e. "boring" shadows.

For whichever process (e.g. normal silver), you want a set Dmax (above f+b that is) regardless of film choice. In the case of normal silver prints this is usually 1.05 for condensed light and 1.20 for diffused light. For alternative processes (VanDyke, Platinum etc.) you want higher Dmax. Some films are less suited for alt. processes as they are difficult to process to the desired Dmax. But as there are very few films to choose from in the sizes wanted, it's foremost a matter of availability. (E.g. APX 100 isn't available in any larger sizes at all, only some 35mm film can be found. The most common films in larger sizes, i.e. larger than 8x10" is FP4 and HP5.)
If you are shooting film and intend to scan it and do the rest in P*shop (This is a very analogue place. ) you should actually go for as low a Dmax as you dare. Somewhat thin negatives are the ones that gives the best results when you scan them.

//Björn

 

[Chuck_P]

1 - yes
2 - yes
3 - yes, some films don't shoulder-off until approaching zone 11 or 12 or higher, this means that they will show some contrast separation in that region; the shoulder is where the highlights start blocking up with increased exposure. But, you must develop the film so that the negative density range is within the printable density that the paper can handle, so it doesn't matter if the film can separate contrast that high on the scale, what matters is where the paper can separate it--usually devoping film to a density range of 1.2 to 1.3 works.
4) - yes, there are 0.3 units of log exposure from one zone to the next

 

[AgX]

Concerning Dmax

As Björn has hinted at, the density should be apt for the following step, printing on paper, as the negative in principle is only an intermediate stage.

However seen the negative on its own, the higher the possible Dmax (with constant gamma, of course), the greater the exposure range.

 

[Anon Ymous]

Concerning Dmax

As Björn has hinted at, the density should be apt for the following step, printing on paper, as the negative in principle is only an intermediate stage.

However seen the negative on its own, the higher the possible Dmax (with constant gamma, of course), the greater the exposure range.

First of all, thanks for the answers
But regarding Dmax and gamma, is it easy to get high Dmax values, while maintaining a normal gamma of 0,55 to 0,6? I have a feeling that higher Dmax pushes gamma to higher values.

 

[Nicholas Lindan]

A long curve at the top right part (shoulder?) means that this film handles highlights nicely. It doesn't overexpose easily

Other way around:

A film that shoulders looses highlight contrast and overexposure is bad news.

A film that goes to a very high exposure (high D-Max) before shouldering preserves highlight detail, but you may have to burn in the highlights to get the detail back.

Films that have no shoulder are the best for highlight detail - the old Plus-X sheet film was 'all toe' and had an upswept HD curve: good for snow scenes and Hurrell-style portraiture. Shadow detail, though, was lousy. The 120/35mm Plus-X films are a different emulsion and do not have this characteristic.

 

[Michel Hardy-Vallée]

I will say something horrible on APUG, but...

If you play with a Photoshop curve, you can start to get SOME idea of what a characteristic curve does. Shadow, midtones, highlight contrast. After all, PS curves are derived from film curves.

But take that with a grain of salt.

 

[gainer]

First of all, thanks for the answers
But regarding Dmax and gamma, is it easy to get high Dmax values, while maintaining a normal gamma of 0,55 to 0,6? I have a feeling that higher Dmax pushes gamma to higher values.

Yes, higher Dmax means higher gamma, or contrast index, unless Dmin goes high with it, but that would be at best inefficient and at worst intolerable because it means high fog.

 

[ic-racer]

CAUTION, "OPINION" TO FOLLOW: The films 'contrast' is best described (for me) by the slope of the middle part of the curve (or various other means), whereas the useful exposure range is the distance on the X-axis from the speed point to the shoulder (or some designated point on the shoulder). Measuring D-max does not tell ME anything additional about the films latitude or contrast or speed. Another way to put it is that the D-max is the asymptote of the shoulder and not what I use as a point to determine the useful latitude. Just an opinion

 

[Nicholas Lindan]

I think you have to be careful here: a film that can achieve a higher DMax through exposure doesn't have a higher gamma but a greater exposure latitude; developing a film to a higher DMax does result in a higher gamma.

 

[Chuck_P]

I will say something horrible on APUG, but...

If you play with a Photoshop curve, you can start to get SOME idea of what a characteristic curve does. Shadow, midtones, highlight contrast. After all, PS curves are derived from film curves.

But take that with a grain of salt.

I'll take that grain-----I've done that and it is a neat way to be able to visually observe the three parts of the curve: toe, straight line, and shoulder.

 

[Anon Ymous]

CAUTION, "OPINION" TO FOLLOW: The films 'contrast' is best described (for me) by the slope of the middle part of the curve (or various other means), whereas the useful exposure range is the distance on the X-axis from the speed point to the shoulder (or some designated point on the shoulder). Measuring D-max does not tell ME anything additional about the films latitude or contrast or speed. Another way to put it is that the D-max is the asymptote of the shoulder and not what I use as a point to determine the useful latitude. Just an opinion

I think I agree. If the value Dmax - Dmin is very large it will probably be useful for projection purposes only. Printing from such a medium becomes tricky I suppose. After all, the paper has a Dmax of itself and if its Dmax - Dmin value is lower than the negative's dodging/burning will/might be necessary. Not a task that everybody likes.

Now, from what I have read/heard the Dmax of a paper can reach up to 2,2. Some won't go over 1,5 (matt papers?). Is that correct?