Characteristic curve and exposure latitude

Correct me if I am wrong, but what I think you are trying to say in broad terms, is that the "charastic curve of the film" describes the speed of each film.

No, not at all. The relative log density scale and basic shape of the curve is the same for example on Ilford's graphs for both PanF+ (ISO 50) and HP5+ (ISO 400). The differences are the angle and length of the straight line portion and the shapes of the toe and the shoulder. So, on these graphs at least, it is unrelated to film speed.

Looking at the characteristic curve only gives half of the overall equation. You also need to consider the limits of the print medium. This varies according to whether you use a diffusion or condensor enlarger and the grade of paper used. There will be a limit to how much of the tonal range given on the negitve can be captured on the print. A low contrast negative will contain a larger tonal range before it reaches this limit and a high contrast negative a smaller tonal range. While it might seem tempting to simply underdevelop the negative to create a shallower slope allowing for a greater tonal range, you also need to consider the aesthetic consquences of a low contrast negative.

Exposure latitiude is the number of stops where the curve is "straight".

For example, if the curve is straight from log 1 to log 4 exposure that is a difference of 3. 10^3 = 1,000 which is about 10 stops (10 stops is 2^10 = 1,024). The actual math is (log 1,000)/(log 2) = 9.97 stops. Keep in mind that the Ilford curve uses relative log exposure so they are not commiting to any give exposure value for any given density - it is all relative.

Present-day colour negative films typically have a tremendously long straight section in the response curve which is why it has so much latitude.

I meant to answer your other question regarding 1.0 density relative to mid-grey.

If you use a condenser enlarger your film density (i.e. actual film density - film base - fog) would typically range from about 0.1 to about 1.2. Mid-grey (18%) is Zone 5, which is in the middle, so the density would be midway between the upper and lower usable density range, or about 0.65. Different films have different curves, and changing developers gives different curves for the same film, that is one of the reasons why the look of the printed picture is different with different film/developer.

Film testing to establish film speed is a piece of cake. Fine tuning the dilution to give the film density you want at the exposure you chose is more bother than I care to endure, but can be done.

Thanks Ian, that explanation is perfect.

Ian, would you like to explain why "exposure latitude is the number of stops where the curve is straight"

Neil, "exposure latitude is the number of stops where the curve is straight" is actually the definition used by film makers (I've probably dumbed down the actual definition since there must be some allowable deviation from a straight line). For many years I thought exposure latitude referred to the total range of exposures that could be captured and differentiated on film. The definition made more sense to me when I realized the limitations of range of negative density that can be printed on paper.

Ian, I'm not familiar with the film manufacturers definition that you refer to: "exposure latitude is the number of stops where the curve is straight". My understanding is that exposure latitude, is the change of exposure from optimum that STILL delivers acceptable results. (I think what you refer to in your quote, is the log exposure range over which the film's response is linear- a bit less than its input dynamic range.) It is therefore subjective, and also dependant on SCENE CONTRAST. As an example, imagine a scene whos range of brightnesses exactly corresponded with the input dynamic range of the film - you dont need to WORRY about a straight line portion, its not important. The correct exposure will place these on the films characteristic curve with the shadows on the toe and the highlights on the shoulder. Any deviation from optimum will result in a relatively poorly exposed negative -because shadows or highlights will be recorded on a part of the films characteristc curve that shows no contrast. Under such exposure conditions the film would be said to show NO exposure latitude. Now, imagine an alternative exposure condition, where the scence brightness range was considerably LESS than the input dynamic range of the film. The correct exposure will place will place the shadows beyond the toe, the mid- tones on the straight line portion, and the highlights before the shoulder. Now in this case, a small variation in exposure is possible before either the shadows or highlights would be recorded before the toe or beyond the shoulder. The exposure can be varied to a degree without degrading the neg, and such a condition therefore gives LATITUDE. Exposure latitude is related to the film contrast, also scene brightness range. The straight line portion isnt really relevant - exposure away from it results in tonal distortion - but we are all used to this. Remember, the paper also has a characteritsic curve: when combined with a film CC, the resulting print shows the familiar reverse S- shape CC with compressed highlights and shadows and increased contrast in the mid- tones.

Neil, we may have to agree to disagree on this issue. I wish I could find the reference to the manufacturer's definition of exposure latitude being the "straight" portion of the response curve. It was also in a quiz on a photo magazine website a while back - intended to clear up misconceptions about what exposure latitude really means.

If I come up with a reference I'll try to contact you offline.

Ian, thanks for offering to supply a reference to the definition of exposure latitude you work with, but my question to you was WHY you think exposure latitude is THE LENGTH OF THE STRAIGHT LINE PORTION OF THE CHARACTERISTC CURVE? and you haven't explained this - other than you think you read it somewhere! I am concerned that you regard the quiz section of a photo magazine as a reliable source of information. I have looked up the defintion of exposure latitude in The Ilford Manual of Photography 5th edition 1958, and this is what it says: "we define latitude as the factor by which minimum camera exposure to give a negative of adequate shadow detail may be multiplied without loss of highlight detail.....exposure range depends upon principally upon emulsion and the degree of development. These two factors also govern latitude, but in addition the latter is dependant opon subject contrast.." You might want to obtain a more recent addition - Focal Press in the UK. The section on sensitometry and tone reprodution should interest you.