Range of useful exposures.
Reversible films and negative films have significant technical differences compared to each other. Negative films, just like in black and white, They generally have a gamma contrast ratio of approx. 0,7. Reversible films, on the other hand, as well as positive materials, are developed to a much higher contrast ratio. Therefore, in both materials, attention should be paid to the range of their useful exposures. With a gamma contrast ratio = 0,7 the range of usable exposures will be between the two limit densities D = 0,1 i D = 2,2. This corresponds to the relationship 1 :1000, while with reversible membranes, with contrast ratio gamma = 1,5 exposures for the same densities relate to each other as approx. 1:25. On the other hand, high contrast reversible membranes are a must, because softer films will give us images that are too flat, completely expressionless. On the other hand, negative films do not require it and the necessary contrast is obtained only in copies made of them.
It practically means that, that we can use the negative film to take pictures of two motifs- or three times poorer in contrasts, and expose it four times longer, than is necessary, without major color distortions (naturally underexposure will give us incorrect and smoky colors, while losing details in the shadows of the image).
Reversible films should be exposed with pedantic accuracy. Normally, motifs without high contrasts tolerate the exposure difference of about half the aperture value, less or more, so that the correct image can only be obtained between two adjacent aperture numbers. So it's understandable, how narrow or wide the choice of possibilities is. Who is very skilled in using a photoelectric light meter, can display them all correctly 36 photos held on 35mm film. It is safer though, especially with more valuable photos, perform them using two or three different exposures, in which we use a different aperture. It is more correct to use a series of different apertures than to choose and use different shutter speeds, because shutter speeds in cameras do not always correspond exactly to their markings.
Even a serious amateur familiar with black and white photography in the field of color photography will act uncertainly, because he doesn't know, what a properly exposed color film looks like, and when unacceptable overexposures occur. Don't forget, that with reversible films, the underexposed material will be too dark, and overexposed - vice versa - too bright. With too short exposure, the dark parts of the photo show some color deviations, while the brighter places show the correct color shades. Then viewing the completely unexposed portions of the film or film, for example their edges, in transmitted light we can most often tell, that they are not neutral gray. Such abnormal film fogging is a result of either development process errors, or they come from not quite good material (bad storage or the like). However, we cannot completely remove a certain low density of color haze, and it occurs in every case. Underexposed color negative film is easy for the trained eye to recognize due to the disappearance of details in the shadows of the image, while X-rays are much harder to tell. If the subject you are photographing contains vivid colors, we recognize this too long exposure by this, that these colors are not pure and distinct in complementary colors, but they show browning. In monotonous objects, such as landscapes, we can recognize this error as a general softening of the contrast. The negative will naturally be darker than normal, but this property is difficult to establish for novice amateurs, because it does not occur to such a strong extent as with black and white materials. On the other hand, a negative considered relatively dark out of caution can still be copied quite well.