Tonal range of B&W contact prints.greenspun.com : LUSENET : Large format photography : One Thread
I've been reading and learning about exposure in regards to B&W film and papers. Most of the material I have read explains that an average scene will have a tonal range of about 128:1 which represents a log range of about 2.1. Furthermore when you take in to account that lens flare can reduce this range by a factor of two (or more), you may be actually dealing with a range of 64:1. This would give you a log range of about 1.8 which now must be recorded onto your B&W film.
From the books that I have read and have been using as a reference has lead me to the conclusion that the film would now compress this image down to about a 32:1 range. This is somewhere about a 1.3 log scale range or in that range. Now as we go to the printing phase the enlargement can be expanded on glossy paper to 128:1 range if everything is working extremely well but most times we are fortunate to get past a 100:1 print.
Now my question is...If there is such a high compression from the beginning image until the negative then how come many people agree " that a 8x10 negative contact printed has a much better tonal range than an enlargement" ? Would not a contact print be just as limited in range as an enlargment because the compression/expansion is controlled by the material involved in the total process ?
Thanks in advance,
-- GreyWolf (email@example.com), March 06, 2001
Why? Because they see what they want to see.
-- Bill (firstname.lastname@example.org), March 06, 2001.
Yes, 8x10 uses the same materials and is subject to the same limitations in terms of paper Dmax etc. However, 8x10 is a large sheet of film. This means the micro gradation is going to be much better. Larger sheet of film=more information to begin with. Micro gradation can be thought of as the contrast characteristics over small parts of the image (gradation is a similar concept - the basic idea is that contrast between objects can vary as a function of their sizes and how close they are to each other). The micro contrast is always lower than the macro contrast. So, if you had 2 pretty large objects, both of which occupied a significant portion of the film, but one of them was twice as bright as the other, you would be talking about the macro contrast between them, which can be pretty good i.e., you stand a good chance of reproducing the tonal relationships between them in a print. However, if you are talking about 2 pretty small objects, both of which occupied pretty small areas on the film next to each other (e.g., the same two objects photographed with a shorter lens from further away), the micro contrast between them is going to be lower and is much more difficult to reproduce in a print. That is, you can do everything exactly the same way and end up with less contrast between the two objects in the print when they are small and close together on the film. This basically demonstrates that micro contrast is always lower than macro contrast. Macro contrast is captured by the characteristic curve. Micro contrast is a function of a number of variables, including inherent contrast or luminance differences in the subject, size on film, film thickness which controls irradiation etc.
Where does format fit into all this? The basic reason micro contrast is always lower than macro contrast is primarily due to the nature of the photographic process i.e., irradiation in the film emulsion etc. When objects are close to each other on film, the spread function results in some of the brightness from thee brighter object spreading over to the darker object, thereby reducing contrast between them. Now, the larger the format, the greater the magnification. In other words, parts of thee scene which are micro contrast on smaller film could well be macro contrast on larger sheets of film. Thus, the better preserved tonality.
On a related note, I suspect this is another reason, LF prints look sharper than prints from smaller formats. Things like irradiation and spread in an emulsion are basic properties of photography. They are components of the spread function. Larger film means that the spread component will be damaging much smaller areas as compared to the scenario where the same spread component can damage much larger areas with smaller film. It also explains why developers which create edge effects are particularly popular with smaller formats, because they create artifacts to compensate for the inherent lower micro contrast in smaller film sizes, thereby creating the illusion of greater micro contrast.
-- N Dhananjay (email@example.com), March 07, 2001.
Grey Wolf - Instead of using theory to try and prove or disprove the existance of something which is right in front of you, why don't you look at the referent itself? In other words, the proper question is: Is a contact print qualitatively different than an enlargement? If so, why? (I think DJ has some good points here about micro- gradation.)
And if you DON'T think there is a difference - congratulations! You will be spared a lot of expense, time, weight and trouble.
Don't get me wrong. There are times when a good enlargement from 4x5 to 8x10 can be almost indistinguishable from an 8x10 contact print. (If you want the differences to be apparent then you might have to have a greater degree of enlargement. To make it obvious, look at the difference between a 35mm printed at 8x10 and an 8x10 contact.) However, to try to use theory to disprove an empirical fact is like the scientists who "proved" that the bumblebee can't fly.
-- Mark Parsons (firstname.lastname@example.org), March 07, 2001.
DJ has an excellent response to the question; I'd like to add and amplify on some of the points. So, FWIW here goes:
1.) I don't think the negative is the limiting factor in the contrast range of negative printing process. Most B&W films have a minimum sensible contrast range of 10 zones. Most modern emulsions are even better and have at least 13. (see "the Art of Photography" by Barnbaum. These extra zones are added on the dense end of the d=loge curve. Tmax 100 is on the order of 15 zones. If you don't believe this take a negative of a landscape with land & sky where the sky comes out blank and burn the crap out the sky. If there were any clouds you'll find that 4-8 times the basic exposure will bring them out. I was sure surprised by this when a printing class instructor showed me this.
2.) the real limiting factor in print contrast range is the print itself; a rather imperfect material. If you think about it, the white can only be 80-90% reflective and the black usually reflects at least 2-4%. The net range is usually about 30 to 1 with exceptional cases at 45 to 1. Now, because the light has to make 2 passes thru the emulsion the blacks can be deeper _but_ you still have the surface reflectance of at least 2% to deal with.
3.) It's obvious that a large negative has more information than a smaller one. This also translates into smoother contrast because even though we say that silver halide photograpy is an analog process it really isn't. It's a somewhat stochastic digital process with lot of dither to linearize it. A silver crystal is either exposed or it isn't. It's not zone 2 or 5 or 9. By averaging out the states of a number of crystals you can get variable representation of the exposure. Using more film and averaging over a larger area allows more graduations. eg. if we assume that we average over an area four times as big in each dimension (going from 35 mm to 4x5) we gain 16 times the graduation in intensity. We could also use this extra resolution to make bigger prints but if we don't then we get much smoother and smaller prints. Edward Weston used 8x10 negs & 8*10 contact prints to get what he wanted.
4.) I haven't thought about why contact prints should be different from indirect prints (ie. using an enlarger) but if you look at the optics involved they are certainly different at the very small, even diffraction limited level. I suspect there is more to it than imperfections in the enlarging lens & stray light in the optical path. I wonder if there's an optical interaction between the silver crystals in the neg and those in the paper. ie., the copying is done at the digital level rather than at the analog level where the information of the negative is averaged by the lens and re-digitized by the paper. (Hmmm, idle musings)
-- Duane K (email@example.com), March 07, 2001.
All lenses have abberations, flare being one of them. Flare reduces contrast, as you point out. Adding the flare of the enlarging lens, reduces your contrast range further. So a contact print, which doesn't have the flare of the enlarging lens, has a higher contrast range,or is less compressed.
Lens contrast (lack of flare) has often been desired over other performance issues, since it allows small changes in tonality to be presented to the film better than a lens with high flare.
-- Charlie Strack (firstname.lastname@example.org), March 08, 2001.
Grey, In practical terms, if you are weighing the differences between enlarging a 4x5 neg to an 8x10 print size and comparing the tonality with an 8x10 contact print, I would probably say to spend your money on something else. I really can't tell the difference between an 8x10 contact print and a 4x5 enlarged to 8x10. 8x10 negs really shine when your print size starts at 16x20 and upwards!
If you're going 8x10, consider an enlarger as well. That's where it gets really exciting.
-- Dave Anton (email@example.com), March 09, 2001.
Thanks to all for contributing towards my understanding. I must apologize for leaving the impression that I was considering acquiring an 8x10 camera. I am not. Simple fact is that I want to understand as much about exposure, negative density, and print making as I am able. To achieve this goal I tend to read quite a bit and also frequently experiment with my equipment and materials. My question was simply a quest for knowledge and nothing more.
-- GreyWolf (firstname.lastname@example.org), March 10, 2001.