Accuracy of exposure

Discussion in 'Black and White' started by raymondc, Feb 14, 2014.

  1. Hi all, just developed 2 rolls, seems ok. It's my 3rd and 4th rolls. I heard that C41 films can be underexposed by 1 stop or over by 2 or 3 stops and you can still get an image out of it but not optimum. What's the story with b/w film? Would be handy if after I got home and then figured oh I underexposed the shot by 1 stop .. or something or I forgot to correct for reprocity.
    Cheers.
     
  2. I'll say in advance that there is no clear cut answer as it depends on alot of things for black and white films. Even with c-41 some films have a wider latitude than others, while some are more contrasty.
    With a developer like pota and the right film you can conceivably get 20 stops out of a film. Most of the time with TMAX or TRI X and d76, normal development its closer to 4 stops. These are rough numbers and agitation, developing time and filters can play with this.
    The best advice I can give you is to expose as as best you can until your familiar with the film. If you know the exact film and developer I sure someone here will know offhand what the latitude is.
    For reciprocity failure I would recommend going to the product home page as they usually have the best advice for compensating.
     
  3. SCL

    SCL

    Ray - with B&W film you obviously want to have optimal exposure, as that affords you more latitude in printing (or publishing). Depending on the type of enlarger head you may be using, you may well want to ascertain whether higher or lower contrast in your negatives works best for printing on #2 paper. But what is optimal for one person may not be for the next. Films display different characteristics depending on how they are exposed and developed...so it is really important for you to know what you want to achieve, and work toward building that with your film choice, exposure and development. With roll film (unlike sheet film) you don't have a choice of developing one frame differently from another....so exposure is really your only variable, meaning you want your exposures on a roll to adhere to a general characteristic of exposure so you have consistency in your output. For most people that means you just have to do a little reading to familiarize yourself with film characteristics and then experiment around with a developer of choice. I belong to the school which suggests not changing more than one variable at a time so you become competent with the results of the process.
     
  4. or I forgot to correct for reprocity.
    Then, you're screwed. Reciprocity is not enough photons to create a stable latent image. Without that, your developer has nothing to develop. Doesn't matter how long the film sits in the soup.
    As to the rest, some would say that the concept of "film latitude" is a myth, that you need to hit your exposures and your development times dead on. Guys like Ansel Adams thought this. Wrote books about it, and how to accomplish it. Other people think that the huge dynamic range of modern B&W films means you can "slide" your image around inside that dynamic range (assuming it fits of course) anywhere you want.
    Me, I've done plenty of experiments that tell me that, for my work and my workflow, Adams is correct. Clearly, YMMV.
     
  5. Optimal exposure is what we aim for, even though we know that we can get away with less or more. It is good to know what to do when, for example, a roll goes one way or the other because of an incorrect meter setting. It is not recommended, however, to make a habit of going under or over. Stephen's advice is worth much.
     
  6. Both C-41 and B&W films have very substantial leeway in exposure, although Mother generally knows best and the best results are usually as the manufacturer says. In B&W it's possible to also compensate in development, although with C/N, it may be easier to do it digitally after scanning carefully.
    Depending on your tolerance, you can usually get far better than 2 stops leeway out of these films.
     
  7. One stop under and two stops over is a pretty safe rule of thumb for most negative films, color or B&W. B&W probably gives you an additional stop in each direction. But as others have said you're still better off to be on the money. The plus-minus latitude is something that saves you when you're off, not something where you should intentionally be off or be sloppy and rely on it saving you. Keep in mind that many scenes have a wide range of brightness within them. Wedding pictures are a classic example, with the white gown, black tux and skin tones somewhere in between. The tux and gown are relying on that minus one/plus two latitude even if you have "correct" exposure for the skin.
     
  8. My advise is to standardize your meter and your development....where densitometer is your friend. Hope camera shutter is accurate (?). Anyway, once you determine the "standard", you can then be free to deviate from it.
    Les
     
  9. I know about optimal .. a friend of mine at the camera club has that tool that meaures the density although practically if a person prefers a lighter or darker image is that tool still correct?
    I just want to know if I made a mistake on that particular frame so I cannot push or pull the entire roll. When can I get "a" image so I know when I have to visit the place again just so I have a record shot.
     
  10. David Vestal did numerous test on underexposing / overexposing a B&W film. Conclusion: -1 / +3 stops range is OK, you won't see any major difference if you know how to print.
    Any tool to "help" you to determine the perfect exposure is a waste. Shoot, develop, print again and again: after a while you will know what to do without any help.
     
  11. The response of a given film to exposure for a given development is described by a characteristic curve, otherwise known as a d-logE curve (d for density, E for exposure, log for logarithmic - log exposure rather than linear exposure gives a more manageable curve shape). This kind of curve looks like this:
    http://en.wikipedia.org/wiki/Sensitometry
    Normal practice will be to place tones of importance on the straight-line part of the curve - the latitude (tolerance) of a film to underexposure is really the degree to which you can place tones on the lower bent part of the curve (called the "toe") and still get a satisfactory rendition. Shadows given less than optimum exposure will end up on the toe and, while still showing some detail, will lack contrast. With b+w photography this can be compensated for by printing on more contrasty paper, with color neg film it can't, and underexposed shadows will look murky and green.
    Any exposure more than the necessary minimum means a certain loss of quality (increase in graininess), but overexposure only becomes a severe problem when highlight tones are pushed up to the top bent part of the curve (called the "shoulder") where increases in exposure produce only a small or almost no increase in density. Negatives like this will never print well - you can force light through overexposed highlights and get some tone in a print, but highlight detail will be impossible to print.
     
  12. "Negatives like this will never print well - you can force light through overexposed highlights and get some tone in a print, but highlight detail will be impossible to print."​
    The density can be reduced with Farmer's reducer, potassium ferricyanide.
     
  13. The density can be reduced with Farmer's reducer, potassium ferricyanide.
    This is a way of reducing density and getting some light through highlights to produce TONE in a print. Farmer's reducer will absolutely not help render highlight DETAIL
     
  14. I don't care so much, the film is over forgiving if you now how to print. That's the reason i come buck from digital to film again.
     
  15. We were all done a disservice by labs when they came up with the terms "push" and "pull". The truth is that these things don't exist, and they don't work. Certainly not as one would think. When I was teaching, I came up with a new adage: "Exposure and Development have nothing to do with each other." I know it's only 98% correct, but it helped my students to think properly about this. As others have already pointed out, if you don't expose enough, no amt of developing will fill in the shadow areas. You can develop for a year and all you might get is extra fog. The highlights (dark areas of b&w neg) are a different matter. Increasing development increases density considerably, and moves the highlights away from the shadows, increasing contrast. B&W film has a nice bit of latitude for the most part. I'd back off a little for the overall density and you'll be fine..
     

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