Correct exposure is different for different kinds of film and very different for digital.
Because of this there is a lot of confusion over what is correct exposure.
Let me explain.
Exposure is the lightness or darkness of a picture. That's the easy part.
The confusion starts when people misconstrue exposure as an absolute.
Correct exposure depends on what you're going to do with any given file or film.
Correct exposure also depends on the subject's lighting ratio (range from light to dark).
Exposure Latitude also is not an absolute. Latitude, or how far off your exposure can be and still be acceptable, depends on all the factors above. Latitude is your room for error in making exposure.
Latitude is broad if your subject has little contrast and if you will edit the image before use.
Latitude is the amount by which your camera's dynamic range exceeds the range of the subject, if and only if you have the opportunity to edit the image before final use.
There is very little latitude if you're using a digital file or transparency as-is. Digital files need to be within 1/3 of a stop if you use them as-is! Luckily most cameras are this good today.
Latitude slims as subject contrast increases. Latitude evaporates when your subject has too much contrast. When this happens there is no correct exposure. It looks bad no matter how you set the exposure. This stumps many beginners. Many times we encounter subjects with too much contrast to be captured without modifying the light.
If there is so little to no latitude today, why do people still discuss it as if it was helpful? Black and white negatives were 99% of photography for the first 150 years of photography. Color only went mainstream in the mid 1970s and digital has only been popular for the past 48 months. B/W negatives have always had broader range than almost any subject. You could overexpose five or ten full stops and get great prints, if you were willing to expose the paper long enough. Old timers take latitude for granted. Digital has no tolerance for overexposure, although, unlike film, it takes a couple of stops of underexposure easily.
Subject Contrast and Lighting Ratio
If the subject's contrast exceeds the brightness range of your digital camera or film, there is less than no latitude and there is no correct exposure! These images look awful no matter how you set the exposure.
An example is photographing indoors and expecting to see both the room's interior and the view out the window.
This confuses beginners. Beginners waste time with hocus-pocus get-rich-quick Photoshop schemes like HDR (High Dynamic Range) compositing to attempt to turn crappy light into a good photograph.
Good photographers modify the lighting to make the subject photographable. Add light to the dark parts with fill flash or other lighting or reflectors, dim the bright sections, or both. Bright windows are dimmed by putting gels (sheets of gray plastic filter material bought at stage and movie lighting supply houses) over windows. Scrims (black mesh cloth mounted in frames) are used to dim the light falling on something outdoors.
This is why you'll see all the things you do on a movie set. That's the lighting and rigging required to make a scene photograph naturally. Shooting in available daylight looks horrible and amateur: shadows in people's eye sockets look completely black on film and make people's heads look like skulls while sun shining off foreheads make everyone look bald. Fill the shadows with fill flash or a white reflector and dim the direct light with a scrim held above and out of the picture for pro results.
If you're using a digital file for direct printing or a transparency for projection or this is easy. Just make it look right.
If the subject contrast was too great there is no correct exposure. You could lose your highlights and your shadows at the same time! At best you might retain either one, but the photo is still awful.
Optimum exposure is more complicated if you're editing the file before printing or scanning or printing the transparency. In these cases you need to understand your use and your medium before you make your image.
It's critical to preserve the highlights. Shadows are trivial to pull out in digital, and vary from transparencies depending on your scanner.
Overexpose the highlights by as little as a third stop in digital and they're lost forever. Slides are much easier: you might have a full stop of wiggle room. Luckily this is easy to avoid in digital: look at a full RGB histogram and you can see if the highlights are lost. Beware: single-color histograms are less than useless, since they usually miss blown out highlights! See my RGB histogram page for more.
This is why I wrote this article.
When I first used a Nikon D1X in 2001 I thought it was awful. If there was an open window in an indoor shot it would underexpose horribly, or so I thought.
I was wrong. The shot looked dark because the D1X was exposing to preserve the highlights. I was being a bad photographer having too much contrast and not doing enough to add fill flash.
Digital cameras are supposed to make images look dark in high-contrast situations. You easily can resuscitate the image in Photoshop's Image > Adjustments > Shadow/Highlight tool.
If the highlights were blown out instead, they're left as white blobs forever.
Many beginners think their cameras are underexposing. Nope. These photographers are trying to photograph under crappy light. Turn on your flash to fill the shadows and see what happens. Magic.
Nikon cameras are very smart. My D70 turns on its "Use Flash" bolt in broad daylight when the contrast is too high. I'll bet you you ignore the "Use Flash" bolt in these conditions. Nikon's flash bolt isn't a low light idiot light: it also warns of excessive contrast!
If you use fill flash and see the bolt blink rapidly right after your shot that means that the flash ran out of power and the image may still be dark. Use a bigger aperture, get closer or use a more powerful flash and you'll fix the problem far better that trying to pull your image out of the mud in Photoshop.
If you're shooting in crappy light and intend to print directly from the files without editing, use the exposure compensation control to dial in as many stops as needed. I've had to use several stops of + (lighten) compensation in these instances.
Another source of confusion is the incorrect use of the Matrix Meter (Evaluative Metering in Canon). Dark filters, like polarizers, often fool the artificial intelligence of these meters and lead to underexposure of light subjects in daylight. More at my Matrix Meter page.
Negative film gets little use today, so I excluded it from the above. It exposes completely differently than anything else.
There is no way to direct-view the exposure of a negative. The closest way to see a negative is to make a "perfect proof," which is a contact print made with the minimum amount of exposure required to make the unexposed film edges print as dark as the paper can get. You determine the correct print exposure by making a test strip. Look at the edge of the unexposed film against the black of the paper. Use the exposure time of the first bar in which the black of the edge of the film matches the black of the paper.
Contact sheets or prints in which the edge of the film looks less black than the rest of the paper have been underexposed, and that tells us that the negatives are also underexposed.
If you find this as helpful as a book you might have had to buy or a workshop you may have had to take, feel free to help me write more with a donation. Thanks! Ken.