There is a recurring complaint in our latest camera reviews which deserves some attention because, unfortunately, more and more cameras get it wrong. This should be obvious, but the reasons cameras have viewfinders it to preview as closely as possible a shot before it is taken. Each viewfinder type provides different advantages in this respect but digital previews from EVFs or LCDs often fail to achieve their potential.
A viewfinder which previews exposure is said to be Exposure-Priority. Optical viewfinders, including those used in digital SLRs, cannot do this at all since the image is formed optically. In the case of modern DSLRs, Live-View provides the opportunity to see an Exposure-Priority preview, yet very few actually do. Of the SLR models reviewed so far, only ones from Canon and Sony have completely implemented this.
The opposite of Exposure-Priority is Display-Priority which shows a bright image regardless of actual exposure. This is useful in certain rare circumstances but in the vast majority of cases, a photographer should know in advance that the resulting image will be improperly exposed. In other words, a bright image on the LCD which results in a completely black frame is useless.
Some discontinued cameras from Sony and Konica-Minolta had an option to toggle between both display modes. Some Canon DSLRs and Panasonic SLDs can also do both based on a custom setting. There also cameras which take the middle-ground approach, going from Display-Priority to Exposure-Priority on the half-press of the shutter-release.
Naturally, there are cases when previewing exposure is impossible. Most notably for flash photography, in which case a camera has to show a relatively bright display hoping that flash illumination will properly expose the scene.
There is another option for visualizing exposure and that is a Live-Histogram. This is, in theory, a histogram which gets updated in real-time and shows the distribution of tonalities. With a histogram, one can tell if parts of the image are likely to be over or under exposed. Indeed, review histograms are extremely useful to judge the adequacy of an exposure.
Well, that is the theory. A number of recent cameras apparently calculate the histogram from the display preview rather than the sensor output. If those displays would be Exposure-Priority, there would be no problem with that and we could not easily see the difference or perhaps not at all. Sadly, for Display-Priority cameras, such live-histogram is mostly useless.
From this, it really seems there is a stronger desire to show a bright image rather than a correct image in the preview! This is truly the reverse of what photographers need. They should see right away if an exposure is off and by how much. This is much more useful than blinking shutter-speed or aperture values which is what most cameras show outside their exposure-range. Results can be acceptable when slightly out of the range, compared to being completely off. Yet, blinking values do not convey this difference at all.