I've started using Photoshop recently and a lot of people mention "blown out backgrounds", and how to stop them — but never explain what they are. Could someone explain?
"Blown out" means the maximum value for all color channels.
For RGB, that means maximum red, maximum blue, and maximum green. This appears to be pure white, even if the color of the subject is not white or a shade of gray.
If you take a baby blue wall and expose it properly, in the resulting image the wall will be baby blue. If you over expose it enough, though, even the green and red channels will reach full saturation and it will look white in the resulting image.
This is because there is only so much red that the red channel can record as a numerical value, so much green that the green channel can record as a numerical value, and so much blue that the blue channel can record as a numerical value.
Imagine the image is grossly overexposed and there's enough green light to give a value of, say, 10,000, but only enough blue and red light to give them comparative values of 320 and 442, respectively. If the maximum value for each color channel is 255 (8-bit raster image format), then all three color channels will have a value of 255 (255,255,255) because that is the highest number each channel can record in 8-bits. Since all three channels are at their maximum values, and all three channels are at the same value, the resulting color when viewed will be pure white.
Now apply this to a photo of an interior room with windows during daylight hours. The room is a lot darker than the outdoors seen through the windows. If we expose properly for the room, the widows will show up as pure white squares and no details of the world outside will be recorded in our photo. On the other hand, if we expose properly for the world outside the windows, the interior room will be almost pure black (0,0,0) with details lost in the noise floor of our camera's sensor.
For examples of how this can work out in different types of photography, please see:
Why is it that when the green channel clips, it turns into blue?
Why is my long exposure shot all white?
How to photograph a room showing both room & view out a window?
How many lumens are required to achieve a pure white background in a 40cmx40cm softbox?
As a product photographer, you tend to actually want blown-out backgrounds and put up extra flashes in order to get them.
A blown-out background is a background that is so bright compared to the subject that the camera renders it as a uniform contourless white area.
The advantage to such a uniformly blown-out background is that it is rather simple to mark and remove such backgrounds (replacing them with transparency) in photo editors and put something else in. However, you usually don't want to overdo blowing out a background before fuzzy boundaries like hairs, and the cutouts tend to work mainly before a different bright background.
It would be my guess that replacing such a photographically blown-out background with transparency and possibly afterwards with a different background was what you were reading about.
This word originates in the age of so called wet process when film, or desk, was exposed in a camera and then developed in a dark room. After development the film is no more sensitive to the light and it was projected on a photo-paper which was developed using same process and you got the photograph. The most common was, and still is, a negative process where the lit parts get dark while the unlit are bright.
The blown out areas are such areas of the scene where the film was overexposed so it turns black and no detail could be retrieved from this area. After development of the photo, the area was plain white with the edges blurred so it looks like it was burned.
In digital era and much more powerful postprocessing this term means the same thing: plain white area in the image.
It can be caused by (assuming it is not intended at all):
- Wrong aperture/shutter setting so the chip cannot register more light. The capacity of the pixel is saturated. Solution: Adjust Aperture/shutter speed.
- Wrong "ISO setting". The amplification setting of the analog-to-digital converter results in values overflowing the highest acceptable value. Solution: Adjust ISO value.
- Wrong flash setup. The flash illuminating the scene was not taken in account and triggers one or both causes above. Solution: Adjust the flash setting and/or adjust apperture/shutter/ISO settings.
- Wrong postprocessing. The filters applied (brightness, contrast, curves, stacking, HDR,...) results in areas where the maximum values are oveflown. Solution: Adjust the filter properties.
The first two bullets must be anticipated when shooting, no matter what camera you are using - the data is lost before storing anywhere - and the scene must be re-shot. The last point is reversible, but it is a good practice to have unprocessed backups. There is also option of using NG filters or polariser.
Blown out background is just subsidiary of the above because it is specific to the backgrounds.
Also note that if you want [the background] to be blown out the best results come with playing with aperture/shutter/flash and using lowest ISO to get the intended effect. This way one can use the whole bit-depth to hold the nuances in shadows.
Blown out essentially means overexposed in photography. It's too bright, and the details for the image highlights are lost. The histogram for the image is chopped, or leaning heavily to the right. There are times when this is done intentionally, for a "high key" effect or to make the background super bright/white to contrast with the image subject. And there are times it is done by mistake, just by exposing the photo incorrectly. So whether it's a good thing or a bad thing depends on what look you're going for.