Are RAWs in bracketed exposure (mostly) redundant?

Question

When I my camera takes RAW+JPEG with bracketed exposure (-1/0/+1 EV), it seems to me that the only difference that could logically exist between the RAW images is the shutter speed.

If I assume that the default 0 EV has reasonable exposure, does this mean that the -1 and +1 RAWs are more or less redundant? Or do they really have a significant amount of extra information that would be necessary if (say) I wanted to create HDR images down the road, or do something else nontrivial?

(Basically, I'm trying to see if I should really keep around all 3 RAW files, since they're so huge.)

Answer 1

The point of bracketing in the context you described is to come up with a higher dynamic range that the one you can get with a single picture because you want or need to do it.

Even if RAW has more bits per channel than JPEG, it still have a limited dynamic range. So depending of the scene you have in front of you, you will want or need to take pictures with different exposures. Moreover, it isn't all about RAW vs JPEG, each camera model has its own capabilities to capture a given dynamic range. Basically, if your histogram shows "burned" high light or completely black shadows, information will be missing with a single shot.

If you think that the 0EV shot has a reasonable exposure, maybe you don't need HDR in the first place (you can even get HDR like result from a single RAW).

Now obviously, the EV difference between two shots will be function of the dynamic range you want to capture, thus function of the scene. And yes, in this case some information will be redundant anyway, as what is in the middle range of picture A (say 0 EV) will also be present in B (say -1EV) and C (+1EV). To limit this, you have to set the bracketing parameter to adequate values.

Regarding the number of pictures you need and the EV difference between each shot, plenty of persons have plenty of advices. What works for me as a "standard" (say 1 2/3 or 2 EV difference with 7 pictures for a given scene) may be inadequate for someone else.

I would like to add that the shutter speed isn't the only variable. Some camera can change ISO or aperture to obtain bracketing (use aperture bracketing only if you know what it does).

Kartick Vaddadi provided an example to illustrated how information is indeed redundant, have a look : https://photo.stackexchange.com/a/67276/26456

Answer 2

Bracketing is useful when the scene exceeds the dynamic-range of the camera. That is it. There is no answer that applies all the time because it depends on the scene.

Answer 3

If I assume that the default 0 EV has reasonable exposure, does this mean that the -1 and +1 RAWs are more or less redundant?

If the exposure is reasonable in both the shadows and highlights — that is, the scene has a dynamic range that easily fits within what the camera can capture — then yes, you're right. Bracketed exposure doesn't do much for you.

Even if the exposure is just one or two stops off at either end, you can often extract enough from a single raw to make a decent composite covering the full dynamic range.

However, if the scene has even more dynamic range than that, having more exposures will help. If there are both very bright highlights and very deep shadows, this may exceed the capability of a single RAW file. In that case, you probably want to bracket even wider than that, taking one central image, and then one to cover the highlights and another the shadows. You may even want to take multiples, getting both the extremes and more in the middle.

Before combining multiple images together for HDR became popular, the primary other reason to bracket was when you weren't sure of which exposure choice you want to make. That makes more sense with film — with digital, you'd normally just watch the histogram, or take a test shot and review.

Answer 4

I've found that bracketing by ±1 or ±2EV doesn't help, but bracketing by ±3EV does.

I did the following experiment:

First, I picked a high dynamic range scene, short of directly shooting the sun. Here, the sun was partially behind thin clouds, and it was uncomfortably bright to look at with the naked eye. In other words, it was much brighter than it looks in the photos below.

For these scene, I took three photos bracketed by 1EV, and did exposure fusion in Photomatix:

I then threw away the -1 and +1 EV originals, and synthesized them from the 0EV one in Lightroom. I then fused all three photos:

I don't notice any difference between them.

I repeated this experiment for ±2EV, first with bracketing:

And then without:

Again, no difference.

I then repeated the test with ±3EV, with bracketing:

And then without:

Please open the photos in separate tabs to view them at full size.

This time, there's a clear difference, both in the colors being off and, in the synthesized version, in a faint rainbow seen on the wall on the right, all the way from the top to the bottom.

The conclusion I drew from this exercise is that even for a high dynamic range scene, exposure bracketing by ±1 or ±2EV doesn't result in a better photo. All it does is waste time and add complexity, like keeping track of more photos, for no benefit. If I bracket, I'll bracket by ±3EV.

This is on the Sony NEX-5R; performance may vary on your camera.

Answer 5

It depends. If the entire dynamic range of the scene can be captured in a single exposure, then it is redundant. If the entire dynamic range of the scene exceeds the capacity of a single exposure, and you wish to capture that entire dynamic range, then you need to bracket.

Be aware that a single 14-bit raw file may contain as much dynamic range information as a a -3, 0, +3 series of 8-bit JPEGs! So shooting a series of raw shots bracketed at -1, 0, +1 includes a lot of overlap, even with digital cameras that can only capture 10-11 stops of dynamic range. The best cameras today can go as high as 13-14 stops at base ISO. As you raise the ISO, any camera's DR will be reduced. For a very wide dynamic range scene you can still get all of the mid-tones without any gaps by shooting a -2, 0, +2 or even -3, 0, +3 series if you are shooting at fairly low ISO (around ISO 800 or less) and center your base exposure value properly.

Answer 6

Let me add the issue of noise to the answers already given. Suppose that the 0 EV exposure is good enough, the shadows are not underexposed, the bright areas are not overexposed, then it's still the case that using the +1 EV exposure (for the parts that are not overexposed) will yield a lower noise picture. By exposing for longer and then adjusting the brightness, to the desired level, you are eliminating noise by averaging. Also, in a HDR picture compiled from different exposures, it's not the case that each pixel originates from only one of the exposures; typically all the exposures will contribute to some degree. So, there is then also an averaging involved with the 3 different exposures, this will then further reduce the noise.

Answer 7

RAW images do not contain all information that could be retrieved from a scene. They simply contain more information than JPEG images. When a photographer chooses to bracket photographs and combine them later, it is usually because the bright and dark areas of their scene are sufficiently different that the camera is not capable of detecting the differences in the dark areas, while also detecting the differences in the bright areas. To detect the differences in he dark areas, the bright areas would saturate the photosites. By bracketing the photographs the camera detects either of the detail in the dark areas, or more of the detail in the bright areas, thus the information contained within the three photographs is different, sometimes drastically. These bracketed images can be combined to create one image that is a reasonable representation of the scene as it is seen by the human eye.

If you were to try to create the same image from JPEGs, it is possible that more of them would be required due to their reduced dynamic range.

Answer 8

You may find you want to take either the +1 or the -1 from the RAW, and you won't know until you sit down to work on it. This is true even if you have bracketed exposures.

If something moves between exposures you can have strange effects with proper HDR. Even if you think everything is still the slightest breeze can do this, or a solid tripod on a less-than-solid floor, someone shifts their weight. Even a subpixel movement can show up. Or how about a bug flying across the scene?

Don't forget you can take your +1 raw and get more shadow detail than you could from the +1 jpg, or the raw imported using defaults.

Finally raws aren't that big, not really. They're 10s of MB when drives are 100s of GB. You could always zip them when you're finished editing as well (I used to do this with the 12-bit files from a scientific camera to good effect, I haven't tried with my .cr2s). I work on the principle of "only delete rubbish".