Why take multiple exposure pictures when you can just change the exposure on a RAW file for an HDR image?

Because it will give you greater dynamic range than a single shot. Let's say you were to take three shots, these three different exposures when blended together will cover a wider dynamic range than a single shot could ever contain. Changing the exposure alone shouldn't cause noise - shooting at high ISO is what causes noise. If you try making an HDR from a single RAW, which is quite possible, you will notice a difference in noise in particular - especially in shadow areas. Taking multiple exposures will help reduce that. Try it - you'll see. Sometimes making an HDR from a single shot is more practical, especially if there's stuff moving in the shot. You really need to learn about the exposure triangle as you seem to be a little confused what exposure actually is: Exposure is a balance between aperture, shutter speed, and ISO sensitivity. For example you can change the exposure using the aperture alone, without changing the shutter speed. Read more here: http://www.cambridgeincolour.com/tutorials/camera-exposure.htm To take good HDRs put your camera on Tv (shutter priority), set auto exposure bracketing if your camera has that function. If your camera doesn't have auto exposure bracketing it's going to be a pain in the 4$$ to do.

You can't "change the exposure" on a RAW image. Yes, I know Abode camera RAW has an "exposure" slider -- that doesn't change the exposure actually (that was fixed when it was taken), all it does is move the black and white points of the exposed image around. Your sensor has a fixed dynamic range -- it can only record so many "stops" of information. If a scene has a range of brightness values bigger than your sensor can record, you can't record all of those ranges of brightness. Period. No amount of tweaking in RAW conversion will give you what wasn't recorded in the first place. You'll either have to shift the exposure towards more detail in the highlights, and lose the darks; or shift it towards more detail in the darks, and blow out the highlights. That's why HDR uses multiple exposures, biased both ways, towards highlights and darks, to combine into a single image that shows more brightness range than your sensor can in one exposure. And perhaps your camera is changing aperture instead of shutter speed? Peace.

To correct what some have said, technically you cannot create a true HDR image from one image. HDR is used to create an image that has greater dynamic range than the sensor of your camera is capable of achieving. A raw image can not possibly have a range greater than what the sensor can record. That's why you take three or more images at over and under-exposure. Creating an "HDR" from one image is not an HDR technique, no matter how many people claim that it is. It's tone-mapping.

It depends on the situation. In some cases it is possible to make an HDR image from one RAW file as in this article http://philip-northeast.suite101.com/how-to-create-an-hdr-image-from-one-digital-photograph-a307717 However, where the extremes of light and dark are larger you need the extra range of light you get from multiple images. The shutter speed should be the only exposure control altered on a set of HDR photos. The aperture remains the same keeping the depth of field the same. http://philip-northeast.suite101.com/digital-photography-tips-on-pictures-for-hdr-photo-software-a267287

Changing exposure on the camera doesn't cause noise unless you are changing the ISO. Generally when doing HDR you change the shutter speed. But you are on to something. RAW has about 1-2 stops of exposure latitude similar to a photographic negative. If that is all you need, then you can create and "HDR" image by processing the same RAW image twice; once for the highlights and once for the shadows. Over processing a RAW file can actually introduce noise into the image. However if you need more than just an extra stop of exposure, the best way to do it is to shoot multiple images.

Will that be enough? I guess not. Be more curious. Exposure level does not depend on shutter speed alone.

As BK said you get much more data for the Radiance file the HDR software creates when you bracket shots, there will be detail in all tones of the image. You should shoot in Raw for the same reason, I usually take 3 frames 2 stops apart and always shoot in Raw. You don't use Tv (shutter priority) as then the camera will change aperture and probably go out of range as there are only a few apertures on any lens, and it will alter depth of field between shots, use AV (Aperture Priority) the camera will then alter the shutter speed with a wide range to choose from and depth of field will be constant on all frames. Are you sure your using auto bracketing and not just using continuous shooting? Not all cameras have auto bracketing. HDR from one image http://www.flickr.com/photos/chris-judge/6612512003/in/set-72157608074352722/ HDR from 3 bracketed images http://www.flickr.com/photos/chris-judge/5579790149/in/set-72157608074352722/ My HDR Flickr file http://www.flickr.com/photos/chris-judge/sets/72157608074352722/ EDIT: The under-exposed image (the darkest one) will always have more noise than the rest as the exposure is closer to the ever present sensor generated noise. Chris