Question about RAW

[Charlie99]

I think there are lots of things you can change that aren't exposure. If you think about what a sensor array sees (as opposed to a single photosite) you can change the intensity and the color profile of the array as well as make a whole bunch of local adjustments, same as in the old darkroom. Exposure, color temperature, white balance, local saturation are all changeable. Now we do it in the computer but it was done for years in the color darkroom.

Bill

I'm pretty sure that BenWilson's point is that you cannot change, after the fact, the amount of light that reaches the sensor. You have the digitized data from the sensors. If the EV setting when taking the photo was too high and you have blown out highlights, no amount of exposure adjustment of the digital data will correct that.

If you are underexposed, while increasing the "exposure" setting in the raw processing software will increase the apparent brightness of the shadow areas, it will also increase the noise. A true change in exposure, done at the time of the photo would increase the amount of light seen by the sensor, which increases the signal to noise ratio.

Using the analogy of films and darkrooms, the manipulations of the RAW file are like making custom adjustments to your prints. But what isn't there in the negative can't be recovered. Hence the name Digital Negative (DNG).

 

[herbdb]

You guys are over my head here. I get what you are saying and agree that we should try to get it right. Sometimes we fail.

Here is a before and after of an underexposed shot. Only the exposure was adjusted (and of course it was saved in a reduced format for the web.) I don't think you can do this with a jpg.

 

[bvanant]

I am not sure that is how the raw file processing in Lightroom works. Clearly if the sensor is saturated then there is nothing you can do, but there are many cases where a JPEG file is totally blown out that can be salvaged very nicely in LR (or other raw file processing software). Look at
Processing RAW Files in Adobe Lightroom
for some examples. In Lightroom you can both change exposure and brightness. Yes you can not change the amount of light that reaches the sensor but you can change (essentially) the sensitivity of the sensor (or the gain).


Bill

 

[Charlie99]

What you can adjust after the fact is the mapping between sensor reading and the brightness in the photo. Somewhat similar to the Curves adjustment in Photoshop.

The advantage of RAW is that the full sensor data (often 16 bits or more per channel for 65,000 levels per channel) is retained. In a JPEG the data is compressed and in a horribly underexposed photo many different output levels from the sensor will be grouped together as just one of the 256 levels in the JPG.

Attached is the underexposed photo from a couple of posts back, with the only tweak being a levels adjustment layer in Photoshop Elements. Note the banding. You can also see lots of JPEG compression artifacts where the JPEG algorithm thought that there wasn't much info in adjacent sections with almost the same level, so it smooshed them together. (Actually, it may be that a lot of the artifacts were introduced when the JPEG was reduced to web size. But the banding is pretty much independent of the lossy recompression of going from jpeg to another jpeg)


In converting the sensor data to jpeg, a lot of information is thrown away and various choices on such things as sharpening are decided. RAW lets you make those choices after the fact.

All I'm saying is that the term "exposure" in reference to RAW conversion is somewhat misleading. To use the analogy of film, the "exposure" control of the RAW converter is similar to changing the exposure time when PRINTING. You are changing exposure times, but the exposure time you are changing is how long the negative is being projected onto the print paper.

 

[herbdb]

Thanks for the link to the Raw Processing article bvanant. I'm on a trip now, but saving the link to study later.

 

[Blackwood]

When shooting in RAW mode, aside from being able to adjust the white balance, what other advantages/benefits are there compared to post processing a jpeg photo with photoshop? Also, what is the difference between a RAW file and a DNG file, any reason to use one over the other?

Thanks

A DNG file is raw and then some. It is specifically adobe's raw format, and conforms to existing TIFF specifications.

Canon records .CR2, Nikon records .NEF, etc.. There are differences in format but in essense they contain the same thing: brightness values for each pixel and camera settings which, with the appropriate color mapping, can be used to raster an image which would be more or less identical to a JPEG produced in camera.

DNG is intended to be platform independent and offer additional functionality which you can find on google.