Exposure for Digital Raw

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“If you expose digital the way you expose film, you run twin dangers of failing to exploit the camera’s dynamic range, and creating exposures whose shadows are noisier than they need to be.” – Adobe Whitepaper – Raw Capture, Linear Gamma and Exposure – Bruce Fraser.

Film was engineered to respond pretty much in the same way as we see, which made perfect sense at the time. While twice the light made the image brighter, it did not make the image twice as bright.

There is a compression factor in the way human senses respond to input which allows us to react over a wide range of values. This allows us to deal with indoor and outdoor light in a manner which is rather different than the change in values that are actually present.

Not so with digital capture. Camera sensors react in a linear fashion, so twice the light means twice the value – literally. This linear distribution of values means that each stop of light captured by your camera is either double or half of the value of the previous. Processed in this manner the image would not look good to us for creating images. So, the distribution of tonal values in a processed file are given a rather strong gamma correction to make the image look like we would expect.

This is more than just behind the scenes data as it is important to understand how the camera exposure relates to the finished image. In raw capture, the first stop at the bright end of the exposure actually contains 50% of the total number of captured photons. Underexposure means throwing away valuable capture information. As each stop darker only contains half the photons, shadow exposure has significantly less information than highlight exposure. Expanding the darker exposure to brighten the file therefore introduces the danger of increased image noise. Compression of information by darkening an image does not have this issue.

We tend to make exposure decisions based on the camera processed jpgs. This is only of real value if you intend to actually use the jpg file. The representation of the raw file by a camera processed jpg file is poor at best. The highlight exposure warning is especially deceiving as clipped jpg highlights may have no bearing on the content of the raw file.

I recommend that you set your camera jpg style settings to minimize the camera processing and not exaggerate the contrast range of the file. This means lowering the settings for sharpening and contrast in order to make the displayed jpg flatter and not scare you into thinking you are overexposing your file.

It is typical of camera meters to protect highlight exposure assuming that you are going to use the jpg file. This can mean underexposure of a raw file by a full stop or more. Losing that valuable highlight information is unfortunate at the very least. This means you need to test your camera and compare the jpg files to the raw files for exposure. If you are truly interested in capturing the best raw file possible, this is very important.

I will be presenting a program on Intermediate and Advanced Photoshop processing on Saturday, May 19th at the Hershey Public Library. The doors open at 9:00 a.m. Cost is $45. Locals (Hershey area) please register and pay the library in advance. If you are not local, please email me and let me know you are coming to the program so I can inform the library of how many to expect.

The program will explore processing in Photoshop that goes beyond common practices. This will include looking at blend modes, BlendIf, advanced selection and masking techniques and other procedures for improving the look of your images.

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