Sunday, September 23, 2007

Clarkvision: Digital Workflow Summarized

I found ClarkVision via Kotke, I've posted about both on Gordon's Notes recently. This ClarkVision description of workflow is noteworth because the author is an uber-Geek (MIT PhD planetary physics), an astronomer, and a super-serious photographer. The combination yields an almost unequalled knowledge of digital photography details.

I also like it because, it's consistent in many ways with my far more modest experience. I do a lot of these things, but I sacrifice quality for speed and portability. Also, use Aperture image editing, not Photoshop.

I don't agree with his approach to metadata (file names, nothing more), but I sympathize with the goal of avoiding lock-in. I'm making a calculated guess that the mass of photo geeks will provide a solution to save my metadata; until now iPhoto and Aperture both support writing applications that could extract most metada (if Aperture were able to export XML/XMP sidecars I'd be almost sanguine ... but, ominously, it doesn't).

Here are excerpts, the article is a great read ... (emphases mine with some inline comments]
Clarkvision: Digital Workflow

... I scan all my film at the full precision of the scanner. Good scanners are at least 12-bits/channel. The output file is 16-bits per channel TIFF images. I do the minimum processing at the time of the scan and save corrections for the photo editor where I have more control. Scan parameters include:

* Straight-line transfer curve.
* Brightness correction only.
* Little to no color correction...

... Digital Cameras. I do both jpeg and raw format output. Jpeg is only 8-bit, while raw on many cameras is 12 bit and a few now have 14-bit output. Raw files are converted to 16-bits/channel TIFF files... [jg. I use RAW in Aperture, no TIFF conversion.]

... In the photo editor

* 1) I only do 16-bit editing. If the starting file is 8-bit (e.g. jpeg), the first step is to convert to 16-bits/channel. [jg. If you work with RAW in Aperture this isn't an issue ...]

o Why? with integer math, there is always round-off error of 1 bit.... If you do multiple editing steps, added errors can result in poor intensity precision with an 8-bit file. This is called posterization. 16-bit editing provides enough precision so that posterization is not a problem....

* 2) Color Space. Check the color space and convert to a wide color space if not already there. I generally use Adobe RGB 1998.

* 3) Adjust Levels. First adjust the brightest portions of the image to your liking using the levels adjustment tool. For example, in a landscape image I might examine the brightest areas of the images, like clouds and adjust the levels adjustment slider so that the brightest parts just reach a value of 254 or 255 on the 8-bit scale typical of the slider tool. ... This levels adjustment is done on the entire image.

* 4) Curves Adjustment. The next step is curves adjustment using the curves tool. Never use Photoshop's contrast and brightness adjustment tools as they are additive. The curves tool, like the levels tool is a multiplicative tool. Multiplicative has the correct math to mimic changes in scene brightness, exposure, or f/stop changes. In the levels tool, the upper slider is used to derive the multiplier, and lower slider is an offset (a subtractive adjustment), and the middle slider changes the multiplier to a 2-part piecewise line multiplier.

* 5) Dodge and Burn selected regions. Select different regions as desired that may be at the limits of dynamic range and dodge and burn to bring them into printable range. For example, bring up shadow detail, or darken clouds. First select the area with one of the selection tools. Next feather it. Feathering makes the selection area a smooth transition to the rest of the image. Once feathered, adjust the level with one of the three following tools:
o Curves tool,
o Levels tool, or
o Shadow highlight tool.

* 6) Increase Image Size. If I'll be making larger prints, I'll interpolate the image to a higher pixel count. I usually use bi-cubic or bicubic smoother interpolation in Photoshop. [jg: this surprises me. It seems a bit magical, but this guy knows his stuff. This one item suggest how sophisticated modern Photoshop is ...]

* 7) "Sharpen." The main tool many use is called unsharp mask, a filter. But unsharp mask does not actually sharpen! The way unsharp mask works is that the image is blurred using a certain radius, and then the original image is differenced with the blurred image, and the result is added back to the original. The effect is to modify contrast around edges. The amount added back is usually controlled by the user (called amount in Photoshop). The effect is actually a change in acutance, not sharpness. But increased acutance gives the appearance of increased sharpness.

However, there are other methods that actually do sharpen. I use Richardson-Lucy image restoration. Photoshop currently does not have this tool.. [jg: Aperture has two sharpening tools, I use Edge Sharpen but I don't know how it works ...]

* 10) Save the file. Depending on my intended use, I save as a 16-bit/channel TIFF files, or convert to 8-bits/channel and save as jpeg. For best quality, 16-tiff files are necessary. I only save as jpeg at highest quality and only when I do not need highest quality. [jg. I never need his quality, so I always save as JPEG, highest quality, for maximal longevity. I work with RAW in Aperture and toss 'em when I'm done. (The Horror!]...

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