Mushroom Duotones

Posted a few duotone images of mushrooms to Flickr. Didn’t mean to spend this morning making these images, but when you get the right subject matter, there is an eerie presence to toned images: Maybe it’s the intersection of image qualities that recall old photography and emulsions hand-coated onto paper coupled with high tech display and printing—or maybe the mushrooms (just shitakes) have an effect on me.

This tutorial on duotones over at Luminous Landscape was useful.

Duotone Soft Proof

Can’t soft proof a duotone in Photoshop. Have to convert to RGB to soft proof; however, the color changes in subtle ways.

One way to compare the images is by checking their histograms.

The duotone image is just a little darker on average (mean 106.97 compared to mean 112.53), and it clearly has pixels in the darkest bins, including a spike of 0% black over on the left edge of its histogram. The RGB image has no black pixels until you reach level 11, about a 4% gray.

Comparison of two histograms

Comparison of RGB and duotone image histograms

You can use Curves adjustment layers to compare the dynamic ranges of the two versions of the image. Option-drag on the black point or the white point in the Curves dialog to see where in the image the darkest and lightest values are found. The duotone reveals solid black (0%) in a few places. The RGB version has no solid blacks. In the light tones, the two are practically the same.
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Lab Enhancements

Using Photoshop’s Lab color mode, you can perform a number of simple image enhancements. For some of these enhancements there are similar RGB operations; however, the results are subtly (and sometimes not-so-subtly) different. The international standard L*a*b* color space from which the Lab mode in Photoshop was derived was constructed to capture the range of human vision. It was based on statistical evaluations of the range of color vision (the “a” and “b” channels)  and of just-perceptible differences in brightness (the Lightness channel).

Several techniques are illustrated by Photoshop actions that you can download. Explanations and a few tips on how to perform the actions manually follow.

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Lab Lightness

Lab color mode can be used for brightness, contrast, and color enhancements in somewhat different ways than RGB mode. Where RGB mode provides a composite channel (RGB) that is composed of red, green and blue channels, Lab mode provides a composite channel (Lab) that is a composite of Lightness, “a” and “b.” The a and b channels encode the color information, while the Lightness channel, as it name suggests, encodes the grayscale values.

Image of a man, showing Lab and lightness channels

Sample image with Lab and lightness channels

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Night Noise

You can reduce the noise in low light photography by taking multiple images and merging them to extract the statistical mean or median value. These statistical operations are available in Photoshop’s Layers > Smart Objects > Stack Mode menu. Here is a clear example of the effects of the mean and median operations on a stack of four similar images.

three images showing noise reduction by mean and median operations

Original, mean, and median images compared

The leftmost image is one of four similar images that were stacked into a smart object. In the middle image, the mean value (average) of the four images is used as the value of at each pixel position. In the right image, the median value (midpoint of the range of values) is used. You can find the full median image here.

The noise reduction is pretty dramatic (you’ll have to click on the image and view it full size to see what I mean). I find the mean image somewhat smoother, visually, than the median image, but the median image has some advantages over the mean.

A man in a yellow rainjacket walked across the view while I was shooting. You can see four images of him in the mean image: logically, one fourth of the pixels  in the stack at those points belonged to the moving man, so he has a ghostly presence. In the median image, he has practically disappeared: the influence of details or noise that appear in only one of the images is much less marked than in the mean image. The median operation is particularly useful for removing momentary details from a statistical composite.

Of course, the reason the original images were so noisy is that I was shooting hand-held at a high ISO (1600). If I had used a tripod and lower ISO with time exposure, I would not have had to resort to statistical operations for noise reduction. Knowing that the image would be noisy, I shot multiple images and used Photoshop’s File > Automate > Photomerge… command to position the images in layers. Details of how to do this can be found in an earlier post, Statistical Blending.

Layered Contrast

Here is a Photoshop technique for contrast that uses layers, one for the lights and one for the darks. Contrast can be adjusted with many commands in Photoshop: Brightness/Contrast, Levels, Curves, Exposure, to name a few. Layered contrast provides certain kinds of control you can’t achieve with the other commands. I’ll describe it step by step for you, by way of explanation, and also provide a downloadable action.

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High Pass Enhancements

Photoshop’s high pass filter can be used with layers to achieve some very useful image enhancements. This post discusses how to increase or decrease contrast along object edges and provides a few downloadable PS Actions. High pass edge contrast enhancement is a standard trick for adding “punch” to images: you probably see it all the time without even realizing it. Edge contrast reduction is a logical consequence of edge enhancement. It could be used as a “softening” filter, but probably qualifies as an “effect,” since it runs counter to expectations for good images. In other words, it’s just waiting for someone to exploit its potential.

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Statistical Blending

You can use statistical blending to render high dynamic range images, to reduce noise, or to create multiple exposure effects in Photoshop (CS4 extended edition). All these techniques require that you have multiple images to start with. For HDR images, you need different exposures of the same subject from a stationary viewpoint. The same is true for noise reduction, only the exposures should be identical. Multiple exposure effects can use any number of different images, all of the same dimensions. In each case, you start by stacking all the images into layers, selecting all layers, and converting them into a smart object. Then you use the Layer > Smart Objects > Stack Mode functions Mean or Median to create a statistical combination of all the images, which you can rasterize. Details after the break.

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