Selective Contrast Enhancement
On January 6th 2009 Jon Allanson gave the first digital presentation of the new year on Contrast enhancement and the use of the High Pass Filter. A short extract from an old video and Peter Henry Emerson indicated that the subject is almost as old as photography itself. But with the advent of Digital photography and PhotoShop the foreground (or background) of an image can be selectively enhanced to improve the impact of an image, and the High Pass Filter gives a route to selectively sharpen elements of an image.
The Layer Mask was the basis of Jon’s Contrast enhancement. Control J was a quick way of producing a Background Copy and he used the Magnetic Lasso tool to do a quick selection. A new Adjustment Layer could call on Levels, Curves, Hue/Saturation, etc to modify the selected area. The aim in most of these adjustments was to enhance the Contrast between areas. Using a de-saturated image, and the Multiply option Jon could (by means of the Brush tool ) selectively rub out the effects of a masked layer. Using the Inverse option during Selections greatly increased the number of options open to him. Jon demonstrated how he easily removed eg troublesome rope strings which spoiled an image.
Jon made brief reference to the File>Automate>Merge to HDR facility in PhotoShop CS2 (and CS3 ) Saving images as RAW files gave a greater dynamic range (say 12 or more bits), than the 8 bits (256 levels) of a JPEG image. One could produce say three JPEG images from the RAW file with EV values say -2, 0, and +2, and then combine the images using the HDR facility to produce an image in which the darkest and the lightest elements are all visible within an improved tonal range image.
Jon is fond of the High Pass filter as a way of selectively sharpening images. The High Pass filter retains edge details to a radius that you independently set. It suppresses the rest of the image with a neutral grey overlay. With the background copy layer selected you change the layer mode to overlay; then choose the High Pass Filter.
You can preview the results as you try different settings.
Barry Pearson proposed the vote of thanks. The talk had been on several levels, the first being the Contrast Control. We had seen some unusual use of standard tools, and there was much scope for experiment. He would now have to practice some of these techniques. He thought everyone would have learned from the talk.
Postscript: Jon has provided a tutorial in PDF format covering much of the material on the Downloads page.