Photoshop CS5 was announced this week and we have seen a wealth of new features for photographers, more so than any other version of Photoshop I can think of. By now you may already have had a chance to see some of the star new features in action such as Content-Aware filling, Puppet Warp and the new Merge to HDR Pro. Tucked away in Photoshop CS5 are a bunch of other smaller features, which nevertheless can be put to good use. In this article I wanted to focus on the new Divide blend mode. It took me a while to work out what it could be useful for, until Chris Cox who helped point me in the right direction by explaining that it could be useful for carrying out what is usually referred to as a ‘flat field calibration’. In other words, you could use the Divide blend mode to remove lens vignetting and stubborn sensor spots.
Some specialist digital capture systems, such as those used in astronomy and microscopy already offer their own software solutions to achieve this kind of sensor clean-up processing. However, with the new Divide blend mode you can carry out flat field calibrations for any type of digital camera system. Be warned that this is a tricky technique to get right. This method may in some circumstances help you cut down on the time spent retouching, providing you are able to capture a valid flat field calibration image close to the time when the other photos were shot.
1 The first step was to capture an image of an evenly lit white surface, exposed so that the center was 100% white. The key thing was to make sure the illumination was even and defocus the camera so as not to pick up any subject texture. It was also important to capture this image at the same aperture as I was going to shoot with since this would affect the definition of the sensor dust picked up by the camera. Since the high-end display I work with has uniform brightness, I got good results by photographing the computer display with a white desktop. I shot at a low ISO setting to keep the noise to a minimum. I also bracketed the exposures so that I had the opportunity to choose the most suitable image – one that was bright enough to apply a Divide blend mode calibration and required minimal further image adjustments to the brightness.
4 Here you can see the modified version where the divide blend mode calibration layer canceled out the lens vignetting and all the sensor spots. The net result was a modified image in which the added layer canceled out the sensor marks that were present in the original and removed any lens vignetting.
5 However, it was important to fine-tune the brightness of this layer using a clipping Curves layer so that the calibration layer in Divide mode precisely canceled out the sensor marks on the layer below. Once I had got the adjustment right for this image. the same setting could be used on other subsequent photos.
6 One of the reasons I chose to include this technique was to show what the new Divide blend mode could be useful for. In all honesty, I think the easiest approach would be to use the Camera Raw Lens Corrections adjustments to take care of the vignetting and use the remove spots tool in Camera Raw to remove the dust marks and synchronize the spotting adjustments across all the other affected photos. This could certainly work well for an image like the one shown here. On the other hand, if you have a problem with a particularly dusty sensor or the dust marks fall on top of areas where it would be tricky to retouch, the flat field calibration technique might just save the day. As you can see in the close-up views shown here, I was able to do a good job of removing most of the spots. However, I was only able to partially remove the big dust mark in the top left corner. It would have been unrealistic to have expected a better result than this for such a large sensor mark.
Adobe Photoshop CS5 for Photographers (Focal Press).
This extract was taken from Adobe Photoshop CS5 for Photographers. This latest update in the Adobe Photoshop for Photographers series will contain 768 pages in full color, plus a DVD containing video tutorials. The book layout has been further improved to make navigation easier and contains updated advice on everything you need to know about using Photoshop, from digital capture to print output, as well as all that is new in Adobe Photoshop CS5.
The title will be stocked in most major bookstores throughout the world and also available to purchase from:
www.focalpress.com as well as through the on-line bookstores: www.amazon.com and www.amazon.co.uk.