11 Mar 2010

The Author

Andy Lim got started in photography after leaving design college, and has given several public talks on the subject of photography. SimpleSLR Workshops Photography workshops from beginners to advanced levels. SimpleSLR Guides Author of best-selling e-book series. Photography Tips Author of useful and practical tips on GoodPhotography.info website. Emotion in Pictures Accomplished professional wedding photographer. His brand attracts clients worldwide with his unique flavor of wedding and portrait photography.


Perfect RAW with Histograms

Histograms are like road maps. You need them in order to see where you’re going. Without them, you’re just guessing.

Ok, I suppose with some experience you can get a pretty accurate guess. But if you’re a perfectionist like me, you’d feel way more comfortable if you knew you nailed the shot with the perfect exposure.

Shooting RAW does give the photographer more latitude in making exposure errors, but lets not use that as an excuse for sloppy exposure judgement. When we shoot in RAW format, we should be even more critical of our exposure choices, in order to bring out the maximum quality possible from the RAW file. Histograms allow you to just do that. Histograms are usually found in digital SLR cameras, or high-end digital cameras. You can also get a histogram reading of your photo in Adobe Camera RAW or other RAW conversion software. What it shows you is a distribution of values from the lightest to the darkest. It tells you whether you captured enough image information in the areas that matter most.

But, don’t they all matter? In most cases, yes, you’d like to have a lot of image information throughout the sweet spots of the image. This is usually from around the lightest highlight areas on the right-hand side to just before the darkest shadow areas on the left-hand side (Photo A, taken from my Phi Phi Island gallery). For many types of images, this is a nice histogram shape to have, with the peaks taking up most of the center area. This indicates a well-exposed image, as there is plenty of image information in the midtones. Usually for these types of images, the shadows and highlights have relatively less image information.

Perfect RAW with Histograms

Nevertheless, a nice shape is subjective. If you are shooting a high-key image, you would like to have most of the image data bunched up on the right-hand side of the histogram. This is because the right-hand side represents the lightest shades within the image. A high-key image is only so because the image consists of mostly light tones. Therefore, a typical high-key histogram would look like the one below. If you tried to force the histogram of a high-key image to look like the one in Photo A (using an image editing program such as Photoshop) you would actually cause the image to look unnaturally dark. That’s because the peaks in the histogram are already where they should be, towards the right-hand side of the histogram.

Perfect RAW with HistogramsPerfect RAW with Histograms

When I take a photograph, I aim for the “foot of the hill” to just touch the right-hand side edge of the histogram. This is called “exposing to the right”. This will give you an image with the best “signal-to-noise” ratio, meaning you get the maximum quality and the minimum noise.

The danger with letting the peaks go this close to the right-hand side is that the image might have some highlight portions burnt out. When this happens, there will be a thin vertical line (or a spike) that appears at the right-hand side edge. This indicates that some portions are burnt out.

Perfect RAW with HistogramsNo need to fret, because this is not a problem when you shoot in RAW format. In Adobe Camera RAW software, you can bring the Exposure slider a bit to the left so that you gradually lower the vertical line until it totally disappears. If you had shot it in JPEG format, you would not be able to eliminate that vertical line because the highlight information on the right-most edge would have been thrown away during JPEG compression.The ability to correct slight exposure judgement errors is one of the strengths of the RAW format. By looking at the histogram, an experienced digital photographer will be able to tell if the image has been properly exposed. In certain images, some spikes may appear near or on the right-hand side edge itself, but is that a badly exposed image? It depends. If the image has a background that contains bright lights or very light-coloured portions, that could be the reason for the right-hand side spikes. If that same image also has got peaks in the middle which are generally more substantial than the right-hand side peaks, those peaks represent the main subject. In Photo B, the lady is the subject, and the histogram peaks in the middle represent her. The spike on the right represents the out-of-focus trees and highlights behind her.

Perfect RAW with HistogramsUsing the Adobe Camera RAW dialog box, we can open up all our images from one folder, evaluate the exposure of each image based on the histogram, and make quick exposure adjustments to each image if necessary by moving the Exposure slider to a point where the histogram looks just right.Within Adobe Camera RAW, there are three separate histograms, one for each color channel, ie. Red, Green and Blue. This enables you to check to see if a particular channel is burnt out, which means that the highlights have been clipped. This detailed overview enables the photographer to quickly adjust exposure to bring back the clipped highlights.

We can then either decide to save these settings in each RAW image (actually Photoshop saves the settings in the Sidecar XMP file, a companion file to each RAW image) by clicking on the Done button, or we can also convert them all into JPEG or TIFF files with the Save button.Modifying the White Balance can also affect the histogram. If an image was shot in Daylight White Balance and the photographer carelessly overexposed certain highlight portions, they may still be able to be recovered using a different White Balance setting, for example, Tungsten. Dragging the Temperature slider (which determines the White Balance setting) to recover lost highlights is one the strengths of the RAW format. Once the spikes on the right-hand side of the histogram disappear, you will have safely recovered your highlights.

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