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- Photography using Digital SLRs

 

Workflow Technique #041

Watch your Histogram

technical note by Uwe Steinmueller

 
 
In the field the main difference using a film or a digital camera is the LCD on the back of the digital camera giving you the ability to evaluate an image right after exposure. So you might think that watching your pictures on that small LCD is a main benefit going digital. Ok, if you try hard you can check sharpness by zooming into the image. The main advantage of this LCD for me is actually to check the exposure of my last photo.
The right exposure is a key to taking quality images. This did not change from film to digital. In some way overexposure even shows more problematic in digital as the sensors tends to clip data above a certain threshold. This means: never overexpose your photos. In this case, the highlights may be lost and recovery is not possible (some techniques to estimate lost highlight details exist though).

Here is help which only the digital cameras can provide: The cameras provide a way to view the result of the last photograph taken and show a histogram of the grayscale values from 0 (black) to 255(white).

Here are four histograms examples (created in Photoshop) that show different characteristics and show the histogram basics.

Histogram 1: Histogram indicating strong overexposure


The highlights (right side of the histogram) have just been lost. This would be a candidate for deletion in all but a few very rare cases. Some might recommend to "burn" the photo in Photoshop. Still that is faking details in the highlights that are not there.


Histogram 2: Histogram indicating potential overexposure

 

Here is only a small spike in the highlights and it very much depends on the photo whether this might be a problem or not. If the spike represents a real pure white or an unimportant spectacular highlight then this photo may be OK. Otherwise, you are in trouble.

In these cases, if the camera has the ability to indicate overexposed areas, I would recommend the user to view the cameras LCD preview (of course with the highlight/overexposure indicator enabled).

Histogram 3: Histogram indicates good exposure

 

Here the highlights are OK. You lost a bit of the dynamic range in the highlight area but Photoshop can correct for this. You should endeavor to get histograms like this. In principle it is best to get as close as possible to the right.

Histogram 4: Histogram indicates strong under exposure

 

Here the data in the shadows are lost. This is not the only problem because digital cameras show much more noise in the shadows and once this image would be corrected extended noise would show up even in the midrange.

“Expose to the right” rule

From the above we learned that the ideal histogram should:

• Not indicate any over exposure or highlights are lost
• Not underexpose as you may lose shadow details but also most importantly get more noise

In conclusion the perfect digital exposure is as far to the right that there is no chance of overexposure. Unfortunately the real truth is not that easy.

The overall shape of the histogram does not matter at all as it merely reflects the tonal distribution of the scene you photographed.

Read the next section and you will understand why it is safer to slightly underexpose than risk any highlight clipping. Especially as some histograms are also hard to read outdoors in bright daylight.

Note 1: There is one additional complication. The right side of the histogram is not evaluated the same with all cameras. Some cameras are more conservative and and show values on the right if there can be still some highlight data be recovered. Other cameras really clip if the values are to the right. You need to learn how to read the histogram of your camera.

Note 2: Also the histogram in your raw converter may be different as different raw converters may have a different interpretation of the EV 0 point.

 


Color channel clipping

We mentioned that most histograms in cameras show the histogram as gray scale levels. But as we deal with color images we have really histograms of all three RGB channels.

Some clipping can even happen in the one or two color channels and it might not be shown in the luminance histogram.

Here is an example from the Canon 10D that demonstrates typical problem with saturated colors. Blue, orange and yellow flowers are good candidates and it is safer not to use up the full exposure headroom to the right.


The histogram in Camera Raw 2.2 shows clearly the challenge in the blue channel.

This overexposure in the blue channel was not shown by the camera histogram as it uses an average of all the three channels (most often they use a so called luminance histogram, where green gets valued higher than red/blue).


In this difficult situation, it would be very helpful if you also had histograms for the three-color channels. The only cameras we know are the Fuji S2, Sigma SD10 amd the new Canon 1D/1Ds Mark II. The 1D Mark II shows a histogram like this one:

The “after” histogram

Most cameras allow you to inspect the histogram as part of the instant preview. Here is an example from the Nikon D70:

Of course, the “after” histogram only helps if you can repeat the shot. Fortunately, in nature photography, this is often the case except for photographing birds and other wildlife. We check the histogram all the time. Once the exposure is right and the light does not change we only check the histogram once in a while.

The “before/live” histogram

Ideal would be to show a live histogram and use it instead of the camera meter. For digicams this dream actually has come to be true. The new 8MP Konica Minolta DiMAGE, Sony F828 and Olympus 8080 all sport live histograms.

Here is a shot from the LCD of the A2:

Implementing live histograms in digital SLRs will be more tricky as the sensor is hidden by the mirror. One thing is for sure refined versions of live histograms is the way to go.

Do we really need the histogram?

What we actually need from the histogram is the information about under/over exposure. If this can be presented with other means then the histogram may get obsolete. For now the histogram is your ticket to a better exposure.

 
 
 
   

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