How to expose to the right

For raw only

The advice is omnipresent: "expose to the right", and it is often accompanied with the suggestion: "watch the histogram". Unfortunately, the latter is useless, and this fact makes exposing to the right rather a lottery.

The A700 displays an RGB hostogram, which is far too very tiny, but that's it, so it has to be accepted.

The problem with the in-camera RGB histogram is, that it does not show clipping; nor does the flashing indicator show real clipping. Those, who have seen true raw histogram already noticed, how far that is from the RGB histogram.

One needs to understand, that the in-camera histogram is based (in case of raw recording) on the embedded thumbnail. Following steps are between the raw data and the thumbnail:

• applying white balance

• de-mosaicing

• conversion from the camera's color space in sRGB or aRGB

• applying saturation

• applying contrast

• applying color tone

• sharpening

• non-linear mapping (some prefer to call in "applying gamma")

Accordingly, what one gets to see on the LCD is very far from the raw data.

Let's see some examples (the raw image is from Imaging Resource).

Following is a crop from the non-demosaiced (but a bit "stirred") image before white balancing:

and after white balancing on the Gretag card:

This is, how it looks in ACR with default settings, after white balancing on the Gretag card:

Now, watch the red clipping indication:

According to the indication, quite much has been clipped. ACR's RGB histogram too shows a very high exposure:

However, is this true?

Here are the histograms based on the unadultered raw data, showing only a tiny clipping, which comes from specular spots. This was an excellent shot (the exposure could have been increased by 1/3 stop without causing more clipping, but that's really not much):

and this is the histogram after WB application and sRGB mapping:

Note that the difference between the last two histograms is caused by the white balance and the mapping. Following histogram shows the result of a linear mapping, i.e. the difference between the raw histogram and this ine is due only to the white balance:

And we did not make any adjustment to the saturation and contrast; the latter has a very strong effect on the distribution.

So, if I wrote all this understandably, then it has to be clear, that the histogram displayed on the camera LCD is far from the true histogram. Accordingly, it can not be trusted re the exposure.

Now, what can one do to get closer to the truth?

1. The de-mosaicing and color transformation can not be neutralized; however, that won't cause a probem in most cases,

2. Sharpening, saturation, contrast, color tone can be neutralized by settings. Sharpening has to be turned down as much as possible, the others are neutralized usually at the midpoint.

3. The mapping can not be neutralized, but it does not matter in this relation. It affects the distribution of the data on the histogram, but it does not cause "going over the edge", i.e. it does not fake clipping, and if it is far from the right edge, then there is really some room there.

However, the gratest factor, the white balance application is still there. There are cameras (Nikons and high-end Canons), which facilitate a neutral setting, but most other cameras do not. However, it's not bad: there is a way to coax the camera's firmware into applying a neutral white balancing (i.e. leaving the color values alone).

I described what to do on another thread and I don't repeat it here. The fellow poster (from another forum), whose idea is the basis for this method probably has developed a WB template for the A700 as well; I am waiting for his answer. In that case all you need to do is loading that template on a card and using as custome WB.

DISCLAIMER

This is ONLY for pure raw recording. The embedded thumbnail will have off-colors and look generally bad, raw+JPEG would be useless. This is the price for the correct exposure.

Oh btw, I named this thread "how to expose to the right". Well, I don't know that; I only know, how to verify it. I am shooting most of the time with exposure bracketing and watching the "neutralized" histogram, which is almost always reliable. Then, if necessary, I apply some exposure bias and shoot again - if the circumstances allow for that.