Overexposing V Underexposing

Started Feb 10, 2013 | Discussions thread
Trevor G Veteran Member • Posts: 6,571
Re: Expose to the Right - not possible without a proper RGB histogram

baobob wrote:

So I have not yet any experience of RAW ETTR with the Fujis

None of us do.

It's not possible to ETTR with any modern Fuji because of the lack of a proper RGB histogram.

You can't do anything regarding exposure with just a luminance histogram, as we have, because we cannot see when a chroma channel is peaking and crushing, or clipping. That will frequently happen before luminance clipping shows on our puny, in-cam histograms.

I've devoted a thread to the topic currently but here is some detail anyway:

Left histogram overlay shows the Luminance channel only, just like the in-cam histogram. Right overlay is a full RGB + L histogram from Silkypix of the same scene.

The in-cam histogram is very deceptive because it only shows Luminance (brightness) information. It can only give an approximate idea of real exposure.

In the very common case shown above, if we use the in-cam histogram it looks as if the scene is under-exposed by about 1EV, but in reality, the R channel is already crushing image detail, as shown by the Silkypix histogram on the right.

Here is the RAW - here is a good reason to shoot RAW, because you can usually claw reasonably over-exposed highlights back from RAW, but not always.

Once again the Fuji Luminance-only histogram is upper left - full RGB+ histogram is on the right, courtesy of Silkypix

properly exposed, it is easy to pick up the extra detail in this, the same image, processed from RAW.

Note for Adobe users: Adobe's Highlight recovery tool allows you to get back much of the crushing you get in over-exposed JPEGs. Silkypix does not. Silkypix does a great job ith RAW.

Important note: The in-cam RGB histogram normally consists of 4 images, one above the other, showing each channel, R G B & L, at once.

I like the Pentax iteration:

But you might prefer the Panasonic version:

Whatever one we get, we need it soon, because this is the upshot of not having one.

You might think that, if the colour is crushed, we could just convert to B&W and not lose anything except the colour.

Wrong...here is the crushed JPEG with all colour turned off:

The JPEG still loses highlight detail from chroma channel clipping or crushing, even with colour turned off

Even just the R chroma channel crushing is reflected back into the Luminance channel, and we get a loss of detail in the highlight areas. Compare with the extra highlight detail in the RAW version below:

The average output of this RAW image has been set to match the JPEG as much as possible

Of course, if you know your histograms, by looking at the L histogram we can see this crushing of the chroma channel causing 2 spikes in the L channel, here reproduced (otherwise check the top image again):

The two spikes on the right side show that chroma channel crushing has been reflected back into the L channel, even though it was nowhere near over-exposure itself.

The matching RAW, after processing, shows none of that problem:

Histogram from RAW output is in great shape, because the chroma crushing was undone in processing and did not affect the L channel.

Please Fuji, may we have a proper RGB Histogram in-camera, like everyone else?

If you agree, would you mind dropping in to this thread and saying something, please?


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Trevor G
Silkypix tutorials at: http://photo.computerwyse.com

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