Why does LOG look flat? A Quick Look

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The Short Answer:

I think part of the confusion is because of the word Log. 

All Log refers to is the type of function applied to the input values before being stored. This function is a logarithm instead of the typical exponent used. And has the characteristic that even as the input value increases dramatically in size, the rate of increase for the output value grows slower.

For example Log2 of a series of numbers counting from 0->1024 would result in numbers from 0->10 but Log2 of a number 0->256 would be 0->8. It grows slower as the numbers get larger which has the benefit of compressing bright parts of the image before clipping.

And that's really all there is to the log part.

But why is it flat?

I think what most people actually wonder about is why does LOG look flat?

The answer is basically made up of a couple pieces.

We are going to start with an OOC version of a photo. (OOC means Out Of Camera, as it not edited by the user)

It looks like a normal landscape photo. 

It's in sRGB color space with the sRGB gamma applied.

The OOC version of the photo


If we display the image with a linear gamma converted from the RAW....

Raw Data converted to linear values

and apply the display gamma curve the image looks flat even though it isn't using a log profile.

sRGB gamma curve applied to the Linear image

There is more to a finished photo then applying the gamma. The camera is doing other steps to improve color and contrast. Editing is an important step.

Often if you are shooting in a LOG profile there is some intention to edit later so those steps to make a more finished image are skipped as they are not as easy for the end user to undo.

The second aspect is familiar to anyone who has edited in something like Adobe RGB before. 

If you shoot in a wider color gamut, but whatever is displaying doesn't know what to do and defaults to the standard profile the image looks muted.

Colors in AdobeRGB incorrectly displayed in sRGB of the OOC camera image.

We can combine both to get a really flat image. This isn't as flat as you see in samples of uncorrected LOG footage, but minimal processing and a larger color space can result in a flat looking image without log being involved. If you can't save a RAW, this might actually be preferable if you intended to edit later.

sRGB Gamma with Adobe RGB Incorrectly Displayed in sRGB

Some manufacturers do list some information about their Log Profiles. For example the functions and color space are documented by Fuji for their F-Log. We can apply the curve, and the same type of display gamut error.

F-log in BT2020 displayed in sRGB

And the end result is that we have the stereotypical flat log look. 

What's import to note is that if a profile for how to get back to the correct look are provided, you should be able to get a correct looking image without needing to do much at all.

( This PDF by Nikon has a great example at the end: https://download.nikonimglib.com/archive4/iJfUz00k72K904BZNQD21iJvnB45/Z7Z6_TG_LUT_(En)01.pdf )

But we can basically do all these steps in reverse to get a "finished" image

Convert FLOG back to linear, re-encode in sRGB gamma, assign it BT2020, convert to sRGB, and then apply an gentle curve "edit" for some improved contrast.

The result is a mostly normal looking photo. There are now some banding and color issues in the darkest and brightest parts since the cumulative error of doing the multiple conversions in 8-bits is showing it's limitation.

But that's basically where the flatness comes from.

REFERENCES

F-log Information:

https://dl.fujifilm-x.com/support/lut/F-Log_DataSheet_E_Ver.1.1.pdf

 https://dl.fujifilm-x.com/support/lut/F-Log2_DataSheet_E_Ver.1.0.pdf

N-log Information:

https://download.nikonimglib.com/archive4/iJfUz00k72K904BZNQD21iJvnB45/Z7Z6_TG_LUT_(En)01.pdf

 



Location: San Jose, CA, USA

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