What the HDR histogram can teach you about your RAW images

Lightroom (LR) and Adobe Camera RAW (ACR) don’t offer a RAW histogram, but there is a new way to get more information about your RAW images: the HDR histogram. And you can use it even if you do not have an HDR monitor or do not intend to process your image as HDR. Just click the “HDR” button in the develop module and watch how the right side of the histogram changes. Detail which is bunched up towards the right side of the SDR histogram can now be properly rendered as it was in the original scene. It is not a RAW histogram, but it will show you much more about the highlight data in your RAW image. **

 

And this full histogram helps illustrate a couple lessons:

We don’t “recover” highlights, we just process them so they fit into the capabilities of our monitors:

Truly “blown” pixels cannot be recovered with RAW processing. RAW processing tools like LR and ACR do not invent highlight detail (though I hope someone invents an AI tool for that to help manage slightly blown skies, that would be quite useful). We can clip pixels in our SDR processing, but if you are able to extract highlight detail from RAW, it was always there.

The HDR vs SDR histogram helps show the difference between truly blown pixels in the RAW vs “highlight rolloff” (compression of the highlights to squeeze them into the SDR range). You can also visualize this information on an SDR display simply by reducing the exposure slider. Everything is now too dark, but it does show what’s possible with your image.

As a side note for those of you who haven’t experienced HDR yet, you can also use this information to help determine if HDR display would help improve your image. For example, if you see color and detail restored when you set exposure to -2, then you you know that 2 stops of HDR headroom would be sufficient to see these pixels properly on an HDR display. Given many displays which offer 4 or even 5 stops of headroom (such as the M1 or later MacBook Pro), you would easily be able to see detail which becomes visible when you slide exposure down to -4.

 

If the untouched RAW doesn’t use much of the HDR range, it is probably under-exposed:

If you are properly exposing to the right (ETTR), your RAW images should frequently look too bright at first (ie require that you set exposure slightly negative when you process them). The reason for this is that brighter exposures have less noise (better signal to noise ratio).

There isn’t an exact point where the HDR histogram tells you your camera would have clipped – it varies by ISO and probably by camera. However, if you consistently see little or no use of the HDR range in the histogram, you are probably frequently under-exposing your images. Your RAW at base ISO can probably offer 3 or more stops of HDR support. So you might have an image that looks properly exposed for SDR, but the HDR histogram might be telling you it could have safely been exposed 3 stops brighter. In other words, your ISO 100 image might have the noise quality of an ISO 400 – 800 image and you could have avoided that with a longer shutter speed.

Note that you will likely see the histogram shows exposure increasing more than you set the slider, not everything in here is based on precise physics (this is still just an HDR histogram based on Adobe processing, not a RAW histogram). This is just a visual way to help understand ETTR principles. You can also watch for predicted clipping when bumping the exposure slider in SDR mode, you just won’t be seeing the same details in the histogram.

Of course, sometimes slower shutters or wider apertures aren’t ideal given other considerations for movement or detail in the image. But if you know the limits, you can reduce noise when possible.

 

** To see an actual RAW histogram:

The HDR histogram helps see more of our image data in LR / ACR. A true RAW histogram would show the mosaic sensor data, typically including two green channels. It wouldn’t have color data and it wouldn’t be responsive to any RAW processing choices (since it would be measured from the original data). This can be helpful to better understand your camera and shooting decisions.

If you want to go a step further and see a true RAW histogram, you might want to check out RAW Digger. I have not personally used it, but have heard great things both about the software and the developer’s excellent support of it.

Greg Benz Photography