We use digital blending to create clean and natural HDR image.
Did you know that blending also recovers shadow detail at the same time?
Yes, good quality shadow detail.
You might think what’s the point of that:
I agree with you that you can also recover shadow detail with the Shadows slider in Lightroom or Photoshop.
But have you noticed something?
That’s right, sensor noise, or simply noise.
This image was taken at Batu Caves in Kuala Lumpur, Malaysia. It’s a dark cave with a gap on the top. It was a high contrast scene and this image is the 0EV of a bracketed exposure.
I shifted the Shadows slider in Lightroom to +100 and zoomed into 500%. The image noise was obvious.
I mentioned signal-to-noise ratio briefly in this post on double processing an image.
What’s signal-to-noise ratio?
All electronic devices have a baseline noise level (like the white noise in radio or the racing ants on TV). For these devices to output something meaningful so we can understand, it needs signal.
In digital camera, the source of signal is light.
When signals hit the sensor bucket on the digital image sensor, they fill the bucket and “dilute” the noise so that the ratio of signal-to-noise is higher.
The signals then get recorded as color information.
That’s A Lot of Technical Information!
You’re absolutely right.
But hear me out because everything will make sense in a minute.
Shadow Has A Lower Signal-To-Noise Ratio
If you think about it, more light = more signals.
Less light = less signals.
So, shadow = less light = less signal = lower signal-to-noise ratio = more noise in the image.
There you have it!
This is the reason why you get noise in the shadows when you brighten it up.
You can read more in Cambridge in Color if you’re interested to learn more about the technical aspect of signal-to-noise ratio in image sensor.
Recover Shadow Details With Blending
The easy way to minimize noise in brightened shadows:
One option is to blend a brighter image exposed for the shadow with an image at normal exposure.
You avoid brightening shadows this way, so you don’t reveal the noise hidden within.
Here’s is a comparison on recover shadow detail by blending (before) and brightening shadow in Lightroom with the Shadows slider (after).
You can see the image with brightening shadow using the Shadows slider in Lightroom has more noise. Both images were shot at ISO 320. You can image the noise would be much more significant with a higher ISO.
You might have noticed from earlier on that I said one option is to blend.
What’s the other?
Expose To The Right To Increase Signal-To-Noise Ratio
It’s another technical term, but relax. It’s pretty straight forward.
Expose to the right or ETTR is a camera technique.
You basically increase your exposure by 1 or 2 stop to brighten up the shadow.
Think about it:
Brightened image = histogram moves from left to right = ETTR.
Why would you do that?
In an article on Luminous Landscape about exposure to the right, Michael Reichmann talked about how over 50% of the tonal values are recorded in the brightest tones of the dynamic range in the digital camera. The tonal values reduce by half as you move down the dynamic range. This means the darkest tones have only a very small amount of tonal information recorded (in his example, 128 of 4096 levels).
This means you should always check the histogram of your image. If the histogram dominates the shadows, you should increase exposure by 1 or 2 stop to move the histogram to the right.
Here’s the trick:
In post-processing, reduce the exposure by 1 or 2 stop to darken the shadow back. This way, the shadows will have less image noise because the tonal values have been recorded at a brighter exposure.
Here’s an example of Frankie. He’s a 3 years old Ragdoll cat and he always likes to sleep by the stairs:).
I took the second image at +2EV and dial the exposure back to 0EV in Photoshop. No post-processing has been done to the images.
You can see the image of the left (0EV) has more noise than then the image on the right (+2EV). You can also see how the histogram has shifted to the right on the +2EV image.
Recover shadow detail with expose to the right is not a quick fix for noise. It requires some planning but it’s the easiest of the 2 options to recover shadow details.
Sometimes ETTR Is Not An Option
But ETTR only works in situation when you don’t have a lot of highlights or whites because there is room for shifting to the right.
There will be times when you just can’t ETTR because the histogram is already so stretched that even a small shift to the right causes highlights clipping.
What do you do then?
I think you already know the answer:
Bracket the exposure to extend the dynamic range of the image. You can read more about dynamic range and how you can use histogram to create better images.
If you’re looking to create high-quality image, especially if you’re thinking of printing your image large, you should consider exposure blending or try ETTR.
It will help recover shadow detail in your image to reduce any visible image noise.
- Digital photography basics: Noise by Ultimate Photo Tips
- “Exposing to the right” exposed as signal to noise ratio by evatotuts+
- Expose right by The Luminous Landscape