Haze and it's "Removal"

First question: what is the technical effect of haze on an image capture?

Second question: what exactly does Haze "Removal" software do to a captured image?

Deep, detailed responses invited, please.

Haze, mist and fog all tend to reduce the contrast, but often more so to the darker parts of the image.

Any increase in contrast tends to partially cancel the effect of the haze. However, rather than increase the contrast globally, it is usually better to just increase the contrast locally using something like UnSharp Mask. With a very small radius (one pixel or less), USM has the effect of sharpening the image, but with a larger radius (e.g. 10 to 50 pixels), it increases the contrast close to edges, and so provides local contrast enhancement.

I don't really know, but I presume haze removal tools use AI or something similar to decide where in the image to apply contrast enhancement for the best haze removal.

An Example: image taken on a foggy day:

Processed with UnSharp Mask to give local contrast enhancement and sharpening (purely to illustrate the effect, I prefer the original):

[deleted]

Thank you both!

I've been looking at various examples and indeed the histograms are indicating contrast effects/manipulation in one form or another.

Still looking ... currently side-tracked into the world of Depth Maps ...

en.wikipedia.org/wiki/Depth_map

Haze it not just a bad thing
Tiny water droplets and dust in the atmosphere will cause “Rayleigh scattering” of light and give rise to haze.
The further away an object is, the more it will look hazy. During the day, haze in the distance looks bluer, brighter and everything will have less contrast.
Haze is, however, something that can be used to good effect to give a perception of depth in an image. Haze is not an “artefact” but is something real and natural.

A painter will often deliberately add haze to create depth in a painting, so why should a photographer try and remove it all ?
Use it in landscape images, for example, to emphasize the layers of receding mountains.
While the foreground is often clear and has lots of contrast, each of the layers in the distance gets brighter and bluer, and finally, they will melt into the sky.
Sometimes I will reduce haze but usually I avoid removing it completely.

Here’s a few examples:

There's a setting in DXO Photolab 6 called DXO Clearview Plus which reduces haze and which is sometimes useful for photos without haze. As others have said about similar things, this is essentially a contrast increaser -- with other magic (and proprietary) ingredients.😀

CORRECTION: I meant, of course increaser, but wrote reducer. Sorry!

David

"reducer"?

It's a thing 😀 though as photographers we may be more used to think of a reducer in a darkroom scenario

As a landscape shooter I find that the blue shift from haze especially in the mountains is more irksome than haze per se

Only the dehaze tools don't reduce contrast - they do increase contrast.

Haze can be thought of as veiling flare that varies with distance.

i dont understand your answer. veiling glare is lens related. ive never heard of "veiling flare"

Jim is using equally popular terminology for the effect, Donald.

Try searching for "veiling flare", should get a hit or two, here's one:

www.cambridgeincolour.com/tutorials/lens-flare.htm.

Not me, Jim - never had or used a darkroom - serious photography for me started with digital.

A misty image:

With a "contrast reducer" applied:

Certainly not haze reduction ...

veiling glare is not the same as lens flare as described in the article. veiling glare is where an image has very bright and very dark light entering the lens and the veiling glare from the lens will not allow total black/dark parts of the image from being recorded at its very darkest. its why a sensor with a lens attached can not capture more than 10 stops of DR, 14 stops is wishful thinking. 😉

NO: reducer. My typo -- CORRECTED in original post.

I stand corrected - any credible references?

irksome?
Would you “correct” the colours of a red sunset?

Both haze and sunsets involve Rayleigh scattering; sunlight travels through the atmosphere and changes as it is scattered.
The path length of the light though the atmosphere for a sunset is longer and the light reaching us will appear more orange or more red.

Haze is more local, the light path-length is shorter and we get a bluer effect on the objects like mountains from that locally scattered light reaching them.
Both are real and both are natural effects.

Would you “correct” the colours of a sunset and remove that redness?
Probably not, and in the same way you could also artistically embrace those bluer colours caused by haze.

As I mentioned above I will also remove some haze in my images, but usually I won’t remove it entirely because it is, after all, a natural effect and can look good.
Also, the editing actions used to remove haze can end up looking very unreal