Can someone explain lens cast by aperture to me

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fferreres Veteran Member • Posts: 6,220
Can someone explain lens cast by aperture to me
1

This is something I've seen Hank and other discuss. I considered an oddity and made some guesses, things that would be subtle, and hardly distinguishable. Then one day, this...same scene, same time, same light, same angle, same lens....same ... white balance:

Stopped down

Wide

Alan WF
Alan WF Senior Member • Posts: 2,903
Re: Can someone explain lens cast by aperture to me
2

Anti-reflection coatings behave differently according to the incident angle. Wider apertures typically have  incident angles that are further from the normal.

Also, wider apertures typically have light passing through the thinner parts of the elements, and so have less absorption, but I don’t think this is dominant. 
Regards,

Alan

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OP fferreres Veteran Member • Posts: 6,220
Re: Can someone explain lens cast by aperture to me

Alan WF wrote:

Anti-reflection coatings behave differently according to the incident angle. Wider apertures typically have incident angles that are further from the normal.

Also, wider apertures typically have light passing through the thinner parts of the elements, and so have less absorption, but I don’t think this is dominant.
Regards,

Alan

Thanks Alan. But would this be this drastic? The jog is even milder, the ProPhoto version in a wide gamut monitor really shows very warm vs extremely cold.

ProfHankD
ProfHankD Veteran Member • Posts: 7,531
Re: Can someone explain lens cast by aperture to me
2

fferreres wrote:

This is something I've seen Hank and other discuss. I considered an oddity and made some guesses, things that would be subtle, and hardly distinguishable. Then one day, this...same scene, same time, same light, same angle, same lens....same ... white balance:

Stopped down

Wide

First off, the stopped down one generally has the colors closer to right... although many of us might actually prefer the warmer rendering, it's basically missing blue. You'll also note that contrast tends to increase when you stop down.

Index of refraction of most materials is a function of the wavelength of the light. Blue rays are deflected more than green or red ones, so getting the red and green marginal rays to the point of focus tends to imply that the blue ones miss their target. I like to think of it as some rays being sent outside the diameter of the glass, and thus being lost rather than collected back in by subsequent element of the lens. It's not uncommon to see a blue fringe around the image circle of a lens, and this might be why. Of course, some of the misdirected blue might also end-up as blue haze in the image.

Stopping down also cuts out the green and red marginal rays, so things get better balanced.

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Rod McD Veteran Member • Posts: 7,317
Re: Can someone explain lens cast by aperture to me

Thanks to OP and everyone responding  - an interesting discussion about a phenomena I haven't come across before

Cheers, Rod

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E Dinkla Senior Member • Posts: 2,221
Re: Can someone explain lens cast by aperture to me
1

That colors are shifting between wide open and one or two stops closed I see in more of my lenses. But the scene you show here + different exposures it seems, may as well trigger some differences in scene interpretation of a default setting for image development, whether in the camera or afterwards.

Met vriendelijke groet, Ernst
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OP fferreres Veteran Member • Posts: 6,220
Re: Can someone explain lens cast by aperture to me

E Dinkla wrote:

That colors are shifting between wide open and one or two stops closed I see in more of my lenses. But the scene you show here + different exposures it seems, may as well trigger some differences in scene interpretation of a default setting for image development, whether in the camera or afterwards.

Met vriendelijke groet, Ernst
No photographer's gear list is complete without the printer mentioned !

I developed from RAW and made both images use the same white balance temperatures. I also tried FastRawViewer and they look the same. The In camera thumbnail (embedded in the RAW) looks the same.

I wasn’t surprised by some shift. Hank has mentioned a few times with some vintage lenses. But I had hardly noticed anything to this intent.

Really, if you display each image fully, not the thumbnail, the photos are day and night with regard to the mood they convey.

OP fferreres Veteran Member • Posts: 6,220
Re: Can someone explain lens cast by aperture to me

ProfHankD wrote:

fferreres wrote:

This is something I've seen Hank and other discuss. I considered an oddity and made some guesses, things that would be subtle, and hardly distinguishable. Then one day, this...same scene, same time, same light, same angle, same lens....same ... white balance:

Stopped down

Wide

First off, the stopped down one generally has the colors closer to right... although many of us might actually prefer the warmer rendering, it's basically missing blue. You'll also note that contrast tends to increase when you stop down.

Index of refraction of most materials is a function of the wavelength of the light. Blue rays are deflected more than green or red ones, so getting the red and green marginal rays to the point of focus tends to imply that the blue ones miss their target. I like to think of it as some rays being sent outside the diameter of the glass, and thus being lost rather than collected back in by subsequent element of the lens. It's not uncommon to see a blue fringe around the image circle of a lens, and this might be why. Of course, some of the misdirected blue might also end-up as blue haze in the image.

Stopping down also cuts out the green and red marginal rays, so things get better balanced.

Hank, thanks. You had noticed and mentioned this before. Do you have any example like you have for CA from the vivitar?

It is clearly visible the shift is very strong here, to not affect the cast a bit, but to make an entirely different shot. This is not just a warmer tone wide open. The closed down shot is NOT what I had perceived. It was very red light indeed.

ProfHankD
ProfHankD Veteran Member • Posts: 7,531
Re: Can someone explain lens cast by aperture to me
1

fferreres wrote:

E Dinkla wrote:

That colors are shifting between wide open and one or two stops closed I see in more of my lenses. But the scene you show here + different exposures it seems, may as well trigger some differences in scene interpretation of a default setting for image development, whether in the camera or afterwards.

Met vriendelijke groet, Ernst
No photographer's gear list is complete without the printer mentioned !

I developed from RAW and made both images use the same white balance temperatures. I also tried FastRawViewer and they look the same. The In camera thumbnail (embedded in the RAW) looks the same.

I wasn’t surprised by some shift. Hank has mentioned a few times with some vintage lenses. But I had hardly noticed anything to this intent.

Really, if you display each image fully, not the thumbnail, the photos are day and night with regard to the mood they convey.

As I said above (and in general WRT this), usually the stopped-down one has the more correct colors. If you separate-out the color channels here, the blue channels actually don't look that different: it's the red channels that differ wildly.

The difference is really contrast, especially on the red channel. If you look carefully, the noise level on the red channel is much higher in the wide open shot than in the stopped-down one. In fact, the red channel is far noisier than the other channels in the wide open shot, but not in the stopped down one.

To me, this suggests that the scene naturally had almost no reds except where the light directly hit -- like the stopped down red channel shows. However, with a wide-open shot containing less blue, the red channel was boosted. It should be much clearer what's going on if you look at the raw directly (before color interpolation and white balancing). In other words, I'm saying your white balance transformation made this more dramatic than the difference really was.

BTW, odd aspect ratio on these.

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OP fferreres Veteran Member • Posts: 6,220
Re: Can someone explain lens cast by aperture to me

I will have to re read the post, but I think I am getting it. ItMs a great idea to inspect the channels, and is readily seen in the shots. I will do that with RawTherapee as I want to understand it more intuitively.

The light source was 100% ambient or indirect except the tree and the close by trunk, but it was about 30 seconds from getting fully dark. So the direct light was as weak as it gets without clouds.

I will also go and observe the mosaic, undeveloped RAW. This will give me an intuition.

I will be paying more attention to this effect. It really can be very dramatic under some circumstances, and it is seldom discussed or observed by anyone. 
Could a lens be optimized in such a way that it has an opposite effect? (Colder wide open)

I may owe you $5000 in free consulting already for all the easy scientific explanations across topics, and I am very grateful for your time and participation in this forum.

ProfHankD
ProfHankD Veteran Member • Posts: 7,531
Re: Can someone explain lens cast by aperture to me

fferreres wrote:

I will be paying more attention to this effect. It really can be very dramatic under some circumstances, and it is seldom discussed or observed by anyone.
Could a lens be optimized in such a way that it has an opposite effect? (Colder wide open)

Certainly, but that seems harder to do. This quickly gets to the concept of APO. Especially on older lenses, these color issues were essentially not modeled when designing the lens. The term APO is supposed to mean corrected for at least 3 colors to agree on focus plane, and many non-APO lenses were only computed for one or two colors, so the blue/red behavior may have been a bit of an out-of-band surprise... especially for lenses intended to be used with monochromatic film that might not have had a very broad spectral response curve.

I may owe you $5000 in free consulting already for all the easy scientific explanations across topics, and I am very grateful for your time and participation in this forum.

No consulting fees here -- especially when I'm talking on the fringes of what I understand (which is the case on this particular topic). We really need somebody with optics design experience to give THE explanation.

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Alan WF
Alan WF Senior Member • Posts: 2,903
Re: Can someone explain lens cast by aperture to me
1

I’m not sure what you’re saying here. Yes, light at different wavelengths takes different paths through optics and has different PSFs in the final focal plane. But which of these impact on color?

On one hand, I can see chromatic aberration causing wavelength-dependent vignetting. For example, the image of the iris can suffer chromatic aberration as seen from the objective; it can pass more or less light at different wavelengths and I can see this causing a color shift. (I’d note that this should be easy to see just by looking at the iris through the objective.)

On the other, if the PSF at one wavelength is more fuzzy at wider apertures, I can again see this causing a color shift.

However, there is a difference, I think.

Consider a lens that gives a fuzzier blue image than a red image (since in my experience chromatic aberration is more of a problem in the blue) and moreover a more fuzzy blue image at wider apertures. This would give a color shift for small features, but not for large features. That is, a grey card would not show a color shift.

Conversely, if the lenses suffers wavelength-dependent vignetting at the iris, I’d expect the color shift to be independent of the spatial scale of the feature.

Any idea which is it?

Regards,

Alan

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ProfHankD
ProfHankD Veteran Member • Posts: 7,531
Re: Can someone explain lens cast by aperture to me
2

Alan WF wrote:

Conversely, if the lenses suffers wavelength-dependent vignetting at the iris, I’d expect the color shift to be independent of the spatial scale of the feature.

Any idea which is it?

I think it is most often wavelength-dependent vignetting, as you called it -- and nearly always the blue is much weaker wide open and reaches normal levels when stopped down. The interesting thing is that it is very common, but usually not very obvious.

My theory on that is along these lines: Color film would clearly shows these color casts, but it was rare that people would shoot anywhere near wide open. Wide open was thought of (and heavily marketed) as primarily about giving a brighter OVF for composition and focusing. Even so, the color casts from most lenses are subtle enough to be difficult to notice unless you have side-by-side shots in the same lighting (perhaps even on the same roll of film, because color did vary a bit with processing). Most lens testing was monochromatic; color tints were not really something people checked for.

Now, however, it's really common to try to shoot images wide open. Perhaps because small-sensor cameras have everything in focus, bokeh are much more appreciated than they were decades ago. Digital cameras also have enough light sensitivity and adjustability to make shooting with a range of aperture settings in the same lighting possible. However, digital cameras don't usually get treated as having a fixed color balance. Thus, even heavy tints tend to disappear as "gray world" auto white balance corrects for them. The result is that color tints turn into color noise biases... again hiding the tint. Incidentally, most cameras significantly alter saturation as a function of ISO, essentially muting colors at high ISO as a side-effect of noise reduction, and most people don't even seem to notice that....

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Tons o Glass 0 Class
Tons o Glass 0 Class Contributing Member • Posts: 758
Re: Can someone explain lens cast by aperture to me

I've noticed this phenomenon after the fact (when processing RAWs and trying to match white balance), but nothing so drastic.

I've not a clue what happened in your shots, especially since I wasn't there observing the light.  Looking at the timestamp there are 44 seconds between the two shots.  On a windy partly cloudy day or during a sunrise/sunset 44 seconds is enough time for the light to change temperature and/or intensity.

What was the stopped down aperture? Would you be open to sharing the raw files to play with?

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OP fferreres Veteran Member • Posts: 6,220
Re: Can someone explain lens cast by aperture to me

Tons o Glass 0 Class wrote:

I've noticed this phenomenon after the fact (when processing RAWs and trying to match white balance), but nothing so drastic.

I only noticed on developing because one looks warm and loving, the other seems scary, ideal for a terror movie. I have never seen anything like it.

Might it just be the lens? It is curious that lens history specialist Marco Cavina was rather fascinated but the huge endeavor to give birth to it. It's quite complicated, almost overdone, for a simple 50/1.8.

Marco has even posted the materials and optical details, and materials for at least 5 of the 7 elements.

I've not a clue what happened in your shots, especially since I wasn't there observing the light. Looking at the timestamp there are 44 seconds between the two shots. On a windy partly cloudy day or during a sunrise/sunset 44 seconds is enough time for the light to change temperature and/or intensity.

There could have been thin clouds filtering some of the light, but both have direct sunlight and I actually have a stream of about 10 shots, 5 with each aperture setting (but some are more blurry).

What was the stopped down aperture?

I didn't take notes. Something like f8. Can we infer from the shutter speed change at least how many stops apart? I try to always use ISO 100, and used ISO 100 for both shots since the only bright part was quite bright, the baby tree.

Would you be open to sharing the raw files to play with?

Yes, if anyone could share which place accepts RAW uploads. Would be nice for DPR to diversify and add a RAW upload service by a reasonable monthly fee, pivot from purely news and reviews to community of gear lovers.

Tons o Glass 0 Class
Tons o Glass 0 Class Contributing Member • Posts: 758
Re: Can someone explain lens cast by aperture to me

fferreres wrote:

Tons o Glass 0 Class wrote:

I've noticed this phenomenon after the fact (when processing RAWs and trying to match white balance), but nothing so drastic.

I only noticed on developing because one looks warm and loving, the other seems scary, ideal for a terror movie. I have never seen anything like it.

Might it just be the lens? It is curious that lens history specialist Marco Cavina was rather fascinated but the huge endeavor to give birth to it. It's quite complicated, almost overdone, for a simple 50/1.8.

Marco has even posted the materials and optical details, and materials for at least 5 of the 7 elements.

I've not a clue what happened in your shots, especially since I wasn't there observing the light. Looking at the timestamp there are 44 seconds between the two shots. On a windy partly cloudy day or during a sunrise/sunset 44 seconds is enough time for the light to change temperature and/or intensity.

There could have been thin clouds filtering some of the light, but both have direct sunlight and I actually have a stream of about 10 shots, 5 with each aperture setting (but some are more blurry).

Hmm... that eliminates changing light as the culprit enough in my mind.

What was the stopped down aperture?

I didn't take notes. Something like f8. Can we infer from the shutter speed change at least how many stops apart? I try to always use ISO 100, and used ISO 100 for both shots since the only bright part was quite bright, the baby tree.

Ah gotcha.  I may have to try this out on a similar scene for myself with my rough Singapore-made HFT Planar (the closest relative I own of your Rollei).

Would you be open to sharing the raw files to play with?

Yes, if anyone could share which place accepts RAW uploads. Would be nice for DPR to diversify and add a RAW upload service by a reasonable monthly fee, pivot from purely news and reviews to community of gear lovers.

Dropbox, Google Drive? Neither may be ideal but I believe they would work.

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OP fferreres Veteran Member • Posts: 6,220
Re: Can someone explain lens cast by aperture to me

Tons o Glass 0 Class wrote:

fferreres wrote:

Tons o Glass 0 Class wrote:

I've noticed this phenomenon after the fact (when processing RAWs and trying to match white balance), but nothing so drastic.

I only noticed on developing because one looks warm and loving, the other seems scary, ideal for a terror movie. I have never seen anything like it.

Might it just be the lens? It is curious that lens history specialist Marco Cavina was rather fascinated but the huge endeavor to give birth to it. It's quite complicated, almost overdone, for a simple 50/1.8.

Marco has even posted the materials and optical details, and materials for at least 5 of the 7 elements.

I've not a clue what happened in your shots, especially since I wasn't there observing the light. Looking at the timestamp there are 44 seconds between the two shots. On a windy partly cloudy day or during a sunrise/sunset 44 seconds is enough time for the light to change temperature and/or intensity.

There could have been thin clouds filtering some of the light, but both have direct sunlight and I actually have a stream of about 10 shots, 5 with each aperture setting (but some are more blurry).

Hmm... that eliminates changing light as the culprit enough in my mind.

What was the stopped down aperture?

I didn't take notes. Something like f8. Can we infer from the shutter speed change at least how many stops apart? I try to always use ISO 100, and used ISO 100 for both shots since the only bright part was quite bright, the baby tree.

Ah gotcha. I may have to try this out on a similar scene for myself with my rough Singapore-made HFT Planar (the closest relative I own of your Rollei).

I'd say almost twin brothers, although some of the glass may be different, and HFT coating will be different than T/T*.

Would you be open to sharing the raw files to play with?

Yes, if anyone could share which place accepts RAW uploads. Would be nice for DPR to diversify and add a RAW upload service by a reasonable monthly fee, pivot from purely news and reviews to community of gear lovers.

Dropbox, Google Drive? Neither may be ideal but I believe they would work.

That was easy...here's the link to the two RAW files:

https://drive.google.com/drive/folders/1wpNMdNXKnvEIOUcCXn88MFGCP787CkAX?usp=sharing

BadExample Regular Member • Posts: 169
Re: Can someone explain lens cast by aperture to me
1

Shout-out to @ProfHankD !!!

Thank you so much for your teaching here. I think I learned more reading this thread than all others combined, AND it didn't end up with the 5 pages of argument over who is technically correct. I am an IT professional, so I know that communication takes effort, especially when you have to word it so less techy people can understand. It doesn't just happen and your responses here and elsewhere are obviously well thought out.

I just wanted to make sure you know that your time and effort here is appreciated. Also, you are very good at 'splainin' stuff to dummies like me. Thank you, sir!!!

~jon

Tons o Glass 0 Class
Tons o Glass 0 Class Contributing Member • Posts: 758
Re: Can someone explain lens cast by aperture to me
1

fferreres wrote:

Tons o Glass 0 Class wrote:

Dropbox, Google Drive? Neither may be ideal but I believe they would work.

That was easy...here's the link to the two RAW files:

https://drive.google.com/drive/folders/1wpNMdNXKnvEIOUcCXn88MFGCP787CkAX?usp=sharing

Thank you! The difference in the thumbnails was drastic. The camera was set to Auto for both of those exposures as opposed to being set to the same white balance. I can confirm that Auto WB behaves this way for me as I stop down a lens. I think the Auto WB algorithm may get pushed hot or pushed cold depending on Axial CA and/or PF or the lack thereof. When white balance is held truly constant, however, I think lenses typically do cool want to cool down a bit as we've noticed when you close them down. I've seen camera-based Auto WB push it one way, and have Capture One push it in the opposite direction though... I still have to digest Hank and Alan's discussion.

What about lenses with shiny aperture blades - they can sometimes cause a loss in contrast as the aperture closes down. Depending on the scene, the white balance an algorithm or our eyes would want to see could get pushed hot or cold if warm or cool colors are bounced around by the aperture blades, respectively.

I popped these into Capture One and let it decide the white balance. It wanted to warm up the f/8-ish shot, and wanted to cool down the f/1.8 shot, but there was still a sizeable gap between the two shots in that regard.

FWIW here's the two shots where I took C1's Auto WB on the f/8-ish shot to be "bible" and set both shots to that, then haphazardly and imperfectly pushed the f/8-ish shot such that the histograms roughly matched.

Exposure was brought down an additional (subractional?) 0.7EV from as shot. The stopped down shot undoubtedly had more contrast and the sunlight also seemed more intense here? I dunno. Maybe the camera didn't want to clip the shadows.

Hmm... which one's cooler now? This one may need to be a little warmer, actually, which is consistent with what we've observed already.

I see that -1.7 EV was dialed when you were shooting. Was it your intention to expose for the sunlight in doing that? My eyes are drawn to it first, that's for sure.

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OP fferreres Veteran Member • Posts: 6,220
Re: Can someone explain lens cast by aperture to me

Tons o Glass 0 Class wrote:

fferreres wrote:

Tons o Glass 0 Class wrote:

Dropbox, Google Drive? Neither may be ideal but I believe they would work.

That was easy...here's the link to the two RAW files:

https://drive.google.com/drive/folders/1wpNMdNXKnvEIOUcCXn88MFGCP787CkAX?usp=sharing

Thank you! The difference in the thumbnails was drastic. The camera was set to Auto for both of those exposures as opposed to being set to the same white balance.

Yes, the thumbnails was quite drastic!

Here's a mistake I did. I copied the WB from one scene to the other. But didn't pay attention, and when the mode is set to CAMERA, it only copies the mode. Thus, it did not port the Temp and Hue to the other image. When I did now moments ago, they look a lot closer, as you show, and if I brighten the wider one a bit (which is a bit darker), they look more even, but this clips a peak of the red channel, which we don't want, as it would affect color. So I lowered the other a bit instead.

Now the difference is less dramatic, although still noticeable.

I can confirm that Auto WB behaves this way for me as I stop down a lens. I think the Auto WB algorithm may get pushed hot or pushed cold depending on Axial CA and/or PF or the lack thereof.

Hank had guessed this is another post, maybe a year ago. But I couldn't follow much why, in practical terms.

When white balance is held truly constant, however, I think lenses typically do cool want to cool down a bit as we've noticed when you close them down. I've seen camera-based Auto WB push it one way, and have Capture One push it in the opposite direction though... I still have to digest Hank and Alan's discussion.

It's still a mystery how the AWB guesses so differently, but in the camera's favor, now looking at the scene much more closely:

  • The stopped down version happens almost a minute later. The sun-line moves about 4cm. You can see this in the lower part of the bright tree trunk. A branch creates a shadow there, and in the f8ish shot it's a bit higher. This caused the grass on the right side to now not get direct sunlight too.This removes a good amount of direct sunlight, and more of the scene is just whatever is reflected thru leaves and trunks. Don't ask me what it means, but probably has an effect. In addition, that lit part is warmer, and being gone, increases the blue perception.
  • Not sure if this affects it, but this particular lens seems B vs RG optimized. The PSF in the blue channel is out of focus when the green and red are in the best focus. I have described and shown examples of how the peak contrast is achieved when the tiny white spot is sharp but with a deep blue aurea around it. By f8 this halo is gone completely. This may have a direct effect into seeing more bluish things in the wide open shot in the blurred areas, maybe the AWB guessing light to be a bit more blue than it is.

What about lenses with shiny aperture blades - they can sometimes cause a loss in contrast as the aperture closes down. Depending on the scene, the white balance an algorithm or our eyes would want to see could get pushed hot or cold if warm or cool colors are bounced around by the aperture blades, respectively.

In this case, I guess it would be minimal. But veiling profile may affect it. I think I guessed this in the thread where Hank commented about AWB.

I popped these into Capture One and let it decide the white balance. It wanted to warm up the f/8-ish shot, and wanted to cool down the f/1.8 shot, but there was still a sizeable gap between the two shots in that regard.

The curves are a bit different. In the wide open one there is less relative blue. The grass that is more lit in one and not that much in another may account for some.

FWIW here's the two shots where I took C1's Auto WB on the f/8-ish shot to be "bible" and set both shots to that, then haphazardly and imperfectly pushed the f/8-ish shot such that the histograms roughly matched.

Exposure was brought down an additional (subractional?) 0.7EV from as shot. The stopped down shot undoubtedly had more contrast and the sunlight also seemed more intense here? I dunno. Maybe the camera didn't want to clip the shadows.

Hmm... which one's cooler now? This one may need to be a little warmer, actually, which is consistent with what we've observed already.

The second one is cooler. But now, I had to make a lot more effort and consider it a guess.

I see that -1.7 EV was dialed when you were shooting. Was it your intention to expose for the sunlight in doing that?

I didn't have time and had ISO 100 dialed, but the f8ish shot would be very blurry as it was already 1/50 wide open, and you see how the light moved in 47 seconds, so I didn't have time. But more importantly, if I increased the ISO, this camera doesn't have dual gain, so I would not gain much vs pushing in post.

The other factor, and this was the first to consider, see the curves. There's a peak of red to the absolute right. If I didn't expose to the right, I would have blown the not much of the highlights, but would have blown the read channel in the main subject that is the trunk and small tree. The surrounding had to look very dark, and the tree shining. I think this photo would display better in an HDR monitor with 10.000:1 contrast. The small tree was magically glowing. And the rest was really really dark. I wanted to keep that in the photo.

Note that the more one brighten the scene, the less and less the small tree fased into a secondary character. If you dial the exposure down, you will see the small tree as main subject.

My eyes are drawn to it first, that's for sure.

Would have been better if it wasn't there. As what really got my attention was that the small tree was the only thing with direct sunlight... plus that trunk.

Thanks for investigating this in more detail. In large part, it was AWB. With still some difference, but nothing that would change the character of the photo that much. There's still more red in the wide open one, consistent with Hank's comment. But much less dramatic.

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