33 replies to this topic

[Princess Leah]

Why do black crows and not white seagulls show more CA in an achromat?

The seagulls are reflecting more light, so why don't I see a violet halo against the sky?

White snow shows plenty of CA, so it can just be down to increased contrast....

[Tony Flanders]

Why do black crows and not white seagulls show more CA in an achromat?

The seagulls are reflecting more light, so why don't I see a violet halo against the sky?

White snow shows plenty of CA, so it can just be down to increased contrast....

Exactly -- it's a matter of contrast. Evolution designed seagulls to be pretty well camouflaged against the sky. I'm not quite sure what evolutionary benefit ravens and crows get from being prominent against virtually any background -- but they are.

[Migwan]

try a bluebird with it's back to the sun, 

[EsaT]

The higher and sharper the contrast, the better it's for chromatic aberration.

Hence the fringing at edges of the Moon and planets, or in bright stars.

In non-sharp content, it's just more blur among the original blur.


 

I'm not quite sure what evolutionary benefit ravens and crows get from being prominent against virtually any background -- but they are.

In birds camouflage seems to be often related to nesting on ground/grass or close to ground:
And even that can apply only to female:
https://en.wikipedia...rn_capercaillie

Another factor is belonging to prey specie, but Ravens and crows are quite big birds and don't have much avian predators.

[Princess Leah]

Do seagulls have some scientific ability to reflect less light than a crow?

 

If the mountains are covered in snow hereabouts, I see plenty of CA in an achromat, even against a light grey sky with little contrast.

I struggle to see why contrast is so important here....

 

Perhaps crows being black, help them to absorb heat in their communal roosts. However Carrion crows are not really known for roosting - it's more the rooks and jackdaws. I'm not sure what Carrion crows gain. I know some birds of prey are reluctant to prey on Rooks/Jackdaws as the whole roost will defend an individual. Perhaps Carrion crows 'give out this message ' to a Pergrine etc.

[Echolight]

I've seen many black birds that have a purple tinge naked eye.

[Bill Weir]

https://epod.usra.ed...at are observed.

 

It’s science.

 

Bill

[gnowellsct]

I've seen many black birds that have a purple tinge naked eye.

Grackles have some kind of optical thing going on in their feathers.  They are classified as "green race" and "blue race."

 

There are many many many many many grackles out in the world.  You may have been seeing the "blue race." 

 

These colors depend on the incidence of the sun etc.  

[Princess Leah]

Please don't make the post be all about Starlings......

[triplemon]

If you have chromatic aberation, only certain colors are "misplaced".

A white seagull is a brighter than the blue sky. So if some "misplaced" blue sky is added to the bright white - it adds up to only a slighty shifted color. The red and green in the birds white is all still where it needs to be.

On the dark crow, you see essentially mostly just the "misplaced" color. As not much is added to it by the bird at all. So the overall color of that bird has about the same color saturation than the misplaced light. Which is a pretty pure blue.

 

Thats why you see any color fringing more pronunced on the darker side of any high contrast edge.

Edited by triplemon, 16 December 2024 - 03:08 PM.

[Princess Leah]

If you have dark branches against a bright sky - you get a good recipe for colour fringing. I'm not sure if you would get more against a dull sky?

 

If you have chromatic aberation, only certain colors are "misplaced".

A white seagull is a brighter than the blue sky. So if some "misplaced" blue sky is added to the bright white - it adds up to only a slighty shifted color. 

Is it not the other way round?

Take a crow for example. You see the silhouette of the wing shape against a blue sky as it flaps it's wings. The unfocused light from the crow invades the sky with violet.

Are we saying the crow is brighter than the sky because it is much closer - that I can accept.

However a seagul reflects more light still, so I would have thought it's 'violet ghost' would be more prominent.

[Princess Leah]

Crows may really be a really dark violet, however I can see strong violet from very dark green branches. Or do branches have a lot of violet in them? (at least compared to gulls )

[triplemon]

Its not the branches or crows that have that color in them, the sky has. That is where the "ultra" blue/near UV comes from that ends up in the wrong place - which happens to be in front of the crow.

 

On a branch you in addition see fringing from both ends of the spectrum - the red from one side and the blue from the opposite side. So narrow dark features often are affected by extra red and extra blue = purple. And most "color corrected" compound lenses are have the fringing for both ends of the spectrum going in the same direction (lateral color) - as the cancellation effect the achromat introduces works out "right" only for the green, it results in "too much red shift" making the red fringing going in the opposite direction as it would go if not corrected for at all, i.e. in the same direction as the blue shift.

Edited by triplemon, 16 December 2024 - 03:58 PM.

[Princess Leah]

Its not the branches or crows that have that color in them, the sky has. That is where the "ultra" blue/near UV comes from that ends up in the wrong place - which happens to be in front of the crow.

 

 

That makes a lot of sense. I accept that. Thanks.

 

However I don't understand why I see a halo AROUND the branches .

 

Do you see how I'm thinking?

[Princess Leah]

I think the penny has dropped.

 

It's happening both ways.

The branches are 'infecting' the sky - priming a base for the CA from the sky to move in.

[Princess Leah]

However that still doesn't explain seagulls leaving no CA trail. Unless seagulls reflect less pigment than crows.

[Princess Leah]

I have it.

In areas of low contrast the exchange of violet is relatively equal and therefore not noticeable. (Equivalent to looking at a white wall).

In areas of high contrast their is an inbalance of violet sharing and therefore it is very noticeable.

[gnowellsct]

Please don't make the post be all about Starlings......

Grackles are not starlings and would resent the implication that they are

[Jon Isaacs]

However that still doesn't explain seagulls leaving no CA trail. Unless seagulls reflect less pigment than crows.

 

The sky is bright, the seagulls are bright, low contrast.  

 

The sky is bright, the crow is very dark, that's high contrast. A darl object silhouetted against the blue sky shows CA...  

 

 

Jon

[Rutilus]

A couple of Jackdaws taken with a Synta 50mm f/3.5 finder scope lens taken against a bright sky.

Sometimes I pick up a bit of C.A., and other times none at all.  

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[Princess Leah]

 

The sky is bright, the seagulls are bright, low contrast.  

 

The sky is bright, the crow is very dark, that's high contrast. A darl object silhouetted against the blue sky shows CA...  

 

 

 

Jon

 

I think most understand this. 

However my point is white branches covered in snow show more CA , than black silhouette branches against a blue sky.

I don't think it's always about contrast, more an inbalance of light energy.

Snow is a bit like Venus during the day.

[Princess Leah]

Grackles are not starlings and would resent the implication that they are

Scientific wise yes. But we often call Starlings, 'grackles' in Ireland.

I believe true Grackles are native only to the Americas.

Edited by Princess Leah, 17 December 2024 - 04:39 AM.

[Princess Leah]

A couple of Jackdaws taken with a Synta 50mm f/3.5 finder scope lens taken against a bright sky.

Sometimes I pick up a bit of C.A., and other times none at all.  

Straight through or angled?

[Jon Isaacs]

I think most understand this. 

However my point is white branches covered in snow show more CA , than black silhouette branches against a blue sky.

I don't think it's always about contrast, more an inbalance of light energy.

Snow is a bit like Venus during the day.

That probably depend on how bright the blue sky is.  

 

Jon

[Princess Leah]

If you look at the horizon near here -there is snow capped mountains against a blue sky. With the naked eye it is hard to make out the outline of the mountains, I would say contrast is very low. Through an achromat the snow gives the sky a violet shadow/outline of the mountains.

Perhaps the eye can't accommodate the brightness of the snow.

 

It is the reverse of Jackdaws and branches through a Nighthawk. Here the brighter sky bleeds violet into the darker forms.

Source 

http://www.scopeviews.co.uk/SV80NH.htm

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