Photoclubalpha

on the park with my dog this morning, plenty birdlife about and loads of gulls, the people feeding the birds tended not to feed the gulls in fact they were throwing the bread away from the gulls towards the ducks, i,ve never really gave it much thought, but these birds are so fresh clean and white, how do they keep themselves so pristine, theya re also easy to take pics of , if i can anybody can.

feed the gulls i say







and heres who was getting all the grub

Living in Cornwall we have a real problem with people feeding the gulls (herring gulls rather then the black headed ones you've shown)- they've become emboldened to the point where they will steal icecream from children's cones and pasties from tourists, and they’re particularly vicious during the breeding season dive bombing anyone who ventures close to their nests!!

Philip

Hi gio67,

I hope our Istanbullian seagulls won't mind that I find your species very interesting-looking. I like the first photo's composition and the pattern of bokeh formed by the water surface as a background.
The duck offers fine details and bright colours. The white, pristine quality of their plumage seems to be really contrasty and a cause of CA in the second seagull photo. The bird is almost outlined by CA and it's apparent even at this size.

Nice catch,

Yildiz

thanks for te comments yildiz and phillp, yildiz this might sound daft but what is CA i work in ventilation and CA means crosstalk attenuation i don,t think thats what it means,

gio67 wrote:thanks for te comments yilditz and phillp, yilditz this might sound daft but what is CA i work in ventilation and CA means crosstalk attenuation i don,t think thats what it means,

Hi gio67,

Sorry, I should have written 'Choromatic Aberration'. If you view your second photo closely on your own computer at 100% full-size or even the one you sent to the forum, you're going to see a purple coloured fringe lining your seagulls' whitest/ brightest parts and that's generally called Chromatic Aberration or CA as used in some photography-related literature.

That purple fringe happens when some lenses cannot cope with the ultra-brightness of the subject against a contrasting background. You can see some on the duck's tail plumage too, where contrasty colours come side by side.

Yildiz

Here are screengrabs of the Choromatic Aberration / purple fringing on the duck's tail and the seagull in flight.

Yildiz

aha!! thanks yildiz, i,m learning every day on here, is their a cure for this or just take pics of dirtier seagulls

gio67 wrote:thanks for te comments yilditz and phillp, yilditz this might sound daft but what is CA i work in ventilation and CA means crosstalk attenuation i don,t think thats what it means,

Actually that description sort of fits

I suppose the cure begins with the particular lens brand and type one uses. The better the lens coating is less or no CA is visible around the subjects that have bright colours and also are under bright lighting conditions. Some new lenses-which are costlier than the kit lenses- introduced new coatings that manage this drawback nicely, at a cost of course.

But lately David Kilpatrick mentioned in one or more of his posts on eliminating CA that there may be very successful ways of getting riid of it during post-processing. Unfortunately I don't remember where the posts are but they probably are in the LENS LORE section. If not, maybe it came up rather randomly in other threads where the subject may have strayed from the actual subject title. He mentions the names of the editting software.

Or one of the experts may mention it here themselves!

Yildiz

thanks yildiz

No problem at all!

One of David's posts on CA-correcting sofware:
http://www.photoclubalpha.com/forum/vie ... 057#p18618

Re: The Zeiss CA's
by David Kilpatrick on Thu Mar 05, 2009 6:56 pm

You just use a program which adjusts for CA, like Lightroom, ACR or Capture1 Pro. CA in this context is simply a different physical pixel size for the R, G and B channels. The correction rescales each channel separately (or two layers at a time) to match their sizes. You can also do this with an automated Photoshop action, if you know the percentage CA and how to resize the R and B layers (normally leaving the G layer alone as the main reference).

David

While the compositions here are all very well, they are also each very odd crops, and that is distracting. Also, the first two pictures look rather soft.

Chromatic Aberration and purple fringes are not necessarily connected. CA is a lens defect caused by different focus points or image scale dependent on wavelength - the lens might be an effective 50mm with green light, but 49.5mm fo blue and 50.5mm for red. The corrections used prefer to bring all three into sharp focus (apochromatic) but may leave some residual difference in size of image per colour. This causes colour fringes. They can be removed by adjusting the size of the R, G and B images later - or in-camera. It may be possible to correct CA perfectly if all three images are very sharp (16-80mm CZ).

The purple fringe is caused by a blend of aberrations including coma (makes the effect appear worse on the outer edges of objects, and towards the edge of the frame) and spherical aberration (makes the effect worse in slightly out of focus areas, and the edges of the frame). It is exaggerated by the 'bucket' effect in which overexposed sensels (pixel wells) can leak into neighbouring ones. Mainly, green pixels spill over into blue and red, which creates the purple tint.

The best way to remove purple fringe is to use CA correction first (in case part of the effect is a CA fringe, which it often is), apply Defringe All Edges using Photoshop ACR or Lightroom, and finally - this works very well - set maximum colour noise removal. If there are still very bad fringes, selective desaturation of the purple colour involved (in ACR or Lightroom calibration/colour controls) may work. Finally, you can set a small Photoshop brush to mode 'Desaturate' and paint along the fringe to retouch it manually.

David

Thank you David.

This sheds a good light on CA and purple fringe occurances, prevention and elimination.

Yildiz