This is a photograph that I have shot outdoors. As you can notice there is a blue glow around the borders of the photo.
I want to know why this happens and how I can remove it.
I use a Nikon d7200.
This photo is ISO 400, shutter speed 1/1250, and f 4.0.
No, this is not chromatic abberation, as others seem to think. Chromatic aberration is a real phenomenon, but not the dominant one here.
The background is grossly overexposed. Lenses aren't perfect, and some small fraction of light that is supposed to be focused on a point ends up in other places. Even if the lens by itself were perfect, a little bit of dust on the lens, and light bouncing around inside the camera will cause some bleeding of the focused image.
Normally this is invisible since the fraction of light that bleeds to elsewhere is very small. The tiny bit of additional light that gets to a pixel by bleeding from elsewhere is swamped by the light that is focused there as intended.
The difference in this case is that the background was so overexposed that the small fraction of background bleeding to nearby pixels is now significant relative to the light from the intended subject for those pixels. The background light was probably blueish purple (sky, perhaps?) relative to whatever was illuminating the much darker forground subject.
To convince yourself this is not chromatic aberration, look closely at the highlights in the jewelry. You do see some haze around each highlight, but that haze doesn't seem to have a particular color. It seems to be the same color as the highlight, just spread out a little. This points to a "soft" lens, or perhaps dust or a thin greasy layer on the front of the lens.
I got a similar effect once by accident by wiping the front of the lens with a cloth that had previously picked up sunscreen off my skin. Unfortunately, this was back in the film days, and I didn't realize what had happened until much later. Inspecting the lens uncovered the thin grease layer on the front, and cleaning it carefully fixed the problem.
One of the problems is blooming, and it depends much more on your light and lens than on your camera model.
If you want a white background, it should be overexposed just enough to not have any detail. If you overexpose more, it will "bleed" onto your model reduce constrast and add unwanted colors.
The crop also shows purple fringing. It would be interesting to look at other parts of the picture with a different focus in order to be sure.
You already lost some information in the picture but you can try to increase contrast and reduce saturation around the model to get a somewhat cleaner picture.
This is chromatic aberration. You can use many methods to remove them. For example, in Photoshop:
Photoshop raw conversion (if you have RAW, of course): after opening file, choose "Lens Corrections" tab, find "Chromatic Aberration", then use sliders "Fix Red / Cyan Fringe" and "Fix Blue / Yellow Fringe".
You have not RAW or other reasons. Photoshop has filter "Lens Correction". The basis is the same as in point 1: the same sliders and the same approach.
Create in Photoshop a new copy of your current layer. Then apply Filter -> Blur -> Gaussian blur with the strenght of 4-6px. Then press right button of your mouse on your layer copy, choose Blending Options and then Colour.
Other photo applications have similar approaches and filters.
The camera lens works by projecting an image of the outside world on the surface of film or digital image sensor. The image is formed by the fact that the lens causes light rays from the subject to change their path. The revised path traces out the shape of a cone of light. We call this action refraction, Latin for to turn inward.
All lenses are plagued by the fact that each color is refracted differently. This is due to the fact that each color has a different wavelength. Violet, with the shortest wave length focuses closer to the lens followed by green, yellow, orange and lastly red. Now image size is a function of the distance from the lens. What happens is, each color forms an image that is slightly different in size. The red image is minutely larger and the violet image is a tiny bit smaller. This results in rainbow-like fringe seen to surround the borders of objects. Technically, this is called longitudinal chromatic aberration.
The English amateur optician Chester Hall, in 1729 made an optical system that mitigated chromatic aberration. He combined a strong positive (convex) lens with a weak negative (concave) lens. Since both display opposite chromatic aberrations, the result is near cancellation. This configuration is called an achromatic (Latin: without color). Today’s cameras and telescopes are spinoffs of his design.
The goal of the lens maker is a “faithful image”. This is yet to be achieved.
