85mm 1.4 L IS preview

tim73 wrote:

Steve Balcombe wrote:

tim73 wrote:

Huh? Chromatic aberration is any departure from ideal behaviour that is dependent on colour.

It's more specific than that. It's the aberration that results from the lens focusing different parts of the spectrum at different distances (LoCA) or at different magnifications (LaCA).

I'm going to have to disagree. The words have a clear technical meaning as they stand, and I don't think it's helpful to ascribe a different or more specific meaning.

For example Brightcolours has his own definition of purple fringes which apparently does not include magenta fringes. Can you reliably distinguish magenta from purple? Of course not.

Funny. There are lenses which, instead of the magenta/green LoCA pairing, have yellow/blue LoCA. In such a case, yes, you can reliably distinguish yellow from purple.

And it is NOT about the difference between magenta and purple.

It is about that PF is a different phenomenon than LoCA.

And aside from that, PF can have different colours. It can range from blue-ish to magenta-ish, that depends on the sensor. PF was more wide spread with early digital cameras. Coatings have been developed to minimise it.

LoCA magenta can be seen anywhere around enough contrasty parts in front of the focus plane. PF, on the other hand, only shows up clearly around overexposed areas.

The magenta from LoCA comes from the lack of green (white - magenta = green (green = yellow + cyan). The green from LoCA comes from the lack of magenta. The colours get focussed on a different plane.

The PF, as far as I know, has to do with too much light reflecting from the sensor glass/microlens/sensor structure and back again from lens surfaces, and the colour(and how it gets interpreted by the sensor) is related to part of the UV spectrum.

Digital camera and lens manufacturers know exactly what PF is, how it happens. The internet, on the other hand, is not entirely clear on it, and that is why there is such a wild variety of claims and confusion with CA about it. You can find many totally different definitions/explanations of PF, and put the links in a post. This does not make things automatically right.

In tis image, you can see the meganta in front, and the green in the back, of the LoCA from this lens. At the bottom of the more over exposed areas, you see a different discolouration (more purple than the magenta LoCA with a different character) mix in with the magenta LoCA. This is PF.

As a scientist (working in optics, as it happens) I think it's very important not to misuse language in this way. The Wikipedia article, unlike this thread, is quite clear.

The Wikipedia article is not "right", it does some guesswork about what is PF. In the past, the wikipedia page was even more wrong about PF.