Sun ghosts

[Gerald]

A few pictorial effects caused by internal reflections of sunlight in a Tamron 28mm F2.8 and Pentacon 29mm F2.8.

The Tamron uses a 5-blade diaphragm while the Pentacon uses a 6-blade. As a consequence, the ghosts produced by the Tamron have pentagonal shape, and the Pentacon, hexagonal.

[Gerald]

I would like to add some interesting technical information I found about the ghosts which are produced when a lens is illuminated directly by the sun.

Today, many people get a little paranoid when sun ghosts appear in an image. On the other hand, those same people love to see pictures with points of light out of focus in what is called "bokeh". Go figure.

A common belief is that the best lens coatings have the ability to eliminate sun ghosts. That is completely wrong! Who says that is a Zeiss scientist, Dr. Hubert Nasse:

"So with the presently available coating technologies the attenuation of the ghost paths is not good enough to cover the possible dynamic range of subjects including the light source and deep shadow. Additional attenuation is required by distributing the light of the ghost path over a larger area of the sensor."

http://diglloyd.com/articles/ZeissZ/ZeissZ-15Distagon-NasseInterview-FlareControl.html

The explanation is very simple. According to Dr. Nasse, a high-quality coating on an optical surface reflects approximately 0.1% to 0.4% of the incident light. This corresponds to an "attenuation" of 8 to 10 stops. Remember we are speaking here about the state-of-the-art of lens coating. Not all optical surfaces are multicoated even in famous and expensive lenses.

Two reflections are required for the sunlight to reach the sensor, so that the total attenuation is 16 to 20 stops. Unfortunately, the ratio between the brightness of the sun and the deep shadows of a picture can get to 27 stops. The conclusion is that the attenuation of the best coatings is insufficient to prevent sun ghosts.

What to do then?

As Dr. Nasse says, the designer of the lens tries to make the reflections of the sun the most out-of-focus as possible. The diaphragm images (see the pictures in the original post) are nothing but the out-of-focus sun's images.

The optical surfaces of a lens are designed to minimize aberrations, but as it turns out they are also important in the formation of sun ghosts. Sometimes, by sheer luck a lens is much better than another in relation to the rejection of sun ghosts. This performance difference may have no relation to the quality of the multicoating, or correction of aberrations.

[luisalegria]

Very nice - and some of these are very nice images !
In the old days these were often used creatively.
For such use I'd say number of blades is less important than whether they appear symmetrical.
A wonky pentagon or hexagon aperture is no problem with a normal image usually, because you dont see it. But in this sort of thing you want a nice looking aperture. Hmm, what about a Komura WA with star shaped aperture ?

[memetph]

The Tamron has a pentagon.
The Pentacon has a hexagon.
What will have a Hexanon?

[PhantomLord]

[pdccameras]

[Rick1779]

[David]

Time to try this with my newly repaired Amica Eyelux and it's square (with tear-drop corners) aperture.

[Lightshow]

Thanks for the discussion, I had always wondered how big a roll coatings had, I had assumed correctly that the shape of the element surfaces were the main factor.

[Gerald]