A question about stars

[mwooten]

Although the rules concerning this particular forum state this is the "forum is for the heavy duty tech discussions." Can someone give a simple explanation as to how/why stars are formed in an image?

I kind of understand how stars are formed in space, and really don't care to know how they are made in Hollywood. I do like stars in my night photos and I've wondered what causes them. Also, are certain lenses more prone to create stars than others?

Thanks,
Michael

 

[Pherdinand]

by diaphragm blades being not perfectly circle-forming but having small corners.

Check it: if you have a lens with less diaphragm blades you get less "corners" of the star.

Also,there was some symmetry thing involved i.e. 5 blades produce 10 points, or such, but i can't recall the details anymore. 🙂

 

[Pherdinand]

i forgot to add, the smaller the aperture you use, the better the stars. Because more pronounced is the effect of the corners, since the irregularity from the corners where blades overlap becomes more important compared to the total area of the opening, than in case of a large lens opening...
Also, night photos are better showing this, since the contrast between the "star" (i.e. a street lamp usually) and the environment (black night sky) is strong.

 

[Gumby]

You can force them to be as you like by using an appropriate star filter... four, six, eight point; forur short and two long points, etc.

 

[Tuolumne]

The classic star effect is caused by the diffraction of light by an object within the optical system. In the case of classic astronomical photos, that is usually the spider which holds the secondary mirror in Newtonian reflectors, hence the four-edged star. (See attached). In the case of photos with general purpose cameras and lenses, it is caused by the edges of the diaphragm in the lens. That's why the more you stop down the greater the star effect. When the diaphragm is completely open, you should get no star effect at all.

/T

 

[mwooten]

Thanks for the answers. Very understandable. Thanks