Re: [APML] Petzval hot spot?

[Chris1011@aol.com]

In a message dated 9/19/2002 12:47:31 AM Central Daylight Time, mschex@verizon.net writes:


> Well I knew I had seen these things before -- here is an example of a 
> front-objective filter that reduces the effects of vignetting.  I can't 
> explain how / why it works -- maybe Roland can help with that.


When it comes to wide angle camera lenses, you have a different animal than a telescope lens. In a telescope lens, the light from all objects go equally through the full aperture, so stopping light in the middle of the lens affects all objects in the film plane equally. It cannot have any effect on vignetting. In a wide angle lens, only a small portion of the front element is used by any one object. The center part of the front lens is used by objects on-axis, and the edges of the front lens is used to image objects at the edges of the field. In this system, you can selectively filter the light on various parts of the front element to eliminate the effects of vignetting. 

For an astronomical lens, you don't really want to filter light to eliminate vignetting. You don't want to throw away light. What you want to do is capture all the light from the entire field of view and bring it to the film plane. When there is a restriction somewhere in the optical train, you cannot get light past it, and will have to live with the loss. Calling this effect a "hot spot" implies that the efficiency in the middle is greater than 100%. In reality, the efficiency at the edges is less than that of the middle, causing the center to look brighter.

Roland Christen