Re: [APML] Petzval hot spot?

[Mike <mschex@verizon.net>]

Hi Chuck,

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.

http://www.bhphotovideo.com/default.sph/FrameWork.class?FNC=ProductActivator__Aproductlist_html___21721___HECF352___REG___CatID=262___SID=F0983FF3220

This is one made by Heliopan:

52mm Center ND 3x Glass Filter - 67mm Front Thread

Many wide angle lenses exhibit fall-off (darkening) at the edges. This can 
be improved with a center filter which has a progression from dark grey in 
the center to clear at the edges. The Heliopan wide angle center filters 
are available in two densities: ND-0.45 (filter factor 3x, or +1.5 stops) 
and ND-0.9 (filter factor 8x, +3 stops).
Most large format lens manufacturers make center filters for their wide 
angle lenses. Heliopan offers an economical alternative for lenses 
requiring a 3x filter. The 8x version is unique and the most powerful on 
the market.

Mike

At 06:05 PM 9/18/2002 -0400, you wrote:
>Thanks Chuck, your argument certainly makes sense.  I am certainly not an 
>optical engineer.  I have never owned one of these lenses but I thought 
>that I had seen / heard of a gradient filter that screwed into the 
>front.   It sounds like it would have had to be close to the image plane 
>in order to even out the illumination.
>
>Mike
> >
> > From: Chuck Vaughn <aa6g@aa6g.org>
> > Date: 2002/09/18 Wed PM 02:44:21 EDT
> > To: <astro-photo@seds.org>
> > Subject: Re: [APML] Petzval hot spot?
> >
> > Mike,
> >
> > > Many of the systems do place the
> > > radial gradient in front of the lens, this does in fact slow down the 
> entire
> > > system however -- but you get an evenly illuminated field.
> >
> > Maybe I'm unable to visualize it but I can't see how it could work.
> > Perhaps someone else can set me straight but let me explain further the
> > way I see it. Feel free to punch holes in my argument because that way
> > I'll know where I err'd. :-)
> >
> > Let's examine an ideal system where the center of the field is 100%
> > illuminated and the edge of the image circle is 50% illuminated and the
> > drop off is linear.
> >
> > A radial gradient placed in front of the film would have be the opposite,
> > i.e., drop the illumination in the center by 50% and the edge 0%. There's
> > an important point to remember here. Each point on the image plane, with or
> > without the filter, sees the sum of all the non-obstructed rays entering
> > the objective. Realizing this, it is easy to understand how the density
> > of the near film filter would have to vary to even out the illumination.
> >
> > What I don't understand is how the same thing could be done with a filter
> > in front of the objective. Going back to my ideal example, the total
> > attenuation of the filter would have to equal 50% in order to reduce the
> > center illumination to 50%. But now the illumination at the edge of the
> > image circle would be 50% from obstructions plus attenuation of the filter,
> > perhaps another 50% depending on just how the obstructions blocked the
> > objective. So now we have 50% illumination in the center and 25% at the
> > edge. We still have the same 50% drop off.
> >
> > In order for this to work the attenuation at the edges of the image circle
> > would have to be zero. This is impossible though because the edges will
> > see rays from a portion of the attenuated objective.
> >
> > Perhaps the easiest way to think about this is a secondary obstruction.
> > That would be step filter in from of the objective, 0% transmission within
> > the obstruction and 100% out of it, yet this has no impact on the
> > illumination of the image plane from center to edge. No circular
> > obstruction placed in the center in front of the objective can alter
> > the illumination curve at the image plane.
> >
> > Chuck <aa6g@aa6g.org>
> >
> >
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> >
>
>
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