How to "add" MTFs?

[T3D john bercovitz <bercov@xxxxxxxxxxx>]

John Bercovitz wrote:
> I've never seen how you do this.  I've just multiplied the contrasts
> together frequency by frequency.  Is that the right way to do it?

John Ohrt wrote:
> As mentioned in an earlier post today, it is my understanding that 
> both resolution and modulation transfer are the inverse of the sqrt 
> of the sum of squares for the distances between lines for the various 
> system components contributing.

I must have missed where you said it applies to MTF also.  Sorry.  I 
can see adding resolutions (eg in lines per mm) in quadrature as being 
at the least a very good approximation, but I can't see through to it 
for adding contrasts.  When I said I would think that contrasts would 
multiply (at each spatial frequency), I was thinking along the lines of 
how you find the transmission of a combined pair of filters by multiply-
ing the transmission of each (in percent, eg) together at each wavelength.  
I guess you don't buy this as being similar?  

Could you illustrate with some numbers how you would add modulations 
in quadrature?  I'm not sure how to approach this.  I guess I follow a 
sample calculation better than an explanation sometimes.

> Comments from the pro's please.

I'd settle for a good rationale.  8-)

Thanks,
John B


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