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Re: [OM] Nope, It missed it by few hundred miles

Subject: Re: [OM] Nope, It missed it by few hundred miles
From: "Julian Davies" <julian_davies@xxxxxxxxxxxxxx>
Date: Sun, 21 Sep 2003 12:36:58 +0100
Er...
My fault, in that I was trying to draw a parallel between two entirely
different results which have the same cause!
Quite correct that film doesn't alias. It fogs.
When you hit the limiting frequency, a fixed uniform sample system begins to
form beating artefacts. Since this is a property of the sampling interface,
the downstream electronics don't know they are artefacts, and can do nothing
about it. Nasty. Digital samplers have to take great care not to exceed this
frequency, however it is defined. There was a debate on the list a while
back about whether Nyquist was right, but that exceeds my ability to follow
or discuss by a country mile. Nonetheless there is a must - not - exceed
frequency in the system which is always enforced by a filter of some kind.
Analogue recording systems don't form beating patterns when a particular
frequency is reached (for the purists this is only true when the recorder
works without modulating a base frequency). They suffer from an increase of
noise with frequency. In photographic film, this is base fog.
Now I don't understand the precise physics of this, but Geoffrey Crawley
published an article in Amateur Photographer making just this point.
If you  think of the ultimate limit of three grains of film capturing two
separated photons (white - black - white bar signal), if the spatial
frequency is higher than the separation, the outcome is One black, two
clear. - by definition if one photon hits a grain, the other must miss the
other two.
It seems that because of the physical construction of film, light scatter in
the emulsion makes "both grey" the result of the miss, and also makes the
limiting frequency considerably lower than the limit case above.
When you have a set of grey film grains, you have fog, assuming that the
levels concerned are far below the levels of light at lower spatial
frequency, i.e. there will be an image recorded on top of the fog.
Since this effect is a rising with frequency one, there is no defined must -
not - exceed point. It just becomes one of the factors in assessing lens
performance, i.e. as we know, the MTF graph does nor reveal all.
Since film formulation is not a static art, it is also a factor in classic
lenses being thought of differently in the modern world compared with their
original production time. Whether it is an enormously important aspect of
the "Contrast Vs Sharpness" debate, I don't know, but bear in mind that it
would be easy to produce lenses with far higher MTF results than are
currently targeted, and would be possible within the price of certain German
manufacturers.

Julian

----- Original Message ----- 
From: "W Shumaker" <om4t@xxxxxxxx>
To: <olympus@xxxxxxxxxxxxxxx>
Sent: Sunday, September 21, 2003 2:26 AM
Subject: Re: [OM] Nope, It missed it by few hundred miles


> Aliasing is an artifact of uniform sampling of information with higher
> frequencies than the sample rate. Low pass (anti-alias) filtering
> minimizes high frequencies from creating lower frequency patterns
> through sampling. Film samples, in the sense that it has grain
> structures, but grains are not uniformly spaced and overlap, so the
> effect of grain randomness is simply the spreading of the noise
> frequency; that is, it is indistinguishable from just the grain noise
> itself and highly unlikely to be affected by lens sharpness. The
> sharpness is simply lost in the grain resolution. Hence, I don't think
> the sharpness of a lens is going to increase or decrease fog, ie.
> noise, from film grain sampling. Light scattering in the lens is much
> more likely to increase fog. I can't figure how an analog camera would
> need anti-aliasing from of an overly sharp lens. I've never heard of
> such a thing. Perhaps I am not understanding something?
>
> Wayne
>



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