Yes, now this is what this chat page is for!
Dan
-----Original Message-----
From: owner-olympus@xxxxxxxxxxxxxxx
[mailto:owner-olympus@xxxxxxxxxxxxxxx] On Behalf Of whunter
Sent: Tuesday, August 19, 2003 8:59 PM
To: olympus@xxxxxxxxxxxxxxx
Subject: Re: [OM] SC vs MC
Ahhh ha.... VERY well written. Vernacular is critical in these
discussions. Contrast is a very confusing term in the sense that it is
cited in both hard science and anecdotal context with incongruent
meanings. Suggest one consider the total signal (in this case visible
light) to be divided into signal and noise or the familiar S/N ratio
concept. Photon scatter both in the atmosphere and the lens, stray and
internally reflected light are components of noise which detracts from
the MTF of the optical system. Polarizers and MC are the two greatest
optical tools for reducing noise in photography. In general, one seeks
to optimize collection of data with the highest S/N ratio; it can never
be 'too good'. Some of the confusion in descriptions arises from
combinations of lenses and film and filter which provide a fixed
'artistic' quality to the output. The term 'contrast' has been used to
describe the results of some techniques. Digital data collection of
the highest S/N ratio now allows 'contrast' manipulation of the data
for desired artistic effect. Once the data is collected, the S/N ratio
can never be improved. Increased signal facilitated by MC improves the
statistics of the image rendered. Signal reflected at the interface
becomes noise. Since statistical considerations of S/N are a square
root function, MC contributes to the contrast of the image by
decreasing noise even more than by enhancement of signal collection.
Bill
On Wednesday, August 20, 2003, at 02:01 AM, R. Lee Hawkins wrote:
>
> Daniel:
> Coatins on lenses have nothing to do with "filtering" the light. The
> coatings are actually interference coatings (the thickness of the
> coating is some fraction of a wavelength of a given color of light,
> typically 1/4 wave of 5550 angstroms for SC coatings). These coatings
> act to increase the transmittance of the lens. Uncoated glass
reflects
> about 40f the light hitting it due to the difference in the indicies
> of
> refraction of air and the glass (1.0 for air, 1.5 for normal glass).
> Put very simply, the interference coating reflects the light that
> reflects
> off the glass back into the glass, thus increasing transmission.
> (In actuality it is more complicated than that.... see the book
> "Thin Film Optical Filters" by Angus MacLeod if you want to know the
> physics behind it all).
>
> The problem is that an SC can only correct well for one wavelength of
> light, since a single coating can only be the proper thickness for one
> wavelength (let's say 5550 Angstroms). Thus, the average
transmittance
> for a single coating is about 990er surface.
>
> If you were paying attention, that last sentence would have made you
> sit
> up and think... hmmm... a lens I use a lot, the 35-105, has 16
elements
> in 12 groups. This means there are 12 x (2 air-glass surfaces/group)
> for
> a total of *24* air-glass surfaces in this lens (you can ignore
> cemented
> glass to glass surfaces, since the cement matches the indicies of
> refraction between the elements).
>
> Light loss by reflection is multiplicative. This means that if the
> first surface transmits 0.99 (or 99%) of the light, the second surface
> will transmit 0.99 * 0.99, or 0.98 (98%) of the light. For a single
> coated lens, the amount of light transmitted by the last surface to
the
> film, assuming 24 air-glass surfaces, is 0.99^24 (0.99 raised to the
> 24th power), or about 79 percent. This is much better than an
uncoated
> lens, which in this case would only transmit 0.96^24, or about 38
> percent of the light to the film.
>
> The even bigger problem is loss of contrast from stray light. That
> reflected light can get reflected back to the film eventually, as an
> out
> of focus image (if you were taking a picture of a point source like a
> star, you would get a lot of out of focus diffuse circles on the film
> which would lower the contrast of the final image).
>
> While SC is obviously better than no coatings at all, let's look at
MC:
> A good broadband (meaning from the blue to the red in the optical
> spectrum) consists of several coatings to maximize transmission in
> several wavelengths. This results in a transmission of something like
> 0.999 (or 99.9%) per surface. For a 24 air-glass surface lens, this
> means 0.999^24, or about 980f the light hitting the lens is
> transmitted to the film. This is a huge gain over SC in throughput,
> but
> more importantly, there is a lot less reflected light bouncing around
> lowering the contrast of your images.
>
>
> That's why you want MC in a (very big) nutshell.
>
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