At 12:57 PM +0000 8/20/03, olympus-digest wrote:
>Date: Wed, 20 Aug 2003 02:01:26 -0400
>From: "R. Lee Hawkins" <lhawkins@xxxxxxxxxxxxxxxxxxxxxxxxxx>
>Subject: [OM] SC vs MC
>
>[snip]
>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.
>
>[snip]
>
>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.
It seems to me we could use the above described difference in transmission as a
test to tell which lenses are and are not multi-coated, so we don't have to
depend on perhaps unreliable marketing materials.
I just tried a simple experiment on a 55mm f/1.2 MIJ Zuiko. The reflections in
this lens are mostly green, with one orange reflection. This using a small
pencil-beam flashlight.
I shined a (claimed 5 milliwatt 535nm) laser pointer through the lens. In the
forward direction, there are no ghosts (spots in addition to the main
blindingly-bright spot), as one would expect if the lens is to have low flare.
In the backward direction, there are something like six spots of varying area
and brightness. In normal photo use, these would have no effect on flare. The
larger the area, the lower the intensity, and it's my impression that all spots
have the same amount of light in them. I would guess that there is one spot
per facing air-glass transition, and that all surfaces have the same kind of
anti-reflection surface. By "facing air-glass transition", I mean where the
laser beam goes from air to glass.
I also tried the same experiment on a 50mm f/1.8 MIJ Zuiko. The reflections in
this lens are all kinds of colors, including white.
I shined the same laser pointer through the lens. In the forward direction,
there are no ghosts. In the backward direction, there are something like four
spots of the same (larger) area and (lower) brightness.
I would guess that the green reflections are from multicoated surfaces and the
orange/red reflections are from single-coated surfaces, and the white
reflections are from uncoated surfaces. Despite the single-coated surfaces,
there are no forward-direction ghosts. I bet that the lenses are designed with
enough multicoated or coated surfaces to simultaneously suppress forward ghosts
and flare and to achieve a desired transmission value (say, 95%), but no more,
as the more coats the more expensive the lens.
Not surprisingly, the more expensive lens (55mm f/1.2) has almost all green
reflections, and the same shade of green at that (showing tight process
control), while the cheaper lens (50mm f/1.8) has all colors (looser process
control) including white (uncoated surface?).
Joe Gwinn
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