I wanted to respond to something in that article. Almost all 35mm SLR
cameras have used plastic focusing screens since the 1970's. The Olympus OM
screens are all plastic, even the oldest ones.
--
Chris Crawford
Photography & Graphic Design
Fort Wayne, Indiana
http://www.chriscrawfordphoto.com My portfolio
http://blog.chriscrawfordphoto.com My latest work!
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On 8/15/08 9:02 PM, "usher99@xxxxxxx" <usher99@xxxxxxx> wrote:
>
> Moose says:
>
> One of the problems with MF on DSLRs, at least small sensor ones, is
> that the finder screens are optimized for brightness over the ability to
> show focus accurately. Oly does offer an alternate screen for the E-3,
> but it is supposedly not user changeable and has no split image focusing
> aid. I suppose after market folks will expand on that.
>
> Moose and Chuck hit the nail on the head a usual.? I ran across an article
> which I excerpted below on focussing screens and manual focus:
>
>
> "When Focus Screens Lie
> Modern focus screens seldom have focus aids. Most just have some markings to
> show AF and/or metering areas. But other than that, they tend to be plain
> "ground glass" screens. The split prism focus aid and the microprism collar
> that were common when I got my first SLR went away long ago. The invention of
> phase detect autofocus swept most of that away. And guess what, the focus
> screens in modern DSLRs really aren't ground glass either. Ther aren't ground,
> nor are they glass. They're typically plastic and their surfaces are made up
> of an a bunch tiny microlenses. Now I'm not an anti-plastic snob. Plastic can
> be a super material. But those pesky microlenses definitely drop a fly in our
> legacy, manual focus, lens ointment.
>
> So what's the big deal? Why should you care about a bunch of little
> microlenses. You can still focus on that screen. Heck - we've all done it. You
> can sit there and see the image go in and out of focus. So what's the problem?
>
> Well, the problem usually isn't immediately obvious. Sometimes these new focus
> screens do an OK job at manual focus. But at other times, they flat out fail.
>
> The problem begins with the desire for nice bright finders in our DSLRs. Most
> people want bright finders. But getting that in a modern DSLR presents
> obstacles. Since most DSLR formats are smaller than 35mm film, most DSLRs use
> more screen magnification than 35mm film cameras. More magnification leads to
> a darker screen. The autofocus sensors (now a standard requirement) need to be
> able to look at the image at the same time you are using the finder to compose
> your picture. So light must be diverted to them for that purpose. The reflex
> mirror that sends the lens image up to the focus screen and your eye is
> partially tranparent to allow some of the light from the lens to find its way
> to the autofocus sensors. That results in less light getting to the focus
> screen and means a dimmer screen. More and more people use slow or moderately
> fast zooms instead of faster primes these days. Oops. There goes some more
> light.
>
> So slower lenses, a smaller format and the needs of autofocus all work against
> the engineer's goal of delivering a bright finder to you. The poor engineer
> has to find solutions, and there are only so many photons coming through that
> slow lens to work with dang it!
>
> So the engineers put microlenses into our focus screens. These microlenses are
> really efficient at passing light at a paricular lens f-number or range of
> f-numbers. For lower priced cameras, the camera engineers assume the user will
> mostly be using lenses with apertures of around f/3.5-f/5.6 - so those cameras
> have screens that are efficient at that range. They expect buyers of more
> expensive cameras to be using lenses that are somewhat faster. Maybe f/2 -
> f/3.5 is more common for that group of buyers. So they use screens that are
> more efficient at that range for those cameras.
>
> This all works pretty well. Most finders are fairly bright. But there's a
> price to be paid when you start doing stuff that the engineers weren't
> counting on. Like mounting that legacy f/1.4 manual focus lens.
>
> I'm not going to try to explain exactly how these microlenses work. That's
> mostly because I'm not fully sure of the details. But the basic idea seems to
> be that they are very efficient at passing light that hits them from certain
> aperture sizes. This works great when you have a lens of that aperture range
> mounted. You get a very bright image since the screen is so efficient at
> passing light at those apertures. But if you mount a faster lens, you don't
> get a proportionally brighter image. The microlenses don't pass that extra
> light very efficiently at all.
>
> Larry J. Clark ran tests on the Olympus E-1 and E-500 using a one degree spot
> meter to measure screen brightness at different apertures and his result show
> the non-linearity quite nicely. Click here to see a graph of the results. More
> details can be found here.
>
> Now from the standpoint of viewfinder brightness, this is no big deal. If the
> screen is bright enough at f/4, then it will also be plenty bright with an
> f/2.0 lens mounted. Maybe not as bright as the more expensive camera that has
> a screen tuned for f/2.8 or faster, but still plenty bright. And it allows you
> to have a pretty bright image with your inexpensive slower lens. That's a good
> thing!
>
> But from the standpoint of manual focus, it isn't necessarily so good. The
> rays from the larger aperture that are being blocked from getting through are
> the rays that show the most misfocus. This means that the focus screen has the
> effect of showing screen focus that is about the same as if you had stopped
> your lens down to f/4!! Now if you are shooting at around f/4 or slower, this
> isn't a problem. The screen focus will be very similar to the focus delivered
> by the lens. But if you are shooting at f/1.4, it's a big deal. You simply
> can't see the point of sharp focus. So when shooting at a large aperture with
> shallow DoF - precisely the place where you tend to need good critical
> focusing capabilities - you have a focus screen that shows too much DoF and
> makes the image appear to be in-focus when it isn't.
>
> Lots of people don't believe this. It is counter-intuitive. And if they never
> focus with fast lenses at fast apertures, they won't be able to notice it. But
> if they mount a legacy 85mm f/1.4 to the typical DSLR and shoot at fast
> apertures, they may soon be wondering if maybe their eyes need checking. After
> all, it sure looked in-focus. It really did! Blink - blink!! "
>
> I then quieried Rachel at Katzeye on how they achieve both a brighter screen
> yet retain the ability to accurately manually focus fast glass:
> I think some have tried Brightscreen screens on the 5D which have helped some
> in this regard but was not a total solution.?
>
> Mike
>
>
> Rachel replied:
>
> "Regarding the brightness and focusing of fast glass, that is a legitimate
> concern and loss of focusing ability is commonly the case with brightness
> enhancement techniques that apply a coating to the screen.? Essentially,
> what those coating techniques do is cut down on the diffusion (and by
> extension, the focusing contrast or 'snap') by smoothing over the surface.
> I don't want to single anyone out by name, but that general idea is the
> traditional brightness enhancing technique that's been around for a while
> and is used by most of the other companies you can find offering brighter
> screens.? Our process is significantly different, however.? We do not coat
> the screen and we do not simply smooth over the surface.? The details of our
> OptiBrite treatment are of course proprietary, but we actually change the
> structure of the plastic at the microscopic level in a very controlled way
> to modify the scattering profile for a specific result.? What this allows us
> to do is maintain the overall level of diffusion so that the focusing
> contrast remains high and focusing of fast glass is still optimal.? But
> within the overall diffusion profile, we can preferentially augment specific
> scattering angles to increase the amount of light that reaches the eye.? So,
> to make a long story short, the OptiBrite treatment we offer on the Katz Eye
> screens does not have a negative impact on the focusing of fast glass as is
> the case with other brightness enhancement processes."
>
>
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