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[OM] Re: digital lenses and manual focusing

Subject: [OM] Re: digital lenses and manual focusing
From: usher99@xxxxxxx
Date: Fri, 15 Aug 2008 21:02:22 -0400
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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