All the AF SLR in the film age did not came with a split screen, it is not
related to whether the screen will be darken with the lens or prism size. I
belive it is mainly due to focusing error in the AF system, if there is
split screen on the camera users can easily see the discrepancy and
complain, it don't have to wait until now. Now with the DSLR, people found
focusing error much easier and camera manufacturers started to implement the
user AF adjustment features (still may not be a total solution).
The brightness of the image inside the viewfinder is mainly affected by the
image magnification (if same lens is used), for instant the OM-4 viewfinder
is 0.85x and E-5-6xx viewfinder is just slightly larger (0.92/0.96x) so it
should only be slightly dimmer. The prism also pay its part but I belive it
is small, even the mirror prism is not that dark. You lost two stops only if
Olympus make a 4/3 camera with 2x mag.
For the lens, not everyone use a 50/1.8, there were lots of slow zooms in
the film age, for example the OM 35-105 is F4.5 at the long end with 16
elements. BTW, the kit lens came with the E-1 is a F2.8-3.5 so lens speed is
not a reason.
C.H.Ling
----- Original Message -----
From: "Moose"
> On 2/9/2010 7:46 PM, Daniel Sepke wrote:
>> John,
>>
>> I think that the lack of manual focus enhanced screens stems from the
>> (inaccurate) assumption that with AF no one wants to have precision
>> manual focusing.
>
> Sure, that's a factor, but there are other factors of optical physics
> and economics that likely play a bigger role.
>
> Consider the standard SLR "kit" back in the days of the OM-1 & 2.
> - f1.8 or faster lens
> - 24x36 mm image isze and thus, focusing screen size, 864 sq. mm
> - Full size glass prism.
>
> How about the ordinary kit DSLR today?
> - f3.5-5.6 zoom
> - ~22x15mm sensor/focusing screen size, 330 sq. mm
> - Either smaller prism relative to image size or penta-mirror.
>
> Oly DSLR kits
> - f3.5-5.6 zoom
> - 17.3x13mm sensor/focusing screen size, 225 sq. mm
> - Smaller prism relative to image size except for single digit models.
>
> Sticking with Oly:
> - f1.8 => f5.6 = about 3 stops. But that's calculated F-stops. Actual
> T-stops for so many elements compared to a MC 50mm prime will be even
> less.
> - image/focusing screen area 1/4 as large = 2 stops
> - less than optimal prism size, maybe a stop, or a bit more? Look at the
> prism hump on the E-3 vs. E-5xx-6xx
>
> So - for the same apparent image size through the viewfinder, a triple
> digit Oly DSLR has about 6-7 stops less brightness than an OM-1. There
> are only three solutions to this problem:
> - Bigger, heavier, more expensive prism and viewfinder optics
> - Smaller apparent image size
> - Brighter screen
>
> On the typical DSLR, it's the screen that does the heavy lifting of
> brightness. Unfortunately, the only ways known to date to brighten the
> focusing screen make it less useful for manual focusing.
>
> As to manual focusing aids on the screen, ever notice how the split
> screen focusing aid on manual focus SLRs blacks out when you push the
> DOF preview button at smaller apertures? The decrease of light and
> change of average angle of incoming light interacts with the tiny prisms
> in a negative way.
>
> So it turns out it's also impossible to design a split screen focuser
> that's much use for the above kit characteristics. It's partly the
> optics of focusing aids and partly the increased DOF of shorter focal
> length, slower lenses. The greater DOf makes seeing when the central
> focal plane is reached visually harder - it just doesn't "snap".
>
> The fact that the E-3 has abandoned interchangable screens suggests to
> me that even the super viewfinder wasn't enough to make focusing aids
> practical.
>
>> I suspect that if they made a split screen or all matte type it would
>> likely be dimmer;
>
> Waaay dimmer.
>> ...
--
_________________________________________________________________
Options: http://lists.thomasclausen.net/mailman/listinfo/olympus
Archives: http://lists.thomasclausen.net/mailman/private/olympus/
Themed Olympus Photo Exhibition: http://www.tope.nl/
|