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.
> ...
>
Moose
--
_________________________________________________________________
Options: http://lists.thomasclausen.net/mailman/listinfo/olympus
Archives: http://lists.thomasclausen.net/mailman/private/olympus/
Themed Olympus Photo Exhibition: http://www.tope.nl/
|