Olympus EVOLT - 8 MP Consumer DSLR (four thirds)

"Stephen H. Westin" wrote in message
...

But the fact that it uses mirrors rather than a prism will have some
effect; Canon 300D has a rather dim finder because of this, as I
recall.

Can anyone quantify how much light is lost by a pentamirror as compared
to a prism?

And how that compares to the dimness resulting from using a ~f4 zoom
rather than a ~f2 fixed lens?

- Len

"Stephen H. Westin" wrote in message
...

But the fact that it uses mirrors rather than a prism will have some
effect; Canon 300D has a rather dim finder because of this, as I
recall.

Can anyone quantify how much light is lost by a pentamirror as compared
to a prism?

And how that compares to the dimness resulting from using a ~f4 zoom
rather than a ~f2 fixed lens?

- Len

(Stephen H. Westin) wrote in message . ..
dj_nme writes:

Stephen H. Westin wrote:
dj_nme writes:

Bill Tuthill wrote:

Mike Henley wrote:


I'm not sure what that means but it says one of the features of this
pop-up flash is that you don't get that lens shadow in the close-up shots.
Here are images with one of them of the flash (it does seem to reach
up higher than usual pop-up flashes)

http://www.dcresource.com/news/newsitem.php?id=2878

Yes, that looks quite good. It's unusual because the SLR mirror
swings to the side, so I suppose this is why there isn't the normal
SLR mirror hump in the middle, on which popup flash is usually set.

That lump on top of an SLR camera usually houses the pentaprism (some
oddball SLRs have a fixed mirror box or porroprisms).
The mirror that you can see from the front (with the lens off)
reflects the image up into the prism.
And of course, Olympus has experience with SLR's with odd finders;
remember the Pen F? This setup is somehow reminiscent of that. And I
think the both use mirrors rather than a prism.

Not true.
The E-Volt uses a porroprism (according to the press release), like
the Pen-F.
I haven't used a Pen-F, so I don't know if it had a good viewfinder.

Right you are. The Canon 300D and Nikon D70 seem to use pure mirrors,
and controversy has ensued,
e.g. http://www.photo.net/equipment/nikon/D70/. Well, the d70 seems
to have a prism:
http://www.nikonusa.com/template.php?cat=1&grp=2&productNr=25214.

snip

It uses a porroprism, which is a series of triangular prisms put
together to flip the image around like a pentaprism.
I think it ends up being wider, but much less tall as a pentaprism for
the same job.

So the only extra loss is the extra air-glass interfaces.

If memory serves correctly, an early full-frame 35mm SLR used a
porroprism in the same way.
I read a reference to this camera on camera history page, but it
didn't have a great deal of detail (just a name that I've
unfortunately forgotten) just stating that it used a porroprism in
it's finder.

There must be a good reason why no other full-frame SLR has them in
their design.
Perhaps bulk?
Or the need to assemble it from pieces, rather than a solid hunk of glass?
I dunno.

Alignment, additional reflections and loss. Space and packaging,
certainly.

So, for the simple (simplicity-seeking) mind, what are you saying? The
E-Volt is good or bad for this?

Stephen H. Westin wrote:

snip

So the only extra loss is the extra air-glass interfaces.


If memory serves correctly, an early full-frame 35mm SLR used a
porroprism in the same way.
I read a reference to this camera on camera history page, but it
didn't have a great deal of detail (just a name that I've
unfortunately forgotten) just stating that it used a porroprism in
it's finder.

There must be a good reason why no other full-frame SLR has them in
their design.
Perhaps bulk?
Or the need to assemble it from pieces, rather than a solid hunk of glass?
I dunno.


Alignment, additional reflections and loss. Space and packaging,
certainly.

Perhaps.
I have two accessory viewfinders for my RF cameras, a Leica Vidom
multifinder and a Steinheil multifinder.
Both use a porroprism to flip the image around.
Considering that the objective lens is so tiny on the Steinheil version
(2mm, 4mm or 6mm diameter depending on focal length), the image seen
through it is suprisingly bright.
I could only assume, given that a typical APS or 135 sized lens usually
has an objective element that is at least 10 (or 20 or 30) times the
diameter as this, then I would expect the viewfinder on such an SLR
camera to be more than sufficiently bright and clear.

Alignment of the porroprism elements would seem like a pretty good
reason against using it a mass-produced item.
Perhaps Olympus has found a short-cut to manufacture them?

Leonard wrote:

"Stephen H. Westin" wrote in message
...


But the fact that it uses mirrors rather than a prism will have some
effect; Canon 300D has a rather dim finder because of this, as I
recall.


Can anyone quantify how much light is lost by a pentamirror as compared
to a prism?

And how that compares to the dimness resulting from using a ~f4 zoom
rather than a ~f2 fixed lens?


If there are two similar cameras from the same maker (one prism, the other
mirror), look at the AF or metering performance at the low end. Difference will
be close to the loss difference between the mirror and the prism versions.

--
-- rec.photo.equipment.35mm user resource:
-- http://www.aliasimages.com/rpe35mmur.htm
-- e-meil: there's no such thing as a FreeLunch.--

"Alan Browne" wrote in message
.. .
Leonard wrote:
Can anyone quantify how much light is lost by a pentamirror as compared
to a prism?

And how that compares to the dimness resulting from using a ~f4 zoom
rather than a ~f2 fixed lens?


If there are two similar cameras from the same maker (one prism, the other
mirror), look at the AF or metering performance at the low end.
Difference will be close to the loss difference between the mirror and the
prism versions.

? Does not the AF system operate on the other side of the focus screen?

Anyway, I looked it up. You lose about half a stop of finder brightness,
assuming that the mirrors are ordinary cheap ones (and they surely are,
otherwise why not have a prism).

- Len

"Leonard" writes:

"Stephen H. Westin" wrote in message
...

But the fact that it uses mirrors rather than a prism will have some
effect; Canon 300D has a rather dim finder because of this, as I
recall.

Can anyone quantify how much light is lost by a pentamirror as compared
to a prism?

In a back-of-envelope calculation, a good aluminized front-surface
mirror could have reflectance about 95% over most of the visible
spectrum. So after the two reflections needed, you wins up with about
90% of the light getting through. Since a prism uses total internal
reflection, the reflectance approaches 100%, but there are two
transparent faces where reflectance is about 0.5% each, assuming good
antireflection coating. So that would come out to 99% of the light
getting through. Probably not a difference you would notics. Assuming,
of course, that everything is clean and properly aligned; dust on the
mirrors would make things worse.

And how that compares to the dimness resulting from using a ~f4 zoom
rather than a ~f2 fixed lens?

Much less, if my calculations above are correct.

--
-Stephen H. Westin
Any information or opinions in this message are mine: they do not
represent the position of Cornell University or any of its sponsors.