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http://www.luminous-landscape.com/re...evisited.shtml

The Viewfinder (Panasonic GH2)

Potentially the most controversial aspect of the GH2 is its electronic
viewfinder. The way that MFT cameras with viewfinders achieve their
small size is by substituting the usual mirror box and prism assembly
for an electronic viewfinder – essentially a small video screen
similar to what one finds on a video camera.

Digicam EVFs tend not to be of very high resolution or brightness, but
the one on the GH2 (and the GH1 originally) is very good indeed. In
fact, I would say that it is an many instances preferable to the
dismal (meant literally) and small optical viewfinders found on the
smaller and usually inexpensive DSLRs.

The EVF on the Sony A55 is also very good (though I haven't had a
chance to do a side-by-side comparison), and similarly I would prefer
these any day to a small and dim optical reflex system, especially
those on cheaper systems that use pentamirrors instead of true prisms.

No – these EVFs are not as bright and clear as a good bright reflex
system, especially one on a full-frame body. But, the trade-off in
size and weight is considerable. Also, the ability to have display
overlays, such as a live histogram, goes a long way to making this new
alternative viewing system attractive.

Frankly, the writing is on the wall. It won't be more than a few years
until the vast majority of new camera model with viewfinders dispense
with prisms and mirrors and replace them with EVFs.

Get used to it. It's not that we as photographers are necessarily
asking for this (though EVF display technology is getting better all
the time). It's just the pressure of industry economics. Price
competition is fierce. Moving mirror assemblies and glass prisms are
expensive to manufacture and assemble. High quality EVFs are not
exactly inexpensive at the moment, but as with all high volume silicon
based products, prices will inevitably fall. When that happens mirrors
and prisms will be relegated to only the high end, where users are
willing to pay for special capabilities.

About 90% of the time I am not displeased using the GH2's EVF. It's
even possible to forget that there's anything different going on. But
in low light things get a bit weird. With a fast lens (like the
remarkable Nokton f/0.95) the view actually becomes brighter than
reality and the final result not what one expects.

On 02/17/2011 10:51 PM, RichA wrote:
> http://www.luminous-landscape.com/re...evisited.shtml

Hmmm...

> to a small and dim optical reflex system, especially
>> those on cheaper systems that use pentamirrors instead of true prisms.

So the viewfinder luminosity is a matter of mirrors

> With a fast lens (like the
>> remarkable Nokton f/0.95) the view actually becomes brighter than
>> reality and the final result not what one expects.

So the viewfinder luminosity is a matter of lenses

Which one is true? Could it be that the entry level DSLR are always
tested with their not-so-fast kit lenses?

And is there any measure anywhere of the actual difference between
pentamirrors and pentaprisms?

--
Bertrand

In rec.photo.digital Alfred Molon <> wrote:

>> And is there any measure anywhere of the actual difference between
>> pentamirrors and pentaprisms?

>I wouldn't know why a pentaprism should be brighter than a pentamirror.
>Mirrors should reflect 100% (or close to) of the light, so it should not
>matter if it's a mirror or a prism.

There is a very slight light loss each time the light goes from
air to glass and glass to air. Mirrors have at least six such
transitions. A pentaprism has only two.

That said, the loss is VERY small and I doubt it is detectable.

--
--- Paul J. Gans

On 18/02/2011 3:20 p.m., Paul J Gans wrote:
> In rec.photo.digital Alfred Molon<> wrote:
>
>>> And is there any measure anywhere of the actual difference between
>>> pentamirrors and pentaprisms?
>
>> I wouldn't know why a pentaprism should be brighter than a pentamirror.
>> Mirrors should reflect 100% (or close to) of the light, so it should not
>> matter if it's a mirror or a prism.
>
> There is a very slight light loss each time the light goes from
> air to glass and glass to air. Mirrors have at least six such
> transitions. A pentaprism has only two.
>
> That said, the loss is VERY small and I doubt it is detectable.
>
These mirrors don't have a glass/air transition, as the glass is
"silvered" on the reflective side. Light loss is probably significant.
How do you come up with six transitions for pentamirror?
I count 3 - from one side to the other side of the angled roof of mirror
assembly, to front mirror.

I'm not sure that modern pro DSLRs with high eyepoint and focus screens
optimised for AF systems and "100% VF" are as "accurate" as some wishful
thinkers may like to believe. Put such a camera on a tripod, look
through the VF at the edge of the image and move your head from side to
side to see what I mean. An EVF would actually be better.

On Feb 17, 8:31*pm, "David J. Littleboy" <davi...@gol.com> wrote:
> "Ofnuts" <o.f.n.u....@la.poste.net> wrote:
>
> > And is there any measure anywhere of the actual difference between
> > pentamirrors and pentaprisms?
>
> It may be coincidental: high end (d)SLRs tend to have big heavy pentaprisms,
> and are designed with 100% coverage and exit pupils large enough that even
> people with thick glasses and long noses can use them comfortably, whereas
> low-end cameras are made with mirror assemblies, and are designed to be too
> small, have less coverage, and tiny scrunched exit pupils that make using
> them unpleasant without contacts and major plastic surgery. So in real life,
> pentaprisms are way better than mirror viewfinders. But the big and easy to
> use vs. small, scrunched, yucky, and dim may have more to do with the
> designs than the technology used to implement the designs. Maybe.
>
> Whatever. Look through a high-end Nikon or Canon at a store some time and
> you'll be unhappy with you midrange dSLR for the rest of your life. Sigh.
>
> --
> David J. Littleboy
> Tokyo, Japan

Odd thing is, mirror systems can be superior to prism systems and are,
in different realms than cameras. Prisms induce chromatic aberration
(something mirrors don't do) and absorb more light than the newest
reflective surfaces of mirrors.

On 2/17/2011 11:29 PM, RichA wrote:
> On Feb 17, 8:31 pm, "David J. Littleboy"<davi...@gol.com> wrote:
>> "Ofnuts"<o.f.n.u....@la.poste.net> wrote:
>>
>>> And is there any measure anywhere of the actual difference between
>>> pentamirrors and pentaprisms?
>>
>> It may be coincidental: high end (d)SLRs tend to have big heavy pentaprisms,
>> and are designed with 100% coverage and exit pupils large enough that even
>> people with thick glasses and long noses can use them comfortably, whereas
>> low-end cameras are made with mirror assemblies, and are designed to be too
>> small, have less coverage, and tiny scrunched exit pupils that make using
>> them unpleasant without contacts and major plastic surgery. So in real life,
>> pentaprisms are way better than mirror viewfinders. But the big and easy to
>> use vs. small, scrunched, yucky, and dim may have more to do with the
>> designs than the technology used to implement the designs. Maybe.
>>
>> Whatever. Look through a high-end Nikon or Canon at a store some time and
>> you'll be unhappy with you midrange dSLR for the rest of your life. Sigh.
>>
>> --
>> David J. Littleboy
>> Tokyo, Japan
>
> Odd thing is, mirror systems can be superior to prism systems and are,
> in different realms than cameras. Prisms induce chromatic aberration
> (something mirrors don't do) and absorb more light than the newest
> reflective surfaces of mirrors.

Another question you will evade:
Which camera prism system are you spouting about?
Where is the scientific backup for your statement?



--
Peter

Alfred Molon <> wrote:

> Actually an EVF is brighter in low light than an OVF, because the EVF
> can amplify the light, while the OVF cannot.

True. But it can only amplify what the sensor delivers,
and delivers quickly (can't expose for, say, a modest 1/15s,
or you'll have lots of fun following the action) while the eye
has all the evolution behind it to cope with the dark at night.
Including longer integration times without impeding following
the action. (Or rather, if your eye cannot follow the action
any more, you cannot blame the camera.)

And of course you loose resolution due to the rather limited
number of subpixels in an EVF.

-Wolfgang

On 18/02/2011 8:07 p.m., Alfred Molon wrote:
> In article<ijknd7$h1k$>, Me says...
>> These mirrors don't have a glass/air transition, as the glass is
>> "silvered" on the reflective side. Light loss is probably significant.
>
> How much exactly?
FWIW, visible spectrum light loss from front "silvered" mirrors seems to
be quoted (google for sources) as being in the range from 5-12%, (X3, as
it's reflected 3 times).
They use prisms in periscopes and binoculars etc for the same reason,
there are much lower light losses through internal reflection in a prism
than with mirrors.

On 2/18/2011 3:27 PM, Paul Furman wrote:
> PeterN wrote:
>> On 2/17/2011 11:29 PM, RichA wrote:
>>> On Feb 17, 8:31 pm, "David J. Littleboy"<davi...@gol.com> wrote:
>>>> "Ofnuts"<o.f.n.u....@la.poste.net> wrote:
>>>>
>>>>> And is there any measure anywhere of the actual difference between
>>>>> pentamirrors and pentaprisms?
>>>>
>>>> It may be coincidental: high end (d)SLRs tend to have big heavy
>>>> pentaprisms,
>>>> and are designed with 100% coverage and exit pupils large enough that
>>>> even
>>>> people with thick glasses and long noses can use them comfortably,
>>>> whereas
>>>> low-end cameras are made with mirror assemblies, and are designed to
>>>> be too
>>>> small, have less coverage, and tiny scrunched exit pupils that make
>>>> using
>>>> them unpleasant without contacts and major plastic surgery. So in
>>>> real life,
>>>> pentaprisms are way better than mirror viewfinders. But the big and
>>>> easy to
>>>> use vs. small, scrunched, yucky, and dim may have more to do with the
>>>> designs than the technology used to implement the designs. Maybe.
>>>>
>>>> Whatever. Look through a high-end Nikon or Canon at a store some time
>>>> and
>>>> you'll be unhappy with you midrange dSLR for the rest of your life.
>>>> Sigh.
>>>
>>> Odd thing is, mirror systems can be superior to prism systems and are,
>>> in different realms than cameras. Prisms induce chromatic aberration
>>> (something mirrors don't do) and absorb more light than the newest
>>> reflective surfaces of mirrors.
>>
>> Another question you will evade:
>> Which camera prism system are you spouting about?
>> Where is the scientific backup for your statement?
>
>
> Interesting point though. Mirror telescopes do not have chromatic
> aberrations; glass lens systems do. But yeah, cheap DSLRs have mirrors,
> high end models have glass. I have no answer to that contradiction.


The mirror and prism are part of the viewfinder. The exposure plane in
both SLRs and DSLRs is from the lens to the medium. (Either sensor or
film.)



Peter

On 02/18/2011 09:25 PM, Me wrote:
> On 18/02/2011 8:07 p.m., Alfred Molon wrote:
>> In article<ijknd7$h1k$>, Me says...
>>> These mirrors don't have a glass/air transition, as the glass is
>>> "silvered" on the reflective side. Light loss is probably significant.

To put that in photographic perspective .88**3 is .68 (roughly
1/sqrt(2)) so that's half a diaphragm stop. To emulate that, set your
lens at max opening minus half a stop, and depress the DOF check button.

--
Bertrand