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Re: DOF preview in OVFs of DSLRs is crippled

 
 
Paul Furman
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      06-21-2008
Paul Furman wrote:
> David J Taylor wrote:
>> Ray Fischer wrote:
>> []
>>> Explain to us how the image seen through the viewfinder of a dSLR can
>>> be different from the image recorded by the sensor given that the
>>> image is produced by exactly the same lens under exactly the same
>>> conditions.
>>>
>>> Perhaps your source doesn't realize that there is a DOF button that
>>> lets one stop down the lens. Perhaps he's incompetant. Perhaps it's
>>> just a mistake.
>>>
>>> But I don't blindly believe things that make no sense without even a
>>> hint of an explanation. You shouldn't either.

>>
>> The explanation was, that the sensor accepts a ray bundle with a
>> larger angle than the viewfinder (if I understood the earlier
>> commentary correctly). In other words, once the lens opening gets
>> wider than f/2.8 (or whatever), the viewfinder will show no smaller
>> depth-of-field. On that particular camera. If you changed the
>> viewfinder things could be different.

>
> That might be part of it, the part I understand is that the screen is
> only partially translucent so some of the image comes through. It's
> awkward to explain... if instead of a ground 'glass' focusing screen, if
> you had an opaque white projection surface and looked at that from the
> other side, that should look correct but you see that image combined
> with a clear view from your eye and your eye has a much smaller sensor
> than a DSLR (well the part used for viewing in this case anyways). Maybe
> I'm missing some terminology, the opening in your eye is much smaller
> than the 42mm opening in a DSLR so it's not capable of using the wider
> angle rays.


Entrance/exit pupil is the terminology I was searching for:
http://en.wikipedia.org/wiki/Exit_pupil
"To use an optical instrument, the entrance pupil of the viewer's eye
must be aligned with and be of similar size to the instrument's exit
pupil. This properly couples the optical system to the eye and avoids
vignetting. (The entrance pupil of the eye is the image of the
anatomical pupil as seen through the cornea.) The location of the exit
pupil thus determines the eye relief of an eyepiece. Good eyepiece
designs produce an exit pupil of diameter approximating the eye's
apparent pupil diameter, and located about 20 mm away from the last
surface of the eyepiece for the viewer's comfort. If the disc is much
larger than the eye's pupil, much of the light will be lost instead of
entering the eye; if smaller, the view will be vignetted."

> It's like holding a video camera up to a DSLR lens, you
> won't get the shallow DOF so people invented a device that projects the
> lens image onto ground glass and the video camera focuses on the ground
> glass rather than focusing on infinity. Sorry, I know I haven't
> explained very clearly.
>
>> I can't comment if that's right in practice (as I no longer have such
>> large aperture lenses).

>
>



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David J Taylor
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      06-21-2008
Paul Furman wrote:
[]
> Entrance/exit pupil is the terminology I was searching for:
> http://en.wikipedia.org/wiki/Exit_pupil
> "To use an optical instrument, the entrance pupil of the viewer's eye
> must be aligned with and be of similar size to the instrument's exit
> pupil. This properly couples the optical system to the eye and avoids
> vignetting. (The entrance pupil of the eye is the image of the
> anatomical pupil as seen through the cornea.) The location of the exit
> pupil thus determines the eye relief of an eyepiece. Good eyepiece
> designs produce an exit pupil of diameter approximating the eye's
> apparent pupil diameter, and located about 20 mm away from the last
> surface of the eyepiece for the viewer's comfort. If the disc is much
> larger than the eye's pupil, much of the light will be lost instead of
> entering the eye; if smaller, the view will be vignetted."


Paul,

Thanks for your comments - whatever the explanation, the DoF preview can
behave in an unexpected way, particularly with large aperture lenses fully
open.

Of course, normally you would use the function to stop-down the lens, so
that you could judge whether your aperture was /small/ enough to keep a
range of distances in focus, so the performance when fully open is not
relevant!

Cheers,
David


 
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Matt Ion
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      06-21-2008
TRoss wrote:
> On Wed, 18 Jun 2008 13:33:39 -0700, Matt Ion <(E-Mail Removed)>
> wrote:
>
>> TRoss wrote:
>>
>>>>>> 5. Optical viewfinder being too dark if you use DOF preview
>>>>> LCD screen completely unviewable in bright light.
>>>> Then use the EVF.
>>> Repeat after me ... DSLRs do not have an EVF.
>>>
>>> If the camera has an EVF, it isn't a DSLR. A camera that has an EVF
>>> has no need for a reflex mirror, which is one defining characteristic
>>> of an SLR camera. The other defining characteristic is a single lens.

>> Tell that to my 40D. You saying it isn't an SLR because it actually has
>> an EVF live-view function? It still has a reflex mirror... and a
>> mechanical shutter... and a single lens.

>
> It has an optical viewfinder, not an electronic view finder. Live View
> is via the LCD screen, a different critter altogether.
>
>>> Besides, an EVF would pretty much be worthless for checking DOF. The
>>> size is too small and the resolution is too low.

>> The 40D's live view has a zoom feature, up to (if memory serves) 10X
>> magnification... basically just displaying a crop from the sensor.
>> Works great for manual focusing when shooting macro. You don't need to
>> see the whole image, just a select area of it.

>
> Again, you're talking about the LCD screen, which is not the same as
> an electronic viewfinder.
>
>>> Also, consider the lifespan of a mechanical shutter is around 100,000
>>> actuations. At 24fps, you would get about 1 hour of movie footage
>>> before shutters start to fail.

>> Do film movie cameras not use mechanical shutters? My old Super-8 cam
>> did... never had to change the shutter in several hundred hours of usage.

>
> Different type of shutter. Your Super-8 camera most likely had a
> rotating shutter, not a curtain shutter like your 40D and pretty much
> every other SLR. I'm aware of only one SLR that used a rotating
> shutter - the Olympus Pen-F.


And this, folks, is why you should avoid broad generalities when you go
on an irrational rant...
 
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Kennedy McEwen
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      06-22-2008
In article <g38vef$25lq$(E-Mail Removed)>, Ilya Zakharevich
<(E-Mail Removed)> writes
>[A complimentary Cc of this posting was sent to
>Alfred Molon
><(E-Mail Removed)>], who wrote in article
><(E-Mail Removed)> :
>> http://www.dphotoexpert.com/2007/09/...-the-cheating-
>> dslr-viewfinder/
>>
>> See the two images of the chessboard. Apparently because of the
>> compromises in the OVFs of DSLRs you see more DOF than you really get.
>>
>> According to the article the only way to get an accurate DOF preview is
>> through a direct video feed from the main sensor to the LCD display.

>
>Apparently, the author has no clue that one should match the
>"effective f-number" of the focussing screen to the wide-open-f-number
>of the lens.
>

On the contrary, the author is apparently well aware of this issue,
which is precisely the whole point of the article.

>The default focussing screen are optimized for kit-like lenses with
>very small wide-open aperture. *Of course* they do not catch light
>from the outside areas of the lens, so give a wrong impression about
>DOF. A matched screen should perform MUCH better.
>

And precisely which SLR manufacturers supply optional focus screens
which are "matched" to f/1.2 glass? Even on the old manual focus film
SLR cameras the darkest screens were only optimised for f/4.

As the author says: "(the 40D) has one of the best screens and best
viewfinders around." There simply are no dSLR focus screens "matched"
to fast lenses. That was the whole point of the article - you can't
trust viewfinder DOF when you need to trust it most, with fast
apertures. Under those conditions, live-view offers significant
advantages.

>But if one forgets this piece of illiteracy, the conclusion of the
>paper holds...
>

Don't confuse practicality with illiteracy!
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
 
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Kennedy McEwen
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      06-22-2008
In article <485b67d3$0$17158$(E-Mail Removed)>, Ray Fischer
<(E-Mail Removed)> writes
>Alfred Molon <(E-Mail Removed)> wrote:
>> Ray Fischer

>
>>>
>>>
>>>
>>>
>>>
>>> >>>>>>http://www.dphotoexpert.com/2007/09/...lr-viewfinder/
>>>
>>> Completely false.

>
>Explain to us how the image seen through the viewfinder of a dSLR can
>be different from the image recorded by the sensor given that the
>image is produced by exactly the same lens under exactly the same
>conditions.
>

That's simple and is explained in the referenced article. It has been
an intrinsic limitation with SLRs for decades, but became more of a
problem when manufacturers switched to ultra-bright viewfinders once AF
became the standard.

In simple terms, the dSLR focus screen should scatter the incident light
equally in all directions to ensure that the image formed on it appears
exactly as it would in the final print, which also reflects light
equally in all directions. If this was achieved then the DOF would
appear on the focus screen exactly the same way as on the final image.
The problem with that is that much of the light transmitted by the
screen then fails to reach the eyepiece, resulting in a dark viewfinder
image. To make the image brighter, the screen is designed to scatter
less, and more light actually transmits directly through the screen,
being directed to the eyepiece by a condenser lens which is often in the
form of a fresnel lens on the rear surface of the focus screen. This
means that what you see through the eyepiece is actually a combination
of the scattered image focussed on the screen and the aerial image
transmitted through it. The observed DOF of the aerial image is
determined by which is the higher f/# between the lens, the eyepiece and
your eye.

The focal length of the lens in your eye is around 17mm, with the pupil
diameter ranging from 8mm in dim light to 1.5mm in very bright light,
making it an f/2.1 - f/11 lens. The eyepiece of the camera, however, is
even higher f/#, since it has a focal length of around 40mm and uses the
same eye pupil - making the limiting f/# of the eyepiece around f/5 -
f/11, depending on the brightness of the image being viewed. This means
that the aerial image ALWAYS has much more apparent DOF than the final
image for apertures faster than f/5, and for apertures faster than f/11
on bright images.

Consequently, depending on the mix of scattered and aerial image that
the focus screen creates, you will get a viewfinder image which can
differ significantly in DOF from the image that the sensor reproduces,
especially with fast, low f-number lenses.

A more technically precise and concise reason for this effect was given
by Ilya: the focus screen does not match the f/# of the lens. However
since NO focus screen available for ANY dSLR matches any lens faster
than f/4, it is actually a criticism of all dSLR camera manufacturers,
not the author of the article in question who is, I know, well aware of
the issues involved.

>Perhaps your source doesn't realize that there is a DOF button that
>lets one stop down the lens. Perhaps he's incompetant. Perhaps it's
>just a mistake.
>

Apparently it is *you* that is incompetent, since all of the issues you
raise are logically explained in the article.

The funniest part of your accusation of incompetence on the part of the
author is that the viewfinder should show **LESS** DOF if the preview
button wasn't used, not **MORE**! Without using the DOF button the
viewfinder image is created with the lens fully open (in this case
f/1.2) while the image is taken by the sensor with the lens closed down
by a third of a stop to f/1.4.

Please stop to think before you call people incompetent - especially
those with a high professional reputation in a field you clearly know
nothing about. Rather than resort to a tirade of abuse of the OP it
would have taken less than 5 minutes with your camera, comparing the
viewfinder image with DOF preview to the final image on the LCD screen
and proved to yourself that he was actually correct, especially so at
fast apertures.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
 
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