An easily overlooked advantage of medium format stereography for handheld viewers is the fact that the 3.4x enlargement imposed by a 75mm viewer lens focused at Infinity allows the medium format stereographer to use the very same f-stops that a 35mm-based stereo viewer would require for sufficient depth of field at the 7x magnification imposed by the shorter focal length lenses used in such viewers.  

In other words, if we�re comparing smaller format viewers to medium format handheld viewers, there is no actual loss of Depth of Field in moving from shorter focal length camera lenses to equivalent focal length Medium Format camera lenses because there is a simultaneous reduction in enlargement factor that offsets what would otherwise be a requirement for smaller Circles of Confusion. 

Show me a 7x magnification MF3D viewer and we can start worrying about a DoF disadvantage.  Until then, there's no need to stop down any further than we would with a 35mm stereo rig.

Mike Davis



 

Mike, I disagree.

> Until then, there's no need to stop down (a MF camera) any further

than we would with a 35mm stereo rig.


I am not following the above statement....


Using the ol stand-by, Hyperfocal formula, which I think is what is
being used here....


Same composure between the two formats, means double the fl for MF at
the same camera position.


Assuming we double the enlargement of the smaller 35mm format to equal
the MF format size, we must reduce the cc of the 35mm in half to achieve
the same Hyperfocal distance. Here is an example.....


35mm format...

40 tfl
8.0 f stop
0.02 cc

33 Hyperfocal distance in ft.



MF Format

80 tfl
16 f stop
0.04 cc

33 Hyperfocal distance in ft.


So using the Hyperfocal formula, and using the same


1) DOF of the scene (same composure)

2) same resolution criteria at near / far point

3) EQUAL size enlarged images


the hyperfocal formula suggests you must stop down 2 stops for MF. (2x
the f stop for 2x the format size). This is true all the way up the
format size ladder. The hyperofcal formula suggest, (in theory) you
will produce equal resolution at the near / far points, when viewed at
the same final print size.


So, DOF is the great "leveler" of resolution between formats - at the
same final prints size. As we all know, stereo photography existence
lies in scenes with a heavy DOF. Without DOF, there would be no
binocular disparity and therefore no reason to shoot stereo.

> The magnification may not be as high, but that lifts us out of the noise of film-grain, and (typically) allows better performance in the optics. Thus MF3d affords us with far more fine detail in the image, more clarity, better resolution, etc.

This is VERY true. Optics performance (aerial MTF) and film performance (MTF of film) is highly dependent on the size of detail of a given subject at the image plane. With equal composure, smaller formats record the same subjects at proportionally smaller sizes at the image plane.


In general, the smaller the detail size on film, the lower the contrast which will be recorded, assuming equal optics. So with equal optics, MF will record higher contrast images at the same final print size....and our visual system / brain equate contrast to resolution. If two images are recorded at the same resolution, but at different contrasts....when comparing the two, we all would choose the higher contrast image as being "much sharper". Contrast is a critical component of perceived resolution. How big of a factor depends on the subject.


However, MF does have a slight resolution advantage, which is at the point of exact focus. The Hyperfocal formula does not consider this fact. MF will resolve about 25% more resolution at the point of exact focus, (at equal final print size) even though the image is recorded at 2x the f stop. Hyperfocal ONLY takes into consideration the near / far point resolutions, it does not consider the MTF of the optics, MTF of the film, subject size and subject contrast. Hence why Hyperfocal distance is best used as a relative reference vs. an absolute.


The real value of MF comes in reduced grain enlargement, which the eye "can" be very sensitive to. This assumes your vision is sufficient to detect such. Min., 1 arc minute, or 20/20 or better. OTOH, an important benefit of 35mm stereo, is the two stops of shutter speed gained from using 1/2 the f stop. (for the same composure, same near / far resolution, same ISO).


So in high DOF scenes, as in stereo, the gains in resolution are heavily reduced by apt. diffraction in the larger formats. The BIG gains of larger formats are realized when you shoot at the same f stops vs. the smaller formats. This is only applicable when there is very very shallow DOF, such as portraits, wildlife, sports, copy work, etc. Unfortunately, not stereo photography..


In my experience, I would suggest the above holds true in practice as well as theory. My 35mm stereo views, using 35mm fl viewing lenses, are "nearly" as sharp as my MF images using 58mm fl viewing lenses..... (equal final size views, within 15%). The most noticeable difference is in the added exploitation of the grain.


Interestingly enough, when I shoot 4x5 stereo, and view in my 4x5 stereo viewer, which has approx. the same viewing FOV as the 35mm and MF viewers (50 - 55 degrees HFOV).... the 4x5 viewing truly demonstrates the "ZERO grain" look we all desire. This comparison uses equal MTF viewing lenses and equal back lighting levels, two critical factors in the comparison.

Boris and Bill,

Thanks for questioning this. I've now done the math I should have done
before I made the post and must retract my earlier contention.

Here's the breakdown:

35mm stereo rig using 34mm lenses (equal to the diagonal of a 24x24mm mask
window)

vs.

6x6cm stereo rig using 68mm lenses (equal to the digaonal of a 48x48mm
mask window)


35mm slides mounted in 24x24mm masks for a 34mm viewer focused at Infinity
- a 7.35x enlargement factor

vs.

6x6cm slides mounted in a 48x48mm mask for a 68mm viewer focused at
Infinity - a 3.68x enlargment factor


If the desired resolution is 5 lp/mm after enlargment....

For 35mm, the resolution captured on film before magnification must be 5 *
7.35 = 36.8 lp/mm

For 6x6cm, the resolution captured on film before magnification must be 5
* 3.68 = 18.4 lp/mm


Several factors impact the resolution recorded on-film, but we're only
talking about defocus at the moment.

Ignoring diffraction, film grain, film flatness, lens resolution, film
resolution, lighting, contrast, subject motion, camera motion, etc., the
CoC diameter required on-film, to deliver the desired resolution after
magnification can be calculated as follows:

Max. CoC Diameter = 1 / enlargement factor / desired resolution after
enlargement

35mm Max. CoC Diameter = 1 / 7.35 / 5 = 0.027mm

6x6cm Max. CoC Diameter = 1 / 3.68 / 5 = 0.054mm

The 6x6cm camera can permit larger CoC diameters to occur on-film to
achieve the desired resolution after magnification because the enlargement
factor imposed by the medium format viewer viewer is not as severe as that
imposed by the 35mm viewer.

Now let's calculate the f-Number each camera would have to use to secure
CoC's no larger than the diameters calculated above - for a scene that
runs from 12.3 feet to Infinity (using standard DoF math):

For a 35mm camera using a 34mm lens (limiting CoC diameters on 24x24mm
masked film chips to 0.027mm to survive a 7.35x enlargment factor to a
6.95-inch square virtual image size in a 34mm viewer: f/5.6

For a 6x6cm camera using a 68mm lens (limiting CoC diameters on 48x48mm
masked film chips to 0.054mm to survive a 3.68x enlargment factor to a
6.95-inch square virtual image size in a 68mm viewer: f/11

I stand corrected!

Thanks,

Mike Davis

Yep, we all get crossed up with this stuff.... its easy...

as a general rule of thumb, every time you double the format, you double
the f stop for the same DOF.

as you correctly mention, this will achieve the same resolution on equal
size final prints, AT the defocused positions

In the end, this equates to faster shutter speeds for the smaller
format... years ago, before I became a big digital user, I would not
think those two stops of ss were that important, but now, I crave them
beyond belief.... as I am so used to just cranking up the ISO on a
digital rig to gain the extra speed.... so IMO, this is another great
advantage of WA lenses, specially on a 35mm rig, as a ~22mm tfl allows
for 4 extra stops of shutter speed vs. a MF normal rig. This is quite
often the difference between getting the shot, and loosing the shot...
this assumes the "golden light" that chrome film really deserves....

Pushing MF is an option, but IMO, the grain becomes more obvious and
removes the biggest advantage MF has... but sometimes you just don't
have an option...

This is why, IMO, a 25MP stereo camera, maybe Siamesing two Sony cameras
is highly desirable option... as John mentioned, grain reduction at
capture... but then, you have to get back to film.... ???

B



Michael K. Davis wrote:

> Boris and Bill,
>
> Thanks for questioning this. I've now done the math I should have done
> before I made the post and must retract my earlier contention.
>
> Here's the breakdown:
>
> 35mm stereo rig using 34mm lenses (equal to the diagonal of a 24x24mm mask
> window)
>
> vs.
>
> 6x6cm stereo rig using 68mm lenses (equal to the digaonal of a 48x48mm
> mask window)
>
>
> 35mm slides mounted in 24x24mm masks for a 34mm viewer focused at Infinity
> - a 7.35x enlargement factor
>
> vs.
>
> 6x6cm slides mounted in a 48x48mm mask for a 68mm viewer focused at
> Infinity - a 3.68x enlargment factor
>
>
> If the desired resolution is 5 lp/mm after enlargment....
>
> For 35mm, the resolution captured on film before magnification must be 5 *
> 7.35 = 36.8 lp/mm
>
> For 6x6cm, the resolution captured on film before magnification must be 5
> * 3.68 = 18.4 lp/mm
>
>
> Several factors impact the resolution recorded on-film, but we're only
> talking about defocus at the moment.
>
> Ignoring diffraction, film grain, film flatness, lens resolution, film
> resolution, lighting, contrast, subject motion, camera motion, etc., the
> CoC diameter required on-film, to deliver the desired resolution after
> magnification can be calculated as follows:
>
> Max. CoC Diameter = 1 / enlargement factor / desired resolution after
> enlargement
>
> 35mm Max. CoC Diameter = 1 / 7.35 / 5 = 0.027mm
>
> 6x6cm Max. CoC Diameter = 1 / 3.68 / 5 = 0.054mm
>
> The 6x6cm camera can permit larger CoC diameters to occur on-film to
> achieve the desired resolution after magnification because the enlargement
> factor imposed by the medium format viewer viewer is not as severe as that
> imposed by the 35mm viewer.
>
> Now let's calculate the f-Number each camera would have to use to secure
> CoC's no larger than the diameters calculated above - for a scene that
> runs from 12.3 feet to Infinity (using standard DoF math):
>
> For a 35mm camera using a 34mm lens (limiting CoC diameters on 24x24mm
> masked film chips to 0.027mm to survive a 7.35x enlargment factor to a
> 6.95-inch square virtual image size in a 34mm viewer: f/5.6
>
> For a 6x6cm camera using a 68mm lens (limiting CoC diameters on 48x48mm
> masked film chips to 0.054mm to survive a 3.68x enlargment factor to a
> 6.95-inch square virtual image size in a 68mm viewer: f/11
>
> I stand corrected!
>
> Thanks,
>
> Mike Davis
>
>