Dr. Kevin Pernicano
Louisville, KY, USA (Home of the Kentucky Derby)

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Hi Kevin,

On Wed, 16 Jan 2008, Dr. Kevin Pernicano wrote:

> http://www.trenholm.org/hmmerk/download.html

You're referencing the out-of-print book by Harold M. Merklinger titled,
The INs and OUTs of FOCUS. He is generally regarded to be the originator
of the idea that focusing at Infinity can offer some benefits over
focusing at the hyperfocal distance when the subject space includes
objects at Infinity in addition to objects that are closer than Infinity.

The page at the link you provided includes this link to a free Internet
edition of Merklinger's "The INs and OUTs of Focus":

http://www.trenholm.org/hmmerk/TIAOOFe.pdf

Many people who read this book somehow come away with the conviction that
focusing at Infinity yields a superior (or at least an equivalent) image
quality to that had when focusing at the hyperfocal distance. This is not
the case. You have to read the entire book to understand that Merklinger
is only pitching his "focus at Infinity" method as a convenient way to
avoid the effort required to focus at the hyperfocal distance, BUT
Merklinger makes it clear that this convenience is had at the expense of
image quality (in foreground sharpness), or at the expense of shutter
speed, or both.

You can find evidence that despite his having invented this idea of just
focusing at Infinity, Merklinger understands that there is useful DoF
beyond the plane of sharpest focus (when you focus at Infinity, you are
not making use of the very precious DoF available beyond where you've
focused) -AND- he understands that CoC diameters can be made small enough
to produce sharp images while hyperfocusing. His book does not at all
dismiss the fact that hyperfocusing really can produce sharp images. Best
of all, his book proves that HE KNOWS his focus at Infinity technique
produces compromised images. Stay with me...

Consider his Rule of Thumb #6 (page 37): "The zone of acceptable
delineation of the subject falls equally in front of and behind the point
of exact focus (not 1/3, 2/3!)."

Which prompts me to ask this question: Why would you want to throw away
the "acceptable delineation of the subject" that's available BEYOND the
plane of focus when focusing at Infinity? Hold that thought as we
continue...

Or his Rule of Thumb #20 (page 71): "The usual depth-of-field scale is
calculated for a 1/30 mm circle-of-confusion. Typical 35 mm films and
lenses are capable of delivering a 1/150 mm standard. To convert an
existing depth-of-field scale to a new (higher, more demanding) standard,
all we have to do is multiply the numbers on the depth-of-field scale by
the improvement factor we desire. To go for that five-fold possible
improvement, multiply all the numbers by 5: Instead of f/2, read f/10.
Alternatively, divide the f-number you are actually using by 5 and look
for that spot on the existing depth-of-field scale: if you are using f/11,
look for the f/2.2 depth-of-field mark. And, if you wish, you can use
different standards for the far limit of depth-of-field and for the near
limit."

That last sentence from his Rule of Thumb #20 reveals that Merklinger
acknowledges that DoF calculations can be tailored to be as aggressive as
we need them - by choosing a Circle of Confusion diameter that smartly
accommodates the anticipated enlargement factor and viewing distance.

But you can also find statements like this:

From page 21: "In general, I have found the results obtained using the
time-honored methods usually yield backgrounds which are on the fuzzy
side."

Here's where he begins to get silly in an effort to sell his novel
focusing method. I have to ask the question: Is he pretending there's only
one CoC diameter we can use for hyperfocusing? If not, there's no excuse
for fuzzy backgrounds when using "time-honored methods!" (See his Rule of
Thumb #20!)

Rule of Thumb #4 (page 36): "If we want anything at infinity to be
critically sharp, focus at Infinity."

That doesn't jive with Rule of Thumb #6! Why would you focus at Infinity
if "The zone of ACCEPTABLE delineation of the subject falls equally in
front of and behind the point of exact focus?" If you find the DoF in the
foreground produces acceptably small CoC's (or disks of confusion, as
Merklinger calls them), why wouldn't these same sized CoC's be acceptable
beyond the plane of exact focus - in the background?

And his Rule of Thumb #23 (page 72): "A gentle repeat reminder: when you
focus at the hyperfocal distance, you are guaranteeing that subjects in
the distance will be resolved no better than your specified minimum
standard. In order to improve upon this, you must focus beyond the
hyperfocal distance."

Here he suggests the possibility that we can use a standard of our own
choosing - "your specified minimum standard" (a la Rule #20) - but when he
suggests that we focus beyond the hyperfocal distance to improve the
sharpness of distant subjects, he NEGLECTS to mention a critical point: If
our chosen standard were aggressive enough to begin with (if we had
smaller CoC's), we wouldn't feel moved to sacrifice foreground sharpness
just to obtain acceptable background sharpness!

And in his Summary, Chapter 11 (page 73): "The traditional depth-of-field
philosophy usually ends with the advice: to maximize depth-of-field,
choose a moderately small lens opening, set the focus to the hyperfocal
distance, and shoot. My parting advice would be a little different. For
typical normal and wide-angle lenses, especially lenses having focal
lengths less than about 50 mm no matter what the camera format, set the
lens opening to somewhere in the 2 mm to 5 mm range, set the focus at
infinity, and shoot. For lens openings larger than 5 mm, and for longer
lenses that tends to mean all normal working f-stops, focus on what is
critically important."

Focusing at infinity can not be done without forfeiting the DoF that
resides beyond the plane of focus (Merklinger's Rule # 6!), thus the
aperture you must chose to adequately resolve foreground subjects when
focusing at Infinity will be smaller than that which could be used if
hyperfocusing instead! Merklinger chooses not to stop down that far and
thus suffers UNSHARP foregrounds. Where does he say that? Keep reading...

Under a photograph of a church with flowers very near in the foreground
(page 60), he writes: "Taken with a 28 mm lens at f/11, infinity focus
provided all the depth-of-field necessary."

Without question, if he's content with the CoC diameters in the foreground
subjects, he could have hyperfocused and been just as content with his
infinity subjects at something like f/8 (where the DoF that resides beyond
the plane of focus could have been pressed into service instead of
wasted!)

And on page 68: "Since working out these details, I find I do a lot of
photography with the lens simply focused at infinity."

How convenient! But don't miss this: Merklinger admits that the
convenience comes at a price...

From page 22: "Objects photographed up close can still be recognized even
if they are a little fuzzy. Objects in the distance may need to be very
sharply imaged if they are to be recognized at all."

From page 48, under the infamous cannon and village picture: "The cannon,
the grass, the gravel, and the trees are clearly a bit fuzzy, but we have
no difficulty in recognizing them." You've got to laugh at that: "clearly
a bit fuzzy".

From page 66: "Experimenting, I learned that with the lens focused at
infinity, things up close still seemed to be adequately sharp."

So Merklinger admits that his infinity focus method produces foregrounds
that are "a bit fuzzy", "a little fuzzy" or "seemed to be adequately
sharp."

Merklinger's method is clearly a compromise that is acceptable only if you
are willing to suffer "fuzzy" foregrounds in favor of sharp Infinity
subjects and a very convenient way to set focus and select aperture. The
fact remains that everything in the shot can be made at least as sharp as
his Nears at a wider aperture (faster shutter speed) than he's using - by
focusing more closely than at Infinity. And, despite his negative comments
about hyperfocal focusing, his own Rules #6 and #20 reveal that he knows
one can achieve acceptably sharp Nears AND Fars by hyperfocusing for a
smaller CoC diameter.

Merklinger's method boils down to this: If you're willing to take a hit in
foreground sharpness and waste the DoF that lies beyond the plane of
focus, you can put convenience ahead of quality by focusing at Infinity
and selecting an aperture that's just small enough to make foreground
subjects only "recognizable". If you want the convenience of focusing at
Infinity AND the foreground sharpness had when hyperfocal focusing, you'll
have to stop down TWO STOPS further than you would with hyperfocal
focusing (because you're throwing away all the DoF that lies beyond the
plane of sharpest focus). Stopping down two additional stops, you'll
suffer exposures that are four times longer and increase your risk of
visible degradation due to diffraction across the entire image (due to a
doubling of Airy disk diameters).

No thanks! I'll stick with using depth of field calculations. The half a
minute required to use a depth of field calculator will not only tell me
at what distance to focus, but will also give me the widest aperture (and
therefore, the fastest shutter speed) capable of delivering the CoC
diameters necessary to support my desired print resolution for an
anticipated enlargement factor and viewing distance.

Mike Davis

Clearly you have spent a lot of time studying this book, analyzing it,
criticizing it, discussing it with others, etc., etc., etc.

I read all your arguments and I remain unconvinced. I don't have time to
argue this issue but as a practical stereo photographer who enjoys
hand-held stereo photography, this book gave me good insights about
focusing.

I see the point that, for scenic/distant shots, having sharp details at
infinity is often more important than having sharp details in the
foreground and I find myself often biased towards infinity focusing,
especially when I use 35mm SLR 2d & 3d cameras and focus by eye (where I
can see the actual focus through the camera). For close ups, I simply
focus at my subject. For many of my shots, in the half a minute required
to use DOF calculations, the subject will walk away.

George Themelis

----- Original Message -----
From: "Michael K. Davis" <zilch0@globalcrossing.net>
> On Wed, 16 Jan 2008, Dr. Kevin Pernicano wrote:

> No thanks! I'll stick with using depth of field calculations. The half a
> minute required to use a depth of field calculator will not only tell me
> at what distance to focus, but will also give me the widest aperture
> (and
> therefore, the fastest shutter speed) capable of delivering the CoC
> diameters necessary to support my desired print resolution for an
> anticipated enlargement factor and viewing distance.

I concur with George's practical approach to focusing. I have simplified my technique further by shooting always at f22 from a tripod. No focusing necessary – the camera is permanently set at hyperfocal for f22. Well, not exactly permanently. For the occasional close-up I estimate the distance to the subject and set focus using the camera's scale. At f22 you need not be precise.

I was mystified, however, by the discussion of a linkage between focus and on-film deviation. To my mind there is none. On-film deviation determines the range of eyeball convergence necessary to view any given stereo slide – whether the image is in focus or not.

Oleg Vorobyoff

Hey George,

By qualifying your position with references to hand-held shooting of
subjects that aren't holding still, you've not encouraged the use of
Infinity-focusing while shooting static landscapes from a tripod.
Merklinger makes no such distinction.

I have no problem agreeing with your choice to abandon traditional
aperture selection and focus techniques for the sake of expedience.
There are many situations where the disciplines appropriate for the likes
of landscape or architectural photography simply aren't plausible.

Merklinger, on the other hand, encourages his readers to focus at
Infinity, allowing the foreground to go "a little bit fuzzy," even when
shooting static landscapes. That's a compromise in image quality I simply
can't condone.

Mike Davis

On Thu, 17 Jan 2008, DrT (George Themelis) wrote:

> Clearly you have spent a lot of time studying this book, analyzing it,
> criticizing it, discussing it with others, etc., etc., etc.
>
> I read all your arguments and I remain unconvinced. I don't have time to
> argue this issue but as a practical stereo photographer who enjoys
> hand-held stereo photography, this book gave me good insights about
> focusing.
>
> I see the point that, for scenic/distant shots, having sharp details at
> infinity is often more important than having sharp details in the
> foreground and I find myself often biased towards infinity focusing,
> especially when I use 35mm SLR 2d & 3d cameras and focus by eye (where I
> can see the actual focus through the camera). For close ups, I simply
> focus at my subject. For many of my shots, in the half a minute required
> to use DOF calculations, the subject will walk away.
>
> George Themelis
>
>
> ----- Original Message -----
> From: "Michael K. Davis" <zilch0@globalcrossing.net>
> > On Wed, 16 Jan 2008, Dr. Kevin Pernicano wrote:
>
> > No thanks! I'll stick with using depth of field calculations. The half a
> > minute required to use a depth of field calculator will not only tell me
> > at what distance to focus, but will also give me the widest aperture
> > (and
> > therefore, the fastest shutter speed) capable of delivering the CoC
> > diameters necessary to support my desired print resolution for an
> > anticipated enlargement factor and viewing distance.
>
>

Michael K. Davis wrote:

>
> Merklinger, on the other hand, encourages his readers to focus at
> Infinity, allowing the foreground to go "a little bit fuzzy," even
> when shooting static landscapes. That's a compromise in image
> quality I simply can't condone.

Merklinger says that if it is critical for your subject at infinity to
be in sharp focus then you should focus on infinity. Remember that
whatever is at the near and far limits will always be less sharp then
whatever is at the distance you focus at. He does not say to just
throw away available DoF. Allowing the foreground to be less sharp is
a necessary compromise when your subject at infinity is more important
than the foreground. If necessary you can crop out the foreground. I
do this all the time by using 6x4.5 stereo mounts, both to get rid of
things that are too near for comfortable viewing and to get rig of
things that fall out of the available DoF.

--
Brian Reynolds | "It's just like flying a spaceship.
reynolds@panix.com | You push some buttons and see
http://www.panix.com/~reynolds/ | what happens." -- Zapp Brannigan
NAR# 54438 |

Hi Brian,

On Thu, 17 Jan 2008, Brian Reynolds wrote:

> Merklinger says that if it is critical for your subject at infinity to
> be in sharp focus then you should focus on infinity.

Merlkinger promotes his focus-at-Infinity technique as if there is no
other way to secure sharp Infinity subjects - a ridiculous contention.

WITHOUT focusing at Infinity, using the 80mm lenses of the TL-120, for
example, it's absolutely possible to create MF3D views of subject spaces
that extend from 13 feet all the way to Infinity - views that are
spectacularly "sharp" - completely devoid of any discernable defocus
throughout the entire subject space, not just at Infinity, when viewed at
3.4x magnification.

Shooting an 80mm lens at f/22 and focusing at 25 feet, circle of confusion
diameters will not exceed 0.037mm from a Near of 13 feet all the way to
Infinity. CoC's this small will support a desired resolution of 8 lp/mm
(more than enough to satisfy the most demanding person) in a 3.4x
magnification viewer (such as the 75mm viewers made by 3DWorld).

Shoot that same subject space with the same 80mm lenses at f/22, while
focused at Infinity and only those subjects that reside between 25 feet
and Infinity will be imaged with CoC's as small as 0.037mm. Subjects
residing at 13 feet will suffer a 2x reduction in resolution due to
defocus.

> Remember that
> whatever is at the near and far limits will always be less sharp then
> whatever is at the distance you focus at.

That's true, but those subjects need not be DISCERNABLY less sharp. If
the CoC diameters at the Near and Far limits of the subject space have
smartly been made too small to be detected at the anticipated enlargement
factor and viewing distance, it's no longer an issue that even smaller
CoC's exist at the plane of sharpest focus and elsewhere within the
subject space between the Near and Far points. If adherance to DoF
calculations fail to produce satisfyingly "sharp" Near and Far point
subjects at a given enlargement factor and viewing distance, the CoC
diameter used to perform those DoF calculations was too large.

> He does not say to just
> throw away available DoF.

It is impossible to make use of the DoF that's available beyond the plane
of sharpest focus when we position that plane at Infinity. When
Merklinger advises us to focus at Infinity when shooting a subject space
that does not lie entirely at the plane of Infinity, he is indeed telling
us to throw away available DoF.

> Allowing the foreground to be less sharp is
> a necessary compromise when your subject at infinity is more important
> than the foreground.

That's incorrect. See my opening comments above.

> If necessary you can crop out the foreground. I
> do this all the time by using 6x4.5 stereo mounts, both to get rid of
> things that are too near for comfortable viewing and to get rig of
> things that fall out of the available DoF.

It wouldn't be necessary to get rid of things that fall out of available
DoF if you had used a combination of f-stop and focus distance capable of
securing sufficient DoF througout the entire subject space. Again, see my
opening comments.

Thanks,

Mike Davis

Michael K. Davis wrote:

> It is impossible to make use of the DoF that's available
> beyond the plane of sharpest focus when we position that
> plane at Infinity.

What about that fifth dimension beyond that which is known
to man? Doesn't the Twilight Zone lay beyond infinity?
This technique would be perfect for taking pin-sharp images
of Rod Serling.


But, removing tongue from cheek, Mike, I'm with you on this one.

If I've taken the time to put the camera on a tripod, I'm
gonna take the couple seconds to spin my DOF calculator and
focus such that my important areas are within the DOF for my
chosen aperture.

If I'm hand-holding, I probably won't be checking my
calculator, but there is no reason for me to ever focus my
stereo cameras at a distance greater than the hyperfocal
distance for my chosen aperture.
--
John Thurston
Juneau Alaska
http://stereo.thurstons.us

Hi Oleg,

At 10:47 AM 1/17/2008, you wrote:
[snip]

I was mystified, however, by the discussion of a linkage between focus and on-film deviation. To my mind there is none. On-film deviation determines the range of eyeball convergence necessary to view any given stereo slide – whether the image is in focus or not.

You're right, of course, that there's no "linkage between focus and on-film deviation."  The post you're referencing didn't describe such a relationship, nor even a relationship between shooting aperture and on-film deviation. 

The Di Marzio-Davis Equation can translate the f-Number indicated by a DoF calculation into the appropriate stereo base one should use to achieve a desired on-film deviation, expressed as a percentage of MAOFD.   For the sake of controlling OFD, that f-Number need  not be used to make the actual exposure (nor must one actually focus where indicated by the DoF calculator) to make use of the f-Number in calculating a stereo base for the purpose of controlling on-film deviation.

The Di Marzio-Davis equation can deploy the f-Number generated by any DoF calculator using a known CoC diameter as a literal measurement of the intrinsic depth of a subject space.   A DoF calculator might indicate the need to use f/16 for a subject space ranging from 18 feet to Infinity, and again recommend that same aperture, f/16, for a subject space ranging from 13 feet to 50 feet - two entirely different subject ranges having identical DoF requirements -AND THUS- identical stereo base requirements -AND THUS- identical on-film deviations recorded in the final stereo views if one makes use of the stereo bases calculated by the Di Marzio-Davis Equation, as intended.

In short, there is no relationship between focus and on-film deviation, but there IS a relationship between the minimum f-Number required to obtain sufficient DoF for a given subject space and the OFD that will be recorded in a stereo view when the stereo base is adjusted as indicated by the Di Marzio-Davis equation.

If one is using a true stereo camera, where the base is fixed, the rearranged derivative of the Di Marzio-Davis Equation can predict the deviation that will be recorded on-film (again expressed as a percentage of MAOFD) for any subject space where the intrinsic depth  is expressed as the f-Number generated by a DoF calculator of known CoC diameter.   Any two subject spaces that happen to require the same f-Number to secure a desired maximum CoC diameter (to secure a desired DoF) will be recorded as views having the same on-film deviation.   No matter where you focus and no matter what apertures you ultimately use to make the exposures, the two views will have identical deviation because both scenes require the same minimum f-Number to satisfy your DoF requirements.

Thanks,

Mike Davis