Heaven knows I don't want to disagree with you Walt, but I do. What you
have said is largely technically correct, but they way you approach it
is remarkably unhelpful in answering the real world question tat started
this thread.
Walt Wayman wrote:
>I knew when I started on this that I'd get both myself and everybody else
>confused. I'm good at that, I guess. Anyway, my final post on this subject:
>
>Leaving out all the irrelevant bits, like that a 24mm lens on a half-frame
>camera has a FOV equivalent to a 48mm lens on a full-frame camera, the
>original question was whether or not the DOF scale on a 24mm lens still is
>accurate on a half-frame camera.
>
>It is, absolutely and unequivocally.
>
And I say it is not, absolutely and unequivocally, when the question is
framed as below.
>The DOF scale is based on the size COC the lens manufacturer deemed
>sufficiently small to be considered "in focus" under normal circumstances.
>
And here is one crux of the matter. What is sharp? What is the viewing
medium? What is the magnification when viewed. How close is the viewer
to the image? Assumptiona about all thes and other factors have gone
into the choice of CoLC, as Oly calls it. Anybody who regularly makes
very large prints knows that the lens markings for DOF are insufficient
to assure adequate DOF. Not all 35mm camera manufacturers use the same
CoC as Oly, by the way.
>The diameter of the COC will be the same no matter what format the lens
>projects an image onto.
>
>
Again, CoC is only meaningful relative to the apparent magnification at
which it will be viewed. If you mean to say that the resolution of image
detail at the sensor plane of parts of the subject at various different
distances from the focal plane will be the same, that is correct.
>If both the half-frame and full-frame images are enlarged to the same degree,
>the COC will be identical.
>
Sure, but so what? Lets say I'm a LF photographer used to making 8x10
contact prints. I go out and take some snapshots with a friend's 35mm
camera using the same focal length as I use for my 8x10. When I get back
the 1x1.5" prints, will the fact that they have the same DOF as my 8x10
prints make their miniscule size any more acceptable? Of course not, I
want to see an 8x10. So you print up an 8x10 of the abdomen of the
subject. Oddly enough, I ask if I might see the whole subject. And we
are off to the FOV with different film/subject size races, which is
where DOF questions get really hard.
>I think the confusion has come in when some folks started talking about
>equal-size prints.
>
But that's what they want! If I'm used to 4x6s from my 35mm camera and
buy a C5050, I'm still going to want 4x6 prints, not 1.1x1.5" prints.
(Interesting how things repeat themselves; the size ratio between 8x10
and 35mm is about the same as that between 35mm and the sensors in many
prosumer cameras.
>Well, duh! To get equal size prints, you have to enlarge the half frame twice
>as much, which makes the COC twice as large. Assuming both full and
>half-frame photographs taken from the same place and of the same scene and
>enlarged to the same degree, the half frame picture is going to be exactly
>like that little 4x5 in. patch in the middle of the 8x10 in. print made from
>the full frame shot, with exactly the same DOF and COC. Blowing the half
>frame up to 8x10 naturally makes the picture fuzzier.
>
>
Quantify please, I want to know how much fuzzier and what I can do to
make the DOF part of the fuzziness the same, please, sir.
>Quoting from the National Geographic Photography Field Guide again, which I
>recommend everyone have a copy of: "The zone of acceptably sharp focus will
>seem more extensive in small prints than in enlargements. A foreground object
>that appears sharp in a 4x6 print may look blurred in a 16x20 blowup and even
>in an 8x10 when you view the picture closely."
>
>So, that's been my basic point all along. The DOF scale is still accurate,
>but when you begin with the handicap of a tiny image and have to blow it up
>more and more, you've got the same problem you would have trying to make a
>good 8x10 print from an old Kodacolor 110 negative.
>
And yet, that is the question being asked. "If I put an OM lens on my
E-1 AND make prints the same size as I did on 35mm, what f-stops will
give equivalent DOF to what I'm used to on 35mm. If I know f5.6 would be
good for this shot on an OM, what f-stop do I use on the E-1 to get the
same apparent DOF on the print?"
I do not, by the way, know the answer. A recent post had a link to a
site that raises the question whether this question for digital sensors
can even be answered using the same criteria and math that has been used
for film.
>It's that darned old silk purse and sow's ear thing again.
>
>
He didn't ask your opinion of his digital sensor - or his mother's
choice in footware, for that matter. :-)
Moose
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