Closest focusing distance for 12x20 camera

Hi there, I was wondering if someone would know how to
calculate the closest focusing distance that a camera would have
if it had a 24 inch bellows (what I believe the Korona 12x20
camera has) and is used with both the 355 G-Claron and 450mm
Nikon. I'm trying to figure out which lens to get for this camera.
I know the 450 covers better, but I may not have much flexibility
for subjects at distance other than infinity. On the other hand,
the 355 should give me more room to work with at less coverage
than the Nikon...

Any help here would be appreciated!!

Thanks very much

Mark
--
p

Mark Baylin wrote:

Hi there, I was wondering if someone would know how to
calculate the closest focusing distance that a camera would have
if it had a 24 inch bellows (what I believe the Korona 12x20
camera has)

Depends one what you mean by close focusing. A
1:1 ratio (life size on the film plane) requires
a bellows extension of at least twice the focal
length. So for a 24 inch max bellows draw you get
1:1 with a 12 inch lens (about 300mm.)

With a longer lens you'll get less. Since the
355mm is your shortest lens, you won't get 1:1
but will get greater than 1:2 (1/2 life size);
probably somewhere inbetween the two when
close focusing.

Sorry, I don't know an equation to calculate
the ratio or "distance," only for bellows extension
factor (exposure correction) which increase the
longer you extend the bellows beginning at about
8x the focal length.

and is used with both the 355 G-Claron and 450mm
Nikon. I'm trying to figure out which lens to get for this camera.
I know the 450 covers better, but I may not have much flexibility
for subjects at distance other than infinity. On the other hand,
the 355 should give me more room to work with at less coverage
than the Nikon...

Any help here would be appreciated!!

Thanks very much

Mark
--
p

Hi there, I was wondering if someone would know how to
calculate the closest focusing distance that a camera would have
if it had a 24 inch bellows (what I believe the Korona 12x20
camera has) and is used with both the 355 G-Claron and 450mm
Nikon. I'm trying to figure out which lens to get for this camera.
I know the 450 covers better, but I may not have much flexibility
for subjects at distance other than infinity. On the other hand,
the 355 should give me more room to work with at less coverage
than the Nikon...


The fundamental formula of optics:

1 / F = 1 / u + 1 / v

F is your focal distance
u is your subject distance
And v is your bellows extention (lens-to-subject)

In your case F = 355mm, v = 24in, and you want to know u

u = Fv / (v - F) (from above)

u = [(355/25.4) x 24 ] / (24 - 355/25.4) = 33.4 in = 2.79 feet

For your 450mm the answer is 5.6 feet, I believe.


Bob G

Bob G wrote:
Hi there, I was wondering if someone would know how to
calculate the closest focusing distance that a camera would have
if it had a 24 inch bellows (what I believe the Korona 12x20
camera has) and is used with both the 355 G-Claron and 450mm
Nikon. I'm trying to figure out which lens to get for this camera.
I know the 450 covers better, but I may not have much flexibility
for subjects at distance other than infinity. On the other hand,
the 355 should give me more room to work with at less coverage
than the Nikon...


The fundamental formula of optics:

1 / F = 1 / u + 1 / v

F is your focal distance
u is your subject distance
And v is your bellows extention (lens-to-subject)

In your case F = 355mm, v = 24in, and you want to know u

u = Fv / (v - F) (from above)

u = [(355/25.4) x 24 ] / (24 - 355/25.4) = 33.4 in = 2.79 feet

For your 450mm the answer is 5.6 feet, I believe.


Bob G

It should be appreciated that at 1:1 magnification the coverage of the
lens will double compared to infinity. A 150 mm lens would cover at
some point with essentially 0 depth of field and an effective f-stop of
very large if you used a long enough bellows pull and got close enough
to the subject. :-)

And v is your bellows extention (lens-to-subject)

Oops! That should read:

And v is your bellows extension (lens-to-film)

Bob G

The fundamental formula of optics:

1 / F = 1 / u + 1 / v



An interesting result (perhaps) from the formula is that no lens can focus on
an object at a distance equal or less than its focal length.


For instance, a 12 inch lens cannot focus on an object 12 inches away.


That's because as u approaches F (the lens-to-subject distance approaches the
focal

length), then [ 1/F - 1/u ] approaches zero and v becomes monstruously
large, that is, the

bellows extension required approaches infinity.


Get a load of that, Turner Bellows!

Bob G

Bob G wrote:
The fundamental formula of optics:

1 / F = 1 / u + 1 / v




An interesting result (perhaps) from the formula is that no lens can focus on
an object at a distance equal or less than its focal length.


For instance, a 12 inch lens cannot focus on an object 12 inches away.


That's because as u approaches F (the lens-to-subject distance approaches the
focal

length), then [ 1/F - 1/u ] approaches zero and v becomes monstruously
large, that is, the

bellows extension required approaches infinity.

If u is less than F, then v ends up negative. As you say, that means
you can't form a real image in that case. But the mathematics still
works, but for virtual images, which are a different matter. The
virtual image is on the same side of the lens as the subject. You see
if from the other side as a magnified version of the subject.



Get a load of that, Turner Bellows!

Bob G

Leonard Evens wrote:

Bob G wrote:
The fundamental formula of optics:

1 / F = 1 / u + 1 / v




An interesting result (perhaps) from the formula is that no lens can focus on
an object at a distance equal or less than its focal length.


For instance, a 12 inch lens cannot focus on an object 12 inches away.


That's because as u approaches F (the lens-to-subject distance approaches the
focal

length), then [ 1/F - 1/u ] approaches zero and v becomes monstruously
large, that is, the

bellows extension required approaches infinity.

If u is less than F, then v ends up negative. As you say, that means
you can't form a real image in that case. But the mathematics still
works, but for virtual images, which are a different matter. The
virtual image is on the same side of the lens as the subject. You see
if from the other side as a magnified version of the subject.

But all this mental abstraction (or math, take your pick)
is making me nuts! I might as well be back in a college
classroom

Still, long time no hear from, Leonard



Get a load of that, Turner Bellows!

Bob G

"Leonard Evens" wrote in message
...
Bob G wrote:
The fundamental formula of optics:

1 / F = 1 / u + 1 / v




An interesting result (perhaps) from the formula is that no lens can
focus on
an object at a distance equal or less than its focal length.


For instance, a 12 inch lens cannot focus on an object 12 inches away.


That's because as u approaches F (the lens-to-subject distance
approaches the
focal

length), then [ 1/F - 1/u ] approaches zero and v becomes
monstruously
large, that is, the

bellows extension required approaches infinity.

If u is less than F, then v ends up negative. As you say, that means
you can't form a real image in that case. But the mathematics still
works, but for virtual images, which are a different matter. The
virtual image is on the same side of the lens as the subject. You see
if from the other side as a magnified version of the subject.


A one-time professor and mentor of mine, Dr. John Pfeiffer, spearheaded the
first science-fiction (literature) class to be taught at a military academy.
To have the curriculum accepted, Dr. Pfeiffer had to present it to a board
of instructors from all departments of the school. Surprisingly, the most
vigorous defense of his curriculum came from the Math department.
Now I see why!

--
Regards,
Matt Clara
www.mattclara.com

"Leonard Evens" wrote in message
...
Bob G wrote:
The fundamental formula of optics:

1 / F = 1 / u + 1 / v




An interesting result (perhaps) from the formula is that no lens can
focus on
an object at a distance equal or less than its focal length.


For instance, a 12 inch lens cannot focus on an object 12 inches away.


That's because as u approaches F (the lens-to-subject distance
approaches the
focal

length), then [ 1/F - 1/u ] approaches zero and v becomes
monstruously
large, that is, the

bellows extension required approaches infinity.

If u is less than F, then v ends up negative. As you say, that means
you can't form a real image in that case. But the mathematics still
works, but for virtual images, which are a different matter. The
virtual image is on the same side of the lens as the subject. You see
if from the other side as a magnified version of the subject.


A one-time professor and mentor of mine, Dr. John Pfeiffer, spearheaded the
first science-fiction (literature) class to be taught at a military academy.
To have the curriculum accepted, Dr. Pfeiffer had to present it to a board
of instructors from all departments of the school. Surprisingly, the most
vigorous defense of his curriculum came from the Math department.
Now I see why!

--
Regards,
Matt Clara
www.mattclara.com