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Formula for pre-focusing



 
 
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  #1  
Old June 11th 04, 11:03 PM
Steve Yeatts
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Default Formula for pre-focusing

Is there a formula to calculate the required distance between the lens board
and the film plane for an object at a given distance from the camera?
Thanks in advance.

Steve (new to 4x5)


  #2  
Old June 12th 04, 05:04 AM
Frank Pittel
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Posts: n/a
Default Formula for pre-focusing

Steve Yeatts wrote:
: Is there a formula to calculate the required distance between the lens board
: and the film plane for an object at a given distance from the camera?
: Thanks in advance.

I don't have an actual formula that I use. I've found that after time I know
about how far the lens needs to be from the film for the different lenses.

Also after focusing the first time with a lens the next shot will be close.


--




Keep working millions on welfare depend on you
-------------------

  #3  
Old June 12th 04, 07:00 AM
Leonard Evens
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Posts: n/a
Default Formula for pre-focusing

Steve Yeatts wrote:
Is there a formula to calculate the required distance between the lens board
and the film plane for an object at a given distance from the camera?
Thanks in advance.


If u is the distance from the lens to the subject, v the distance of the
lens from the film plane, and f the focal length, then

1/u + 1/v = 1/f.

So

v = fu/(u - f)

In words, multiply the subject distance by the focal length, and then
divide that by the difference between the subject distance and the focal
length.

But this requires some additional qualification since the lens has an
extent of its own. The subject distance is measured from a point called
the front principal point and the film distance is measured from a point
called the rear principal point. For most lenses, these points are both
close to the center of the lens which in turn is close to the lens
board. But for lenses of telephoto design, both these points may be far
in front of the lens. For many wide angle lenses, the rear principal
point is a short distance in back of the lens board. Usually lens
specifications tell you what the rear flange focal length is. If you
know that and the focal length, you can determine the position of the
rear principal point.


Steve (new to 4x5)



  #4  
Old June 12th 04, 06:33 PM
John Emmons
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Posts: n/a
Default Formula for pre-focusing

by the time you figure out the formula, you could have set up, focused and
exposed the film...;^)

Steve, the best "formula" to use for focusing is to get under the dark cloth
with a focusing loupe and get the scene sharp. Check the near and far, all
the corners, etc.

Keep in mind that you probably won't be able to get the entire scene in
focus with the lens wide open, you'll want to stop the lens down and let
your eyes get accustomed to the light, with a bit of practice you'll be able
to focus.

The math that Leonard uses is undoubtedly correct, but I think you'll find
that in practice it's much better to actually see the image get sharp on the
ground glass.

John Emmons

"Leonard Evens" wrote in message
news
Steve Yeatts wrote:
Is there a formula to calculate the required distance between the lens

board
and the film plane for an object at a given distance from the camera?
Thanks in advance.


If u is the distance from the lens to the subject, v the distance of the
lens from the film plane, and f the focal length, then

1/u + 1/v = 1/f.

So

v = fu/(u - f)

In words, multiply the subject distance by the focal length, and then
divide that by the difference between the subject distance and the focal
length.

But this requires some additional qualification since the lens has an
extent of its own. The subject distance is measured from a point called
the front principal point and the film distance is measured from a point
called the rear principal point. For most lenses, these points are both
close to the center of the lens which in turn is close to the lens
board. But for lenses of telephoto design, both these points may be far
in front of the lens. For many wide angle lenses, the rear principal
point is a short distance in back of the lens board. Usually lens
specifications tell you what the rear flange focal length is. If you
know that and the focal length, you can determine the position of the
rear principal point.


Steve (new to 4x5)





  #5  
Old June 12th 04, 07:08 PM
Leonard Evens
external usenet poster
 
Posts: n/a
Default Formula for pre-focusing

John Emmons wrote:
by the time you figure out the formula, you could have set up, focused and
exposed the film...;^)

Steve, the best "formula" to use for focusing is to get under the dark cloth
with a focusing loupe and get the scene sharp. Check the near and far, all
the corners, etc.

Keep in mind that you probably won't be able to get the entire scene in
focus with the lens wide open, you'll want to stop the lens down and let
your eyes get accustomed to the light, with a bit of practice you'll be able
to focus.

The math that Leonard uses is undoubtedly correct, but I think you'll find
that in practice it's much better to actually see the image get sharp on the
ground glass.


This is the second response expressing a similar sentiment. I would
agree that in practice one wouldn't focus by setting the film to lens
distance based on the subject to lens distance and calculations. That
seems rather obvious. But there are certainly other reasons for wanting
to know the formula. The most common is when you want to know if you
can focus at a certain distance with the maximum bellows extension your
camera allows, but there are others. Steve's original request was quite
simple and didn't suggest he intended to use that as a primary method of
focusing, although perhaps his subject title could be interpreted that
way. Generally, when someone asks a question like this, I assume he
has a reason for wanting to know the answer, and, if I know the answer,
I respond. Let's give him the benefit of the doubt instead of lecturing
him about what he should be doing.

By the way, my answer is correct.


John Emmons

"Leonard Evens" wrote in message
news
Steve Yeatts wrote:

Is there a formula to calculate the required distance between the lens


board

and the film plane for an object at a given distance from the camera?
Thanks in advance.


If u is the distance from the lens to the subject, v the distance of the
lens from the film plane, and f the focal length, then

1/u + 1/v = 1/f.

So

v = fu/(u - f)

In words, multiply the subject distance by the focal length, and then
divide that by the difference between the subject distance and the focal
length.

But this requires some additional qualification since the lens has an
extent of its own. The subject distance is measured from a point called
the front principal point and the film distance is measured from a point
called the rear principal point. For most lenses, these points are both
close to the center of the lens which in turn is close to the lens
board. But for lenses of telephoto design, both these points may be far
in front of the lens. For many wide angle lenses, the rear principal
point is a short distance in back of the lens board. Usually lens
specifications tell you what the rear flange focal length is. If you
know that and the focal length, you can determine the position of the
rear principal point.


Steve (new to 4x5)






  #6  
Old June 12th 04, 11:58 PM
Vladamir30
external usenet poster
 
Posts: n/a
Default Formula for pre-focusing

John said:

Steve, the best "formula" to use for focusing is to get under the dark

cloth
with a focusing loupe and get the scene sharp. Check the near and far, all
the corners, etc. . . .


With all due respect, I disagree with this advice. It's often difficult to
stop down to the taking aperture and tell whether everything is "sharp" or
not. Many of us photograph early in the morning and late in the evening or
in fog , rain, cloudy conditions, or other times when the light isn't very
bright. Then we're stopping down to at least F22, maybe F32, F45 or
whatever. At those apertures and that kind of light I'm sometimes lucky if I
can even compose an image on the ground glass much less stop down to the
taking aperture and tell whether everything in the scene is sharp or not.
And even in bright sunlight there often are shadow areas where the light is
dim.

So many of us to resort to something other than just looking at the ground
glass to tell whether everything looks sharp or not. I use the method
discussed in Tuan's article on focusing the view camera found at
www.largeformatphotography.info but there are other methods too that IMHO
are more practical than trying to figure it out from the ground glass.


"John Emmons" wrote in message
...
by the time you figure out the formula, you could have set up, focused and
exposed the film...;^)

Steve, the best "formula" to use for focusing is to get under the dark

cloth
with a focusing loupe and get the scene sharp. Check the near and far, all
the corners, etc.

Keep in mind that you probably won't be able to get the entire scene in
focus with the lens wide open, you'll want to stop the lens down and let
your eyes get accustomed to the light, with a bit of practice you'll be

able
to focus.

The math that Leonard uses is undoubtedly correct, but I think you'll find
that in practice it's much better to actually see the image get sharp on

the
ground glass.

John Emmons

"Leonard Evens" wrote in message
news
Steve Yeatts wrote:
Is there a formula to calculate the required distance between the lens

board
and the film plane for an object at a given distance from the camera?
Thanks in advance.


If u is the distance from the lens to the subject, v the distance of the
lens from the film plane, and f the focal length, then

1/u + 1/v = 1/f.

So

v = fu/(u - f)

In words, multiply the subject distance by the focal length, and then
divide that by the difference between the subject distance and the focal
length.

But this requires some additional qualification since the lens has an
extent of its own. The subject distance is measured from a point called
the front principal point and the film distance is measured from a point
called the rear principal point. For most lenses, these points are both
close to the center of the lens which in turn is close to the lens
board. But for lenses of telephoto design, both these points may be far
in front of the lens. For many wide angle lenses, the rear principal
point is a short distance in back of the lens board. Usually lens
specifications tell you what the rear flange focal length is. If you
know that and the focal length, you can determine the position of the
rear principal point.


Steve (new to 4x5)







  #7  
Old June 13th 04, 02:24 PM
Leonard Evens
external usenet poster
 
Posts: n/a
Default Formula for pre-focusing

Vladamir30 wrote:
John said:


Steve, the best "formula" to use for focusing is to get under the dark


cloth

with a focusing loupe and get the scene sharp. Check the near and far, all
the corners, etc. . . .



With all due respect, I disagree with this advice. It's often difficult to
stop down to the taking aperture and tell whether everything is "sharp" or
not. Many of us photograph early in the morning and late in the evening or
in fog , rain, cloudy conditions, or other times when the light isn't very
bright. Then we're stopping down to at least F22, maybe F32, F45 or
whatever. At those apertures and that kind of light I'm sometimes lucky if I
can even compose an image on the ground glass much less stop down to the
taking aperture and tell whether everything in the scene is sharp or not.
And even in bright sunlight there often are shadow areas where the light is
dim.

So many of us to resort to something other than just looking at the ground
glass to tell whether everything looks sharp or not. I use the method
discussed in Tuan's article on focusing the view camera found at
www.largeformatphotography.info but there are other methods too that IMHO
are more practical than trying to figure it out from the ground glass.


I entirely agree with your comments. Even in bright light, I find it
difficult to see much of anything if I stop down below f/16 or f/22. It
is true that my retina is not that sensitive to dim light after 60 years
of myopia, but I once posted a question about this and found that many
people agreed with me that it is very difficult to evaluate depth of
field by stopping down to the taking aperture.

Steve Simmons has suggested the use of screen brighteners and a loupe.
I have a Maxwell screen which is about as bright as anything you can
find, and I still can't see much of anything when I stop down to the
taking aperture. The use of a loupe is more involved than one might
think. In effect, you are magnifying the picture. If you used a 10 X
loupe to view a 4 x 5 transparency, you would expect less DOF than you
would see with a 4 X loupe. So your choice of loupe involves implicit
assumptions about how much the final image will be enlarged and how it
will be viewed. As a result, it is going to be a individual thing.
One person may find that a certain procedure, developed after years of
experience, may work well for him/her in evaluating DOF, but it doesn't
follow that the same procedure will work for someone else. The
advantage of the focus spread methods, as described by Tuan at
www.largeformatphotography.info, is that they are more objective and
less dependent on the individual.

Having said that, let me add that it doesn't hurt to stop down as far as
your eyes will allow you to check that DOF is in the right ball park and
extends equally from front to rear. Experience will help you estimate
how much more you will get if you stop down further. In my case, I
usually use f/11 to f/16 for that purpose. If you can get to f/32, all
the more power to you.



"John Emmons" wrote in message
...

by the time you figure out the formula, you could have set up, focused and
exposed the film...;^)

Steve, the best "formula" to use for focusing is to get under the dark


cloth

with a focusing loupe and get the scene sharp. Check the near and far, all
the corners, etc.

Keep in mind that you probably won't be able to get the entire scene in
focus with the lens wide open, you'll want to stop the lens down and let
your eyes get accustomed to the light, with a bit of practice you'll be


able

to focus.

The math that Leonard uses is undoubtedly correct, but I think you'll find
that in practice it's much better to actually see the image get sharp on


the

ground glass.

John Emmons


  #8  
Old June 19th 04, 11:38 PM
Richard Knoppow
external usenet poster
 
Posts: n/a
Default Formula for pre-focusing


"Leonard Evens" wrote in message
news
Steve Yeatts wrote:
Is there a formula to calculate the required distance

between the lens board
and the film plane for an object at a given distance

from the camera?
Thanks in advance.


If u is the distance from the lens to the subject, v the

distance of the
lens from the film plane, and f the focal length, then

1/u + 1/v = 1/f.

So

v = fu/(u - f)

In words, multiply the subject distance by the focal

length, and then
divide that by the difference between the subject distance

and the focal
length.

But this requires some additional qualification since the

lens has an
extent of its own. The subject distance is measured from

a point called
the front principal point and the film distance is

measured from a point
called the rear principal point. For most lenses, these

points are both
close to the center of the lens which in turn is close to

the lens
board. But for lenses of telephoto design, both these

points may be far
in front of the lens. For many wide angle lenses, the

rear principal
point is a short distance in back of the lens board.

Usually lens
specifications tell you what the rear flange focal length

is. If you
know that and the focal length, you can determine the

position of the
rear principal point.


Steve (new to 4x5)


Its not hard to find the principal points of a lens. If
you know the focal length its only necessary to focus the
lens exactly at infinity and measure one focal length from
the focal plane toward the lens. By definition that is the
principle point. To measure the front or first principal
point turn the lens around and again focus it for infinity.
Again, measure from the focal plane toward the lens, that
will be the first or front principal point and is the point
to measure object to lens distance to.
If there is no handy target far enough away to
approximate infinity you can focus by autocollimation. You
need a small mirror and a card with a small hole in it, and
a flashlight to place behind the card. Place the mirror over
the lens. Place the card in back of the lens, adjust the
card (or lens) until an image of the illuminated hole is
reflected back to the card. It should be near the hole. When
this image is sharply focused the lens is exactly at
infinity focus from the card. In a view or press camera you
can usually get sufficiently good accuracy by placing a
small light source over the ground glass near the center and
focusing the lens on that.
Since you know the focal length and infinity focus
position you can calculate the amount the lens must be moved
from infinity focus and use that to adjust for some closer
distance. Its still necessary to know the location of the
front principal point so you can measure the distance.

For any lens the distance from the infinity focus point for
any closer distance is:

X = f^2 / u - f

Whe
X = extension from infinity focus
f = focal length
u = subject distance



--
---
Richard Knoppow
Los Angeles, CA, USA



  #9  
Old June 20th 04, 02:39 AM
Leonard Evens
external usenet poster
 
Posts: n/a
Default Formula for pre-focusing

Richard Knoppow wrote:

What you say below is correct. But there is one minor problem for
perfectionists. You have to know the actual focal length, which may be
different from the nominal published focal length.


Its not hard to find the principal points of a lens. If
you know the focal length its only necessary to focus the
lens exactly at infinity and measure one focal length from
the focal plane toward the lens. By definition that is the
principle point. To measure the front or first principal
point turn the lens around and again focus it for infinity.
Again, measure from the focal plane toward the lens, that
will be the first or front principal point and is the point
to measure object to lens distance to.
If there is no handy target far enough away to
approximate infinity you can focus by autocollimation. You
need a small mirror and a card with a small hole in it, and
a flashlight to place behind the card. Place the mirror over
the lens. Place the card in back of the lens, adjust the
card (or lens) until an image of the illuminated hole is
reflected back to the card. It should be near the hole. When
this image is sharply focused the lens is exactly at
infinity focus from the card. In a view or press camera you
can usually get sufficiently good accuracy by placing a
small light source over the ground glass near the center and
focusing the lens on that.
Since you know the focal length and infinity focus
position you can calculate the amount the lens must be moved
from infinity focus and use that to adjust for some closer
distance. Its still necessary to know the location of the
front principal point so you can measure the distance.

For any lens the distance from the infinity focus point for
any closer distance is:

X = f^2 / u - f

Whe
X = extension from infinity focus
f = focal length
u = subject distance




  #10  
Old June 22nd 04, 02:55 AM
Richard Knoppow
external usenet poster
 
Posts: n/a
Default Formula for pre-focusing


"Leonard Evens" wrote in message
...
Richard Knoppow wrote:

What you say below is correct. But there is one minor

problem for
perfectionists. You have to know the actual focal length,

which may be
different from the nominal published focal length.


Its not hard to find the principal points of a lens.

If
you know the focal length its only necessary to focus

the
lens exactly at infinity and measure one focal length

from
the focal plane toward the lens. By definition that is

the
principle point. To measure the front or first principal
point turn the lens around and again focus it for

infinity.
Again, measure from the focal plane toward the lens,

that
will be the first or front principal point and is the

point
to measure object to lens distance to.
If there is no handy target far enough away to
approximate infinity you can focus by autocollimation.

You
need a small mirror and a card with a small hole in it,

and
a flashlight to place behind the card. Place the mirror

over
the lens. Place the card in back of the lens, adjust the
card (or lens) until an image of the illuminated hole is
reflected back to the card. It should be near the hole.

When
this image is sharply focused the lens is exactly at
infinity focus from the card. In a view or press camera

you
can usually get sufficiently good accuracy by placing a
small light source over the ground glass near the center

and
focusing the lens on that.
Since you know the focal length and infinity focus
position you can calculate the amount the lens must be

moved
from infinity focus and use that to adjust for some

closer
distance. Its still necessary to know the location of

the
front principal point so you can measure the distance.

For any lens the distance from the infinity focus point

for
any closer distance is:

X = f^2 / u - f

Whe
X = extension from infinity focus
f = focal length
u = subject distance



Its not difficult in principle to measure the exact focal
length, but it does require some precision, especially for
shorter lenses. With the above formula one can determine the
focal length from the extension required for a known
distance or from magnification. The easiest is the set the
lens for unity magnification since the focus extension from
infintity to 1:1 is exactly one focal length.
It may be easier to measure the extension for a known
distance than it is to set the image for exactly 1:1.
However, knowing the exact distance requires knowing the
location of the princpal points and that requires knowing
the focal length unless one has access to an optical bench
and a nodal slide. So, the unity magnification is perhaps
the most practical method. The lens is focused for infinity
with either a very distant target or by autocollimating as
above. Then the lens is set for exactly 1:1 image to object
size. The displacement of the lens from the infinity focus
point is one focal length. A double check is the total
distance from focal plane (ground glass) to object. At 1:1
it is exactly four times the focal length.
The principal points are found by measuring one focal
length back toward the lens from the focal plane at infinity
focus. The first or front principal point is found by making
the same measurement with the lens turned around. These
points are somtimes useful to know for calculating focus,
depth of field, exposure correction, and magnification.

--
---
Richard Knoppow
Los Angeles, CA, USA




 




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