bellows extension aperture 'correction vs. effective f-# change with accessory lens

"MurrayatUptown" wrote in
message
om...
If one uses an accessory lens (say, a close-up or Telek
type) on a
view camera to change the f.l., I assume the apparent of
effective
f-#'s for that lens will change.

I'm not sure if there are then two factors that alter
exposure???

The above scenario is not exactly what I did, but it is
the essence of
it so you understand my question.

What I actually did was take a Polaroid 150 shutter/lens
(=130 mm w/
EV 10-17 combinations). I put a -4.5 cell from another
Polaroid (110B
spare parts) on the back of this lens,and now have
approximately 320
mm f.l.

The shutter speed component of the Polaroid EV numbers
remains
constant, but the f-# contribution changes - I assume I
have to
correct for bellows factor by virtue of the new extension
for 320 mm
vs 130, and the additional glass may have changed the
effective
aperture.


Can I for example, measure exposure on the ground glass at
130 mm
without the extra lens, then re-set focus for 320 mm with
the extra
lens, remeasure the new light level and thus know how much
exposure
correction is needed for the sum of the two factors?

Any other suggestions?

And, say, how does one add a 'no-spam, etc' to the
displayed email
address on posts? Google only let me use a real email
address.

Thanks

Murray

It depends on how you use the lens. Close-up lenses are
negative elements which shorten the focal length of the lens
they are applied to. That also effectively increases the
speed. If the lens is used for a distant object the stop
must be calculated on the basis of the ratio of the normal
focal length to the focal length for the combination.
When ued for close up photography the increase in speed
may compensate the "bellows factor"
at commonly used distances. The bellows factor still exists.
For precise exposure you must calculate the effective speed
of the combination of normal and close up lens and then
apply the dsitance correction. Where these lenses are used
on folding cameras with fixed minimum focusing distance the
bellows correction and speed increase often just compensate
each other. Since this is the usual application for these
lenses correction is usually not necessary and the stop dial
for the lens will read the "effective" stop for the close up
work.


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

"Richard Knoppow" wrote in message
...

It depends on how you use the lens. Close-up lenses are
negative elements which shorten the focal length of the lens
they are applied to.

Are you sure they are negative? I think they are actually +diopter lenses.

There are usually more than 1 way to describe physical effects. Some people
say close up lenses do not alter the focal length of the lens they are
applied to but rather "trick" the lens to "believe" that infinity is closer,
up to a distance equal to 1000/diopter mm from the close up lens (when
prime lens focused at infinity, the distance gets closer if the prime lens
is focused closer than infinity), therefore allowing to focus the prime lens
closer than it is capable of san the close up lens.

Guillermo

"Richard Knoppow" wrote in message
...

It depends on how you use the lens. Close-up lenses are
negative elements which shorten the focal length of the lens
they are applied to.

Are you sure they are negative? I think they are actually +diopter lenses.

There are usually more than 1 way to describe physical effects. Some people
say close up lenses do not alter the focal length of the lens they are
applied to but rather "trick" the lens to "believe" that infinity is closer,
up to a distance equal to 1000/diopter mm from the close up lens (when
prime lens focused at infinity, the distance gets closer if the prime lens
is focused closer than infinity), therefore allowing to focus the prime lens
closer than it is capable of san the close up lens.

Guillermo

I agree - the 'Close-Up' lenses I have seen were all (+), i.e., +1,
+2, +3 diopter.

The Telek ones I should have kept & hastily sold on eBay were (-), and
they were 'tele-converters' of a sort.

Murray

The so-called 'close-up' ones I have seen have all been (+).

The Telek ones I had and let slip away were all (-), and described as
increasing f.l. (which makes sense mathwise).

How I am using them, experimentally, as usual. The 130 mm lens with an
approximately -4.5 (determined by resulting 310-320 mm f.l.) 2-element
cell stuck on the back will cover the better part of an 11x14 glass,
but with plenty of blur...we shall see.

Murray

The so-called 'close-up' ones I have seen have all been (+).

The Telek ones I had and let slip away were all (-), and described as
increasing f.l. (which makes sense mathwise).

How I am using them, experimentally, as usual. The 130 mm lens with an
approximately -4.5 (determined by resulting 310-320 mm f.l.) 2-element
cell stuck on the back will cover the better part of an 11x14 glass,
but with plenty of blur...we shall see.

Murray

The so-called 'close-up' ones I have seen have all been (+).

The Telek ones I had and let slip away were all (-), and described as
increasing f.l. (which makes sense mathwise).

How I am using them, experimentally, as usual. The 130 mm lens with an
approximately -4.5 (determined by resulting 310-320 mm f.l.) 2-element
cell stuck on the back will cover the better part of an 11x14 glass,
but with plenty of blur...we shall see.

Murray

Hi Murray!

see http://medfmt.8k.com/mf/diopter.html (search for "Bob Gurfinkel" who
wrote a shutterbug article on using + and - diopter elements with a
formula to calculate equiv. focal length effects etc. ) hth - regards bobm
--
************************************************** *********************
* Robert Monaghan POB 752182 Southern Methodist Univ. Dallas Tx 75275 *
********************Standard Disclaimers Apply*************************

Hi Murray!

see http://medfmt.8k.com/mf/diopter.html (search for "Bob Gurfinkel" who
wrote a shutterbug article on using + and - diopter elements with a
formula to calculate equiv. focal length effects etc. ) hth - regards bobm
--
************************************************** *********************
* Robert Monaghan POB 752182 Southern Methodist Univ. Dallas Tx 75275 *
********************Standard Disclaimers Apply*************************