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

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

"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.

No aperture compensation needed for those type of lenses. You can think of
them as "reading glasses" for your lens. Take a pair of reading glasses and
you will notice there is no change in brightness when you see thru them (if
there is any, it is caused by any coating the glasses may have).

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.

Yes

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?

DOn't know which 2 factors you talk about. That said, measuring would work.
Or for a rough estimate the correction needed in stops would be:
K = 6.644 * LOG(320/130) = 2.6 stops
or if you prefer an exposure time correction factor:
T = 2 ^ K = 2 ^ 2.6 = 6

Guillermo

"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.

No aperture compensation needed for those type of lenses. You can think of
them as "reading glasses" for your lens. Take a pair of reading glasses and
you will notice there is no change in brightness when you see thru them (if
there is any, it is caused by any coating the glasses may have).

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.

Yes

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?

DOn't know which 2 factors you talk about. That said, measuring would work.
Or for a rough estimate the correction needed in stops would be:
K = 6.644 * LOG(320/130) = 2.6 stops
or if you prefer an exposure time correction factor:
T = 2 ^ K = 2 ^ 2.6 = 6

Guillermo

"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.

No aperture compensation needed for those type of lenses. You can think of
them as "reading glasses" for your lens. Take a pair of reading glasses and
you will notice there is no change in brightness when you see thru them (if
there is any, it is caused by any coating the glasses may have).

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.

Yes

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?

DOn't know which 2 factors you talk about. That said, measuring would work.
Or for a rough estimate the correction needed in stops would be:
K = 6.644 * LOG(320/130) = 2.6 stops
or if you prefer an exposure time correction factor:
T = 2 ^ K = 2 ^ 2.6 = 6

Guillermo

"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.

No aperture compensation needed for those type of lenses. You can think of
them as "reading glasses" for your lens. Take a pair of reading glasses and
you will notice there is no change in brightness when you see thru them (if
there is any, it is caused by any coating the glasses may have).

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.

Yes

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?

DOn't know which 2 factors you talk about. That said, measuring would work.
Or for a rough estimate the correction needed in stops would be:
K = 6.644 * LOG(320/130) = 2.6 stops
or if you prefer an exposure time correction factor:
T = 2 ^ K = 2 ^ 2.6 = 6

Guillermo

Hello Guillermo:

I realize there is more than one Guillermo in photography, but I
wondered if you're the same one from pinhole work...

I shouldn't have said Telek...probably no big deal, but it's a
2-element cell from a Rodenstock-Ysarex lens.

I came up with 2.5 stops...I might have estimated 310 last time.

Thank you

Murray

the 2 factors I assumed existed were

1) bellows extension

2) optical change with the 2 element cell causing a different effective aperture #

Murray

the 2 factors I assumed existed were

1) bellows extension

2) optical change with the 2 element cell causing a different effective aperture #

Murray

"MurrayatUptown" wrote in message
om...
Hello Guillermo:

I realize there is more than one Guillermo in photography, but I
wondered if you're the same one from pinhole work...

Yeah it is me.

I came up with 2.5 stops...I might have estimated 310 last time.

2.5 stops, 2.6 stops, close enough!

the 2 factors I assumed existed were

1) bellows extension
2) optical change with the 2 element cell causing a different effective
aperture #

Those 2 factors you mention are just one factor. They are, tho, 2 different
ways to look at what is happening.

Guillermo

"MurrayatUptown" wrote in message
om...
Hello Guillermo:

I realize there is more than one Guillermo in photography, but I
wondered if you're the same one from pinhole work...

Yeah it is me.

I came up with 2.5 stops...I might have estimated 310 last time.

2.5 stops, 2.6 stops, close enough!

the 2 factors I assumed existed were

1) bellows extension
2) optical change with the 2 element cell causing a different effective
aperture #

Those 2 factors you mention are just one factor. They are, tho, 2 different
ways to look at what is happening.

Guillermo