About central (diaphragm) shutters at high speeds/large apertures

Hello,

I have a old 6x6 TLR (Zeiss Ikon Ikoflex), it has a mechanical central
shutter (Compur Rapid).

I remember that there is an issue with the exposure at high speeds and
large apertures with this kind of shutter, because of the time needed
for the shutter to fully open (and to fully close).

This raises two questions:

-- when we select a speed of 1/125 (for instance), how does the
shutter is supposed to behave ? If it starts opening à t=0, does it
starts closing at t=1/125s ? Or does it actually start closing
*before* that, so that it is fully closed at t=1/125 ?

-- typically, how fast were these shutters to go from full closure to
full aperture ?


And finally: are there some tables around that give the exposure
corrections for that ?


Thanks,

--
pehache

In article .com,
pehache-tolai wrote:

Hello,

I have a old 6x6 TLR (Zeiss Ikon Ikoflex), it has a mechanical central
shutter (Compur Rapid).

I remember that there is an issue with the exposure at high speeds and
large apertures with this kind of shutter, because of the time needed
for the shutter to fully open (and to fully close).

This raises two questions:

-- when we select a speed of 1/125 (for instance), how does the
shutter is supposed to behave ? If it starts opening à t=0, does it
starts closing at t=1/125s ? Or does it actually start closing
*before* that, so that it is fully closed at t=1/125 ?

-- typically, how fast were these shutters to go from full closure to
full aperture ?


And finally: are there some tables around that give the exposure
corrections for that ?


Thanks,

--
pehache

There's a good discussion and some hand-drawn diagrams showing how a
Compur shutter works he http://rick_oleson.tripod.com/index-88.html
He doesn't give numbers, except for shutter speeds, but shows the cam
mechanism that controls the shutter blades' opening and closing speeds.
Also that there are usually several mechanisms governing different speed
ranges.

--Nicco

pehache-tolai wrote:

I have a old 6x6 TLR (Zeiss Ikon Ikoflex), it has a mechanical central
shutter (Compur Rapid).

I remember that there is an issue with the exposure at high speeds and
large apertures

Small aperture.

with this kind of shutter, because of the time needed
for the shutter to fully open (and to fully close).

This raises two questions:

-- when we select a speed of 1/125 (for instance), how does the
shutter is supposed to behave ? If it starts opening à t=0, does it
starts closing at t=1/125s ? Or does it actually start closing
*before* that, so that it is fully closed at t=1/125 ?

The shutter speed is roughly the time between the shutter being half open
and half closed again.

-- typically, how fast were these shutters to go from full closure to
full aperture ?

That varies a bit per brand and type, and shutter size.
But perhaps as much as 1 ms.

And finally: are there some tables around that give the exposure
corrections for that ?

Nope.
The effect is small. And variable with aperture.
So no tables needed, nor really possible.

On Aug 16, 11:32 am, pehache-tolai wrote:
Hello,

I have a old 6x6 TLR (Zeiss Ikon Ikoflex), it has a mechanical central
shutter (Compur Rapid).


And finally: are there some tables around that give the exposure
corrections for that ?


The 1958 Ilford Manual of Photography gives the
following table. It simply calls it the performance
of a "good between the lens shutter," but it is
very likrly from a Compur-Rapid because the 250th
speed is almost as inefficient as the 1/500th
speed - a characteristic of the old Compur-Rapid
shutters which used an extra spring for top speed.

The Synchro-Compur shutters which were contemporary
to the 1958 Ilford Manual were actually quite
a lot better and had the modern speed sequence.

Marked Effective Exposure time in milliseconds
Speed
f/3.5 f/5.6 f/8 f/11 f/22

1/10 100 103 105 108 110
1/25 40 43 45 48 50
1/50 20 22 24 26 27
1/100 10 12 13 14 15
1/250 4 5 6 6.5 7
1/500 2 2.5 3 3.5 3.8

If you are shooting negative film, there is
no need to worry; an extra bit of exposure
does no harm at all.

If you are shooting slide film, the difference
is serious at 1/500 and 1/250. It is arguably
significant at lower speeds. On a more modern
shutter the problem is only likely to be
serious at 1/500th of a second.

Peter.
--

On 16 août, 18:11, Niccolo Machiavelli
wrote:

There's a good discussion and some hand-drawn diagrams showing how a
Compur shutter works he http://rick_oleson.tripod.com/index-88.html
He doesn't give numbers, except for shutter speeds, but shows the cam
mechanism that controls the shutter blades' opening and closing speeds.
Also that there are usually several mechanisms governing different speed
ranges.


Thanks, that's pretty interesting.

Indeed, I have to select the 1/500 speed *before* cocking if I want
it. Now I better understand why (this enables the use of the booster
spring).

--
pehache

On 16 août, 18:23, "Q.G. de Bakker" wrote:

I remember that there is an issue with the exposure at high speeds and
large apertures

Small aperture.

Yep, that's what I see now from the next answer, that the shutter is
calibrated to give correct exposure at large apertures...


with this kind of shutter, because of the time needed
for the shutter to fully open (and to fully close).

This raises two questions:

-- when we select a speed of 1/125 (for instance), how does the
shutter is supposed to behave ? If it starts opening à t=0, does it
starts closing at t=1/125s ? Or does it actually start closing
*before* that, so that it is fully closed at t=1/125 ?

The shutter speed is roughly the time between the shutter being half open
and half closed again.


OK. Close to my first hypothesis, then...


And finally: are there some tables around that give the exposure
corrections for that ?

Nope.
The effect is small.

Not that small, more than 1 IL in the worst cases...

And variable with aperture.

That's why a table may help :-)

--
pehache

On 17 août, 07:07, Peter wrote:

The 1958 Ilford Manual of Photography gives the
following table. It simply calls it the performance
of a "good between the lens shutter," but it is
very likrly from a Compur-Rapid because the 250th
speed is almost as inefficient as the 1/500th
speed - a characteristic of the old Compur-Rapid
shutters which used an extra spring for top speed.

The Synchro-Compur shutters which were contemporary
to the 1958 Ilford Manual were actually quite
a lot better and had the modern speed sequence.

OK. So as a first approximation, I think I can use the following table
for the Compur-Rapid, just assuming that the 1/250 is as bad as the
1/500 of the table, the 1/100 is as bad as the 1/50 of the table, and
so on...

Marked Effective Exposure time in milliseconds
Speed
f/3.5 f/5.6 f/8 f/11 f/22

1/10 100 103 105 108 110
1/25 40 43 45 48 50
1/50 20 22 24 26 27
1/100 10 12 13 14 15
1/250 4 5 6 6.5 7
1/500 2 2.5 3 3.5 3.8

So the shutter is calibrated to give the right effective exposure at
large aperture, which makes sense. Small apertures are used mostly
when the body is on a pod and when we have time to think about a
correction :-)


If you are shooting negative film, there is
no need to worry; an extra bit of exposure
does no harm at all.

Sure...


If you are shooting slide film,

I do shoot slide film (B&W slide film), that's why I worry about that.


the difference
is serious at 1/500 and 1/250.

and probably already at 1/100 with a Compur-Rapid...

It is arguably
significant at lower speeds. On a more modern
shutter the problem is only likely to be
serious at 1/500th of a second.

I guess that on modern electronically-controlled shutters, the
corrections are even handled by the system to keep the effective
exposure always correct ?

--
pehache

On 17 août, 10:00, pehache-tolai wrote:
On 17 août, 07:07, Peter wrote:



The 1958 Ilford Manual of Photography gives the
following table. It simply calls it the performance
of a "good between the lens shutter," but it is
very likrly from a Compur-Rapid because the 250th
speed is almost as inefficient as the 1/500th
speed - a characteristic of the old Compur-Rapid
shutters which used an extra spring for top speed.

The Synchro-Compur shutters which were contemporary
to the 1958 Ilford Manual were actually quite
a lot better and had the modern speed sequence.

OK. So as a first approximation, I think I can use the following table
for the Compur-Rapid, just assuming that the 1/250 is as bad as the
1/500 of the table, the 1/100 is as bad as the 1/50 of the table, and
so on...


Sorry, I think I missed your point. What you actually say is that this
table IS likely for a Compur-Rapidv (and not for a better shutter).
(my english is not perfect and I read it too quickly at first).

So I can use it as is...

--
pehache

On Aug 17, 1:32 am, pehache-tolai wrote:



This raises two questions:

-- when we select a speed of 1/125 (for instance), how does the
shutter is supposed to behave ? If it starts opening à t=0, does it
starts closing at t=1/125s ? Or does it actually start closing
*before* that, so that it is fully closed at t=1/125 ?

-- typically, how fast were these shutters to go from full closure to
full aperture ?

And finally: are there some tables around that give the exposure
corrections for that ?


From the Kodak Professional Photoguide, 1981:
quote
Corrections for faster blade shutter speeds and small apertures

Blade shutters at their faster shutter speeds and small lens apertures
tend to give more exposure than is indicated by the shutter speed, f-
number combination. This is not a manufacturer defect but is due to
the inertia of moving parts combined with the geometry of the lens-
shutter-diaphragm structure. It will usually occur in daylight
photography with fast films and will cause overexposure of up to 1
stop. You can find the correction factor to apply in these situations
by running an exposure series on a fast transparency film such as
Kodak Ektachrome 200 (Daylight). This effect does not occur with
focal-plane shutters. The following table gives an example of the type
of correction you may find necessary.

(cut and paste the following to notepad so you get a monospaced font)
Lens aperture
Speed 1/125 1/250 1/500
f1.4-5.6 None None None
f8 None None 1/2stop
f11 None 1/2stop 2/3stop
f16 and smaller 1/2stop 2/3stop 1stop
/quote


Aren't you glad some of us never throw away old manuals?
;-)

"Noons" wrote in message
oups.com...
On Aug 17, 1:32 am, pehache-tolai wrote:



This raises two questions:

-- when we select a speed of 1/125 (for instance), how does the
shutter is supposed to behave ? If it starts opening à t=0, does it
starts closing at t=1/125s ? Or does it actually start closing
*before* that, so that it is fully closed at t=1/125 ?

-- typically, how fast were these shutters to go from full closure to
full aperture ?

And finally: are there some tables around that give the exposure
corrections for that ?


From the Kodak Professional Photoguide, 1981:
quote
Corrections for faster blade shutter speeds and small apertures

Blade shutters at their faster shutter speeds and small lens apertures
tend to give more exposure than is indicated by the shutter speed, f-
number combination. This is not a manufacturer defect but is due to
the inertia of moving parts combined with the geometry of the lens-
shutter-diaphragm structure. It will usually occur in daylight
photography with fast films and will cause overexposure of up to 1
stop. You can find the correction factor to apply in these situations
by running an exposure series on a fast transparency film such as
Kodak Ektachrome 200 (Daylight). This effect does not occur with
focal-plane shutters. The following table gives an example of the type
of correction you may find necessary.

(cut and paste the following to notepad so you get a monospaced font)
Lens aperture
Speed 1/125 1/250 1/500
f1.4-5.6 None None None
f8 None None 1/2stop
f11 None 1/2stop 2/3stop
f16 and smaller 1/2stop 2/3stop 1stop
/quote


Aren't you glad some of us never throw away old manuals?
;-)

An interesting thread. But how important is all this given that the sutter
speeds of mechanical shutters, especially older ones, are probably not
precisely accurate to begin with. Remember reading somewhere a report that
found many 1/500 speeeds on several mechanical shutters to be closer to
1/300- 1/400. So it's probably not worthwhile tryng to make very minor
exposure adjustments anyway.