About central (diaphragm) shutters at high speeds/large ap..

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.
 >> Stay informed about: About central (diaphragm) shutters at high speeds/large ap.. 

On Aug 16, 11:32 am, pehache-tolai <pehach....RemoveThis@gmail.com> 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.
--
pirwin.RemoveThis@ktb.net
 >> Stay informed about: About central (diaphragm) shutters at high speeds/large ap.. 

On 16 août, 18:11, Niccolo Machiavelli
<Machiave....DeleteThis@communo.firenze.it> wrote:
>
> There's a good discussion and some hand-drawn diagrams showing how a
> Compur shutter works here: 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
 >> Stay informed about: About central (diaphragm) shutters at high speeds/large ap.. 

On 16 août, 18:23, "Q.G. de Bakker" <q... DeleteThis @tiscali.nl> 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
 >> Stay informed about: About central (diaphragm) shutters at high speeds/large ap.. 

On 17 août, 07:07, Peter <pir....TakeThisOut@ktb.net> 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
 >> Stay informed about: About central (diaphragm) shutters at high speeds/large ap.. 

On 17 août, 10:00, pehache-tolai <pehach....TakeThisOut@gmail.com> wrote:
> On 17 août, 07:07, Peter <pir....TakeThisOut@ktb.net> 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
 >> Stay informed about: About central (diaphragm) shutters at high speeds/large ap.. 

On Aug 17, 1:32 am, pehache-tolai <pehach....TakeThisOut@gmail.com> 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?

 >> Stay informed about: About central (diaphragm) shutters at high speeds/large ap.. 

"Noons" <wizofoz2k.DeleteThis@yahoo.com.au> wrote in message
news:1187358066.390095.261210@l22g2000prc.googlegroups.com...
On Aug 17, 1:32 am, pehache-tolai <pehach....DeleteThis@gmail.com> 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.
 >> Stay informed about: About central (diaphragm) shutters at high speeds/large ap.. 

Recently, Norm Fleming <nfleming.TakeThisOut@ms.umanitoba.ca> posted:
>
> 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.
>
Hmmm. A couple of things come to mind. The variance in exposure times in
this thread were *worst* at 1/500th, so it is possible that what you
recall was reporting the same phenomenon. Also, 1/350 (the intermediate of
your estimates) would be about 1/2 stop off. Given a choice between being
1/2 stop or 1 stop off, I'll take the lesser of the two, thank you!

The bottom line is that being a mechanical issue the variance will be
repeatable, therefore it can be tested, measured and compensated for in
use. I see that as a Good Thing compared to some other issues.

Neil
 >> Stay informed about: About central (diaphragm) shutters at high speeds/large ap.. 

On 16 Aug, 17:32, pehache-tolai <pehach... RemoveThis @gmail.com> 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 is a common misconception about this. The shutter speed is
really a measure of the amount of light it allows to fall on the film.
The true time it takes will be longer as the shutter has to speed up
to open and speed up to close.
 >> Stay informed about: About central (diaphragm) shutters at high speeds/large ap.. 

On Aug 17, 4:00 am, pehache-tolai <pehach....DeleteThis@gmail.com> wrote:

> 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

The problem can only arise if you are using fast film (Say EI 100
or faster) in full daylight. The shutter makers didn't think you
would normally be doing that, and certainly not with reversal
film. If you were shooting 32 speed Ektachrome, you would
never run in to the problem - at 1/500 second you would be using
f/4 under full sun.

Nowadays EI 100 is a normal speed for reversal film, and if you
shoot under full sunlight, you may have to keep shutter efficiency
in mind.

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

I've never shot B&W reversal in a still camera. What film and
processing are you using?

Peter.
--
pirwin.DeleteThis@ktb.net
 >> Stay informed about: About central (diaphragm) shutters at high speeds/large ap.. 

On 17 août, 15:41, Noons <wizofo... RemoveThis @yahoo.com.au> wrote:

>
> (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?
>

Sure !!

--
pehache
 >> Stay informed about: About central (diaphragm) shutters at high speeds/large ap.. 

On 18 août, 11:53, RolandRB <rolandbe... DeleteThis @hotmail.com> wrote:
>
> There is a common misconception about this. The shutter speed is
> really a measure of the amount of light it allows to fall on the film.

Yes... but at which aperture ?

It appears from the tables posted by other contributors that the
shutter speed is usually calibrated for the full aperture and that
corrections are needed for small apertures.

--
pehache
 >> Stay informed about: About central (diaphragm) shutters at high speeds/large ap..