Q: Exposure correction for leaf shutters

"RolandRB" wrote:

I am wondering if some kind person who has this guide can send me a
scan.


Faster to type it

f stop | 1/125 | 1/250 | 1/500
f/8 | None | None | -1/3
f/11 | None | -1/3 | -2/3
f/16 | -1/3 | -2/3 | -1

Remember, _reduce_ exposure by amount indicated. You might want to test it,
though. I don't recall ever having exposure problems with modern leaf
shutters (Fuji 645, Mamiya 7, even post war Rolleiflexes).

--
David J. Littleboy
Tokyo, Japan

RolandRB wrote:
Thanks for that. I'm seeing this problem on my old folders when I use
f11 at 1/250th sec. for sunny conditions and I'm a fairly good judge
of shutter speed. The shots I take are overexposed by between a third
stop and a half stop which really shows up on slide film.

It may also be that the shutter is slower than the marked speed due to age,
i.e. lack of lubrication or being out of adjustment, or exaguration (I don't
think I spelled that correctly) by the manufacturer.

Many 35mm SLR's from Japan in the 1950's and 1960's 1/500th was really 1/300th,
and I believe many cheaper leaf shutters where similar.

The main exception would be Kodak leaf shutters, by now they are all slow to
unusable without being taken apart, the old grease removed and re-oiled.
If they were done properly, even in the 1960's they won't need it now.

If you have a CRT Television set you can photograph the screen and count the
scan lines, but those are on their way out.
Geoff.
--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM

Geoffrey S. Mendelson wrote,on my timestamp of 1/07/2009 7:39 PM:


If you have a CRT Television set you can photograph the screen and count the
scan lines, but those are on their way out.

Turn off progressive scanning and it works the same with a LCD screen. At least
mine does.
But I don't count the lines, I just watch through the back while working the
shutter: each successively higher speed should produce a band of lighted screen
roughly half the width of the previous.

"Geoffrey S. Mendelson" wrote:
RolandRB wrote:
Thanks for that. I'm seeing this problem on my old folders when I use
f11 at 1/250th sec. for sunny conditions and I'm a fairly good judge
of shutter speed. The shots I take are overexposed by between a third
stop and a half stop which really shows up on slide film.

It may also be that the shutter is slower than the marked speed due to
age,
i.e. lack of lubrication or being out of adjustment, or exaguration (I
don't
think I spelled that correctly) by the manufacturer.

Many 35mm SLR's from Japan in the 1950's and 1960's 1/500th was really
1/300th,
and I believe many cheaper leaf shutters where similar.

Good point. Interestingly, the Fujiblad leaf shutters only go up to 1/400.

If you have a CRT Television set you can photograph the screen and count
the
scan lines, but those are on their way out.

Science is always making such wonderful improvements on things. Sigh.

--
David J. Littleboy
Tokyo, Japan

On 01-07-09 08:40, David J. Littleboy wrote:
"Geoffrey S. wrote:
RolandRB wrote:
Thanks for that. I'm seeing this problem on my old folders when I use
f11 at 1/250th sec. for sunny conditions and I'm a fairly good judge
of shutter speed. The shots I take are overexposed by between a third
stop and a half stop which really shows up on slide film.
It may also be that the shutter is slower than the marked speed due to
age,
i.e. lack of lubrication or being out of adjustment, or exaguration (I
don't
think I spelled that correctly) by the manufacturer.

Many 35mm SLR's from Japan in the 1950's and 1960's 1/500th was really
1/300th,
and I believe many cheaper leaf shutters where similar.

I've measured my 1970's Sonnars to just shy of 1/500. (1/450 ish)

Good point. Interestingly, the Fujiblad leaf shutters only go up to 1/400.

If you have a CRT Television set you can photograph the screen and count
the
scan lines, but those are on their way out.

Science is always making such wonderful improvements on things. Sigh.

That's consumer technology. Science always leaves room for other
measurements. I've used a photocell and oscilloscope to verify leaf
shutters, for example.

RolandRB wrote:

The number of lines will surely give you the total open time of the
shutter - not its "exposure time" which is based on the "amount" of
light let in and not the total open time of the shutter. Part of the
total open time of the shutter will be the blades moving to the open
position and the blades moving back amd during those blade movement
times then only about half the light will be allowed through, at full
lens aperture, because, on average, the lens is half closed during
these blade movement periods. To give a simplified example then if the
blades took 2 ms to open and then immediately, when fully open, closed
again, also taking 2 ms, then the shutter is open for 4 ms but it only
supplied 2 ms worth of full light at full aperture and so the exposure
time is 1/500th sec. If the aperture of the lens is just a small part
in the middle then it will be allowing full light through, in this
case, for most of the 4 ms and so the effective exposure time is
1/250th sec. Hence, in this simplified example, if you were using an
f16 stop with a shutter speed of 1/500th sec then you would have to
stop down by a full stop to give you the correct exposure for that
aperture.

That's exactly it: shutter efficiency is the problem at fast speeds and
small apertures.
Not something you measure by simple trying to count how many scan lines you
can see (a giant leap in science and technology indeed!) through the lens.

Though too slow shutters, due to old grease, add to the problem.

And different models/makes may be different too.
So the best thing would be to do exposure tests, and try to determine the
required correction from exposed and processed film.

On 01-07-09 11:40, RolandRB wrote:

Underexposed slide film is better than overexposed so at f11 and
1/250th sec then half a stop underexposed might be a good guess.

I used to shoot slide film a little over exp for scanning, so bracketed
for nominal and over.

OTOH, my 80 f/2.8 is a bit sluggish so a nominal setting yielded
slightly underexposed Velvia:

http://photo.net/photodb/photo?photo_id=6046198&size=lg

.... which the Nikon 9000ED scanner had little trouble with though the
shadows are completely dead.

"Alan Browne" wrote in
message
...
On 01-07-09 08:40, David J. Littleboy wrote:
"Geoffrey S. wrote:
RolandRB wrote:
Thanks for that. I'm seeing this problem on my old
folders when I use
f11 at 1/250th sec. for sunny conditions and I'm a
fairly good judge
of shutter speed. The shots I take are overexposed by
between a third
stop and a half stop which really shows up on slide
film.
It may also be that the shutter is slower than the
marked speed due to
age,
i.e. lack of lubrication or being out of adjustment, or
exaguration (I
don't
think I spelled that correctly) by the manufacturer.

Many 35mm SLR's from Japan in the 1950's and 1960's
1/500th was really
1/300th,
and I believe many cheaper leaf shutters where similar.

I've measured my 1970's Sonnars to just shy of 1/500.
(1/450 ish)

Good point. Interestingly, the Fujiblad leaf shutters
only go up to 1/400.

If you have a CRT Television set you can photograph the
screen and count
the
scan lines, but those are on their way out.

Science is always making such wonderful improvements on
things. Sigh.

That's consumer technology. Science always leaves room
for other measurements. I've used a photocell and
oscilloscope to verify leaf shutters, for example.


This will give you the actual graph of the shutter
opening. The correction for efficiency depends partly on the
size (mass) of the shutter blades and on some other factors
but in Compur and similar shutters can be figured by
assuming the opening and closing times combine to about
1/1000 second. The marked shutter speeds are based on the
equivalent exposure at maximum aperture. Since the shutter
is not fully open for part of the time the effective
exposure at small stops will be greater than the marked
time.
Opening and closing times are constant in most shutters
to this error becomes greater as the shutter speed
increases. On small and medium Compur shutters, with marked
top speed of 1/500, the effective exposure time at smaller
stops is around 1/400th providing the shutter is clean and
working properly. This difference is of no consequence for
B&W or color negative but might be for reversal films.
Note that focal plane shutters also suffer from
efficiency errors but these go the other way, that is, the
effective shutter speed is higher (exposure less) than the
marked speeds. The correction depends on the amount of
diffraction at the edges of the slit and vary with the
distance from the slit to the film and the angle of the
light beam from the lens. Hence it varies with stop, focal
length, and distance of the lens from the shutter. The
difference caused by loss of efficiency can be significant
in large FP cameras like the Speed Graphic at the top speeds
but can also be significant in some 35mm cameras. For this
reason measuring the shutter speed using a simple shutter
tester requires some care. One often finds Speed Graphics
with the shutter tension adjustments cranked way up because
someone was trying to get a 1/1000th second reading with a
simple tester. Unless the light source is adjusted
correctly the top speed may read as low as 1/800th on the
tester. Similarly a measurement on a leaf shutter at its top
speed will read low even though the shutter is operating
correctly. Typical total open time (speed measured at the
center of the aperture) will be about 80% of the marked
speed at the top speed.


--
Richard Knoppow
Los Angeles, CA, USA