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Songyun
10-Dec-2009, 10:14
I got a Calumet shutter test, just testing a few shutters.
I found between 1 and 1/60 most reading are within 1/6 stop but when I hit 1/250, or 1/400 the reading is way off. Is that because of the shutter or the shutter tester?

Bob Salomon
10-Dec-2009, 10:23
A shutter is accurate if it is 30% of the marked speed. Except for a shutter like the Rollei Linear Motor electronic shutters most mecanical shutters: Compur, Copal, Prontor, Seiko, will be may be more then 30% at their fastest speed. This is true for these shutters regardless of the format used. In other words, a Hasselblad Prontor shuttered lens will be quite a bit off of a true 1/500.

Songyun
10-Dec-2009, 10:41
If I shoot @ 1/500 I may have very good chance to have under/over expose my slides, is that right?
BTW, I remember shooting 35mm format, there is a rule of thumb that if you hand held a camera the shutter speed should at least the inverse of the focal length of the lens. Does that hold for LF hand held cameras?

Bob Salomon
10-Dec-2009, 12:03
If I shoot @ 1/500 I may have very good chance to have under/over expose my slides, is that right?
BTW, I remember shooting 35mm format, there is a rule of thumb that if you hand held a camera the shutter speed should at least the inverse of the focal length of the lens. Does that hold for LF hand held cameras?

over. The shutter will not get to 1/500.

BetterSense
10-Dec-2009, 12:22
Keep in mind that most leaf shutters will measure significantly slower than their "effective" speed even when they are in adjustment. A 1/500 shutter can be expected to measure 1/400 or less when tested with a shutter tester--this does not mean it is 'running slow' because it takes a finite amount of time for the leafs to fully open and close. This is compensated for with a slighly longer open-time. With leaf shutters at high speeds, shutter speed is also a function of aperture.

Bosaiya
10-Dec-2009, 12:35
Kentucky windage.

Songyun
10-Dec-2009, 13:14
Keep in mind that most leaf shutters will measure significantly slower than their "effective" speed even when they are in adjustment. A 1/500 shutter can be expected to measure 1/400 or less when tested with a shutter tester--this does not mean it is 'running slow' because it takes a finite amount of time for the leafs to fully open and close. This is compensated for with a slighly longer open-time. With leaf shutters at high speeds, shutter speed is also a function of aperture.
in high speed situation
the smaller aperture, the slower shutter speed?

Drew Wiley
10-Dec-2009, 13:42
I've had very good luck with the Calumet tester, both for large format lenses and small
cameras. The readings reflect real world film testing. Every single Copal shutter I've
tested, new or old, was way off at the highest speeds. But I've had Compur shutters
which were spot on, even for analagous focal lengths, but otherwise hated because of
their higher vibration. I came to the conclusion that most of this is in fact due to the
respective shutter designs. Almost no one uses the higher speeds with a view camera
anyway, and if they do, a strobe is probably involved and will be the primary determinant of the exposure anyway. I carry a tally of all my shutters, and would
consider anything a third stop off to be of consequence with color tranny film. About
once a year I retest each lens I frequently use. They don't seem to vary much with
age.

Dan Fromm
10-Dec-2009, 14:56
in high speed situation
the smaller aperture, the slower shutter speed?No, its the other way 'round.

If you want to know whether a shutter will overexpose, shoot reversal film with it.

Alan Butcher
11-Dec-2009, 05:39
Are you using the Calumet instructions, especially important for the higher shutter speeds?

--
Alan

MIke Sherck
11-Dec-2009, 07:02
I don't recall ever using a shutter speed faster than 1/100 with a large format lens. How common is using a faster shutter speed, anyway?

Frank Petronio
11-Dec-2009, 07:11
If you shoot short depth of field stuff -- or are pretending you are cool with your Aero-Ektar -- you'll use the faster speeds outdoors in Sun, at least that is what I've heard, having not seen the Sun for a few months here in Rochester.

I usually just count 1/250 and 1/500 as half a stop slower. The worst shutters in this regard are the old American ones like the big Acmes, I doubt they ever got faster than 1/50 even if marked higher.

Ivan J. Eberle
11-Dec-2009, 07:25
It's probably not just an artifact of your Calumet tester; springs do weaken with time and top speeds may not have been all that honestly rated. Shutter propagation time can be observed on my shutter tester and my Copals tend to be slower than the speed rating on the fastest speeds. As others have indicated, +/- 30% is tolerable (1/3 stop).

It rarely matters how accurate speeds higher than 1/125 are with LF. These days few shoot press camera style with transparency films (fewer still are current fast ISO transparency films in sheet sizes that'd make it possible) where exposure is all that critical. Largely why high speeds persist since that press era is they function as an ambient light excluder when shooting sync'd flash. If you have a particular scientific technical reason to need a precise shutter at higher speeds, you'll perhaps be better served in a smaller format where quartz oscillator and microprocessor controlled shutter timing is commonplace.

If shooting landscape or location, for instance, what you might rather be more concerned with is the consistency between readings at the same speed, most of these type exposures being between 1/30s down to 8s (timed with a cable release and Mississippi One Mississippi Two is close enough beyond a couple of seconds). Probably worth testing at different temps. A gunky or damp shutter will more readily hang up in the cold.

Songyun
11-Dec-2009, 07:41
Thanks everyone, my intention is to shoot 45 hand held (say with a 150mm lens) I don't know how fast the shutter should be. Assume in 35mm format I should have speed faster than 1/150s, I don't know if that applies to LF or not.

Bosaiya
11-Dec-2009, 07:53
Thanks everyone, my intention is to shoot 45 hand held (say with a 150mm lens) I don't know how fast the shutter should be. Assume in 35mm format I should have speed faster than 1/150s, I don't know if that applies to LF or not.

That really depends on your expectations. The nice thing about LF is that you are not as likely to make the same type of enlargements you would with a mini-cam. So whereas 1/150 might be necassary to achieve the results you are after in 35mm you might find 1/60 perfectly adequate for 4x5.

I recently found myself shooting at 1/15, which is pretty low, I normally shoot handheld at 1/60 and don't experience an unappealing amount of blur. The subject matter lends itself to a certain amount of dynamic appeal so I'm not too worried, your needs may be completely different.

BetterSense
11-Dec-2009, 10:18
I can shoot my speed graphic at 1/30 or maybe even with no detectable handshake to 8x10 ish enlargements. Subject motion of course, is another problem. I just use my FP shutter if I need faster shutter speeds.

Songyun
11-Dec-2009, 10:44
so 1/60 is a good start point to hand held a 150mm lens, sounds good. At least all the shutters I have tested are good at 1/60.

BetterSense
11-Dec-2009, 11:42
It probably depends on how much coffee you drink, how heavy your camera is, and how lucky you are. 1/125 should be a piece of cake, though.

Mick Noordewier
11-Dec-2009, 11:50
Are you using the Calumet instructions, especially important for the higher shutter speeds?

--
Alan

Alan,
I've got a Calumet shutter tester, but no instructions. What are the instructions for higher shutter speeds?
-Mick

Jim Graves
11-Dec-2009, 11:57
One other factor for shutter speed is usage. I've found that when a lens has not been used for awhile it is considerably slower in the first few firings and continues to speed up through 10 firings or so. This is easy to observe with a timer.

When I go out to shoot now I try to remember to fire the shutters at least 5-10 times before actually shooting my first picture. I tend to use fairly old shutters though, so the effect might be exaggerated with my equipment.

Len Middleton
11-Dec-2009, 12:01
Songyun,

As well as not enlarging the image to the same degree (as mentioned above), if you have been used to using SLR type cameras in smaller formats, you also had watch your shutter speeds at the slow end because of "mirror slap" from the SLR mirror. Tripping a Hasselblad in a large quiet church, almost seems like firing off a shotgun in there...

Unless you are using something well out of the mainstream in LF, you will not have to deal with that vibration inducing SLR mirror mechanism.

Hope that provides with some additional perspective,

Len

Bob Salomon
11-Dec-2009, 13:17
"you also had watch your shutter speeds at the slow end because of "mirror slap" from the SLR mirror"

But the mirror doesn't come down till after the exposure so that won't cause vibration and some cameras dampened the mirror on the way up to eliminate slap at that end.

Len Middleton
11-Dec-2009, 13:53
Bob,

I cannot say how big an impact that might be in real life.

I do know that I much prefer using my Leica M2 rangefinder in low light at slow shutter speeds rather than my Nikon F2, or my brother's 'Blad, and not just related to the low light focusing issue.

I have no empirical data, so really that is just a "soft and fuzzy" feeling rather than fact based.

If you have some facts or other information, I would be pleased to see them, so that I can have a better informed opinion.

Sincerely,

Len

Bob Salomon
11-Dec-2009, 13:59
Bob,

I cannot say how big an impact that might be in real life.

I do know that I much prefer using my Leica M2 rangefinder in low light at slow shutter speeds rather than my Nikon F2, or my brother's 'Blad, and not just related to the low light focusing issue.

I have no empirical data, so really that is just a "soft and fuzzy" feeling rather than fact based.

If you have some facts or other information, I would be pleased to see them, so that I can have a better informed opinion.

Sincerely,

Len
Take a look at how Rollei dampened the mirrors in the SL66 series, SLX and 6XXX series cameras. They had pneumatic dampeners in the mechanism.

Len Middleton
11-Dec-2009, 17:04
Bob,

With your extremely broad knowledge you have me at a significant disadvantage.

The only Rolleis I have seen close up, are a little compact 35mm (with the collapsible 40mm Sonnar) I had and my old TLR Rolleiflex, neither of course having a moving mirror. And no doubt as you indicate one could install mechanisms at extra cost to dampen the vibration, if cost was not the primary customer selection criteria.

On the other hand, I suspect that many of us used less sophisticated equipment that did not provide that benefit. And in fact to eliminate it as a potential issue, Nikon (F & F2) and others provided a mirror lock up.

In the later designed 35mm SLR's did they find other less expensive solutions (e.g. foam at top of mirror box) to dampen vibrations, as I noticed that mirror lock up was much less common in later year?

Thank you for your insights,

Len

BetterSense
11-Dec-2009, 19:34
I think it was much less common later because it was never that big of a deal to begin with. Or maybe it was a bandaid for early SLRs that weren't engineered such as to 'gate' the shutter slap out of the exposure window. Such slap is only an exposure problem if it has active modes in the exposure window. I know my OM2n's mirror has no effect on the exposure..I have fired it hanging from the strap at speeds of down to 1/4 and the shots are sharp. Then again, it's a very well-engineered camera.

neil poulsen
11-Dec-2009, 23:15
I've never been too concerned as to whether or not a shutter was accurate. My biggest concern is whether a given speed is consistent. Knowing that it's consistent, I can f-stop correct for any lack of shutter speed accuracy.

With that said, it's rare that I find a Copal or Compur shutter has an inconsistent shutter speeds. For example, I find on many shutter speeds for these shutters that the range (largest to smallest) is within 1/20th of a stop. Usually, the range is within 1/10th of a stop.

On faster shutter speeds, one can see it go both ways. A fastest speed can be fast or slow, inconsistent or consistent. The important thing is to test speeds and know how they behave.

Ben Syverson
12-Dec-2009, 00:21
On faster shutter speeds, one can see it go both ways. A fastest speed can be fast or slow, inconsistent or consistent. The important thing is to test speeds and know how they behave.
I'm building an LF shot logger with built-in exposure calculator for the iPhone, and one of my big debates was whether to include a per-speed compensation for each lens. In other words, you'd be able to enter into the app that your 1/250 speed on one lens was a stop slow.

I decided against it because it was ultimately too easy to abuse. Unless someone has done very methodical testing, it's hard to know exactly how far off each speed might be. If someone got it wrong, they would wind up blaming me! And ultimately, any user who knew absolutely that a certain shutters' speed was off by a certain amount would be advanced enough to take that into account manually.

domaz
15-Dec-2009, 10:13
How does temperature affect shutter speed in mechanical shutters? I figured it would slow them down- but I just developed a sheet I took in very cold weather that was underexposed even though I was pretty sure of the exposure (added 2 stops because of the snow etc..). Can cold actually speed up the shutter?

Jorrit
16-Dec-2009, 07:52
http://www.beeldvangst.nl/extimg/copal0-speed.png

On my first 90mm f/5.6 in Copal 0, I've found the 1/500 to be around 1/270.
It doesn't bother me (yet), and I did find some interesting shutter "recoil" at high speeds. I'm wondering if that could be used to some benefit. In practice it would likely be like a weaker second exposure, with a delay.

I also found the working aperture to be of no influence on the speed. 1/270 stays 1/270. I've tested from wide open to closed. The pic above is a cropped screen capture from Audacity using a photo transistor. So from the graph it looks like the aperture only has influence on the width of the first pulse.

Jack Dahlgren
16-Dec-2009, 10:32
http://www.beeldvangst.nl/extimg/copal0-speed.png

On my first 90mm f/5.6 in Copal 0, I've found the 1/500 to be around 1/270.
It doesn't bother me (yet), and I did find some interesting shutter "recoil" at high speeds. I'm wondering if that could be used to some benefit. In practice it would likely be like a weaker second exposure, with a delay.

I also found the working aperture to be of no influence on the speed. 1/270 stays 1/270. I've tested from wide open to closed. The pic above is a cropped screen capture from Audacity using a photo transistor. So from the graph it looks like the aperture only has influence on the width of the first pulse.


I'm having trouble understanding what those pulses (especially the ones to the right) mean. What is it you are measuring and how? Voltage level through a photo-transistor? What is your setup? I've been thinking of building my own shutter tester and am curious about what you have put together.

Jorrit
16-Dec-2009, 11:51
I'm having trouble understanding what those pulses (especially the ones to the right) mean. What is it you are measuring and how? Voltage level through a photo-transistor? What is your setup? I've been thinking of building my own shutter tester and am curious about what you have put together.

Ah sorry, yes the y-axis is voltage (in unknown scale), and x-axis is time.
It's this (PDF) (http://static.photo.net/attachments/bboard/004/0044cW-10288684.pdf) circuit belonging to this (http://photo.net/large-format-photography-forum/0044cW) thread. You basically hook the transistor to the mic input on your soundcard, cock shutter, shine a bright light on the lens, release, and use a sound capture prog to do the measuring.
To be honest, I didn't build it myself, I was lazy and ordered one on Ebay after searching for "shutter speed tester"

The phototransistor seems only to register changes in input signal. So a continuous light will yield a flat line, as will continious absence of light.
First spike is shutter open, second negative spike is shutter close. The third and fourth spikes can only be another open-close sequence. I assume it's a sort of recoil from the "main" release. It only happens at high shutter speeds. When checking a slow 1 second speed, there are no after-spikes, so to speak.

venchka
16-Dec-2009, 12:04
The secodary spikes vary with aperture. That's curious.

Jorrit
16-Dec-2009, 12:36
The secodary spikes vary with aperture. That's curious.

http://www.beeldvangst.nl/extimg/copal0-f5_6-3retries.png

Above is a new capture: tried 3 times @1/500, with the same aperture @wideopen. So by keeping aperture the same, the result is also variable. In both width and amplitude.
So far for a reliable hidden Copal multi exposure feature.

GPS
16-Dec-2009, 12:47
...
First spike is shutter open, second negative spike is shutter close. The third and fourth spikes can only be another open-close sequence. I assume it's a sort of recoil from the "main" release. It only happens at high shutter speeds. When checking a slow 1 second speed, there are no after-spikes, so to speak.

Or rather the other spikes have their source somewhere in the electronic circuit itself...

Jorrit
16-Dec-2009, 13:45
You're right to succesfully attack my rather too strong proposition that the other spikes "can only be" such-and-so. I got carried away by the possibility of a secret undocumented 1/500 = multi-exposure feature :)

GPS
16-Dec-2009, 14:32
Didn't want to attack, just knowing that mechanically it would be next to impossible to have such a recoil... :-)

BetterSense
16-Dec-2009, 15:36
I also found the working aperture to be of no influence on the speed. 1/270 stays 1/270.

I think you are not thinking about this fully. Your photodiode probably turns on as soon as the shutter begins to open. The fact is that it takes a finite amount of time for the aperture blades to FULLY open. During the time in which they are in the process of opening, the effective aperture of the system is not the full marked aperture, because the shutter blades are blocking the aperture partially. At small apertures, this effect will be negligible, but at full apertures the shutter blades can block the aperture a significant amount of the time during their opening-and-closing. Thus even though the "shutter speed" may not change with aperture, the amount of exposure the film gets WILL change with different apertures, and thus sometimes high shutter speeds are deliberately slow, in order to give the expected exposure at all apertures, with the penalty of slight overexposure at smaller ones.

Jorrit
17-Dec-2009, 05:00
Your photodiode probably turns on as soon as the shutter begins to open. The fact is that it takes a finite amount of time for the aperture blades to FULLY open.
[...] thus sometimes high shutter speeds are deliberately slow, in order to give the expected exposure at all apertures, with the penalty of slight overexposure at smaller ones.

I agree with you that the shutter needs some time to fully open.
Let's look closer at the following graph. Copal 0 shutter set at 1/500, aperture at f/5.6.

http://www.beeldvangst.nl/extimg/copal0-zoom-500.png

My proposition is that the first pulse (measuring some 1/3000s) is the traveltime of the shutter going from closed to open. The phototransistor seems to only register CHANGES in light. So while the shutter is traveling, the phototransistor registers changes in light, and at the end of the 1/3000s doesn't so much anymore. The line is more or less flat. Meaning no changes in light = open shutter.
Now the duration of this more-or-less flat line is some 1/290s.
After that, the transistor starts to register changes again butin the oposite direction: the shutter is closing.

So even if we almost entirely exclude the 2 shutter travel times, the shutter stays open a lot longer than the advertised 1/500, leading to over-exposure at any aperture value.

The weakness I see in my own proposition is the lack of information about the y-axis. Are the values on a lineair scale? A log scale? Or perhaps an exp scale? That matters, because I've placed the white line at the end of the first pulse at an arbitrary position.
So we don't know for sure whether the end of 1/3000s is really a more-or-less open shutter. I asume it to be around f/5.6. But i can't tell. It may very well be still f/16 or something.

The second set of open-close is another story that intrigues me.

Struan Gray
17-Dec-2009, 08:28
Jorrit, I don't know where your second pulse is coming from, but like GPS I suspect it is electronic in origin.

There are two things you need to realise to understand your curves. First, that your sound card is not DC-coupled. In fact, it includes a circuit designed precisely to reject DC. This is because DC is bad - sometimes very bad - for audio amplifiers and loudspeakers/headphones. This is why you only see changes: your detection setup can only measure changes.

Second, the response time of the phototransistor is limited, as is the input circuitry of your sound card. In an ideal world you would be able to take your traces and integrate them to find the instantaneous light level on the detector, but the limited bandwidth of the detection system makes that impossible (even if you added some ND to avoid the saturation you now have).

As others have said, a leaf shutter takes a finite time to open and close. A plot of the light intensity versus time will not be a perfect square pulse, but will have sloped lines at the beginning and end. At full aperture, the leaves only just reach the edge of the aperture stop in time to turn around and start closing again: the pulse shape becomes, roughly, a triangle. The total exposure is the integrated light intensity over the whole pulse. Since the area of a triangle is 0.5*base*height, it follows that the start and end times for a triangular pulse need to be twice as far apart as they would be for the same exposure from a square pulse from some ideal shutter that could open and close instantaneously. It therefore makes perfect sense that your measurement - which only accurately measures the start and stop point of the shutter sequence - gives a shutter speed that appears to be a factor of two out.

At small stops the shutter blades don't move in any different way, but they clear the aperture stop much faster. The same slope is there on the ends of the pulse, but now the blades spend most of their time moving outside the clear hole of the aperture, so the slopes are just a small correction to the total exposure. If you had a true plot of light intensity vs time, a small stop just cuts most of the top off the triangle seen at wide apertures to form a flat-topped pulse with sloped ends. There is very little difference between that and an ideal pulse, and the exposure is longer than it 'should' be - again, by a factor of two or so.

The marked shutter speed can only be correct for one stop. Convention seems to be to make the marked speed that which gives the same exposure at full aperture as a perfect shutter that can open and close instantaneously, even if that speed does not correspond to any actual timing within the shutter. How this convention was arrived at I don't know, but you can find discussions of the effect going back to the C19th.

In your original traces, all the information about these effects is hidden in the flat-topped saturation of your measurement system. If you add some ND, or turn down the brightness of your test light, you should see changes in the shape of the curves as you change the aperture. Remember though that the shape may be largely determined by the bandwidth of your phototransistor/sound card combination, so the changes may still be subtle. The same phototransistor and a real oscilloscope should show the effect clearly though.

If you want to google, these effects come under discussions of shutter "efficiency".

Ivan J. Eberle
17-Dec-2009, 09:11
With the sound-card testers using the graphing in a sound editor like Audacity, it's rather simple then to average the mean opening and closing times, e.g. and measure only that range. Easiest way to do this I've found (assuming opening and closing propagation times are the same at either end) is to measure from the first instance of opening to the first instance of closing.

For critical use, it's probably best to reduce the aperture (or the distance of the light source) so that the signal from diode doesn't swamp the input ADC, and remains within range of the graph.

I bought an $8 phototransistor for my assembling own sound-card tester because it has a much faster on-off response than the cheapest ones. Don't have the spec handy at the moment but IIRC it's switching time is on the order of a few microseconds. (The resolution of the A/D recording sampling chosen in Audacity will be more of a limiting factor here.)

Jorrit
17-Dec-2009, 09:24
Struan, thanks for your pointers and insight.

Ivan J. Eberle
17-Dec-2009, 09:41
While I do like the ring of Struan's elegant answer much better than my own oversimplification, I don't yet know what the practical application of this is for quick and dirty shutter testing.

My limited understanding (admittedly imperfect) is that the taking aperture won't matter as much as it might have back in the 19th Century, because leaf shutters are now effectively placed at the nodal point of the lens-- with the light bending around the iris blades there to form a full and not truncated image, so that we're only changing intensity, not significantly truncating the image forming area described by the opening and closing of the shutter as might happen with a Packard type.

My own testing with the graphing seems to confirm this at different apertures, at least that the timing is unaffected and that the mean opening and closing times are unaffected.

But if I'm mistaken on this point I eagerly await someone to educate me further.

Jorrit
17-Dec-2009, 10:32
Ivan, I didn't see your post before I posted my reply. Sorry about that.

From a quick test just now, things seem to improve after desaturating or unswamping the signal. From my quick unscientific testing it seems that the afterspikes or shutter recoil as I so vividly imagined, seem to be an ill after effect of the swamping.
The after-spikes disappear with an unclipped signal.

I just realised that my test light will be perceiced by the phototransistor as a small point source, because I'm using a rather wide angled lens.
Perhaps we can get a better impression of the exposure when the entire field of view of the lens is covered with - say a softbox. Only then something can be said about it's influence on exposure. I'm not sure my modeling lights are strong enough though, but perhaps a quick sheet of paper will do just fine instead.

bobwysiwyg
17-Dec-2009, 11:30
I've been following this thread with interest. I use the photoelec and Audacity to check my shutters, including the focal plane shutters in my Nikons. It was interesting that of all my Nikons, the only one that exhibited the 'after spike' was my older Nikon F. I never could explain the cause to myself, yet it didn't seem to have any relationship to the true accuracy (or exhibited exposure) of the shutter.

rdenney
17-Dec-2009, 15:37
I just realised that my test light will be perceiced by the phototransistor as a small point source, because I'm using a rather wide angled lens.
Perhaps we can get a better impression of the exposure when the entire field of view of the lens is covered with - say a softbox. Only then something can be said about it's influence on exposure. I'm not sure my modeling lights are strong enough though, but perhaps a quick sheet of paper will do just fine instead.

The photo-transistor system isn't designed to measure intensity. It only measures switching, and any intensity effect there might be is filtered out by the DC filter on the audio input. That was Struan's main point. Thus, you can't measure light intensity.

To do a proper test of shutter efficiency, one needs to test the total light coming through the lens as the shutter opens and closes, integrating the total exposure over that time. There are, I expect, shop-grade shutter testers that do that (in addition to measuring opening and closing curtain speeds for focal-plane shutters). Given that few of us use the fastest speeds or the widest apertures, it's probably something for technicians to test and not required for those of us who just want to know if our shutters are way off.

I have tested a range of focal-plane shutters using a Calumet tester, and by positioning the sensor I was able to determine if the shutter open time varied across the frame. These were largish shutters and doing so was not that easy. It required using a DC light source that didn't pulse (as AC light sources do) and attenuating it sufficiently to just exceed the trigger threshold of the tester. But the Calumet tester uses the same technique as the transistor tester, with the output coupled to a micro-second timer that is started on the positive pulse and stopped on the negative pulse. Those testers cannot measure leaf shutter efficiency very easily, but they sell for a lot because they are useful nonetheless.

Rick "who rarely uses speeds faster 1/50 or 1/60 where it just doesn't matter" Denney

Jack Dahlgren
17-Dec-2009, 15:49
While I do like the ring of Struan's elegant answer much better than my own oversimplification, I don't yet know what the practical application of this is for quick and dirty shutter testing.

My limited understanding (admittedly imperfect) is that the taking aperture won't matter as much as it might have back in the 19th Century, because leaf shutters are now effectively placed at the nodal point of the lens-- with the light bending around the iris blades there to form a full and not truncated image, so that we're only changing intensity...

That is correct. With a wide aperture you are changing intensity over the duration of the shutter opening. This directly determines how much light is transmitted while the shutter is open and thus the effective exposure of the film.

This is why Struan mentioned integrating your measurements - in essence summing the light intensity over the time that the shutter is open.

Imagining the shutter as an iris might make it easier. As it opens over a period of time, the intensity goes from 0 to the maximum. Then depending on the shutter speed it lingers open for a time (longer for long shutter speeds, maybe almost nothing at short shutter speeds). If you plot the intensity over time it will be a mound of some kind with a pointy top for the shortest shutter speed and a flatter one for long shutter speeds. In the short shutter speed case, the total amount of light passing through the lens is less than it would be if the sides of the mound were not sloped. Unfortunately, a shutter is mechanical so it takes a material amount of time to travel the distance.

Now one way to shorten the time it takes to go from fully closed to fully open is to shorten the distance that the shutter has to travel. One way of doing this is using a smaller aperture. As soon as the shutter blades are clear of the aperture, the shutter is effectively as open as it is going to get, even if it has not physically reached its limit. The same goes for closing.

From looking at your charts though, I'm not really certain it is a big problem. The amount of time that the intensity is varying (your first pulse) appears to be pretty short in comparison to the duration of the exposure. I don't think you can trust it at all for measuring that opening time, but it seems like it is close enough to rule out that as being something you have to compensate for in your exposures. It is easy enough to be a half or a whole stop off in an exposure due to changing light, mis-measurement, reciprocity or other factors. I think it is interesting but not really significant for normal use.

Since it is a slope, you could pretty simply average it out by measuring from the peak of the pulse to the peak of the other pulse instead of from the start of the first to the finish of the last. But as pointed out, there are other unknown things going on in the devices you are using to measure it, so it may still have some error.

GPS
17-Dec-2009, 17:11
...

The amount of time that the intensity is varying (your first pulse) appears to be pretty short in comparison to the duration of the exposure. I don't think you can trust it at all for measuring that opening time, but it seems like it is close enough to rule out that as being something you have to compensate for in your exposures.

...

I think it's correct to say so.
One way to see it photographically (with no electronics involved) is to expose a piece of film put at much a shorter distance behind the lens than is its focal length (or put the film just a few mm of the lens exit pupil). In this way the film will get exposed just on a illumination circle not too wider than the lens rear diameter. Make a short (the shortest) exposure with the widest aperture and develop the film. You can see then if the center of the illuminated circle has much a different exposure than the edge of it. The difference would be given by the time the shutter blades were taken to open completely and to close completely. All the exposure in between those 2 actions would have no influence on the illumination difference between the edge and the center of the film (provided you didn't over saturated the film with the light).
I think the difference must be negligible.

Struan Gray
18-Dec-2009, 02:00
The amount of time that the intensity is varying (your first pulse) appears to be pretty short in comparison to the duration of the exposure.

The variation time is almost certainly set by the DC-rejection of the sound card. I doubt it will have much to do with the shutter blade travel time. Traditionally, audio circuits are handwavingly specced at 15 Hz - 20 kHz, but with an unspecified computer sound card the position and sharpness of the lower frequency cutoff is an open question.

I have tested some shutters with a lab-grade photoreceiver (from New Focus (http://www.newfocus.com/products/?navId=3&theView=modelGroupDetail&productLineId=3&productGroupId=207&modelGroupId=1053), if anyone cares). These are designed to give an analogue signal proportional to the average intensity falling on them, and have a known, and more than adequate, measurement bandwidth. I didn't have a storage scope with a data link, so I can't show you any traces here, but they were pretty much as expected.

The measurement is largely insensitive to exactly what you have as a source object, or the focus distance, but convention says that apertures are defined for on-axis light from infinity. You do need an extended source if you're trying to do radiometry, but for measuring the light intensity as the shutter opens and closes even a point source will do. Similarly, the focal length is not a big issue.

The only exceptions are if the light is bright enough to locally saturate the detector, or if wild bokeh varies the shape of the light spot so that parts of it miss a small area detector. The former requires a non eye-safe laser, which I hope nobody here is daft enough to try and use for this application; the latter is I suppose possible, but anyone experimenting with such lenses is likely to welcome any further contributions to out-of-focus-wierdness from the shutter inefficiency.