Why do leaf-shutters 'fail' at high speed?

[Max Power]

I have been messing about with a Canonet for a while now, and have noticed a tendency towards over-expose at 1/500 and f16. What escaped me was that it was only in circumstances demanding these two parameters that I was getting over-exposure. Under all other situations demanding 1/500, the exposure was perfect (thus 1/500 is not slow).

This morning I was re-reading my copy of the 'Kodak Professional Photo Guide' and I noticed a blurb about compensating for leaf-shutter geometry at high speeds. The blurb stated that at small apertures and high shutter speeds, one must always compensate by up to one full stop. Apparently, this is an inherent weakness of leaf shutters.

This strikes me as being a bit strange...Can someone explain to me why this happens? Why is this not a problem with focal-plane shutters?

Thanks,
Kent

 

[Konical]

Good Morning, Kent,

As I understand it, there are two major factors involved.

First, the shutter blades must open completely, so that they don't block light coming through the lens at the wide apertures. that means that the center area has light coming through longer than the edge area. The small iris opening at f16 or so tends to mean a little overexposure at higher speeds. At lower speeds, the opening and closing times are a relatively small percentage of the total open shutter time.

Second, a lot has to happen in a very short time with a short exposure: open shutter blades; come to a dead stop, close shutter blades. That's a lot to ask of a spring-driven device made with thin, delicate parts. No wonder many leaf shutters are a bit slow at higher speeds, especially as they age and any lubricant becomes either more ineffective or actually begins to impede the moving parts. In contrast, the focal plane shutter operates somewhat like a scanner at the high speeds; the travel speed doesn't change much, but the slit between the curtains is reduced. Even so, focal plane shutters are also a bit slow at high speeds.

If that doesn't all make sense, be patient. Someone else will probably explain it more clearly.

Konical

 

[stephen]

Leaf shutters work by a series of blades separating to create a circular hole, that increases in size as the shutter opens - i.e. the opening increases to the maximum diameter during the first part of the exposure, remains open for most of the exposure time, and then closes down again.

If you are using a wide aperture, the time during which the shutter isn't fully open doesn't matter too much, as it is equivalent to exposure using a small f stop. When the lens is closed right down, the effect is that the shutter is fully open for much longer, simply because the hole allowes the full diaphragm opening to be uncovered.


Focal plane shutters are variable width slits in a roller blind, and expose the frame piecemeal - hence the distortion that can occur with moving subjects. The matter is explained better in various books, usually under "shutter efficiency".

 

[rbarker]

Seeing stars

It's a function of the geometry of the shutter blades. As a leaf shutter starts to open, it forms something of a star shape, and then continues to open to the full circle, then closes again, reforming the star shape. The marked shutter speed can be thought of as the average time for the whole process. At small apertures, the effective shutter speed is longer, as only the center part is "seen" by the aperture.

Focal plane shutters are also marked with the effective shutter speed, based on the average time the shutter is either fully open (the max sync speed), or has a faster effective shutter speed as the slit passes across the film gate.

 

[Max Power]

It makes perfect sense!!!

You lot really are incredible!!!
I was hoping to get at least one answer that I could understand. I got three!

Thanks very much, I understand now, and what is more, I can now make practical use of this knowledge.

Cheers!
Kent

 

[Sjixxxy]

I'm just glad that it wasn't just in my head that when I used any of my canonets in conditions that need 1/500th & f/16, that the negatives seemed to be a bit dense.

 

[Max Power]

So why leaf shutters, then?

Please permit me a follow-on question, then

Why bother with leaf shutters then? Is it because 75 years ago, they were the only way to go? Is it because you can incorporate shutter and aperture elements in one small package?
Why are they still used (there must be some advantage, no?) If focal-plane shutters can move at faster speeds, why not simply use them?

Thanks,
Kent

 

[Andy K]

In the short time I've been using a non-compact camera with a leaf shutter (Voigtlander Vito CLR, just over a week), the biggest advantage I've noticed is it's very, very quiet.

 

[Ed Sukach]

Why bother with leaf shutters then? Is it because 75 years ago, they were the only way to go? Is it because you can incorporate shutter and aperture elements in one small package?
Why are they still used (there must be some advantage, no?) If focal-plane shutters can move at faster speeds, why not simply use them?

A focal plane shutter exposes a given point on the film for a shorter time. In operation, when the shutter is activated, the "first" curtain starts to move across the film opening. At a given time, the "second" curtain follows. That is not necessarily "faster". As the "slit" (actually the area separating the first and second curtains ) moves across the film, of necessity, there must be some distortion of the form of a fast moving subject. There is a specific time when the second curtain starts its movement only when the first curtain has uncovered the entire frame ... that is called the "mechanical" shutter speed, and it is the only speed where the light from a very fast "electronic" flash will reach the entire frame.

With a leaf shutter, electronic flash is suitable - synchronized - at any speed.

 

[Konical]

Good Evening, Kent,

As Andy says, most leaf shutters are quieter than focal plane shutters, although much of the noise with the latter is often the up and down mirror action, especially with medium format cameras. One notable exception is the shutter on my Fuji GW67 rangefinder camera which is, for some strange reason, much louder than one would expect.

The second major advantage to a leaf shutter is that it will X-synch at any setting.

Konical

 

[Jim Chinn]

The other advantage to a leaf shutter is that it is usually a seperate unit from the camera and in the case of LF lenses seperate from the lens itself. This allows it to be used with a variety of lenses and camera bodies. The focal plane shutter is part of the camera body. When the shutter craps out, the body is junk unless the shutter can be repaired.

Does anyone know what is the typical number of cycles for a leaf shutter such as a copal? If I remember correctly Nikon F-5s had an advertised cycle life of 100,000. That may be high. Anyone know off hand how many times does a typical focal plane shutter cycle before failure? (cycle meaning an opening and closing of the shutter)

 

[rbarker]

Leaf X-sync

As a follow-on to Konical's point about leaf shutters doing X sync at all speeds, the advantage may not be immediately obvious. The benefit comes through when trying to balance fill flash with daylight, where the shutter speed is used to control the ambient contribution to the total exposure. Some MF focal plane shutters, for example, sync at 1/60, which is very limiting.

 

[Dean Williams]

Does anyone know what is the typical number of cycles for a leaf shutter such as a copal? If I remember correctly Nikon F-5s had an advertised cycle life of 100,000. That may be high.

I would think that 100,000 cycles would actually be low for a high end 35mm SLR. Years ago I had an FT3 and remember the average shutter life as being 80,000, and this was Nikon's low end camera at the time. Somewhere, maybe here on APUG is a post about a guy running his D70 through 119,000 cycles (in three months!) before it gave out. It is based on the N80, definitely a middle of the road model.

 

[Woolliscroft]

Why bother with leaf shutters then? Is it because 75 years ago, they were the only way to go?

There were plenty of focal plane shutters 75 years ago. Another plus with leaf shutters is that they produce less shock than FP, so they can be easier to hand hold at slow speeds and can even produce slightly sharper results on a light tripod.

David.

 

[dr bob]

There were plenty of focal plane shutters 75 years ago. Another plus with leaf shutters is that they produce less shock than FP, so they can be easier to hand hold at slow speeds and can even produce slightly sharper results on a light tripod.

David.

I have seen this disputed many times over the Internet for some time. It is very true. After early years of b&w work I succumbed to color print w/lab processing while raising a family and traveling professionally. In circa 1980, I went through a bit of crisis in my photography. It just wasn’t up to my pre-collegiate and premarital 35mm transparency work.

Careful examination and comparison of the products of the two periods revealed that the later negative images were just not as clear as the previous. The early work was accomplished with a Zeiss leaf shutter camera and the latter with a Zeiss focal plane SLR. I began to wonder? Did all that baking and shaking going on in the SLR affect the image?

To test this theory, I set up a tripod to make a photograph of a local church using good ol’ Kodak Kodachrom 64 (I recollect). The cameras were identically set and tripped with a cable release. The results confirmed: leaf shutters make clearer pictures even when mirror-lockup is employed. The slides are still here somewhere but I really don’t want the trouble of digging through a couple of thousand to find them – no filing system then (and not much now).

I got better and now do only b&w (again) and little 35mm work.

 

[Helen B]

Ed wrote:
'There is a specific time when the second curtain starts its movement only when the first curtain has uncovered the entire frame ... that is called the "mechanical" shutter speed...'

Well, I've learned something there. I've never heard that term used for that meaning. Is it common? Not knowing the correct terminology I'd just have called it the 'synch speed', and I'd have used 'mechanical speed' for a speed that doesn't need a battery. They may, of course, be designed to be the same speed.

Best,
Helen

 

[wiseowl]

I think that being pedantic "Sync speed" is a more accurate term than "mechanical speed", after all most if not all of the earlier focal plane shutters were fully mechanical. I would go for "maximum synch speed", as you can of course synchronise at lower speeds.

Just MHO

I guess it's worth mentioning that some modern flash/camera pairings give sync at all speeds with focal plane shutters using something called FP mode. I guess this works by strobing the flash, but I've no real tech info on it.

Martin

 

[stephen]

I guess it's worth mentioning that some modern flash/camera pairings give sync at all speeds with focal plane shutters using something called FP mode. I guess this works by strobing the flash, but I've no real tech info on it.

Martin

And if we go back to the days of flashbulbs, there were special FP bulbs which gave a longer peak output to enavle synch with FP shutters. My old Exakta has X, M and FP synch...

 

[rbarker]

I guess it's worth mentioning that some modern flash/camera pairings give sync at all speeds with focal plane shutters using something called FP mode. I guess this works by strobing the flash, but I've no real tech info on it.

Yes. Metz has a flash that syncs to 1/1000 for the Leica M7, and strobing is how they do it. (Earlier Leica Ms have a sync speed of 1/50, which is terribly limiting.) They call it "high speed syncronization".

 

[Ed Sukach]

Yes. Metz has a flash that syncs to 1/1000 for the Leica M7, and strobing is how they do it. (Earlier Leica Ms have a sync speed of 1/50, which is terribly limiting.) They call it "high speed syncronization".

Olympus was, as far as I know, the first to introduce a "long duration" electronic flash. It fires a burst of individual flashes for a period of 1/60th of a second - the time it takes for both curtains to move across the film frame. In the OM-4T, that would work at a shutter "slit-speed" of 1/2000th second. Unfortunately all that "stobing" takes a lot of power, so the guide number of each individual burst is really low.

The "Synchronization" speed, where the film is totally open to the image, is also the speed where the shutter defaults when the batteries fail or are removed ... without electricity it is "mechanical" ... in the case of my Olympus OM-4, 1/60th second.

 

[Max Power]

There were plenty of focal plane shutters 75 years ago. Another plus with leaf shutters is that they produce less shock than FP, so they can be easier to hand hold at slow speeds and can even produce slightly sharper results on a light tripod.

David.

Actually, this makes sense...In physical terms, the movement of a leaf-shutter exerts an equal force over 360 degrees on a flat plane whereas a focal-plane shutter does a 180 degree movement on that flat plane; thus there must be some transfer of movement in the opposite direction.

...And so his education in all things photographic continues!!!

Thanks all,

Kent

 

[Charles Webb]

X sync

X sync means simply that the flash contacts close and trip the strobe when the shutter reaches it's maximum opening. F sync. works a bit differently.
The flash bulb does not excite or reach it's point of maximum illumination instantly, it must begin receiving current 20 milli seconds before the shutter
reaches it's maximum opening to be able to deliver the greatest amount of light through the wide open shutter. There is no delay in X sync. when the
shutter reaches its widest opening and before it begins to close the strobe dumps the capacitor stored energy to the tube.

Have a good day.
C Webb

 

[Roger Krueger]

Some more points for (and against) leaf shutters:

They're cheaper than focal plane shutters. Fixed lens cameras almost always have leaf shutters. But with interchangable lenses, it's cheaper to build one shutter, in the body, than one in every lens. That's why there are so few interchangable lens leaf shutters in 35mm, and those there are almost all have the leaf shutter and rear elements fixed to the body, only the front elements interchange, limiting lens design freedom immensely. It's also a pain to build the mechanical linkage allowing the body to cock the lens.

Bigger than 35mm the expense, physical shock and sync limitations of focal plane shutters are much magnified, so there are many interchangable-lens leaf-shutter systems.

They require much less force/time to cock. Leaf shutters have for decades been used in MF SLRs where the shutter is open for viewing, closes when the button is pressed, reopened for exposure after the mirror is up, closed after exposure, and then opened again for viewing after the mirror is back down. FP shutters don't need to do this on an analog camera because they're behind the mirror, which is good, because they can't be re-cocked quickly enough.

The problem arises on digital cameras where one wants continuous viewing on the LCD monitor. None of the FP-shuttered digitals (all the interchangable lens ones) can use the LCD as a viewfinder--thus, ironically, this useful feature is found on even the cheapest digicams, but it's missing on the expensive ones.

The cocking force issue also led to a special breed of leaf shutter: the Press shutter. These shutters are cocked just before use by the force of triggering them.

Against them, it seems they are much more prone to needing a CLA once or twice during their life; FP shutters are more likely to work fine right up to their final demise.

 

[zenrhino]

Why bother with leaf shutters then?

For me it's that they're super quiet. My canonet is my "invisible man" camera for street shooting. No mirror slapping up and down to be heard. Plus since there is no mirror, there's less vibration and that lets you shoot handheld at slower speeds. I routinely shoot my canonet at 1/15th second and get nice sharp shots.

 

[David H. Bebbington]

Another point in favor of leaf shutters (I believe) is that the motion of all the shutter components is totally concentric, thus (at least theoretically) any vibration caused by shutter leaf motion is self-cancelling, making the minimum potential vibration level zero. If you do get a bad vibration level with a leaf shutter, in my experience it's down to a stiff cable release or release lever.

Merry Christmas to all,

David