What are the physical limits that determine a camera's flash sync speed?

Question

We know that Flash Synchronization Time represents the shortest time we can use with active flash in order to avoid the formation of a black stripe in our photos. So, it is very good if it is quite brief.

For instance, in my Zenit 122 camera, it is 1/30s and it is very difficult to take free-hand pictures that are not blurred. In my Nikon D3500 it is 1/200s, which is much more manageable.

It is good that this time, for a certain camera, is the least possible. But how can camera manifacturers reduce it? Intuitively, I think that there is a limit due to the fact that it is unavoidable that when a shutter curtain is ending, the other one has already started. And I do not understand how different cameras can have different synch times, since the mechanism behind it is the same for all.

Answer 1

1. You don't need a very short flash sync time for pictures that are mainly lit by the flash because the real exposure is the duration of the flash which is very short (for fill-in the problem is different).

2. What sets the shortest time during which the whole focal plane is exposed is the travel speed of the shutter curtain. And a fast curtain speed generates more mechanical stress on the curtain itself and the whole shutter mechanism, so you have to use more high-tech or expensive materials. Your Zenit is a camera designed with materials available 20 years ago.

Answer 2

Before electronic flash became the norm, we used flash bulbs. These were a one-time use bottle of sunlight. Typically they consisted of a glass envelope filled with oxygen. The bulb contained a filament similar to an ordinary tungsten lightbulb. The tips of the support wires upon which the filament was mounted, were tipped with phosphorous. The glass bulb was stuffed with aluminum wool.

Camera shutters are subsets of pocket watches. They contain springs and gears and a switch. Most early cameras were fitted with a shutter mounted just behind the front lens group. This was known as a between-the-lens shutter. Pressing the go button caused the spring loaded leaves of the shutter to open quickly, remain open for a timed interval, and then quickly close. This scheme is not too efficient. When the shutter opens and closes, the blades of the shutter act like a variable aperture. In other words, the shutter runs up and down the f-numbers as it opens and closes. Only at full open was the f-number equal to the pre-set aperture. Typically they operated at about 60% efficiency.

Between-the-lens shutters were popular because the clock-work mechanism allowed nearly perfect flash synchronization. These flash bulbs needed a warm-up time. The shutter switch was closed 20 milliseconds before the shutter reached full open. This was called “M” (for medium) synchronization. The electricity heated the filament. This ignited the phosphorous. The burning phosphorous ignited the aluminum wool. All this took 20 milliseconds (1/50 of a second). Another flashbulb design skipped the aluminum wool, instead using a heavy dose of phosphorous. This bub needs only 5 milliseconds (1/200 of a second) to reach peak brilliance. This was called “F” (fast) synchronization.

The between-the-lens shutter is limited to a top speed of about 1/800 of a second. Plus, every lens needed one. Thus it is impractical for cameras that feature interchangeable lenses. Focal plane shutter solves two issues. 1. It is at the rear of the camera so lenses can be used without built-in shutters. 2. The focal plane shutter can obtain shutter speeds in the 1/1000 + second range. Its disadvantages: 1. To synchronize it with a flash is challenging. Special flash bulbs are needed. This is because the focal plane shutter works by uncovering a slit in a curtain. The go button is pushed, a spring loaded curtain travels the span of layout of the rectangle (film or digital sensor) format size. The curtain travel time is sluggish. The curtain features a narrow slit. As the curtain travels, the slit covers and then uncovers only a portion of the format opening. In other words, the shutter speed is the time of travel of just the slit width. This time is just a fraction of travel time. Because the curtain travel time is long, special flashbulbs were needed. These were called “FP”. They worked by lengthening out the time the aluminum wool used to fully burn. In other words the blitz length was increased to accommodate the sluggish curtain movement time.

The advent of the electronic flash changes the mechanism of synchronization. The electronic flash needs no warm-up time. The duration of the electronic flash is a blitz about 1/800 or even 1/2000 of a second.

The between-the-lens shutter adapts; the switch is closed when the shutter reaches maximum opening (no warm-up time zero delay). This is called “X” synchronization.
The focal plane shutter of the modern SLR and has a big synchronization problem. The electric flash blitz must happen when the shutter is fully open. This means the slit width must be the same size as the format rectangle. Only a slow shutter speed will work. At slow shutters the slit width is super wide. For your camera this is 1/30 of a second. A more modern focal plane shutter with more powerful springs and lighter curtain can synchronize at far faster shutter speeds. Typically this will be about 1/125 of a second, some faster some slower.

Answer 3

What are the physical limits that determine a camera's flash sync speed?

• The distance the shutter curtains must travel. This can be as far as 36mm for horizontal travel shutters, such as those found in many 35mm film cameras, to as short as 14.5mm for a Canon APS-C digital camera with a vertical travel focal plane shutter.
• The g-forces that the shutter curtain mechanism can tolerate when accelerating up to speed, which affects the speed with which the shutter curtains transit the sensor or film gate. Different materials vary in how much acceleration they can tolerate and remain dependable for a very long time or large number of actuations.
• The combination of the two factors above, distance and speed, determine the total transit time of the shutter curtains.
• The level of accuracy with which a flash sync signal can be initiated. Any inconsistencies from one shot to the next must be allowed for in the amount of time the shutter curtains must remain open.
• The latency of the flash between the time it receives a "fire" signal and when it has released the majority of its energy. When an electronic flash fires, the energy it releases is not uniform from start to finish. It begins at zero, builds very quickly to a peak, and then tails off more gradually. Typically, flashes are measured by the amount of time their output is above 50 percent of peak (T.5) and 10 percent of peak (T.1). The total time between the "fire" signal and the end of T.1 is most useful for measuring the duration of studio strobes, which tend to be longer than the duration of camera mounted speedlights or smaller portable flashes.

This can all be summed up in this paragraph at the Wikipedia article for Flash (photography):

The time available to fire a single flash which uniformly illuminates the image recorded on the sensor is the exposure time minus the shutter travel time. Equivalently, the minimum possible exposure time is the shutter travel time plus the flash duration (plus any delays in triggering the flash).

So how does that apply to your Zenit 12?

I do not understand how different cameras can have different synch times, since the mechanism behind it is the same for all.

Short answer: While the concept is the same, the actual mechanisms used are not the same for all cameras with focal plane shutters. Far from it.

Your Zenit 122 film camera appears to be an ABS plastic-bodied update of the earlier Zenit 12 which had a metal body. The basic design of these cameras goes back to the 1950s. So while your camera was introduced in 1990, most of its design parameters are from a much earlier time.

The Zenit 12 has a fully mechanical horizontal travel focal plane shutter with cloth curtains. This is reflected in the limited range of available shutter durations: 1/30 to 1/500 second in whole stop steps. Since the Zenit 122 has the same range of shutter times available, it's pretty safe to assume it also has the same or a very similar shutter mechanism.

Horizontal travel shutter curtains were near universal in 35mm cameras that used the same mechanical motion of a lever for advancing the film to also cock the springs in the shutter mechanism for the next exposure. Both the film and the shutter curtains were moved in the same direction at the same time by the same movement of the film advance lever. Only once 35mm film cameras started using shutter curtains made of multiple metallic blades (to allow vertical clearance of the viewfinder prism placed almost directly above the light box) cocked with electric motors that were free from the need to move in the same direction as the mechanical motion of a film advance lever did the transition to vertical travel shutters take place. Eventually even the springs cocked by small electric motors were replaced in high end cameras with very precise electronically controlled servos that move the shutter curtains in real time as the exposure occurs.

Cameras with a mechanically operated horizontal cloth shutter and fully mechanical flash sync mechanisms have several distinct disadvantages compared to more modern cameras with vertical travel shutters using metal curtains and solid state electronics controlling both shutter movement and flash sync.

• The shutter curtains must travel further.
• The shutter curtains can not accelerate as quickly. They are both heavier and less tolerant of rapid acceleration. They are driven by springs that are expected to last a long time and thus not pushed to their absolute limits.
• The combination of the above two factors conspire to make the shutter transit time much longer than the typical 2.5-3.5 milliseconds (1/400 to 1/280) of modern cameras. Transit times of around 10-20 milliseconds (1/100-1/50) were typical with spring driven horizontal travel cloth shutter curtains. Rubber (and later, teflon) coated cloth curtains were more durable and allowed cameras such as the Pentax K-1000 and Canon AE-1 to have an X-sync speed of 1/60!
• The flash sync signal was also initiated by spring powered mechanical motion that was triggered by the first shutter curtain reaching the fully open position. The time it takes a spring to move a striker that generates a 'spark' of electricity (to ignite a chemical flashbulb) while closing a circuit (for electronically triggered flashes), is very short, but it is longer than the time required by a solid state relay.
• Cameras designed in the middle of the 20th century were used with a wide variety of flash types. Chemical flash bulbs¹ and electronic studio strobes of the time were much slower to reach peak energy than even modern studio flashes, which can take as long as five or six milliseconds (1/200 - 1/160) to reach the end of T.1.
• Since the flash sync speed of the Zenit 12 is also the camera's longest shutter duration, other than "Bulb" mode, there's no option to extend the exposure time by an accurately measurable amount to accommodate slower flash technologies.

On the other hand, modern digital cameras enjoy several advantages at the same points:

• The vertically travelling shutter curtains do not have to travel as far. Even "full frame" cameras only need shutter curtains to move 24 mm. In APS-C cameras they must only cross around 14.5 mm to 15.9 mm.
• Lightweight, yet very strong, metal alloys or composite materials are used for the blades and actuation arms of multi-section shutter curtains. They can handle high g-forces better and are easier to accelerate.
• Transit time around 3 milliseconds (1/333) is typical. Even very low priced budget models, which tend to have smaller sensors, rarely have transit times longer than about 3.5 milliseconds (1/285). The most expensive "flagship" models optimized for fast handling can achieve transit times slightly shorter than 2.5 milliseconds (1/400).
• Solid state electronics not only allow faster communication of the flash sync signal, but because electronically controlled shutters are also more consistent in their exact transit times from one actuation to the next, the beginning of the process can be initiated earlier by anticipating when the shutter curtain will be fully open and beginning the sequence so that the "fire" signal is sent to arrive at the hot shoe or PC connector very shortly after the first shutter curtain is fully open.
• Modern cameras are designed with the idea that they will be used with modern flash technologies. Small speedlights can typically do a full power flash in around 1 millisecond (1/1000). Lower power releases are even shorter. But beyond that, a much wider variety of available shutter settings, typically in 1/3 stop increments all the way from the camera's sync speed to as long as 30 seconds, rather than 1/30, allows for them to be used with flash technologies much slower than what would be required at the camera's rated sync speed. If a camera has a flash sync speed of 1/250, it's trivial to extend the shutter duration to 1/200, 1/160, 1/125, etc. to allow time for a "slow" studio flash to reach the end of T.1.

_{¹ From the Wikipedia article cited above: In the past, slow-burning single-use flashbulbs allowed the use of focal-plane shutters at maximum speed because they produced continuous light for the time taken for the exposing slit to cross the film gate. If these are found they cannot be used on modern cameras because the bulb must be fired before the first shutter curtain begins to move (M-sync); the X-sync used for electronic flash normally fires only when the first shutter curtain reaches the end of its travel.}

Answer 4

Your intuition is correct in that a flash that only fires once must fire after the first shutter curtain is fully open and before the second curtain begins to close. Higher sync speeds are achieved by strobing the flash as the two shutter curtains move across the frame together. As you can imagine, this requires very precise timing, and only became possible with the advent of microcontroller-based cameras. Some SLRs (even relatively low-end models from back in the film days) can use this method to sync at speeds up to 1/8000s.