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.