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When referring to flash exposure, what does "max sync speed" mean?

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  • \$\begingroup\$ other question should be edited to include "what is sync speed" \$\endgroup\$
    – Canon Gangsta
    Jul 28, 2010 at 18:52
  • \$\begingroup\$ @Gangsta: Is the current wording "What exactly is flash sync speed" not sufficient? \$\endgroup\$
    – jrista
    Jul 29, 2010 at 0:41
  • \$\begingroup\$ The question is written as "Flash Sync Speed: Flash sync speed : buying decision factor ?" which doesn't match the content. This question is purely asked for definition of sync speed, not diluted with : buying decision factor" This question will likely get more search engine hits because it's asked as a person would ask it in a query. \$\endgroup\$
    – Canon Gangsta
    Jul 29, 2010 at 1:55
  • \$\begingroup\$ @Gangsta: I understand your point now. However, terminology was off a bit. The question is titled "Flash Sync Speed...", but asked starting with "What exactly is flash sync speed". I think both questions will be indexed by search engines equally enough, but I'll leave this one as is. \$\endgroup\$
    – jrista
    Jul 29, 2010 at 2:03

3 Answers 3

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With mechanical shutters on most common DSLRs, they actually consist of two shutter curtains, that move in the same direction.

At slow shutter speeds, one will open, and then, after some time, the other will close. As these are mechanical devices, there is a maximum speed at which they can reliably move. With really fast shutter speeds only a narrow band is exposed to the image through the lens at any one time as the second curtain begins to close before the first curtain has fully opened. The faster the shutter speed, the narrower the opening between the two curtains as the second one chases the first one across the focal plane. The transit time of each shutter curtain is the same for all available shutter speeds in most modern cameras. It is the time interval between the movements of each that determine the exposure time. Even though each point on the sensor may only be exposed for as short a time as 1/8000 of a second, it still takes the curtains around 1/400-1/200 second (depending on the camera's design) to move across the entire sensor and take the picture.

The sync speed is the fastest speed at which the entire sensor is exposed to the light through the lens at the same time.

High Speed Sync is available on some flash/camera combinations which allow flash to be used at faster shutter speeds. This is achieved by strobing the flash to coincide with distinct bands of the sensor, such that the entire picture is correctly exposed. As an example, if the shutter speed were to allow at most half the sensor to be visible, the flash would strobe twice - once for the top half, and once for the bottom.

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  • \$\begingroup\$ Spot on, one brief suggestion. Perhaps give 'mechanical shutters on most DSLRs' their name: focal plane shutters. The answer applies to any focal plane shutter, from a speed graphic to a leica M9, so might as well be inclusive! \$\endgroup\$
    – ex-ms
    Jul 28, 2010 at 21:54
  • \$\begingroup\$ @matt I did consider that, but wanted to emphasize to anybody unfamiliar with the subject that it was something physical, without having to go off and bing/google/etc. "focal plane shutter" \$\endgroup\$
    – Rowland Shaw
    Jul 29, 2010 at 11:39
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    \$\begingroup\$ good point. What about something along the lines of: ...most common DSLRs (called "focal plane shutters")... with a link to relevant definition? Kill two birds with one stone? Or too much in one answer? \$\endgroup\$
    – ex-ms
    Jul 29, 2010 at 20:21
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It refers to the maximum shutter speed where both the sensor is fully exposed to the light. Shutters consist of a rear and front curtain. For shutter speeds faster than that, the rear curtain trails increasingly close behind the front shutter before it reaches the other end.

One benefit of sync speed that has nothing to do with flash is the amount of distortion that you will get with very fast moving subjects. It is the same as the rolling shutter effect of DSLR video slow sensor readouts, but their effective speeds are much much slower. I am not sure how much the improvement is, if anyone else knows.

A flash works by emitting one quick burst of light. If you manage to shoot a shutter speed faster than the sync speed, part of the image will likely be unexposed to the flash.

Some flashes have HSS, which emit many pulses of light. There are some disadvantages vs a higher sync speed, like inability to stop motion using flash (on the extreme end, often you use HSS at fast enough shutters anyway) and lowered max flash power

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Sync speed is the fastest shutter speed that the camera can physically swing the mirror up and open the shutter curtains in sync with the flash.

Selecting a sync speed over the max sync speed will capture the shutter curtains or mirror in the exposure as it fails to "get out of the way" in time for the flash exposure. It appears as a uniform dark area. Select a lower shutter speed below the shutter max sync and adjust aperture or flash power to compensate.

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    \$\begingroup\$ The mirror doesn't really have anything to do with sync speed. The mirror has to be flipped up entirely out of the way (except with a pellicle mirror system) before the shutter opens, regardless of shutter speed. \$\endgroup\$
    – Jerry Coffin
    Jul 28, 2010 at 19:23

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