Slow sync & high shutter speed

[henrykoh]

Need to ask a newbie question: With the mounted flash (5600, etc), is high shutter speed above the flash sync speed possible when rear curtain flash mode is selected?

[TME]

Originally Posted by henrykoh Need to ask a newbie question: With the mounted flash (5600, etc), is high shutter speed above the flash sync speed possible when rear curtain flash mode is selected?

U mean if HSS (High Speed Sync) is possible when the flash is on rear curtain mode? Yes it is... Rear curtain simply forces the flash to fire just before the rear curtain closes... nothing to do with HSS... if the shutter speed is still faster than the X-sync speed, then HSS will still kick in...

[henrykoh]

Hi TME,
Thanks for your reply. I just bought the 3600 flash and HSS works with slow sync but it cannot work with rear curtain at least on my A200. Tested and proven not to work conclusively. Cheers.

[tankm]

Tried on the A700. Shutter speed max at 1/200 (with SSS) and 1/250 (without SSS) in REAR SYNC mode.

Originally Posted by henrykoh Hi TME, Thanks for your reply. I just bought the 3600 flash and HSS works with slow sync but it cannot work with rear curtain at least on my A200. Tested and proven not to work conclusively. Cheers.

[Clockunder]

If the shutter speed is faster than the max. sync speed of the camera, it's no longer "slow sync" already leh. It's high sync actually.

When the shutter speed is faster than the max. camera sync speed, the rear curtain starts to close even before the front curtain finishes dropping down.

At high shutter speed faster than camera's max. sync speed, if the HSS flash only starts firing when the front curtain has fully dropped down, then it's too late already as the rear curtain has already started dropping down and covered some part of the frame, causing the upper part of the frame not having exposure to the flash.

If the flash starts firing when the rear curtain starts to drop, then the lower part of the frame will have more exposure to the flash than the upper part.

So the only way for consistent exposure throughout the frame in HSS, the flash must start firing when the front curtain starts dropping and finishes firing when the real curtain has fully dropped down ..... i.e. very much like the flash (at HSS) simulating continuous consistent ambient light during high shutter speed in bright daylight.

This explains why High speed sync and rear curtain is not "possible"/available for shutter speed faster than the camera's X-sync speed.

[TME]

Originally Posted by henrykoh Hi TME, Thanks for your reply. I just bought the 3600 flash and HSS works with slow sync but it cannot work with rear curtain at least on my A200. Tested and proven not to work conclusively. Cheers.

It's a limitation of either the flash or the body... HSS with rear curtain is possible on higher end bodies...

[TME]

Originally Posted by Clockunder If the shutter speed is faster than the max. sync speed of the camera, it's no longer "slow sync" already leh. It's high sync actually. When the shutter speed is faster than the max. camera sync speed, the rear curtain starts to close even before the front curtain finishes dropping down. At high shutter speed faster than camera's max. sync speed, if the HSS flash only starts firing when the front curtain has fully dropped down, then it's too late already as the rear curtain has already started dropping down and covered some part of the frame, causing the upper part of the frame not having exposure to the flash. If the flash starts firing when the rear curtain starts to drop, then the lower part of the frame will have more exposure to the flash than the upper part. So the only way for consistent exposure throughout the frame in HSS, the flash must start firing when the front curtain starts dropping and finishes firing when the real curtain has fully dropped down ..... i.e. very much like the flash (at HSS) simulating continuous consistent ambient light during high shutter speed in bright daylight. This explains why High speed sync and rear curtain is not "possible"/available for shutter speed faster than the camera's X-sync speed.

When in HSS mode... don't care front or rear curtain, the flash will fire in pulses once the shutter opens...

[Clockunder]

Originally Posted by TME When in HSS mode... don't care front or rear curtain, the flash will fire in pulses once the shutter opens...

As explained earlier, when in HSS mode, it’s neither front curtain nor rear curtain.

Front and rear curtain modes basically tells the flash when to start firing that one burst of light (lasting only may be between 1/10,000 to 1/50,000 second).

When the shutter speed is the same as the camera’s X-sync speed, the front and rear curtain modes give identical results because the start times of flash firing are the same in both modes (i.e. When shutter speed is at X-sync speed, the rear curtain will immediately start to fall once the front curtain is fully dropped. In front curtain modes, the flash will fire immediately when the front curtain has dropped fully but when the shutter speed is at X-sync shutter speed, this happens to be also the same time as in rear curtain modes in which the flash will fire just before the rear curtain begins to fall).

The more the shutter speed is slower than X-sync speed, the greater is the difference between these 2 modes as the difference in the start of the flash firing becomes greater. For e.g. the difference in results between front and rear curtain modes is more at 1/30 than when at 1/125.

Front and rear curtain modes are only relevant when the whole frame is at some point fully exposed and this means only when the shutter speed is not faster than the X-sync speed of the camera.

When the shutter speed is faster than X-sync speed, only a part of the frame is exposed at any point in time and a single burst of flash light will never be able to add light to the whole frame. This is when High Speed Sync comes in.

At High Speed Sync, the flash basically sents out pulses of light continuously such that the subjects are illuminated nearly continuously throughout the exposure duration. Since only a part of the frame is exposed when the shutter speed is faster than X-sync speed, the flash must start firing immediately when the front curtain starts to drop and finish firing only when the rear curatin has fully dropped so that the whole frame gets the same exposure.

As can be seen from the above, both the front curtain and rear curtain modes tell the flash to start firing too late and are irrelevant in HSS.

In essence, you can’t tell the camera when to start firing the flash in HSS and therefore there is no curtain setting when HSS is used. For some cameras, due to the reasons above, it is explicitly stated in the camera’s user manual that it cannot be set to “rear curtain” when HSS is used (actually, more precisely, there is no curtain settings possible/available when in HSS). This answers thread-starter’s question.

(For some camera systems, when the flash is set to HSS mode but the shutter speed happens to be slower than X-sync speed, it will automatically revert back to use normal flash mode for that shot …i.e. single flash burst. The above of course applies to mechanical shutters. Electronic shutters is another story.).

[eow]

a bit long... but clockunder is rite,
hss doesn't work in rear nor in slow synch mode.

[s00n]

Just add to this question,
Why my HSS will be switch off once I tilt up the flash?
My setup (7D + HS3600)

[TME]

Originally Posted by s00n Just add to this question, Why my HSS will be switch off once I tilt up the flash? My setup (7D + HS3600)

Yes... HSS works only when the flash head is pointing forwards 90 degrees...

[sulhan]

HI Henry,

Welcome to the alpha mount users forum here in CS.
While having all these discussions on if the HSS is there or not etc, I'm wondeirng if you could provide a simple description of effect that you would like to obtain from your setup.

Approaching it this way is better .

All that you know the effect that you are trying to get is not achievable anyway using the inteded mode but achievalbe in another mod .

Rgds,
Sulhan.

[henrykoh]

Originally Posted by sulhan HI Henry, Welcome to the alpha mount users forum here in CS. While having all these discussions on if the HSS is there or not etc, I'm wondeirng if you could provide a simple description of effect that you would like to obtain from your setup. Approaching it this way is better . All that you know the effect that you are trying to get is not achievable anyway using the inteded mode but achievalbe in another mod . Rgds, Sulhan.

Thanks for the welcome. I asked because that my set up when shooting macro but i guess just slow sync will suffice. it is really not an issue, just a newbie question.