Flash with high shutter speeds?

[WayneF]

HSS isn't so trivial.

Here's something to think about.

The normal maximum shutter speed for flash involves the flash only firing once after the shutter opens. But the shutter is mechanical... so this takes time. The shutter has two "curtains" (doors). One slides open (typically from top to bottom) and the other slides shut (also from top to bottom). If it were to only use one curtain which slid open (top to bottom) and then closed (bottom to top) then the pixels along the bottom row would technically get a longer exposure than the pixels on the top row -- and that's no good. Hence... two curtains on the shutter.

HOWEVER, it does take some time to slide the curtain completely open. The flash must not fire until the shutter has finished opening... and must fire BEFORE the shutter begins closing. If it fires too soon or too late then you get a flash when part of the sensor was covered by a shutter curtain and that's no good.

If your max flash speed is normally 1/160th then it means your camera takes 1/160th of a second to open the shutter, in that millisecond of time the flash will fire, and then the second curtain will immediately begin closing. The exposure will actually take TWICE the amount of time (1/80th second) but since the first row to be uncovered as curtain #1 opens is also the first row to be covered up again as curtain #2 closes, no single pixel is exposed for more than 1/160th second.

Here's the problem with flash at high speed.

If you want to take a faster photo... say 1/320th... then the first curtain has to start sliding open. When the first curtain is only 1/2 way open, the second curtain actually starts closing. It is effectively exposing just a slit of light which sweeps across the sensor. No single pixel is exposed for more than 1/320th of a second EVEN THOUGH the mechanical speed of the curtains does not technically change. They move at the same speed. The only difference is the delay from the time the first curtain starts to move until the 2nd curtain starts to move and this changes the size of the "slit" that sweeps across the shutter.

And therein lies the problem with "flash". If the flash fires just once at any point during that exposure... only the part of the image exposed by the slit gets the benefit of the flash. Everything else was shaded by the partially closed curtains.

To compensate for this problem, some camera and flash combinations are able to support a mode called "high speed sync". When this mode is enabled, the flash will fire multiple times as the curtain moves. Just HOW many times it fires, depends on the shutter speed.

It was good up until here, and then, sorry, but it goes bad.

If you use 1/320th (half the exposure time compared to the 160th sec in which just a single pulse of light would have worked), the flash has to fire twice. The first time when the top half of the image is exposed, the second time when the bottom half of the image is exposed. These two have to be perfectly synchronized to avoid overlap.

This creates a new problem for the flash. The flash can't dump the full power of light when it flashes because it needs to reserve some power for that second flash of light. This means that each of the two pulses of light can only be HALF as powerful as compared to what the flash could do with a single pulse of light. This cuts into your power and you have to consider the impact on your shots.

If you increase the speed again... say now we're at 1/640th, now the flash has to pulse 4 times. No single pulse of light can be more than 25% of the total power of the flash. At some point you're forcing the flash to pulse 8 times... or 16 times... and the effective power of just 1 pulse is getting low to the point where the flash coverage is not very great.

To compensate for that... you need multiple flashes.

With the Canon 600EX-RT you can cluster them (up to 16 I think... it may only be 15). In any case... clustering them and getting them to all do HSS at the same time (synchronized high-speed synch -- which sounds redundant but isn't) allows you to combine the power of an array of flashes so that you STILL get the power output that a single flash could deliver in just one pulse... but you get to do it at very high shutter speeds.

Bottom line... yes, you can do high-speed sync.... but you may need many flash guns to pull it off depending on your subject. There is no free lunch. :-/

That is not how HSS works. The flash does NOT fire twice at 1/320, nor four times at 1/640, nor with multiple flashes. You could use multiple flashes ganged just to get the power level back up.

HSS converts the one flash to be a continuous light, like the sun or an incandescent bulb is continuous. Meaning it is on before the shutter opens, and goes off after the shutter closes (seen by the shutter, it is continuous). To do this, it runs at literally several thousand flashes per second, all run together... Fast, to be Continuous. Essentially becomes like a continuous desk lamp in the hot shoe, for the duration of the shutter travel time.
And HSS mode then also has change to trigger the flash before the shutter opens (continuous light has to be on then) and turn it off after the shutter closes. This keeps it on for the entire focal plane shutter travel time, even for a narrow slit like 1/8000 second. Where ever that little slit is, the flash is on.

It has to run at low power to be able to do all this flashing.. like around 2.3 stops down, which is like only about 20% power level (probably still suitable for fill levels however). And this mode is no longer a speedlight, continuous light has no motion stopping ability whatsoever. All we have now is the shutter speed.

Of course, a continuous light into a fast shutter speed like 1/8000 second is reduced to be five stops down (1/32) of the illumination that 1/250 second would see. Shutter speed is really hard on continuous light. Pretty tough conditions, esp when it is already only 20%.

But the beauty is, for continuous light, the Equivalent Exposure concept returns... Meaning, we can open aperture to compensate, so that 1/8000 second f/2.8 is Equivalent Exposure of 1/250 second at f/16 (except 1/250 may be regular mode, not HSS mode, depending on camera model, so would not be Equivalent). But Equivalent for both continuous sun and for continuous flash. It is none too much flash power, but can work if the distance is not great, esp at fill levels.

It would seem that there was some uber-speed on the far end where the flash would remain lit throughout the exposure... even if unreasonably fast, and given that it's likely to vary by flash, I wonder how fast that would be.

The problem is that regular flash is a strong peak pulse, gradually tapering off to nothing. As the narrow slit travels over the frame, the flash is falling to nothing, not exposing the frame equally. This is not always bad in daylight, with the sun filling in, we may not always notice, if the flash subject is in the bright area. Bad news if it is dark though.

Maximum shutter sync speed means the shutter is 100% open, and while the flash is mighty short, it simply does not matter so long as it occurs while the shutter is fully open.

The cameras doing 1/500 second sync (like D40) are NOT focal plane shutter, but instead use an electronic shutter built into the sensor chip. CCD chips have to disable the sensor to shift data out anyway, so this can be simply used as a free shutter, activating for only say 1/500 second. The electric shutter is fully open for the flash, but a focal plane shutter is mechanical, at finite speed. There are other downsides to it, but it can sync flash at any speed.

 

[robbins.photo]

^^^^^^^^^ An OUTSTANDING explanation Tim!!! Bravo!

Ok granted, that was a first rate, top notch explanation on the whole process. But really I'm just here for the free lunch.

There is no free lunch. :-/

Ahh crap.

 

[Gavjenks]

AFAIK, HSS flash isn't something that a camera body has much to do with. It's a property of the flash more so.

Even a 60 year old large focal plane shutter view camera can benefit from HSS modern speedlights. You would just put your flash into manual HSS mode if it has it (mine does), and then set the mechanical camera to whatever setting is designed to go with "long duration" flash bulbs (the old equivalent of HSS, the only difference in-camera is that it simply triggers the electric impulse at the start of the exposure).

It would seem that there was some uber-speed on the far end where the flash would remain lit throughout the exposure... even if unreasonably fast, and given that it's likely to vary by flash, I wonder how fast that would be.

The output of a flash follows a curve. So although it is actually pretty easy to imagine circumstances where especially studio strobes would still be outputting "a lot" of light over the entire exposure, it won't be a CONSTANT amount of light, so you'd end up with a weird gradient across the image, even though all parts of it are somewhat lit.

In some cases, you could go SO fast that the gradient isn't very noticeable, though. Like for instance, some of the cheaper or older style hulking monolight studio strobes set to their slowest power might take 1/300th of a second to output 90% of their light, and if you pair that with a 1/8000th second shutter at the least sloped part of the output, the gradient of that portion of the curve would probably be pretty unnoticeable. metering would be difficult though, and you'd have to very intentionally be doing this, almost just for the novelty of it... because that's such a bizarre group of settings and equipment that it would probably never happen by accident.

 

[Gavjenks]

Also note that to some extent, HSS can be replaced with more power + a neutral density filter in most situations. Since the most common reason to want to use a faster shutter is to drown out ambient light, you can do the same thing by cranking up the flash 3 stops and then adding a 3 stop ND filter. Flash exposure remains equal and ambient goes down by 3 stops.

HSS is more convenient, but I'm just saying if you find yourself in a situation where you don't have it, or it is undesirable for some reason, the above alternative could often save your butt, or be maybe cheaper if you plan it ahead of time in some cases.