how does the hss work

8/13/2019 How Does the HSS Work

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TUTORIAL: HIGH-SPEED FLASH SYNC

August 2, 2010

HIGH-SPEED FLASH SYNC / AUTO FP .. VS .. NORMAL FLASH

There have been a number of questions about high-speed flash sync (HSS), and how it affects the

output from your flash. There were also some questions asked about high-speed flash sync with

this recent post where we tried to reverse-engineer a photo.

I decided to do a series of comparison photos, so we can actually see what happens before, at and

beyond maximum flash sync speed. And we can also see what happens with high-speed flashsync. To do this, I set up very simple portrait lighting using a single speedlight and a large

umbrella. A simple white paper-roll backdrop, and our model, Rachel. Here is the setup in my

dining room
http://neilvn.com/tangents/high-speed-flash-sync/http://neilvn.com/tangents/high-speed-flash-sync/


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The large (60) umbrella to my left, and a small reflector to the right to add a little bit of

fill. Ideally the reflector should be larger and closer but this isnt so much a portrait session as

it is about photographing sequences of images at various shutter speeds. I stayed close to the

umbrella keeping it as close to the lens axis as possible so that the light is as even as wecould manage with a single speedlight setup. Of course, the reason for it only being one

speedlight is that we can now observe its behavior.


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For all the images in this article, the speedlight was set to give full output in manual

mode. This way we arent bringing in the uncertainties of TTL flash. TTL flash is an automaticmetering mode, and there will be variations in output if we change our composition. Manual

flash is consistent and predictable. I worked at full output so that we can more clearly see some

of the effect of going to high-speed flash sync.

Initially, I triggered the flashgun / speedlight with radio transmitters. PocketWizard Plus II units,to be exact. They dont allow high-speed flash sync. So we can now see what will happen

then. For the final sequences where we go to high-speed sync (HSS), I used an on-camera

speedlight as a Master, to fire the Slaved speedlight mounted with the umbrella. The Masters

output was disabled, so were just dealing with one speedlights effect.

In the spirit of keeping much of the info on this site system-agnostic, well look at how the

Nikon D3 and the Canon 5D works.

HOW DOES THE CAMERAS SHUTTER WORK?

Before we get deeper into this, we need to cover some basics first how the cameras shutter

works. The focal plane shutter found in D-SLRs, consist of two curtains that open and

close. They open and close with a certain timing, to open the sensor / frame to light. When theshutter is tripped, the first curtain opens, revealing the sensor (or frame) to the light, and then the

second curtain closes. The time interval between the first curtain opening, and the secondcurtain closing, is the shutter speed. It can be 1 second, or it can be 1/60 of a second, or as short

as 1/4000 of a second.

Looking at the top part of the diagram, we can see the timing of the flash pulse. With normal

flash sync (shown here as Low Speed flash sync), the flash is a near-instantaneous burst oflight. (Around 1/2000 of a second, although this varies on the design of the flashgun, and how

the flash pulse is controlled.) For description here, lets just regard the flash burst asinstantaneous. Therefore we need the entire frame open so that the entire scene / subject is

revealed by the light from the flash. Otherwise, part of the scene / subject will be obscured by

one of the shutter curtains. (Well see this in another sequence of images here.)

There will be some shutter speed which will be the highest shutter speed at which the first

curtain has just stopped moving, and the 2nd curtain has not moved yet. One notch of a shutterspeed over this, and you will have part of one of the shutter curtains obscuring the sensor /

frame. It is one of the essential things we need to know about, and understand, when we deal

with flash photography.)


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With high-speed flash sync a truly amazing bit of engineering the flashs output is released

as a rapid series of light pulses. The flash is now effectively continuous light over a very short

duration. Now, when we go over maximum flash sync speed, the flash is released as that shortperiod of continuous light, and we can take our shutter speed much higher than maximum flash

sync speed.

This change from a high-energy near-instantaneous burst of light (normal flash), to the short

period of continuous light (high-speed flash sync), does imply a loss of effective power. It

makes sense in that a lot of the light from our flash will just hit the shutter curtains, and wont hitthe actual sensor. In other words, much of the output from the flash in high-speed flash sync

mode will be lost.

And this is where the rest of this article comes in

MAXIMUM FLASH SYNC SPEED

So lets look at how the flash output appear using old school radio slaves which dont allow us togo to high-speed flash sync. This is how studio-type shoots are usually set up .. various flashes

and light modifiers set up, and balanced .. and then tripped by radio slaves. Normal flash

sync. In other words, no high-speed flash sync.

Here is the first sequence, using the Nikon D3. The next sequence is with the Canon 5D. Iwanted to show that the behavior of normal flash sync and maximum flash sync speed is

universal for all focal plane shutters found on D-SLRs.


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The Nikon D3 (like most of the bigger Nikon D-SLRs) have a maximum flash sync speed of

1/250th. Below that, and if there is not much ambient light, the choice of shutter speed has NO

effect on the flash. We just need the entire sensor / frame to be open to the blitz of flash. This

can be at 1/8 or 1/60 or 1/125 as long as it is at, or slower than maximum flash sync speed.

However, as we can see there, at 1/250 we start to see the edge of the one shutter curtain. This is

due to propagation delay. (More about this in a short while.)
http://neilvn.com/tangents/images/flash/hss/Nikon-pocketwizard-lrg.jpghttp://neilvn.com/tangents/images/flash/hss/Nikon-pocketwizard-lrg.jpg


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Here is the same sequence for the Canon 5D. The 5D has a maximum flash sync speed of 1/200

and we see the same effect with the higher shutter speeds obscuring the flash exposure.

btw, if for some reason you want to see slightly larger versions of those, click on the two sets of

images for the larger side-by-side version.
http://neilvn.com/tangents/images/flash/hss/Canon-pocketwizard-lrg.jpg


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PROPAGATION DELAY

As you could see there with the sequence of images for the Nikon D3 and the Canon 5D even

though you might be working at maximum flash sync speed you might still get the edge of a

shutter curtain. This is due to something called propagation delay.


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For these images we were using radio transceivers to trip the flash. (Pocket Wizard Plus II

units.) As mentioned earlier, they are simple devices that just trigger the speedlights. There is

no intelligence there between the camera and speedlights.

So this is where slight synchronization errors can creep in. Where we are working right now at

maximum flash sync speed, were on the very edge of any specific cameras capabilities. So

when we trip the shutter, the camera has to fire the transmitter mounted on it; which then tripsthe receiver connected to the speedlights / flashes; which in turn fires those speedlights /

flashes. There is a whole chain of events that take place within finite time. And that is where

any slight synchronization error will show up just like it does here with the edge of the shutter

curtain showing.

This is a common-place occurrence, and not just inherent to the two cameras I used here. Any

studio shooter will have experienced this problem and will know to use a shutter speed lowerthan maximum flash sync speed when shooting in the studio. A shutter speed like 1/125 or 1/100

or 1/60 is fine when working in the studio where ambient light levels are low. The shutter speed

has no effect then on the flash output, so any shutter speed lower than maximum flash sync speed

is fine in the studio.

On-location though, I do use maximum flash sync speed, even if there is the chance ofpropagation delay. The reality of on-location photography is that we are much much less likely

to see the effect of flash exposure at the edge of the frame. For example, I mainly do portrait

photography on location. My subjects are usually more centrally placed. Hence, propagationdelay doesnt affect me. I just shoot at maximum flash sync speed then to give me the most

efficiency from my flash.

HIGH-SPEED FLASH SYNC (HSS) / AUTO FP

Okay, finally we get here. Lets see what happens when we go past maximum flash sync speed,with HSS enabled.

Here is what the Nikons do:

(Actually, this is what pretty much everycamera does that has HSS capability.)


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Here is what the Canon 5D does. Very similar to the Nikon D3, with a slight quirk around the

point we get to maximum flash sync speed.
http://neilvn.com/tangents/images/flash/hss/Nikon-hss-lrg.jpghttp://neilvn.com/tangents/images/flash/hss/Nikon-hss-lrg.jpg


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The Canon 5D, and the Canon 5D mark II, both have the same quirk at maximum flash sync

speed. If you are at max flash sync (1/200), with or without HSS, the output is affected

differently. This is something I mentioned in an earlier post on maximum flash sync speed. It is

just something to be aware of with the 5D bodies.

So lets look at the implication of those two sequences of images.
http://neilvn.com/tangents/images/flash/hss/Canon-hss-lrg.jpghttp://neilvn.com/tangents/images/flash/hss/Canon-hss-lrg.jpg


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The moment we go over maximum flash sync speed, our output from our flash drops

considerably. It makes sense:

If we are in normal flash mode, then the flash is an instantaneous burst of light. We just need our

entire frame / sensor to be open, for flash exposure to be consistent from edge to edge.

The moment we go into HSS mode, then the flash output is essentially continuous light. And continuous light is affected by shutter speed choice. Think of ambient light. If we change our

shutter speed, we change our exposure. This is exactly what happens with flash in HSS mode.

LINEAR RESPONSE OF HIGH-SPEED FLASH SYNC

As mentioned, since HSS flash acts like continuous light, it should have a similar linearresponse So lets see what happens when we change the aperture in relation to the change in

shutter speed.


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And there is the linear response, clearly to be seen.

1/500 @ f4 1/1000 @ f2.8 1/2000 @ f2


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This is also obvious then why we need to be at maximum flash sync speed when working in

bright lightand trying to overpower the sun with flash. (At, or just below, maximum flash sync

speed.)

For these examples, we had 1/250 @ f11 which would, for ambient light only, translate to

1/2000 @ f4 as an example. Yet, here we have an equivalent of 1/2000 @ f2 when going to HSS.

According to the examples here, we lose about 2 stops in comparison, if we were considering

ambient light. eg, going from 1/250 @ f11 to 1/500 @ f4

Now, when we go even higher, the linearity starts to flatten out. Instead of 1/4000 @ f1.4 it

would appear that we lose about 1 third of a stop of light, and have to open up by dropping the

shutter speed slightly to 1/3200 to get the same brightness. But this could also be partly due to

working at wide open aperture on this lens.

Lets see if the same linearity exists for the Canon 5D:

Indeed, there it is:


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.. with the same tapering off in the linearity when we go super-high on our shutter speed. I had

to open the shutter speed by a third of a stop to get the same exposure again. Although, similarly,

this might be partially due to working at full aperture on the Canon 85mm f1.8


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THE IMPLICATIONS OF HIGH-SPEED FLASH SYNC

So the output drops considerably in HSS mode. This implies several things:

1. When we work in bright ambient light, then we have a sweet spot at (or just below) maximum

flash sync speed, if we want the most efficiency from our flash. This is because the highershutter speed implies a wider aperture .. and this wider aperture allows our flash to reach further,

or work less hard for the same exposure.

2. If we want correct flash exposure with HSS flash, then we need to move our flash much closer

to our subject if we are using flash as the dominant source of light. Or we need to be aware that

our flash will be merely fill light, since the output is affected dramatically.

3. If we want to control our depth-of-field, we are much better off using neutral density filters,

than going to HSS mode.

4. You cant overpower the sun by going to HSS. If anything, you should not be in HSS whenyou are dealing with bright light. You need to do something entirely different to overpower the

sun with flash. This idea that you go to a much higher shutter speed to control the available light

when you use flash, is one of the biggest fallacies Ive come across on the various photography

forums. It simply does NOT work that way. The next section will show exactly why.

5. Because of the loss of effective power with HSS, you need to double up, or quadruple up on

speedlights to compensate if you are shooting in bright light. Alternately, you can move the

flashgun much closer, and use direct off-camera flash, as with this photograph. This is Aleona,one of our models at the flash photography workshops, leaping into the air. I wanted to

completely freeze any movement, and went to a high shutter speed.


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settings: 1/2000 @ f3.2 @ 400 ISO;

Manual flash at full power from an off-camera Slaved speedlight, controlled wirelessly with the

on-camera flash set to Master.

how does the hss work

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