High Speed Sync Flash - Canon

[jeffde]

Hi
Just recently have tried a few high speed sync flash photos when the subjects are in bright sunlight.
Sunday was meeting with a couple whom i photographing their wedding in November outside on the farm.
Unfortunatley it appears that they will be standing in the sun during the ceremony so took some photos in the sun - harsh at midday....
#1 is normal Apertur Priority F4.5 SS 1/250 28mm on a 24-105mm F4 lens

#2 Swithed to high speed sync and result
F4.5 ss 1/1000 40 mm on a 24-105mm F4 lens 34 seconds later.

A big difference you might say.....
#1
58560

#2
58561

BTW nothing done in PP (LIghtroom) as these are rejected shots...

[judybee]

wow, this makes a huge difference. thanks for sharing.

[MarkChap]

Number 1 is way over exposed because the camera will only sync at 1/250 with the flash on the hot shoe.
You would have to close down your aperture to compensate for the shutter speed, had you shot this at 8 your exposures would have been very similar, all be it with an increased depth of field.

Had you removed, or turned off, the flash, your camera would then have metered the scene correctly and not been so over exposed.

[jeffde]

Hi Mark,
I agree but in using the flash the idea is to fill some (you never get them all) of the shadows on the faces - and this is the idea behind the high speed sync (so i have read) - still new to me.
The first statement is not entirely correct as the camera has synched at 1/1000with the flash on the hotshoe... (ableit using the handy little H button on the flash)

[PH005]

Great tip. Thanks.

[Riverlander]

Jeff
The fastest shutter speed at which your Canon can synch with the flash is 1/250 sec. That is to say the flash fires once exactly when the shutter is fully open. That is the "maximum" synch speed.

In high speed flash the flash fires more than once as the shutter curtains move across the sensor - thus covering the full image piece by piece. Obviously if the flash fires more than once in 1/100 sex or less, then it cannot fire at full power each time. However it must fire at the same power each time or you will get banding in the image.

Therefore you will get reduced effect on shadows etc. So if your desire is to illuminate the shadow areas, then you are bette off using 1/250 or slower, at a smaller aperture value.

Not all things are as simple as they first appear.

[jeffde]

Hi Graham
I ddidn't say it was simple nor do i really understand it - having only done it a couple of times. - Your explanation along with Mark's does help me a bit as well.
The tutorial i saw on this (Digital Photography School i think from memory ) though really showed that for outside portraits in bright sunlight when you had little or no control over positioning (ie: weddings) that it was a great help - I think it also said that it would only give you a higher shutter speed than synch ie 1/250 if it was required.
The major benefit is that it makes the background darker than you subject and you avoid overexposure of the background which happens alot when using fill flash unless you compensate -
So theoratically you can set the flash to high synch leave your apeture at what you want and then let the shutter speed be determined by the camera - instead of individually metering each shot??

As i said i'm new to this feature...

[Wayne]

The only issue with shooting at 1/250 sync speed is that as said already, you need to stop down until you get the exposure right, with the downside then being large DOF. To get shallow DOF in such bright conditions when using flash for fill, you either need the Hi-speed sync, or some ND filter to cut the light entering the lens to a level where your meter shows 1/250 or slower shutter speed is required for a correct exposure.

[Scotty72]

Not sure about the 50D but, I know that with both my 40D and 7D (so I assume the 50D would be about the same); you can get away with a non sync speed of 1/320 th with NO banding and at 1/400 their is just the tiniest hint. So you could remotely fire a full blast (not lots of mini pulses).

Food for thought.

[Wayne]

One thing to think about, and I have no idea about Canon 430's or 580's but with Nikon speedlights, the lower the flash power, the shorter the flash duration, so basically the shorter the duration, the faster shutter speed you can use without the rear curtain blocking the light as it is closing.

If blasting away at 1/1, then you will have a longer flash duration, and need a slower shutter speed to avoid the rear curtain blocking the flash.

The SB-900 for example, flash duration is as follows;

1/880 sec. at M 1/1 (full) output
1/1100 sec. at M 1/2 output
1/2550 sec. at M 1/4 output
1/5000 sec. at M 1/8 output
1/10000 sec. at M 1/16 output
1/20000 sec. at M 1/32 output
1/35700 sec. at M 1/64 output
1/38500 sec. at M 1/128 output

So even at 1/1 you should still be able to shoot with a shutter speed around 1/800 before the rear curtain starts to block the light and showing as a dark band in the frame, and by 1/16 power output, a shutter speed of 1/10000 (the max shutter speed on bodies is usually around 1/8000).

I'll do some testing and post the results.

[Riverlander]

Wayne's ideas sound fine but they are wrong - maybe his tests will shopw it.

At any shutter speed faster than about 1/250 sec the sensor is not fully exposed at any instant in time. The curtains follow one another and only a "slit" of the sensor receives light as they pass. Therefore one flash will not suffice.

Not all things are as simple as they first appear.

[Wayne]

Well in trying to test it seems that when using a speedlight either on or off camera and using the Nikon CLS (pop up flash) to trigger the flash, the body won't sync higher than 1/250 unless you enable hi-speed (FP) sync mode regardless of manual, or iTTL being set on the speedlight.

I wonder how it would go using a studio light or PW's to fire when the body doesn't know to restrict the shutter speed to 1/250s.

Riverlander, I remember now reading about the curtains following each other as a slit so to speak at high shutter speeds, so would you think that a flash of long duration may be enough to allow the 2 curtains to make their pass of the sensor and expose the whole thing?

[Riverlander]

would you think that a flash of long duration may be enough to allow the 2 curtains to make their pass of the sensor and expose the whole thing?
Of course. Why bother, when you could just as easily use a floodlight or video lights or something?

[Tannin]

Riverlander, I remember now reading about the curtains following each other as a slit so to speak at high shutter speeds, so would you think that a flash of long duration may be enough to allow the 2 curtains to make their pass of the sensor and expose the whole thing?

Indeed. But flashes don't stay on for that long. Is this just a failure to design them that way (because HSS does the job just fine, so why bother changing stuff), or because such a long-duration flash would generate too much heat and damage the flash gun, or because it would burn way too much battery power? My guess is that all three apply. In addition, I think (but do not know for certain) that the way a flash gun produces less than 10% power is NOT to fire a "half-intensity" flash of standard duration, it is to fire a full-intensity flash of shorter duration. (Your figures above seem to confirm at this.)

Now, back toi the OP's problem.

Jeffde, the first thing to do is forget about high-speed sync. I don't mean do not use it, I simply mean that apart from allowing a greater range of shutter speeds, it works EXACTLY THE SAME as ordinary flash. So, from the point of view of setting your exposure, just ignore HSS - expose as per normal flash.

If your camera and flash gun can do this, I reckon the best thing is to always set up for HSS no matter what the circumstances. HSS every time, If your shutter speed drops below 1/250th, you don't actually need it, but who cares? Just set it to HSS and forget about it. Simple! Simple is good!

Now you can use any exposure you like (within reason) and the flash won't care if that is f/8 and 1/125th or f/2 and 1/2000th. (I'm not sure what the actual upper limit for HSS is, but I've certainly used it successfully up around 1/2000th with my 1D III and 580EX II.)

Now, to business.

First, you need to tell the flash not to blast away and make everything too bright - you want fill flash, not actual in-the-dark flash photography - so set some negative exposure compensation. Any amount you like to start with, you can adjust it later.

Second, you need to tell the camera to under-expose just slightly. One stop under might be a good starting point. Again, you are going to fire a terst shot or two and adjust to taste.

How are you going to under-expose| You can either:

1: use aperture priority and set (say) -1 stop of exposure compensation (Note - ordinary EC, not flash EC - they are different)

2: use full manual exposure and go a little bit under the "correct" value

Either method works. In practice, you will usually find that method #2 (full manual) is easier. Yes, it sounds more complicated, but it actually works out no worse, and often a bit easier. Use whatever aperture will give you the depth of field you want, then set the shutter speed so that it's not quite enough for correct exposure.

Now, take a test shot. You must look at the background and the subject individually, because we are exposing them dfifferently. We are using natural light to expose the background (the flash is not powerful enough to reach that far, or not enough to matter), but we are using BOTH natural light AND flash for the subject.


Background too dark? Set the shutter speed a little bit longer to let more light in.

Background too bright? Set the shutter speed a little bit faster to darkern it.

Subject too dark? Back off on the -ive flash exposure compensation.

Subject too bright? Use more -ive flash exposure compensation.

Test again, adjust again if required, and you are ready to rock.

Enjoy!

[jeffde]

Tony and all. Thanks for all this info.
I suppose for weddings I will set on AP on high sync and go from there.
I tend to not like using manual for weddings as with all thats happening i tend to concentrate on the composition as much as possible and adjust Aperture depending on the effect i want (Ie DOF).
As Tony said if the SS drops below 1/250 it doesn't amtter normal flash operation applies.
I've seen on some other sites that's what others do.
The cut and dry is that i think this is a great feature i wish i had investigated more a year or 2 ago - would have made a big difference to some of my shots in bright sunlight.

What i really like is the amount of IP (Intellecutual Property) on AP - its truly amazing the amount of knowledge members have on this site....

[Trev]

thanks for the comparison shots - educational in themselves

ta
Trev