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01-28-2010, 03:06 PM   #1
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Does this seems correct? HSS

Hello guys,

I finally came about testing both my K10D / Kx HSS capabilities with the AF540.

First I tested just basically shooting the computer desk, and it actually worked fine, and I went all the way up to 1/4000th !! I think it was impressive..


But then I got this small kitchen fan, placed a piece of paper on one of the blades, and started shooting from 1/90 up... I have no idea how to explain this, but it seems counter intuitive to have had the following results:


[trying to remember imageshack again, so bear with me!]

1/90:


1/125:


1/180:


1/250:


1/350:


1/500:



Why is that only the speeds BELOW Xsync got frozen, and the higher speeds, it seems like it dragged?

I suspect it had to do with the fact the flash is "washing" the image thru the HSS... but if this is the case then, whats the point of HSS?

Can someone explain?


Thanks in advance,

PS: both KX and K10D had the same results.

01-28-2010, 03:28 PM   #2
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If I remember reading correctly, HSS uses several small bursts of light, instead of single bursts when you're using x-sync or under shutter speeds. That should account for your results, but I would wait for someone who's more knowledgeable on the matter to chime in
01-28-2010, 03:43 PM   #3
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Could you also post the aperture and ISO beside each photo? Those values are needed to figure out what's going on.
01-28-2010, 04:27 PM   #4
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QuoteOriginally posted by ma318 Quote
Could you also post the aperture and ISO beside each photo? Those values are needed to figure out what's going on.
ISO and aperture values are not needed to figure this out.
Use your eyes and look at the pictures.
They are multiple exposures.
SystemA has it right.
Each burst of the flash captures the moving fan blade in a different point of it's travel.
It's a neat effect, one that hadn't occurred to me as a side advantage of HSS.

01-28-2010, 04:33 PM   #5
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QuoteOriginally posted by ma318 Quote
Could you also post the aperture and ISO beside each photo? Those values are needed to figure out what's going on.
these values most certainly are not needed...


OP, remember that after 1/180, your shutter moves as a slit across the sensor,

the faster the shutter the speed, the narrower the slit

since the fan spins pretty fast, the slit moving down up (or up down?) you record the paper at every interval, creating motion.

that is why going above sync speed is not always the correct approach, you have to know what you are photographing.
01-28-2010, 05:04 PM   #6
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sort of like how, on the car forums, people always ask "what color is your car? we need that before we can help you."
01-28-2010, 05:17 PM   #7
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Those info are needed to better explain what's going on....

I notice all your shots were taken at ISO 200 and f2.8 with shutter speed 1/90 or faster. I think at these settings indoor, there should not be enough ambient light to light up any of your photos. They would all had to be lighted by the flash.

Typical fan speed is 1200 RPM which means 20 revolutions per second or 1 revolutions per 1/20 second or 360 degrees per 1/20 second. To freeze the motion of the fan, you probably don't want the fan blade to move more than 1 degree during exposure. And the fan will move 1 degree in 1/(20X360) or 1/7200 sec.

Your flash at minimum power will flash for a duration of 1/20000 sec and maximum power for 1/1200 sec.

For your three photos at or below the sync speed, the flash happened to flash at close to minimum power. Thus the fan blades were all frozen by the flash firing at around 1/20000 sec to 1/10000 sec. Note the shutter speed would not have affected the flash exposure. It would only affect the ambient exposure which in this case would be very dark and have no effect on your photos.

In high speed sync mode, the flash fires continuously, many thousands of times per second. All these pulses of light essentially merge together into one long "pulse" that stays on the entire time the shutter is open. So for HSS, the light from the flash kind of becomes the "new ambient light". This resulted in capturing the fan movement. With HSS, the fan movement will have to be freezed by the shutter speed. You can actually see less fan blur as the shutter went up to 500. You should get less blur at 1/4000. But still not be as good as the ones you got at or below sync speed with the flash.

(I think I said in another post that HSS would fire the flash a discreet small number of times. I think I was wrong. It actually flash thousand of times in a second.)

Last edited by ma318; 01-28-2010 at 09:57 PM.
01-28-2010, 05:44 PM   #8
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Yeah!

QuoteOriginally posted by ma318 Quote
Those info are needed to better explain what's going on....

I notice all your shots were taken at ISO 200 and f2.8 with shutter speed 1/90 or faster. I think at these settings indoor, there should not be enough ambient light to light up any of your photos. They would all had to be lighted by the flash.

Typical fan speed is 1200 RPM which means 20 revolutions per second or 1 revolutions per 1/20 second or 360 degrees per 1/20 second. To freeze the motion of the fan, you probably don't want the fan blade to move more than 1 degree during exposure. And the fan will move 1 degree in 1/(20X360) or 1/7200 seconds.

Your flash at minimum power will flash for a duration of 1/20000 sec and maximum power for 1/1200 sec.

For your three photos at or below the sync speed, the flash happened to flash at close to minimum power. Thus the fan blades were all frozen by the flash firing at around 1/20000 sec to 1/10000 sec. Note the shutter speed would not have affected the flash exposure. It would only affect the ambient exposure which in this case would be very dark and have no effect on your photos.

In high speed sync mode, the flash fires continuously, many thousands of times per second. All these pulses of light essentially merge together into one long "pulse" that stays on the entire time the shutter is open. So for HSS, the light from the flash kind of becomes the "new ambient light". This resulted in capturing the fan movement. With HSS, the fan movement will have to be freezed by the shutter speed. You can actually see less fan blur as the shutter went up to 500. You should get less blur at 1/4000. But still not be as good as the ones you got at or below sync speed with the flash.

(I think I said in another post that HSS would fire the flash a discreet small number of times. I think I was wrong. It actually flash thousand of times in a second.)
That sounds as if it could be believable, I think!

01-28-2010, 06:25 PM   #9
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I hope its correct. If not, I welcome other explanations. Its an interesting "problem".
01-28-2010, 06:38 PM   #10
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QuoteOriginally posted by ma318 Quote
I hope its correct. If not, I welcome other explanations. Its an interesting "problem".
No, I think it sounds like a plausible explanation. It's just over my head.
01-28-2010, 06:48 PM   #11
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Just think back to the diisco days and the strobe lights. Or a car timing light.
01-28-2010, 07:11 PM   #12
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Here is someone's detailed explanation on HSS. Its for Canon but should work the same for all brands.

Flash Photography 101, Chapter 4 - Guide Numbers and High Speed Sync - Canon Digital Photography Forums

Here is someone whom had confirmed what the OP has found with HSS.

High Speed Shutter vs. Ordinary Flash Sync


Conclusion is that you can not use HSS to freeze very high speed motion - especially high speed circular motion. This applies to all brands' HSS.

Last edited by ma318; 01-28-2010 at 07:47 PM.
01-28-2010, 08:38 PM   #13
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Couldn't we just turn off the fan ?

Sorry...could not resist.

Thanks for the fine explanation ma319.

Last edited by bogiesbad; 01-28-2010 at 09:29 PM.
01-28-2010, 09:20 PM   #14
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Thanks for raising this topic. I had noticed this when taking a photo of a hummingbird once - and I had not considered that I could get a better capture by dropping my shutter speed to x-sync.
01-28-2010, 10:38 PM   #15
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QuoteOriginally posted by bogiesbad Quote
Couldn't we just turn off the fan ?
That certainly would be the most practical "thinking out of the box" solution!
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