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Flash sync

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HuntedDragon
17 Feb 2010 - 9:04 PM

hi all, just been reading through too many articles here and there and came across something that made me think for a bit, 'the importance of the correct flash sync speed'..

as far as i'm aware, if the flash goes off before or after the shutter opens and closes then it's a resulting 'black image', understandable because the light would have come and gone when the shutter was closed...

if it goes off to 'close' to the opening or shutting then only a part of the flash will be exsposing. so i was thinking the flash to camera sincronising was just a matter of the two things coinsiding at the same time...(so why such a slow shutter speed like 1/60 or even the 1/200'th? surely you could really up the speed with a flash).

the question being why is the sync speed so crutial? (i'm thinking the setting in camera for 1/60 or 200 under sync speed) (the faster the shutter the darker the slower the lighter, with or without flash).

did i just read it rong?

Last Modified By HuntedDragon at 17 Feb 2010 - 9:07 PM
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steve_kershaw
17 Feb 2010 - 9:19 PM

you need to try and seperate your thinking towards flash and ambiant light,

your flash duration is faster than your maximun shutter speed, above 1/8000th, the reason you have a maximum sync speed is to do with how the camera talks to the flash, bit it will sync from bulb to 1/200th (depending on your camera)

if you are using just your flash to light your subject then, the shutter speed is not important, as the flash duration is so fast it wont make much diferance, but typicaly you would use 1/125th to controle camera shake, you apature is important because this in conjunction with your power output from your flash will determin your exposure

typicaly you wont use just your flash to expose your shot, but a combination of flash and ambiant light, setting your camera to automatic will usually select around 1/60th f5.6 ish this is to allow more ambiant light with the lower shutter speed 1/60th and enough flash power to help reduce camera shake,

steve_kershaw
17 Feb 2010 - 9:36 PM

just to add (after reading your post again lol)

if you set your camera to M, Program TV, AV, then (depending on your menu settings) you can increase your shutter speed and it will not work properly

for instance you are taking a shot outdoors with a light backdrop, you want to iluminate your subject, the meter reading is showing a shutter speed of 1/350th, (asuming an xsync speed of 1/250th) when you take the shot you will only iluminate the top 3/4 of the shot with the flash, though the rest of the shot will be exposed by the ambiant light,
if you increase your shutter speed to 1/500th the you will only iluminate the top 1/4, and above 1/500 none of the shot will be iluminated by the flash, resulting in a bright background and dark subject,

decreasing your ISO, and increasing your apature will help keep your shutter speed down to within your xsync speed

HuntedDragon
17 Feb 2010 - 9:38 PM

hmm thank you for that, but a further question,
'a maximum sync speed is to do with how the camera talks to the flash' is this in the respect to how fast the electronic signal takes to reach the flash..to say set up E-TTL or something along those lines?

i'd agree with the apature being important...espetialy in bright light as well as for depth of veiw lol.

HuntedDragon
17 Feb 2010 - 9:42 PM

steve...by top 3/4's do you mean 3/4's of the shutter open time from when it opens, or just the top of the subject?

discreetphoton
discreetphoton Site Moderator 93453 forum postsdiscreetphoton vcard United Kingdom20 Constructive Critique Points
17 Feb 2010 - 9:48 PM

If you are shooting full-flash (ie. don't want any ambient light to register) then using the sync speed is the most efficient use of power.

At maximum sync seed, the camera cuts out more of the ambient light than it does at slower speeds, so the flash exposure doesn't have to climb over any highlights to register properly.

steve_kershaw
17 Feb 2010 - 9:51 PM

Its to do with the timing of the flash, not the duration, its the internal workings of the camera

The 3/4 coverage is called banding, (may be worth a google)

basicly its to do with the shutter stopping some of the flash reaching the camera, If you set your camera on M and make shure your flash is enabled in the settings, take an image in a dark room so there is litte ambiant light, set the shutter speed to 200 (apature is not important) and you will see a full image lit buy your flash (you may need to set manual focus) increase a stop at a time and you will see a black band appear at the bottom of your image, untill at above 1/500th your image will be completly blacy,

HuntedDragon
17 Feb 2010 - 9:52 PM

discreet photon...so the sync setting is the optimum for a brighter flash?

sorry about the further questions guys... i literaly picked up a camera and played with the dials n numbers and had a look at the images.. then i started reading..

HuntedDragon
17 Feb 2010 - 9:54 PM

ahhh, thank you steve

dlegros
dlegros  12217 forum posts England
17 Feb 2010 - 10:02 PM

I know you said you've read lots of articles, but try the Wikipedia entry on flash synchronisation.

In essence, most SLRs use a focal place shutter which is a pair of curtains covering the sensor (or film!).

To expose the sensor to light the first curtains opens and travels across the sensor.

To end the exposure, the second curtain then travels across the sensor covering it.

At slow speeds, the 1st curtain opens exposing the sensor completely then the 2nd curtain closes.

To get high speeds, the sensor is not fully exposed, instead the 1st curtain partially opens, then the 2nd starts so only a small section of the sensor is exposed to light.

The sync speed on your camera is the fastest speed at which the entire sensor is exposed at once.

As a flash emits light for a incredibly small amount of time (down to 1/40,000 or so) if your shutter speed is above the sync speed, only the open slit between the edges of the curtains is exposed to the light from the flash so you only see the effect of in a section of the image.

HTH

steve_kershaw
17 Feb 2010 - 10:06 PM


Quote: so the sync setting is the optimum for a brighter flash

no
the flash duration is arount 1/8000th of a second, for just flash output to light the subject the shutter speed makes no differanse (below the sync speed,)

the shutter speed controlls the ambiant
the apature controlls the ambiant and flash
the iso controlls the ambiant and flash

HuntedDragon
17 Feb 2010 - 10:41 PM

ahahhh, thank you all..

mad-dogs
mad-dogs  122201 forum posts England
17 Feb 2010 - 10:52 PM

Flash speeds for a Nikon SB-900

* 1/880 sec. at M1/1 (full) output
* 1/1100 sec. at M1/2 output
* 1/2550 sec. at M1/4 output
* 1/5000 sec. at M1/8 output
* 1/10,000 sec. at M1/16 output
* 1/20,000 sec. at M1/32 output
* 1/35,700 sec. at M1/64 output
* 1/38,500 sec. at M1/128 output

If you use a shutter speed of 1/8000 you will not get the benefit of full power from your flash.
Your sync. speed is where your shutter is synchronised to be open when the flash is triggered.
Many digital cameras can synch up to 1/8000, but you do not get the full flash output as you can see from the table.

HuntedDragon
18 Feb 2010 - 12:42 PM

canon lists at up to 1/200..thats really slow.

Last Modified By HuntedDragon at 18 Feb 2010 - 12:44 PM
justin c
justin c  104526 forum posts England36 Constructive Critique Points
18 Feb 2010 - 1:06 PM


Quote: Canon lists at up to 1/200..thats really slow.

They'll sync at any speed with the appropriate flashgun. You just enable High Speed Sync on the flashgun.

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