Max shutter speed with flash

[Alis]

Why is that when I use the flash on my camera, the shutter speed only can go up to 1/200? I know that if you can go higher you probably do not need flash, but what is the reason you don't have that option? I sometimes like to increase the flash output and increase the shutter speed to get a sharper image, why not? This almost looks like one of those universal constants in physics; wondering why...

[Colin Southern]

The shutter on your camera has two curtains. Below your x-sync speed (usually around 1/200th or 1/250th) both curtains open at the same time and the sensor is completely exposed. Above the X-Sync speed one curtain moves before the other so you get a traveling "slit" of light hitting the sensor (needed because shutters just can't physically move fast enough for higher shutter speeds).

This creates a problem for flashes because the flash itself normally only lasts around 1/2000th of a second - so if it goes off and your shutter speed is, say, 1/500th then only a portion of the sensor would be exposed - which is why you see portions of the frame blacked out when people exceed their X-Sync speed.

One way around it is to use high-speed-sync (Canon name for it - don't know the Nikon name) - rather than have a single 1/2000th of a second flash it actually pulses the flash so that the scene stays illuminated for the entire time the curtains are moving - but the tradeoff is that you get reduced range from the flash with the higher the shutter speed, the less the effective flash output.

Does this help?

[ajith.rajeswari]

Hi Colin,

Please ignore if this is a stupid question.

I read in an article sometime back that some of the new digital camera instead of a having physical shutter between the shutter and sensor there is some circuit breaker which stops the flow of signals from the sensor to the processing engine, if this is the case will it improves the flash sync speed as the virtual shutter is fast enough?

e.g Camera Casio EXILIM Pro EX-F1 (not sure 100%)

Thanks
Ajith

[Colin Southern]

Electronic shutters are common with point & shoot type cameras, but with one noteable exception that I know of, never really made any in-roads into SLR territory.

As far as flash photography goes (where the light from the flash is the primary source of illumination) shutter speed is pretty much irrelivant (below X-Sync speed). Case in point - if I put a flash on my camera and sat in a totally dark room - I could leave the shutter open for as long as I wanted; exposure would only occur when the flash went off - so in that respect it doesn't matter if the shutterspeed is 1/200th or 1/4 - it's only when ambient light also enters the exposure that things get more interesting - which is usually why we'd want to go above X-Sync speed (ie to minimise ambient light).

At the end of the day, high-speed sync works OK most of the time - things only start to get "interesting" when you need to shoot above X-Sync speed and HSS isn't available (perhaps when you've got the flashes off camera being triggered by a pocket wizard).

We don't seem to get much discussion of flash techniques here - happy to go into it a bit deeper if anyone wants to know anything specific about them.

[Davey]

which is why you see portions of the frame blacked out

I presume this is the same with 35mm film slr too as well as digital? The reason I ask is a while back I had a few images have blacked out portion of frame. I cannot remember many details but always wondered what caused it as I never had it before or after, just on those. I remember it was indoor shots in funny lighting and of a night so I turned the flash down from full belt as worried about glare and set the shutter longer than normal and thought even though was over what I'd been advised to keep it under I thought it might work. It didn't but I never worked out the black barring across a few images so wondered if could be this. Sorry if this is painfully obvious and I'm asking silly questions.

[Colin Southern]

I presume this is the same with 35mm film slr too as well as digital?

Yes - exactly the same.

Most shutters open from top to bottom (less distance to travel) - so typically the under-exposed portion runs horizontally, starting at the bottom.

It's not so much a problem when the flash is camera mounted as the cameras are smart enough to over-ride faster shutter-speed attempts, so the problem usually shows itself when triggering studio strobes or off-camera flashes via radio triggers - especially the former when you may need to set shutter speeds down closer to 1/125th (due to timing issues with the strobes, even though the camera can handle something faster).

Again - especially in a studio setting - it's usually not a problem as very little ambient light usually makes it in - thus the strobes are very successful at freezing motion (if you want to test how much ambient light is getting in then just fire off a shot inside at 1/200th - say F5.6 (with strobs off) and see what you get - it's usually pretty close to a completely black frame).

[Alis]

Above the X-Sync speed one curtain moves before the other so you get a traveling "slit" of light hitting the sensor (needed because shutters just can't physically move fast enough for higher shutter speeds).

So, is it fair to say that when we set the exposure on let's say 1/2000, the total time during which the "slit" is sweeping the sensor is actually more than 1/2000 and at least for a fast moving subject, the sensors will "see" the subjects in different positions? If so, how does this relate to more sharpness at higher shutter speeds?

[Raycer]

One way around it is to use high-speed-sync (Canon name for it - don't know the Nikon name)

Note for Nikon users, its FP (focal plane) flash.
D50, D70, D100 has combined electronic and mechanical shutter and can sync up to 1/500
the cameras after that can only sync up to 1/250.

[atvinnys]

note also that this might only be allowed by the body with an external flash...
my rebelXT doesn't let me set the shutter speed more than 1/200th with the internal flash.

(was gonna try my external flash, but batteries ran out...)

[Alis]

note also that this might only be allowed by the body with an external flash...
my rebelXT doesn't let me set the shutter speed more than 1/200th with the internal flash.

(was gonna try my external flash, but batteries ran out...)

Well, i tried it with an external flash last night on high-speed sync and at high shutter speed and got some stunning results indoor, exactly what I was trying to do before! As Colin mentioned, the range reduces and I guess the cycling time of the flash increases. But if you have a lot of spare batteries, I guess this is a good option for some special situations.

[Colin Southern]

So, is it fair to say that when we set the exposure on let's say 1/2000, the total time during which the "slit" is sweeping the sensor is actually more than 1/2000 and at least for a fast moving subject, the sensors will "see" the subjects in different positions? If so, how does this relate to more sharpness at higher shutter speeds?

You're on to it

It doesn't affect "sharpness" per sec - but you can get a phenominon called "Focal Plane Distortion" - Wikipedia have a photo which demonstrates this well ...

http://en.wikipedia.org/wiki/File:Fo...istortions.jpg

The same effect can do things like give you oval wheels on racing cars etc.

[Colin Southern]

note also that this might only be allowed by the body with an external flash...
my rebelXT doesn't let me set the shutter speed more than 1/200th with the internal flash.

(was gonna try my external flash, but batteries ran out...)

Yes - you will be able to go above 1/200th with an external flash attached.

[Colin Southern]

Well, i tried it with an external flash last night on high-speed sync and at high shutter speed and got some stunning results indoor, exactly what I was trying to do before! As Colin mentioned, the range reduces and I guess the cycling time of the flash increases. But if you have a lot of spare batteries, I guess this is a good option for some special situations.

Great stuff

If you're X-Sync speed is, say, 1/200th or 1/250th - and you're using HSS of FP mode with a shutter speed of around 1/250th to 1/1000th then normally you wouldn't notice too much difference.

Some flashes can chew through batteries pretty quickly - that's why I don't use internal ones. There are a few options, I use an Al Jacobs "Black Box" (www.aljacobs.com) - it's basically just an external lead-acid battery connected to the flash via a Quantum "fake battery" adaptor.