Jump to content

What happens when the flash duration is shorter than the shutter speed on a leaf shutter?


paulrumohr

Recommended Posts

The Qflash T4D has a flash duration of something like 1/320 sec at manual full power.

 

Since Hassy lenses sync at 1/500, the shutter isn't open for the entire flash at this setting.

Using my light meter, I notice that going between 1/2 and full power on the QFlash

doesn't yield a whole stop when the shutter speed is set to 1/500 on the meter.

 

I'm suspecting that the net effect is just simply a less effective flash. I don't believe there

would be any difference in the action stopping effect for moving subjects.

 

But would there be some strang kind of vignetting? My first instinct would be to say no,

because of the shape of the iris as the leaf shutter closes down (relatively round).

 

I'm planning to test it out today, but any predictions? Any other ill effects I'm not

considering?

Link to comment
Share on other sites

Paul,

 

The flash being shorter duration than the shutter time is what flash sync is all about. Many modern flases have a duration in the 1/2000 range - the important thing is that you do not exceed the shutter sync speed of your camera. If you do, you will get partially or improperly exposed frames.

 

Some neat effects can be had by using fill flash in low light environments where there is movement. For example, some neighbor kids were shooting hoops on my driveway late one evening and the sun had been below the horizon for about 20 minutes. To shoot without flash, I would have had to bump the ISO up to at least 400... possibly 800. Instead, I left the ISO (it's a digital SLR) at 100 and used fill flash. To expose the background, the camera (in program mode) set the shutter speed down around 1/30th of a second. The on-camera flash (Canon 550EX) metered to expose the subject (the boys jumping for the hoop). The resulting image was really neat: the boys and the ball, as well as the background, were sharp. The background was about 1-1/2 stops below the subjects. The slow shutter speed and movement caused a rather eerie halo around the subjects - in essence, the subjects as exposed by the natural light/slow shutter speed combo (longer duration than the flash time). The sharp version of the boys was what was exposed by the flash - very short and, therefore, sharp. You can see this effect in photos on Photo.net and other sites from time to time.

Link to comment
Share on other sites

Flash duration of 1/320th sec is longer than shutter speed duration of 1/500th sec.

 

Think about it. The shutter isn't staying open for the full duration of the flash pop. The shutter is only staying open for a fraction of the strobe pop.

 

1/500 is shorter period than 1/320. Take your calculator and divide 1 by 500 then 1 by 320 and you'll see what I'm talking about.

 

You've mentioned it correctly when you state:

 

"Since Hassy lenses sync at 1/500, the shutter isn't open for the entire flash at this setting."

 

Since the shutter isn't open for the full duration of the flash pop, you're going to get less exposure.

 

About the vignetting, nope, that's not how leaf shutters work. Think about a continuous light source, the Sun, and the fact the shutter when actuating goes from a small hole to a larger hole, then back to a smaller hole when taking a single exposure.

 

The only thing you'll see is hat you've already seen, less exposure from strobe pop than you'd expected.

Link to comment
Share on other sites

A leaf shutter takes time to open; then there is the dwell time; then there is the time to close. What this means is that the shutter speed is longer at F16; than F2.8; when a fast 1/500 sec is used. One can see this on a shutter tester. The shutter will be a good bit longer than 1/500; at F16; than at F2.8 .
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...