Veteran Member Join Date: Sep 2007 Location: Bay Village, Ohio USA |
HSS is a neat tool, but its lousy for stopping action. In effect, it turns the flash into a continuous light source, by flashing repeatedly. Its good when you need a high shutter speed to overcome ambient light.
A while back on this or another forum, someone posted comparison pictures. Using a Pentax AF540-FGZ, in normal flash mode, with a shutter speed of 1/180, they were able to freeze the blades of an electric fan. In HSS mode, with a shutter speed of 1/500, the blades showed quite a bit (about two inches) of movement and were blurred almost to invisibility.
A focal plane shutter, as used in all Pentax dslrs, there are two curtains, an opening, or leading cutrain and a closing, or trailing curtain. When you trip the shutter, the leading curtain opens. After the appropriate time, the trailing curtain closes. At speeds of 1/180 and slower, there is a brief period during which the entire sensor is uncovered. At faster shutter speeds, however, the trailing curtain begins closing before the leading curtain is completely open, resulting in a moving slit across the sensor. The faster the speed, the narrower the slit.
This maximum speed, which is 1/180 on all Pentax dslrs, is called the X-synch speed.
If you have, or can borrow a film slr, you can easily demonstrate what happens if you take a flash photo at a shutter speed above the x-synch speed. Get a cheap roll of drugstore film, set the shutter speed to 1/1000, turn your flash on and take a photo. When you have the film developed, you will see that probably less than half the negative has been exposed.
The flash duration on most speedlights in normal mode is usually about 1/1000 second or faster. The lower the power setting (if the flash has a power setting), or the closer the object is, the shorter the flash duration. Many go as fast as 1/30,000 or even 1/50,000 second at the lowest power setting.
In other words, the flash duration, not the camera shutter speed, controls the exposure. So, at 1/180, the flash duration may be 1/5,000 second, which will stop most action. The problem comes in when you are shooting in bright enough ambient light that you can get a recognizable image using 1/180 and whatever aperture you're using.
That's where HSS comes in handy. It allows you to shoot faster than 1/180, while still maintaining the same aperture. It does this by flashing the strobe several times, as that moving slit travels across the sensor. Each flash exposes a different area of the sensor. In this case, the camera shutter controls the action-stopping, not the flash. So, unless you can shoot at the camera's top shutter speed of 1/4,000 second, you probably won't get that frozen water droplet you're looking for.
The good news is that that Yongnuo 460 can be cranked down to, I believe, 1/16 of full power. At that power setting, I believe the flash duration is 1/30,000 second, which will stop even a speeding bullet (the problem there is triggering the flash while the bullet is still in the lens's field of view).
A flash tube is not like an incandescent light bulb; it can't be dimmed. The only way to control its light output, is to control how long it is on. This is done by sophisticated electronics that quench the flash prematurely.
If you want to freeze water drops and such, set things up with the water source only a couple of feet away from the camera. Set the flash to its lowest setting and experiment with triggering the flash. Triggering it by hand, be prepared for a lot of misses.
Many photographers who try this find it easier to focus manually, turn the room lights off, trigger the shutter at a very long speed (1 second, e.g.) and trigger the flash manually, using the test button on the flash. There's no camera made that can autofocus that fast, and you'll be shooting on a tripod, so you don't need SR. The biggest obstacle with ultra-high speed photography, is triggering things at precisely the right moment.
Paul Noble
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