[kenyee]

You may be able to get 1/180 if you have your shutter speed adjustment set to 1/3 stops rather than 1/2, so I am willing to be wrong on this.

Yep. This.
I run w/ 1/3 stops usually and 1/180 is on my rear dial...

[Ben_Edict]

I was using shutter speed 1/60.
I change the shutter speed to 1/160 and the result is similar (not underexposed).
Does the flash know that the camera's shutter is set to 1/60 or 1/160?
It seems that the flash auto-compensate for faster shutter speed.
Is this right?

The fastest shutter speed is 1/160. Above that, the flash won't fire.
What's the reason behind this?

cheers

The shutter speed is secondary for flash photography, as long as it is long(!) enough to allow for a fully illuminated image. You see, a camera shutter located at the focal plane, as is the case with our Pentax DSLRs and all other DSLRS and 35mm SLRs (film), has 2 curtains. One is closed over the sensor or film. When you take an image, this first curtain travels out of the way (either horzontally or vertically) and makes place for the light to reach the sensor or film. Then the second curtain follows and closes over the sensor - the exposure ends.

This only works for long exposure times up to 1/180s in the case of Pentax (some other camera may reach 1/250s and older film cameras will only reach 1/60s for a fully open curtain shutter)

At shorter exposure times (say, anything shorter than 1/180s, namely 1/150s, 1/500s etc.) the curtain's mass will prevent such a fast action: 1st curtain out of the way, 2nd curtain closing. The shutter would fall apart due to the acceleration and breaking forces.

Thus engineers apply a trick: the first curtain starts to move out of the sensor plane and the second curtain follows immediately. What you now get, are two curtains travelling across the sensor with some distance between them. This "distance" is the travelling slit. The width of the slit depends on the shutter speed: the shorter the speed, the smaller the slit.

What that has to do with flash photography?

Easy enough: in flash photography the light will usually be provided by the flash alone. It usually has a short duration of 1/1000s at max, more often much shorter. To illuminate the whole sensor it is clear, that the sensor must be fully open to the flash light, aka the shutter speed must be so long, that the first curtain is gone and the second curtain is just not closing again. So this is 1/180s with Pentax. This is then called the sync-time - but any longer shutter speed will do just fine.

If you use a shorter exposure time (because you are taking images in bright daylight), you have a problem, because the flash duration is so short, light will only fall through the small travelling slit onto your sensor. You will only find a small strip of the image than properly exposed by the flash.

(Now, let us forget HSS (High Speed Sync) for the time being, as only a couple of flash guns support this anyway.)

As 1/180s is the shortest time a flash can fully illuminate the sensor, Pentax decided to simply cut off*the whole flash synching for any shorter exposure times. That is understandeable to a degree, but can be very annoying, because it prvents us from using GPS devices in the hot shoe for automatic ge-tagging.

Longer expsoure time allow also the use of flash. But for the flash illumination the shutter speed is irrelevant, But at longer speeds the ambient light may play a more and more significant role for the overall exposure, if you are not shooting in near total darkness.

I am already too long and stop here!

Ben

[bodhi08]

Yep. This.
I run w/ 1/3 stops usually and 1/180 is on my rear dial...

Actually, 1/3 stop increments did not give me 1/180s. It is the 1/2 stop increments.

[bodhi08]

Thanks a lot for all the replies.

As 1/180s is the shortest time a flash can fully illuminate the sensor, Pentax decided to simply cut off*the whole flash synching for any shorter exposure times. That is understandeable to a degree, but can be very annoying, because it prvents us from using GPS devices in the hot shoe for automatic ge-tagging.

Longer expsoure time allow also the use of flash. But for the flash illumination the shutter speed is irrelevant, But at longer speeds the ambient light may play a more and more significant role for the overall exposure, if you are not shooting in near total darkness.

I am already too long and stop here!

Ben

Ben:
Thanks for the long and detailed explanation. You save me reading few books on flash photography

I like to use the longest exposure time that I can get away with. Because I like to capture the ambient light (thus the atmosphere) especially at night.

That is why I really like the slow-speed sync feature.

I can illuminate the model(s) and still get the ambient light while people walking by will be blurred. And I can use the pop-up flash to do it.
It saves me some weight and space in the bag

cheers

[Wheatfield]

Actually, 1/3 stop increments did not give me 1/180s. It is the 1/2 stop increments.

This is correct.

[kenyee]

Actually, 1/3 stop increments did not give me 1/180s. It is the 1/2 stop increments.

Whoops...forgot I set it to 1/2 stop increments for HDR...gives you the max bracketing range too :-)