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04-22-2010, 05:49 PM   #16
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It locks the shutter at the x/sync speed, thereby ensuring that twiddling the rear dial by accident won't screw up a shoot.

04-22-2010, 11:40 PM   #17
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X mode - why and the history of it.

This is a hangover from film days. A long time ago, my first 35mm camera had an x mode - it was necessary to synchronise the shutter for electronic flash - the more usual mode was to use flashbulbs at slower speeds - high speeds (up to 1/180s) were no good for flashbulbs as they took a measurable amount of time to reach peak output before burning out.

Electronic flash is virtually instantaneous and was used at higher speeds than flash bulbs.

Also, and this is important, the electronic flash was only triggered when the shutter had fully opened, hence a special synch mode shutter switch was brought into action when x mode was selected.

If the same 1/180 speed was used in normal mode (i.e. non synch mode), the shutter would trigger the flash as it was still opening (as it needed to do for flash bulbs). This would mean that the electronic flash would fire off as the shutter started to open and no illumination would be recorded.

Today, since we all use electronic flash, the x mode is a redundancy as modern cameras all trigger the flash after the first curtain has opened but before the second curtain starts to close.

At speeds faster than 1/180s, the two shutter curtains move as a slit across the sensor - at or below this speed, the sensor is fully exposed before the second curtain closes.

Since they move as a slit, any single flash pulse will show up as a bar of light across an otherwise dark image.

High speed mode uses a series of flash pulses (a stream of light) and in this way illuminates the whole scene as the narrow slit between first and second curtains moves across the sensor.

About 30 years ago, the max flash speed was only 1/90s as the curtains moved across the long dimension of the 35mm frame (i.e. horizontal direction). Modern cameras have the curtains moving vertically, which enables doubling the synch speed to 1/180s.

So it is not an arbitrary decision by Pentax - it is a design limitation caused by the SLR shutter design.

I foresee a time when the shutter will be a sheet of LCD material (or similar) with instantaneous black/clear response instead of the old fashioned mechanical shutters still being used.

When that happens, any shutter speed will be able to be used with any flash.

Some folk have learned to rely on X mode instead of setting speed in Tv or M modes. It is a user preference, not a requirement of the camera. Technically, the X mode is redundant.

The KX does not have an x mode but of course the synch speed of 1/180 is available through the other modes.

I hope this explanation will clear things up.

Best wishes.
04-23-2010, 05:45 AM   #18
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Is there a radio trigger that has HSS capability for pentax/k20?
04-23-2010, 07:45 AM   #19
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QuoteOriginally posted by jlaubza Quote
About 30 years ago, the max flash speed was only 1/90s as the curtains moved across the long dimension of the 35mm frame (i.e. horizontal direction). Modern cameras have the curtains moving vertically, which enables doubling the synch speed to 1/180s.
To refine the timeline a little - vertical run metal focal plane shutters were availlable in the late sixties (usual synch speed 1/125), which is more like 40+ years. Boy, that makes me feel old

04-23-2010, 07:46 AM   #20
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QuoteOriginally posted by hoanpham Quote
Is there a radio trigger that has HSS capability for pentax/k20?
Not that I'm aware of. I know Canikon has a few options, but they are all fairly expensive (Radio Poppers, and some variant of a Pocket Wizard)

I went cheap with a set of Cactus V4 radio triggers. No E-TTL/HSS, but I'm glad I got to learn more about manual flash.
04-23-2010, 10:50 AM   #21
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Maybe I'm alone here, but when I first read the specs on the soon-to-be-released K10D, I was quite excited about the X mode.

I generally shoot manual in two scenarios:
1) low-light, long-exposure stuff, where you pretty much have to, and
2) manual off-camera flash stuff, where you pretty much have to.

So one night, I'm out shooting 15-second nighttime exposures. Next day, I go in and do some studio stuff. I put the dial on M and I have to spin the dial 35 notches to get from 15" to 1/180. That scenario happened to me over and over until I got my K10D, and I'm happy as a clam.
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