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01-10-2011, 05:54 PM   #1
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Rear curtain and slow shutter speed flash sync question for the 645D

So there I was yesterday afternoon, sitting in my lounge room gearing up for a shoot in the CBD of Perth City last night and thought I'd better make sure I know exactly how to enable rear curtain sync.

Bugger

In full manual mode I've got the choice of Flash on or red-eye reduction on and that's it. In P, AV and another mode (forgotten) I get the added choices of X-sync (slow speed) or X-sync with red-eye reduction.

Reading the manual shows me I can have rear curtain using a one or two dedicated Pentax flashes and enabling it with the flash, but unless I'm mistaken and have missed something, with the 645D I can't enable a rear curtain flash sync from the camera to fire a manual flash on the rear curtain.

I read up on X-sync and the manual is very light on telling you how to turn it on and in which modes you can use it, but nothing on what it is and why you'd use it.

In Manual mode and a 3 second exposure I wanted to fire a manual flash via a generic radio trigger (Cactus V4) at the end of the 3 seconds and could not make it happen.
Going into Av mode and selecting X-sync made no difference what so ever to the regular flash setting, X-sync selected via the flash button on the rear, 3 second exposure from the F3.5 I had set, flash fired right at the start when the trigger fired.

Regular flash on setting lets me fire the flash at a slow shutter speed when X-sync isn't an option, so what's X-sync there for and what's it do that regular flash on/off doesn't?

As it turned out I gve 1600ISO a go and just used a bit of fill flash fired normally and LOVE the results - pics to follow later, probably next week, as I got home around 11pm, am caught up with family stuff today and have another shoot tomorrow then a weekend away for a birthday celebration coming up.

01-10-2011, 07:49 PM   #2
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This really confused me when I got the flash a couple of weeks ago. I am used to setting it on the camera.

I have used the X-sync mode mainly out of convenience when using my strobes. I believe it stop the camera from going faster than 1/125...then again I maybe completely missing the point of this mode.
01-10-2011, 08:08 PM   #3
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As far as I can tell - using the X mode just locks the camera at 1/125th ....usefull for my studio work but useless for last night when most shots were around the 1/30th of a second mark.

The X-sync flash option that you can access from the Flash mode button when in P or Av mode didn't seem to do anything different to the regular Flash On option - so guessing these come into play when using a Pentax Flash that can "talk" to the camera and is otherwise useless for other flashes or strobes.

That said - It's not a big issue for me, no regrets, and I'm still LOVING the 645D ... just a mild dissapointment that some rear curtain work I wanted to try won't be happening now.

@ Pentax if you're listening, another thing to add to the list for future firmware updates:
- Tethered shooting ability
- Higher flash sync speed (even if it's just a bump up to 180th or 250th)
- Rear curtain sync Flash Mode to work with any generic flash/strobe.
01-11-2011, 07:40 AM   #4
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I really doubt that they can change the sync speed with a firmware, if they can then that would be awesome.

01-11-2011, 08:22 AM   #5
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On the K10D/K20D, X-sync just locks the shutter at the max sync speed.
I rarely use it. I just use full manual mode and set aperture/shutter to what I want for flash exposure, and ambient exposure respectively. Anything above max sync speed won't fire the strobes.
Also AFAIK, the rear curtain sync is tied to the flash (i.e., not something you can set on the body.

And no, they can't change max sync speed w/ firmware, though they could allow it to fire at all shutter speeds so people can get black bars on part of their images (but they probably didn't allow that so they don't have to field complaints from people for "broken" cameras ;-)

p.s., can't believe you're using Cactus triggers w/ a 645D :-O
01-11-2011, 05:04 PM   #6
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QuoteOriginally posted by kenyee Quote
p.s., can't believe you're using Cactus triggers w/ a 645D :-O
Why not - they work!

I've had my V4s for ages now, they've been knocked off tripods, dropped, kicked and generally abused more than should be reasonably expected and they still keep working. They're one of the most long lived and reliable pieces of kit when it comes to accessories that I've ever had the pleasure to use.

I mainly use strobes with manual settings, so just need something to fire them - no wireless control of lighting is needed for the stuff I do. When I do use flash it's manual units, never liked TTL and the like, so again the V4s do what I need them to do - just fire the flash when I hit the shutter.

I've used a range of more expensive triggers when hiring other studios or tying gear that friends have loaned me - yet if/when the V4s die, they'll be replaced with another set.

As for firmware Vs Sync speed - I've yet to meet a software engineer that'll admit something can't be fixed/improved by an upgrade .... so I'll put the challenge out there and see if the gurus at Pentax can't prove us wrong
01-27-2011, 07:47 PM   #7
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UPDATE

I borrowed a friends Metz flash that she bought for using with her K20D. I forget the model number but it is a Samsung/Pentax specific unit and one of the higher end models.

I tried it on the 645D and it works perfectly - I had access to TTL, hi-speed sync, slow-speed sync, rear curtain sync ..... all worked as they should and the shots were exposed pretty well.

On TTL HSS (High Speed Sync) I tried 1/250th and it worked fine (no black bars), haven't tried faster yet.

Rear curtain is a bit weird in that there's an initial low power flash at the start of the exposure, then a "proper" flash at the end on the rear curtain ... or is that the "norm"?

Pretty happy with that and will now chase down a Metz unit of my own to add to the kit. If I can then get all this to work off-camera I'll be even happier
01-28-2011, 07:59 AM   #8
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It occurs to me that the way Pentax dedicated flash units (as well as units from other manufacturers that implement Pentax protocols) do rear curtain sync may be by inserting a carefully timed delay, which they can do because they know the shutter speed setting and shutter characteristics, avoiding the need for a physical sensor associated with the rear curtain. Which would explain why it's not available at the camera.

Tim Wilson
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01-28-2011, 08:43 AM   #9
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QuoteOriginally posted by Warped Quote
Rear curtain is a bit weird in that there's an initial low power flash at the start of the exposure, then a "proper" flash at the end on the rear curtain ... or is that the "norm"?
What you're seeing is probably the normal pre-flash with today's pseudo TTL.
Instead of having a TTL sensor to cut off flash power like they did in the old days, a low power preflash is used to grab a quick snapshot of the scene to figure out how much full power flash to add to it...that's why the cameras today have big lookup tables of scenes to guess how to set the power level to...
01-28-2011, 11:36 AM   #10
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QuoteOriginally posted by kenyee Quote
What you're seeing is probably the normal pre-flash with today's pseudo TTL.
Instead of having a TTL sensor to cut off flash power like they did in the old days, a low power preflash is used to grab a quick snapshot of the scene to figure out how much full power flash to add to it...that's why the cameras today have big lookup tables of scenes to guess how to set the power level to...
Oh, that's what they are doing. Some studio strobes have a feature to not fire on the first flash and I was thinking why not just turn off the red-eye feature in the camera instead.

Last edited by tuco; 01-28-2011 at 03:03 PM.
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