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10-25-2012, 09:42 PM   #1
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Q shutter speed with flash

I don't understand why there's a problem with the Q and flashes with the toy lenses. Can someone please explain it? WHY is the flash sync speed limited when using the electronic shutter? Why didn't Pentax build the shutter into teh body instead of the lenses for that matter.

Charles.

10-25-2012, 10:00 PM   #2
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QuoteOriginally posted by ChopperCharles Quote
Why didn't Pentax build the shutter into teh body instead of the lenses for that matter.
This would have been great. Kind of in line with the body side SR. <Scratching head.>
10-25-2012, 10:26 PM - 1 Like   #3
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I don't think the slow sync speed problem arises from anything other than platform design choices.

Leaf shutters have some nice properties, so choosing to use a leaf shutter system is a viable choice. One benefit is the cavity doesn't need to host a shutter. Hence, smaller body and smaller SR mechanism. For a body like the Q, this is a big deal. Pardon the pun. Other benefits exist, as well as other problems being introduced. But the choice to make leaf shutter lenses isn't inherently bad, just different.

Toy lenses and adapted lenses generally don't have a leaf shutter. So the camera needs to use an electronic shutter. But the electronics involved aren't instantaneous. The sensor output has to be scanned in batches to get all the data out.

Check out this picture by JohnMFlores, which shows the electronic shutter in action: Visions of Jello | Flickr - Photo Sharing!
You can see 10 independently scanned sections, each section providing a small horizontal band taken at one moment in time, then it waits a fraction of a second, then it moves on to the next band, leaving a vertical smear in-between bands. You can see the darkening bands prominently in the sky and the effects of the time dilation at the boundaries of successive scans where vertical blurring occurs. The perspective of the white lines indicate that the sections are read from image-top to image-bottom in sequence. Each mini-exposure lasts 1/800 of a second, just as specified in the exif data, but the entire scan obviously took longer, probably 1/13 of a second as described below.

I think its safe to assume that 1/13 of a second is enough time to populate the whole image frame in a buffer to permit scanning an entire frame. Anything faster has to be done in banded batches.

Anyway, you've got to expose at 1/13 or longer to ensure that the flash exposure is stored in each scanned band.

Eventually the electronic shutter will improve. The Q50 might have a "global shutter" as is fantasized by film geeks who also suffer from the jello effect even at faster shutter speeds. Ultimately, to make a small body the shutter had to be moved to the lens, so non-shuttered lenses are required to operate without a mechanical shutter, so the sensor has to fall back to a slow speed that permits scanning the entire frame.

I don't think Pentax is going out of their way to harm anybody. Slow toy-lens sync is a consequence of an important design choice.
10-25-2012, 10:41 PM   #4
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That really didn't explain anything. How does the electronic shutter work, and why doesn't it work well? I don't see the difference in having light hit a sensor for a certain number of milliseconds and then having the light vanish, vs telling a sensor "I've seen enough, turn off now".

Charles.

10-25-2012, 11:06 PM   #5
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The answer was a little buried in the reply, so here's a couple of references to explain it further: Shutter (photography) - Wikipedia, the free encyclopedia and http://caspegroup.com/How%20an%20electronic%20shutter%20works%20in%20a%20CMOS%20camera.pdf
10-26-2012, 11:29 AM   #6
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Okay, I get it now, the sensor can't read fast enough.... but then how can it support 1/2000 speeds without flash? I don't get that, if it can do 1/2000 without it should be able to do clear all pixel columns in less than 1/13th of a second. But that aside, my Sigma flash has high speed sync. It reduces the output, but why not support that for all flash speeds?

Charles.
10-26-2012, 12:34 PM   #7
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Hi Charles,

The problem is that each of the columns (or rows) of pixels have to clear, expose, then dump their charges in sequence. Don't forget that they are sending information to the LCD up to the time the shot is taken. With a mechanical shutter, they all start off in the dark, so only need to expose and dump. The exposure takes place while the shutter is open, then the dump can proceed even after the shutter closes. AFAIK, they have yet to manage to set up a CMOS that can clear the whole sensor at the same time, then "turn off" all the pixels at the same time.

At high shutter speeds, the shutter is essentially a slit that moves across the frame of the sensor, the shutter is never completely open, so with the short burst of light, only a portion of the sensor would be exposed. Your Sigma gun fires multiple flashes to get high speed sync so the light lasts as long as the shutter "slit" is moving across the frame. Here's an explanation of how it works:

High Speed Sync for Flash at Any Shutter Speed

Scott
10-26-2012, 02:48 PM   #8
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Yes, I'm aware of how the high speed sync works. Why can't they implement it on the Q though for the toy lenses?

Seems like that would be the perfect solution.

Charles.

10-26-2012, 07:34 PM   #9
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The problem is probably the flash unit. In high speed mode the flash has to flicker a little longer than your camera's highest flash sync speed. DSLRs have at least 1/125 or higher flash sync. This is what their external flashes are designed to work with. The Q is 1/13. This means the flash would have to flicker more than 4X as long. No flash has been made to do that. Could it be done? Probaby, but there would be a great reduction in total power output/distance and more heat to deal with.
thanks
barondla
03-18-2014, 05:21 AM   #10
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Any way around that 1/13 yet? I need built in flash and i work with manual lenses.
03-20-2014, 11:26 AM   #11
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QuoteOriginally posted by Pancolart Quote
Any way around that 1/13 yet? I need built in flash and i work with manual lenses.
The only way is to use the original Pentax K-Q adapter.
Since it has its own leaf shutter you can shoot adapted lenses at up to 1/250 sec. (with external flash) and 1/1000 (with internal flash)

Last edited by crewl1; 03-20-2014 at 03:15 PM.
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