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02-12-2008, 03:27 PM   #1
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k10d P/Tv flash oddity

I first noticed this with my new Sigma EF530-DG and thought it was a deficiency of that flash, but turns out the same thing happens with the built-in flash: Tv mode doesn't consider flash output in its setting of lens aperture. For example, say P mode chooses 1/60th of a second at F/8. Turn it to Tv mode and set 1/60th of a second, and the aperture will go to F/2.8 and start blinking to indicate that it wishes to go faster than the lens will allow.

Either way, the P-TTL ends up setting the flash to the appropriate power, but still, it's a bit weird. Is there any explanation for this, or is it just a quirk?


(I'm using fixed ISO so that's not an issue here.)

02-12-2008, 04:07 PM   #2
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QuoteOriginally posted by mattdm Quote
Tv mode doesn't consider flash output in its setting of lens aperture.
You are correct, it doesn't. And that's to be expected.
In P or Av mode in normal flash sync mode, the camera would set the shutter speed to X-Sync or below shutter speed (according to 1/(focal length) rule) to enable flash sync while avoiding camera shake. This has priority over ambient light metering. If you use "Slow Sync" mode, then it would slow the shutter speed way down to meter the ambient light correctly.

In Tv mode, you are supposed to be able to set the shutter speed freely. So it no longer takes the flash sync into account. It will only meter the ambient light.

It is actually easier if you use M mode to set the aperture and shutter speed directly, while using the EV meter to meter the ambient light to your desired EV.
02-12-2008, 06:49 PM   #3
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QuoteOriginally posted by nosnoop Quote
In Tv mode, you are supposed to be able to set the shutter speed freely. So it no longer takes the flash sync into account. It will only meter the ambient light.
Doesn't that make Tv mode pretty much useless unless you happen to want the lens wide open?

Hmmm. Is there a way for the flash output to be automatically adjusted via P-TTL to the metered EV while setting the aperture and shutter speed manually?
02-12-2008, 07:20 PM   #4
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QuoteOriginally posted by mattdm Quote
I first noticed this with my new Sigma EF530-DG and thought it was a deficiency of that flash, but turns out the same thing happens with the built-in flash: Tv mode doesn't consider flash output in its setting of lens aperture. For example, say P mode chooses 1/60th of a second at F/8. Turn it to Tv mode and set 1/60th of a second, and the aperture will go to F/2.8 and start blinking to indicate that it wishes to go faster than the lens will allow.

Either way, the P-TTL ends up setting the flash to the appropriate power, but still, it's a bit weird. Is there any explanation for this, or is it just a quirk?


(I'm using fixed ISO so that's not an issue here.)
There has been a lot of discussion about the camera not thinking like you do, specifically related to flash exposure.

in any automatic mode, the camera tries to get the best flashless exposure, and then fires the flash usually at minimum power.

In auto ISO mode it will take flash shots at ISO 1600 in an attempt to get an exposure in a dark room.

I simply dont bother with automatic functions in flash. I set to manual, and adjust the apature to get the DOF I want, shutter is usually maxed out to 1/180 (K10D) or 1/150 (*istD)

SOmetimes there can be too much automatic functions. It would be nice if the camera looked at the flash, knew the power, and could set apature and ISO to use the flash power to maximum, but I don't think the communications work that way

02-12-2008, 08:59 PM   #5
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QuoteOriginally posted by mattdm Quote
Doesn't that make Tv mode pretty much useless unless you happen to want the lens wide open?
In dim light, yes. Or if you want to "drag the shutter" and use slow shutter speed to get good ambient exposure.

But don't forget the use of flash during the day. Tv mode is quite useful during daylight fill flash and high speed sync.

QuoteQuote:
Hmmm. Is there a way for the flash output to be automatically adjusted via P-TTL to the metered EV while setting the aperture and shutter speed manually?
This is exactly what "M" manual mode does! You can set any aperture or shutter speed, and the P-TTL would be metered correctly regardless (within range). You can fine tune the P-TTL flash output using flash exposure compensation.

So in M mode, you don't need to worry about the flash. Just select your aperture and shutter speed to your liking. Use the EV meter as a reference for the ambient light metering. Say you want the ambient light to be 1 1/2 stop under the flash exposure, you adjust until the EV meter shows "-1.5", then fire. The flash would expose correctly, while your aperture and shutter speed combination would determine that the ambient light is exposed at 1.5 stop under.
02-12-2008, 09:04 PM   #6
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QuoteOriginally posted by Lowell Goudge Quote
SOmetimes there can be too much automatic functions. It would be nice if the camera looked at the flash, knew the power, and could set apature and ISO to use the flash power to maximum, but I don't think the communications work that way
That is not always desirable as you would get a properly lit subject against a dark or pitch black background.

What the camera is trying to do is to get a good ambient light/flash light balance. Fuji F series P&S does this as well. So it prefers using flash as a "fill" rather than the sole source of illumination (thus the setting of high ISO and wide aperture). And it does give you a more pleasing good ambient light background flash shot.

And with K20D's very usable ISO 1600, it gives you even more flexibility is controlling ambient/flash light balance.
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