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05-16-2010, 10:02 AM   #1
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flash overexposes at < F2.8 ???

Hi All,

I just bought a K7 with a 50mm f1.4 FA and a Metz 48af. I'm at a loss to explain something, hope someone here can straighten me out.

I get overexposure with my lens opened up past f2.8 or so, here is what I'm doing:

1. Find a spot that meters 1/160 f1.4, about 15-20 ft away.
2. Av mode, iso 3200, no exp compensation.
3. Flash in P-TTL with 50mm selected.

It's burned out at f1.4, gradually gets better as I stop down, perfect from f2.8 on.

I guess there could be a minimum flash output, and I'm fixed at f1.4 and in this case the shutter speed cant go up, but it seems to me it ought to be able to take flash output right down to zero if need be. Or possibly the pre-flash is too much at this sensitivity?

Dont see myself at f1.4 and ever using a flash actually, thats not what I bought the lens for, but want to understand this.

Last time I did any photography was 15+ years ago when I was doing weddings, burned myself out with it and havn't picked a camera up since until now. Man have things changed, cant believe what this little monster will do and what I used to lug around and have to fiddle with constantly. AF is a godsend to these old eyes, I'm lovin it.

3200 at f1.4 with results like this, bwahhaahhaa, I only dreamed of such things back then.

Appreciate any help understanding this.

Thanks,

Larry

05-16-2010, 10:11 AM   #2
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Does the flash even support ISO 3200? My AF540 only goes to 1600.

05-16-2010, 03:52 PM   #3
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Hi Jeff,

Yes, spec says 6400.

Hopefully I'll get a chance to play with it some more tonight.

Thanks,

Larry
05-16-2010, 05:33 PM   #4
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Bump to this question, it is something that has happened to me (not necessarily at f/1.4) but it seems like the lowest flash output could still be greater than the minimum required, therefore overexposing.

Anything more concrete and how to avoid this?

Thanks

BB

05-16-2010, 07:27 PM   #5
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Use lower ISO. It just doesn't make sense to use ISO 3200 when the flash is so strong.
05-16-2010, 11:16 PM   #6
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My guess is that it's indeed the minimum flash duration.

Your Metz has a GN of 35 (meters) at ISO100 and 50mm. At ISO3200 your guide number will be around 200 allowing a max. flash distance of roughly 70 meters at f/2.8 (200/2.8). Minimum flash distance (for this flash, see page 72/73 of the manual) is about 10% of this so around 7 meters.

In my opinion, the only valid reason to use ISO3200 is if you indeed have to lighten up a subject that's far away. But you will not use a 50mm lens for that.
05-16-2010, 11:24 PM   #7
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Every flash has a range min - max output. There is no such thing as "to take flash output right down to zero."

For most flashes, the min output is about 1/10 of the max output.
05-17-2010, 02:48 AM   #8
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QuoteOriginally posted by larryroohr Quote
Hi All,

I just bought a K7 with a 50mm f1.4 FA and a Metz 48af. I'm at a loss to explain something, hope someone here can straighten me out.

I get overexposure with my lens opened up past f2.8 or so, here is what I'm doing:

1. Find a spot that meters 1/160 f1.4, about 15-20 ft away.
2. Av mode, iso 3200, no exp compensation.
3. Flash in P-TTL with 50mm selected.

It's burned out at f1.4, gradually gets better as I stop down, perfect from f2.8 on.

I guess there could be a minimum flash output, and I'm fixed at f1.4 and in this case the shutter speed cant go up, but it seems to me it ought to be able to take flash output right down to zero if need be. Or possibly the pre-flash is too much at this sensitivity?
Larry, you have too high an ISO setting and too fast an aperture for the flash. It cannot dial down its output enough - no flash could at these settings. Only when you stop down your aperture, the flash's output will be compensated by the smaller aperture. You will get the same effect with any flash at short distances and you need to either use lower ISO or - as you have done - close the aperture.

Ben

05-17-2010, 07:21 AM   #9
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Thanks everyone,

It occurred to me last night that for any ttl system the delay through the sensor-processor-serial I/O-flash would be a fundamental bottom end to how fast you could shut off the flash, whether thats it or not I dont know, probably are other good reasons for a minimum as well.

Guess I just won't do that anymore
05-17-2010, 08:32 AM   #10
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QuoteOriginally posted by larryroohr Quote
Thanks everyone,

It occurred to me last night that for any ttl system the delay through the sensor-processor-serial I/O-flash would be a fundamental bottom end to how fast you could shut off the flash, whether thats it or not I dont know, probably are other good reasons for a minimum as well.

Guess I just won't do that anymore
High power transistors/IGBTs are slower than small-signal ones. "slow" being in terms of ten thousandths of a second, but still - there's a minimum turn-on/turn-off time of the power electronics.
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