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10-14-2007, 11:13 AM   #1
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Exposure whith flash bounce

Hello

I recently upgraded my old Pentax PZ-20 to a K100D. At the same time I bought a AFG-360 flash.

I am a novice at using an external flash and especially at bouncing the flash.

Some shot I take with bouncing the flash are underexposed. some others are perfectly exposed.

The flash is set in PTTL mode. I was using P mode on the camera and I have linked the AE point to the AF point like it is suggested on this forum to get proper exposure when using the flash.

So the two questions I have are: 1) do I have to increase exposure compensation on the flash when bouncing? (I thought that in PTTL mode this would not be necessary) and 2) any good pointers on good informations on flash boucing technique?

thanks all.


Last edited by magenois; 10-14-2007 at 11:15 AM. Reason: typo
10-14-2007, 11:39 AM   #2
baw
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Bounce flash..

I have the same setup and it gives good results normally.
You have to realise that when bouncing via eg. the ceiling only part of the light reaches the subject. The flash can only deliver it's maximum rated power, and when that is not enough under exposure will result

Same for long distance flash, when maximum power is not enough there is nothing P-TTL can do to regulate.
10-14-2007, 12:55 PM   #3
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What baw says is correct.

If you make a small bounce card held on with a rubber band I think you'll have more luck, it will fill in shadows a bit and stop so much power being wasted in bounce mode. I use a 4cmx9cm white plastic strip mounted vertically behind the head, and may only have one or two cm showing, but it makes a big difference, faster recycling and more natural photos and it's adjustable in intensity by sliding the plastic up or down.
10-14-2007, 01:14 PM   #4
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QuoteOriginally posted by baw Quote
I have the same setup and it gives good results normally.
You have to realise that when bouncing via eg. the ceiling only part of the light reaches the subject. The flash can only deliver it's maximum rated power, and when that is not enough under exposure will result

Same for long distance flash, when maximum power is not enough there is nothing P-TTL can do to regulate.
I understand that. however, the subject was about 6 feet away and I was bouncing the flash on a standard height ceiling (8 feet). I dont really know but I doubt that the AF-360 was not powerfull enough in such a situation.

thanks for the input

10-14-2007, 01:35 PM   #5
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The manual way to calculate is distance/guide number=diafragma goes for bouncing also but. As distance you need the distance the light travels and open one or two extra stops depending on how reflective the bounce surface is.

Guido
10-14-2007, 10:34 PM   #6
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BAW is correct - I was shooting a party with an AF540 with a 50mm f/1.4 and a low but poorly reflective roof and a diffuser (Stofen) on the flash and I started at f/3.5 for reasonable DOF and had to go back to f2.8 to get a reliable exposure on more distant shots. This was mostly at ISO 100.

Bounce and diffusers rob an enormous amount of light so I needed the 540 to work at this low ISO.

Rod
10-15-2007, 05:54 AM   #7
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I don't think a lack of sufficient flash power is the problem here. After all, he said the subject was only six feet away and any decent flash unit should be able to handle that distance. I believe he is talking about the Pentax AF-360 FGZ (never heard of an AFG-360). If so, with a guide number of 119 feet, that flash should be more then powerful enough to bounce to a distance of six feet on anything other than an extremely dark or extremely tall ceiling (or unless hindered even further in another way, such as an additional diffusing device).

Instead, perhaps it might be wise to look at a couple of the more common flash issues. Is your flash unit aimed past your subject, magenois? At a subject distance of only about six feet, the flash would have to aimed nearly straight up to bounce light back successfully onto the subject. If you tilt the flash head too far forward, the light from the flash would travel over and fall behind the subject, leaving your subject in the darkness in between.

A flash unit throws around a lot of light, often making nearby objects too bright. This can easily fool a meter averaging an overall scene into underexposing your intended subject. Try framing your subject more tightly, or using the spot metering mode.

Finally, when tilting your flash head, are you using the preset positions? These positions are often marked on the flash and can be felt by small clicks as you move the head up and down. Some flash units will only work properly (make the necessary internal electrical contacts) in these preset positions, and I suspect P-TTL uses these positions to calculate flash exposure (only a suspicion, not proven).

If none of these work, let us know and we'll keep trying other ideas.

stewart

Last edited by stewart_photo; 10-15-2007 at 06:08 AM. Reason: clarification
10-15-2007, 06:56 AM   #8
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QuoteQuote:
I believe he is talking about the Pentax AF-360 FGZ
Correct.

QuoteQuote:
Instead, perhaps it might be wise to look at a couple of the more common flash issues. Is your flash unit aimed past your subject, magenois? At a subject distance of only about six feet, the flash would have to aimed nearly straight up to bounce light back successfully onto the subject. If you tilt the flash head too far forward, the light from the flash would travel over and fall behind the subject, leaving your subject in the darkness in between.
I cant recall if it was tilted straight up, this might actually be my issue.

QuoteQuote:
Finally, when tilting your flash head, are you using the preset positions?
Yes I was.

QuoteQuote:
If none of these work, let us know and we'll keep trying other ideas.
Thanks for the very informative replies. I'll do some more testing this week and I'll let you know if I dont still get it right.

Marc

10-15-2007, 07:25 AM   #9
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Bouncing flash is kinda like playing pool, you are sending the light up at an angle and that angle determines where the biggest portion of the light will land. fwiw I usually put +1 ev on the flash if I am bouncing, altho normally I chimp a bit.
10-15-2007, 08:34 AM   #10
baw
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One more thing that can help:

when bouncing zoom the flash manually to it's longest setting, thus "concentrating" the light more. It will be spread more than enough by the bouncing surface already.
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