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04-18-2013, 10:34 PM   #1
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How to meter ambient light exposure when a flash is mounted?

When shooting indoors with a flash I want that its using will be as seamless as possible. So the main goal is to come to a balance between the flash and the available light.
When the flash is turned off, one can press the green button (in camera's 'M' exposure mode) and there will be a correct exposure for an ambient light (for example, 1/25, f3.2, ISO800). After switching the flash on and making a shot there will come a relatively good blending between the available and flash light (the flash is in the P-TTL mode).
However, if the flash is turned on and I press the green button again, there will be absolutely different exposure (for example, 1/125, f4.5, ISO800). The exposure of the foreground in the image will be OK (because of the P-TTL) but the backround will be too dark.
Now all I can do is 1) frequent switching off/on the flash or 2) making a lot of test shots when the flash is always switched on.

Is there any faster way to get the right exposure for an ambient light when using a flash?

P.S. I read a lot of complaints about P-TTL. As to me, it works great! I get right exposures almost all the time even with a bounced flash.
Because of the complaints I bought a Metz 58 AF-2 with an 'Auto' mode. Nevertheless, I find P-TTL's behaviour much superior to the 'Auto'.

04-19-2013, 12:45 AM   #2
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Which camera?

With most of the K line (such as my K5) you set the flash mode in camera to "Slow Speed" synch to do what you want. With that setting, the camera meters as if no flash is on board and sets the shutter accordingly but fires the flash anyway, during the exposure. You have to be careful with this as the shutter will remain open for the full metered time and if it is dim, you will get slow shutter speeds and camera motion blur even though the flash fires. Check shutter and aperture as if for no flash and set-up as if you are shooting without flash. I also rotate the flash head up or to the side to avoid over-exposure.

On the K5, you enter the Flash menu by pressing the lower part of the back circular controller. The setting is "Slow" with a jagged arrow under it. I imagine it is similar on other models.
04-19-2013, 12:51 AM   #3
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In the old days, we used to use a separate hand held light meter, which you walk about with and take readings from where ever you want.

I've still seen me use my Gossen recently, when I'm struggling to get it right, rather than guess and the trial and error approach.
04-19-2013, 12:54 AM   #4
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Try to select the slow sync flash program. Use the flash button on the four-way button pad.
The normal flash program underexposes the ambient in weak light, perhaps that's what you experience here.
The slow sync flash program aims for "correct" ambient exposure at all light levels, even if it yields really
slow shutter speeds.

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--Anders.

04-19-2013, 01:03 AM   #5
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Oh, thank all you so much for the hint!
Yes, it must be the "Slow sync" mode when one presses the flash button on the four-way button pad (I have a Pentax K-5 IIs camera).
I'll try this mode in the evening.
04-19-2013, 08:42 AM   #6
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Well, in the 'M' mode the 'Slow sync' option is not available. It becomes available only in the 'Av' mode.
I made some shots in the 'Av' but it's not what I am looking for.
As in the previous example, the right exposure should be 1/25, f3.2, ISO800 + P-TTL. In the 'M' + green button the exposure is 1/125, f4.5, ISO800 (or 1/160, f3.2, ISO800). In the 'Av' mode the exposure is 1/100, f3.2, ISO800.
So in the 'Av' mode the exposure is closer to the ambient light but it is not spot on. The background is much brighter. Moreover, in the 'Av' mode it's impossible to get the right exposure (= to the ambient light) at all. In the 'M' mode I can at least make some test shots and then change the settings to my liking.
Do you know any other ways I can easier balance the flash with the available light?
04-19-2013, 10:56 AM   #7
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While "P-TTL" works fine for most flash shooting, for this type of balanced ambient plus flash shooting, I use Av, switch the flash from "P-TTL" to "Auto" and set the camera flash exposure compensation down 1 to 1.5 stops using the wheel in the flash set-up menu.

The P-TTL mode will try to provide the best exposure using the flash. Auto should just provide enough light to match the camera settings. I have the same flash unit as you and it works well for fill. I almost never have the flash pointed straight ahead, preferring to angle it up 45 to 60 degrees.
04-19-2013, 11:33 AM   #8
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I also try to bounce my flash when it is possible.
As for the Av, I guess switching from P-TTL to Auto won't make any difference regarding the background. I shot today a big chair before a window. It was well lit outside and I wanted that both the chair and the landscape outside the window would be well balanced.
Av mode (with the Slow Speed enabled) gave something like 1/100, f3.2, ISO200. The chair's exposure was OK, but the background was very overexposed. In order to get the well balanced exposure of the background it had to be something like 1/180, f5.6, ISO80. When I switched to the camera's M mode and dialed that exposure, everything came out perfect. I chose the right exposure for the background, P-TTL took care of the foreground exposure.
As you see, P-TTL or Auto always applies to the foreground exposure while I am talking about the ambient light exposure that a camera chooses itself and can't be changed by the flash impulse in any way.

04-19-2013, 12:48 PM   #9
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I see what you are doing now. I think you are going beyond what any camera or flash automatic mode can do on its own. Shooting against a window and trying to balance the interior to what you can see through the window is a case where I'd recommend using "M" and shooting a few test shots to get each part of the image correctly exposed and then combining what you found into one shot, manually setting the exposure. It seems that is what you are doing and it is working.
04-20-2013, 04:27 AM   #10
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QuoteOriginally posted by abmj Quote
I see what you are doing now. I think you are going beyond what any camera or flash automatic mode can do on its own. Shooting against a window and trying to balance the interior to what you can see through the window is a case where I'd recommend using "M" and shooting a few test shots to get each part of the image correctly exposed and then combining what you found into one shot, manually setting the exposure. It seems that is what you are doing and it is working.
Maybe you are right that no cameras can do this but it's a pity anyway.
It would be great if one could meter an exposure of available light correctly even when a flash is switched on. Maybe I should try a spot metering mode but I doubt it will work anyhow.
04-20-2013, 08:38 AM   #11
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Frankly, in this day many professionals and advanced amateurs would leave the flash off for such "through the window" shots and use HDR instead to balance exterior and interior light levels. But that is a technique many of us don't like. It seems you either love it or hate it.
04-21-2013, 05:43 AM   #12
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In my experience pseudo-HDR from a single image works better than a convertion from multiple images but I don't implement it often though.
As for the flash, no HDR can subtitute it when subjects move fast. My example with a chair was just a test. I am going to use flash primarily when taking photos of people. In this case only a flash can help to balance differently lit areas of an image.
04-21-2013, 07:17 AM   #13
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Use a separate reflective light meter at the window, manually adjust your flash to the same guide number for your subject. All that said, remember that you will in all likelihoods still run into a white balance problem. Flash is close to average daylight conditions, but daylight is rarely so obliging.
04-21-2013, 11:09 AM   #14
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Using a separate light meter is too clumsy. It's much easier to turn off a flash, an internal light meter of a camera will show a correct exposure (it may be needed to zoom to an ambient light source/use a spot metering). Then memorize that exposure, turn on the flash and then manually select the memorized exposure. The exposure of ambient light will be correct.
The whole question is - why does a camera's metering change when one switches on a flash (or why is there no mode to leave the 'without a flash metering' even when a flash is switched on?
04-21-2013, 12:26 PM   #15
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You answered your question in your original post with this thread - you are using P-TTL. I am pretty sure a P-TTL flash is telling the camera the flash's preferred apertures. If you use fully manual flash, and fully manual exposure, I'll bet your settings won't change.
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