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I took a photo in P mode, with the flash on:

enter image description here

I noted the exposure it used — 1/3s at F2.8. I then switched to A mode, set the exposure to F2.8, and took a photo. The camera chose the same shutter speed (as one would expect), but the photo ended up subtly brighter (please open each photo in a new tab and switch between them to see the difference):

enter image description here

I then switched to S mode, set the exposure to 1/3s, and took a third photo, which also ended up being brighter than the one in P mode:

enter image description here

Why is this happening? The settings are the same in all three cases — F2.8, 1/3s, ISO 1600, flash on, flash exposure compensation -2, slow sync, regular exposure compensation 0, matrix metering, manual focus and RAW format. The only thing that changed was the mode — P, A and S. All three photos were shot using a tripod, from the same position and with the exact same framing. All three are shot using the Sigma 19mm F2.8 prime lens on the NEX-5R, and with a remote control to avoid camera shake.

BTW, this doesn't happen when the flash is off. I took another series of photos (albeit of a different scene), and I got identical photos in P, A, S and M modes. Only when the flash is on does there seem to be a change in exposure between modes.

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  • \$\begingroup\$ Canon DSLRs meter flash to be a main light in P mode and fill light in Av/Tv mode. Perhaps Sony makes a similar distinction? \$\endgroup\$
    – Imre
    Commented Aug 6, 2015 at 10:15
  • \$\begingroup\$ But I set it to slow sync mode for all the photos. \$\endgroup\$
    – Kartick Vaddadi
    Commented Aug 7, 2015 at 2:53
  • \$\begingroup\$ Slow sync affects timing of the flash, not metering (i.e. "when", not "how much"). \$\endgroup\$
    – Imre
    Commented Aug 7, 2015 at 6:39
  • \$\begingroup\$ I thought that in slow sync mode, the flash is used as a fill light and not as a main light. Isn't that the point of slow sync -- to capture the background? How would that happen if the flash is used as the main light? \$\endgroup\$
    – Kartick Vaddadi
    Commented Aug 8, 2015 at 3:56

1 Answer 1

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I think logic will tell you what the problem is:

When all three modes are used without flash, the image looks the same. When all three modes are used with flash, the images are different.

So, let's investigate the flash.

Now, what makes a flash work?

Well, we have a power source, a xeon bulb, and a large capacitor.

More than likely, your batteries didn't die in between shots or deplete enough for that to be a problem, so let's rule that out.

This leaves the xeon bulb and the capacitor.

Well...the xeon bulb is expected to last a very long time and if it is fired with the same voltage constantly, it should provide very constant results. If it didn't then no one would purchase electronic flashes as they'd be unreliable.

That means it is could be the capacitor. Now, you saw the light telling you that the flash was ready before you fired; however, just because the light is on, that doesn't tell you what the charge of the capacitor is. I'm more than willing to bet that there were different levels in the capacitors each time it was fired.

If you could, try repeating this expierment, but change the order: A, S, P or some other variation and compare those pictures.

It could be that or...in P mode two lights aren't on. One on the building and one on a lamp post.

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  • \$\begingroup\$ If the capacitor was charging to different levels, that falls under the same category of "no one would purchase" them. It could be the two lights, as you said. \$\endgroup\$
    – Kartick Vaddadi
    Commented Aug 6, 2015 at 3:23
  • \$\begingroup\$ On most camera/flash, the "ready" light of the flash is "on" when the capacitor is charged about 80 %. When triggering the camera, mechanism/algorithms are able to detect when the "correct" amount of light has been delivered. Those can stop the flash from giving more light by rapidly switching the power to a new route (sometime it can even return the power to the capacitor for a new flash use). Maybe on your camera it can be the source of subtle difference. Can you try again and wait 5 seconds after the flash light to take a picture ? Do you have "pre-flash" before the real one ? \$\endgroup\$
    – Olivier
    Commented Aug 6, 2015 at 10:56
  • \$\begingroup\$ I think I did wait 5 seconds after the flash light, but let me try again. How do I tell if pre-flash is enabled? There's no setting for that on my camera (NEX-5R). \$\endgroup\$
    – Kartick Vaddadi
    Commented Aug 8, 2015 at 3:59

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