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I was clicking some images through an opening recently. The opening had been intentionally constructed to ensure that only the camera's lens passed through and thus the flash-LED was physically blocked by the barrier through which the opening had been made.

Now I clicked images of the object with flash-on and then clicked an image of the same object with flash-off, and there was a drastic difference in my images. The images with flash-off were quite bright while the ones with flash-on were dull. Why does such a thing occur? And which image is more accurate, with flash-off or flash-on?

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    You should include example images in your question to help people answer accurately. – Harry Harrison Jul 10 '16 at 15:18
  • I intended to but the data is stored on my work PC in my office. I cannot access it at the moment. I shall update though as soon as possible for me to. – Ekdeep Singh Lubana Jul 10 '16 at 15:20
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Your camera will have anticipated the extra light provided from the flash, and adjusted the exposure to compensate. Because this extra light has been blocked out. The exposure is now wrong.

Additionally, when the flash fires, your camera is likely to use a different white balance setting to accommodate the difference between the flash light and the other lights in the scene. This shouldn't result in a different brightness, but may lead the colours in the image to look very different.

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    Corollary: If you shoot in RAW, you might very well get the same visual result after making the white balance the same. – Nayuki Jul 11 '16 at 5:42
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    @Nayuki That depends on the shutter speeds used for each shot. If the blocked flash shot was at 1/60 second (often the slowest speed many cameras will use when in Aperture priority mode when an active TTL flash is attached) and the ambient light shot was at 1/8 second then the dimmer 1/60 second shot will have a much lower signal to noise ratio and less total color information (because more of the the shadows will be lost in the noise). – Michael C Jul 14 '16 at 5:14
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Assuming the flash is normally automatic TTL mode, so that its light is metered to take care of itself. Then TTL is metered so that the TTL flash power level is adjusted to provide a correct flash exposure.

We don't know which camera we are discussing (brand, and a compact or DSLR, etc?), but speaking of camera more sophisticated than the minimum, then indoors without flash, the ambient exposure of course typically may need a slow shutter speed. But when flash is used, most cameras will enforce a Minimum Shutter Speed With Flash, often about 1/60 second (because we are using flash and don't need slower). If metering without flash was say 1/8 second, then this ambient will be underexposed three stops at 1/60 (but not to worry, we are using flash instead). This faster 1/60 limit is not about any proper exposure, it is simply an arbitrary minimum limit, because we are using TTL flash instead (the camera gives up on the dim ambient, and plans to use flash instead).

Check your images EXIF to see what actually happened. It will show all details. I think you will see a slow shutter speed and a faster shutter speed, for the reason just above.

So this is no concern if the flash is not used, when the example 1/8 second will be used as was properly metered. But the camera does not know that you used the flash, but blocked its light. So you got a faster shutter, about 1/60 second (insufficient for ambient), but you blocked the flash (insufficient for flash). Don't do that. :)

Which image is most accurate is of course the one with the better actual exposure, the one with the better light that came out to be the best picture

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