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I have a Canon EOS 2000D DSLR. I recently tested its flash. Gives ugly shadows, but that is easily remedied by turning a low-cost white A4 paper into a flash diffuser.

However, the power of the flash is not that great. Thus, I have been planning to purchase a removable flash for the camera.

I also tested the red-eye prevention feature of the DSLR. It works excellently, even without the A4 paper diffuser. Apparently, the flash fires once before the main flash, and this causes the iris of the eye to close, thus preventing red eyes.

Now, if I purchase a removable E-TTL II flash, will the pre-flash red-eye prevention work also with the removable flash?

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    I’m not sure. But using a speed light...bounce the light or use any number of big diffusers and red eyes become a thing of the past anyway. – Hueco Feb 3 at 17:44
  • You can also put the flash on some stand and use it remotely controlled from your camera. – xenoid Feb 3 at 23:09
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A removable flash will not create red-eye if you are using it off camera... which I guess it is the idea in the first place.

A red eye is produced because the flash is illuminating directly into the inside of the eye reflecting the color back to the lens because they are so close together.

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An external flash, even on the top of your camera has more distance to the lens, so it is more difficult to produce red-eye.

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But more specific to your question, no. The pre-flash of a TTL will not cause the same effect as a pre-flash for red-eye reduction. The eye needs some time to close the pupil as a response of the intense light, let's say 1/2 - 1 second.

A TTL preflash is too fast for the pupil to respond to it.

  • Some systems do have a preflash-based redeye reduction mode for their speedlights. Your answer explains why that might be necessary, but — does the Canon system have this feature? – mattdm Feb 3 at 17:58
  • I expanded a bit my answer. – Rafael Feb 3 at 18:01
  • Although less likely, you can get redeye with an external flash, particularly one mounted on the hot shoe. The angle of reflection equals the angle of incidence. If the eye's retina to pupil axis bisects the angle between the flash and lens and the pupil is sufficiently dilated, red-eye can still be an issue. – Michael C Feb 4 at 0:35
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    That eye diagram is missing a lens. And the human retina is not built like a mirror, so one should rather assume diffuse reflective behavior. The eye is an optical system with the retina as sensor, and the overlap of the real images of flash and camera lens determines how much flash-lit retina the camera lens gets to see. Apart from the degree of lateral bleedover, that depends on input separation angle, iris size, and the eye not properly focusing on camera lens and/or flash. – user81797 Feb 6 at 16:43
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Your understanding of how Red Eye Reduction works on the EOS 2000D is incorrect. Your camera has a lamp on the body near the shutter button that lights up briefly before the photo is taken. There is an option in the menu system to disable it.

You won’t need Red Eye Reduction with an external flash because the flash head is sufficiently high above the lens so that the light is not reflected in the eyes causing “red eye”.

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  • Indeed, I found the lamp in my camera! I'm sorry I can't accept two answers, but have an upvote! – juhist Feb 3 at 18:07
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Apparently, the flash fires once before the main flash, and this causes the iris of the eye to close, thus preventing red eyes.

Some subjects have an unfortunate reaction to the pre-flash causing not just the iris of the eye to close. Red-eye prevention tends to be several flashes. Digital camera TTL variants tend to work with a single-preflash for metering purposes.

Depending on how well an external flash is integrated with the camera, it may have working TTL of the pre-flash kind. Red-eye preflashes are rarely needed with external flashes since the part of the retina lit by the flash tends to be not visible from the camera's lens unless the flash is in close proximity to the lens. In considerably dark environments with a wide open iris, there might be a bit of overlap, though.

  • TTL uses multiple flashes when off camera flashes are optically controlled such as with Canon's wireless E-TTL system prior to the 600EX-RT or Nikon's CLS. One pulse is the metered "pre-flash", then several very fast pulses communicate the desired power level to each wireless flash. – Michael C Feb 4 at 0:30
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As a note aside:

Apparently, the flash fires once before the main flash, and this causes the iris of the eye to close, thus preventing red eyes.

That's only part of what happens. What also happens is that the subject's attention focuses on the flash and causes the eye's lens to reflexively focus for the flash. That gives the flash a focused image on the retina that does not overlap with the view of the lens coming from a similar distance and thus "seeing" only a sharp camera-lens shaped spot on the retina.

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I now purchased a flash (Canon 430EX III-RT) and tested it. When the external flash is attached, it seems like the camera doesn't even try to use the red eye reduction LED (although red eye reduction was turned on in the camera menu). At 3.5 meters distance, I see no red eyes.

The flash was attached to the camera hot shoe, and was pointing in the same direction as the lens. No flash diffuser was used.

Apparently, Canon has figured out that the distance between the external flash and the lens is so great that red eyes are not a danger given the distance. Thus, the red eye reduction LED does not turn on with an external flash, even if red eye reduction is turned on.

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