Why isn't infrared AF assist beam the default when available (instead of irritating strobing)?

Question

I have an entry level Canon camera with a built-in flash.

When I try to photograph people at night the camera fires some pre-flashes to help with focus. While I do understand its necessity, some of my subjects are confused by these flashes thinking that I actually take the photo.

I bought an external flash and one of the reasons for buying it was that I read on the specs that it has a red light that can help with focus.

I put it on the camera, and while shooting the flash still fired pre-flashes! I finally found a setting in the flash the enables the red lamp in order to focus.

But I am curious why this is not the default setting? The pre-flashes drain the battery and confuse my subjects. The red light is much more inconspicuous and at least so far it has always locked on focus.

So do I miss something if I enable this setting permanently so that only the red light is used for focus assist?

As an added advantage the red light will always work even if the flash head is turned to the ceiling or to the wall.

Update:

Camera is 700d and the Flash is 430IIIEx

The setting I am talking about is described in the manual Personal setting F05 - AF-assist beam emission method

Setting 0 - default -Small series of flashes fired by flash

Setting 1 - Infrared Infrared AF-assist beam aiming at the center AF point is emited

My question is essentially why setting 0 is the default and not 1? Also what do I miss if I select permanently setting 1

Answer 1

The 430EX III is a bit "special". Canon builtin flashes optionally use a series of preflashes for focus assist, but Speedlites up until this point have instead only used the (less confusing/irritating) IR focus assist beam. Perhaps as a cost-cutting (or market segmentation) measure, in the 430EX III the IR assist is only available when using the center AF point, and preflashing is used otherwise. Because of the limitation to the center AF point, the IR beam is not enabled by default.

Please refer to eg. this thread for details, including nice pictures showing the narrower-than-before beam pattern.

Answer 2

Two reasons:

• The narrow infrared beam of the 430EX III limits the usefulness of the IR assist to the center focus point.
• The IR assist has a more limited range than the brighter pre-flashes.

There are also other sources of pre-flashes, even when IR focus assist is selected. The modeling flash feature on many Canon models is one. Using the E-TTL automatic flash on all digital EOS cameras is another. So is red-eye reduction.

Answer 3

I think your assumption "When I try to photograph people at night the camera fires some pre-flashes to help with focus." is mistaken.

The series of flashes prior to the shutter opening to take the photo are usually enabled or disabled by the 'red-eye reduction' option, which intentionally fires the flash several times on low power to make peoples' irises contract. The pre-flashes are not done to assist with focus. Large, dilated pupils at night often produce a red-eye effect which is not desirable.

You haven't posted which model camera so I can't help with specifics, but check the user manual for the red-eye reduction setting and disable it if you prefer.