Is using a large aperture with flash uncommon?

Question

I found How can I use wide aperture with fill flash? and How can I use flash with large apertures? while trying to find a solution for using a large aperture to get DOF with the flash setup I have.

The answers seem to suggest that its difficult with standard equipment, but possible with a few options, one of which is highspeed sync, but according to an answer

"This does, however reduce the power output of the flash, by roughly two stops, so there's a game of diminishing returns going on."

My question is, is the use of a large aperture common (or not) when using flash in professional (or amateur) photography. I am curious as if it was a common technique, I would think there would be more options available, or more equipment offering it as standard (or is this a false assumption)?

Answer 1

I'd say that it's common enough, since low-depth-of-field portraits are fashionable. However, you have to distinguish between two situations: in a studio or other controlled situation where flash is the primary light, vs. outdoors where you are trying to overpower the sun.

As the questions you've linked suggest, the studio situation is easy, but if you need to balance or compete with very bright ambient light, physics dictate that there is only one answer: a brighter light. And because that is difficult, the easiest way to get the flash to be relatively brighter is to reduce shutter speed, and then that runs into all of the problems of high-speed sync.

Even if people really, really wanted to do this all the time, those things are still hard. So, it's sort of a chicken and egg problem. On the one side, there's incentive from people who find the look useful/interesting and probably do want to do it more, but that's balanced by counter-pressure from the inherent difficulty and expense. Basically, I don't think there's anything significant at work beyond those things, and it's likely that the marketplace is roughly at the equilibrium point between them. In fact, I'd go so far as to say that the options which are available (powerful flashes with HSS) are actually reflective of your assumption — the technique is popular enough to support what is on the market, and not significantly more or less.

Answer 2

Yes, flash usages and aperture are independent of each other. I shoot weddings and 90-95% of my flash usage is with wide open or near wide open apertures.

What is confusing you is specifically the case of shooting outdoors where you want the flash to fill in with power equivalent to daylight. In that situation, the shorter your shutter speed, the less daylight is let in and the more impact your flash has on the scene relative to the sun.

The problem is that cameras have something known as the maximum sync speed. This is the fastest shutter speed for which there exists a time when the shutter is fully open. A DSLR shutter is composed of two curtains, first one curtain opens and then the other curtain closes. When you are shooting a slower shutter speed than the sync speed, the first shutter opens fully before the second shutter begins closing. The flash can go off any time in this window (normally either right after the first curtain opens or right before the second curtain closes).

When shooting faster than the sync speed, the second shutter begins closing before the first shutter finishes opening. This means that there is no moment in time where the shutter can go off and fully expose the image. To avoid a dark band, HSS or high speed sync has to be used. High speed sync works like a strobe and produces rapid lower power flashes over the duration of the shutter. Since the flash has to discharge multiple times however, it can not discharge as much power overall, and thus some overall flash power is lost.

Making adjustments to aperture or adding ND filters won't make a difference on the impact of sunlight vs flash though as both also limit the amount of flash power that reaches the sensor as well, so it makes no difference at all on aperture selection. You select aperture for the desired depth of field and choose flash power and shutter speed based on the desired lighting.