Shutter Speed, Aperture & ISO

If you read anything about aperture, shutter speed and ISO in relation to flash, you’ll probably come across this somewhere:

• Shutter speed controls ambient ⛅
• Aperture controls flash ⚡
• ISO controls both ⚡ + ⛅

This is not totally true.

It should be:

• Shutter speed controls ambient ⛅
• Aperture controls flash and ambient (both) ⚡ + ⛅
• ISO controls both ⚡ + ⛅

(Except for High Speed Sync flash, which we’ll cover in a later lesson).

Shutter speed

(Controls ambient light ⛅)

A flash or studio strobe emits a near-instantaneous light, much faster than any shutter speed duration.

Where most cameras can only go as fast as 1/200 second when combined with flash (see “Sync Speed” below), the flash itself can be 1/1000 or 1/30.000 second for example. (See image)

Therefore, regardless if your shutter speed is 1/200 second, 1/60 second, or 1 full second, the only moment your scene will be influenced by your flash is that 1/1000 second or less when your flash goes off. So the shutter speed has no influence on the flash exposure. The shutter merely must be open when the instantaneous flash pulse happens.

What does this mean in the real world?

At fast shutter speeds, there isn’t enough time for ambient light to get into your camera and on to your sensor. So as long as you keep your shutter speed fast enough, ambient light won’t influence your exposure. How fast this fast enough exactly is will depend on your environment. You’ll need a much faster shutter speed in sunlight than in a dark basement to keep the ambient light out. (See “Sync Speed” below)

Of course, when you make an image like this (fast shutter speed, no flash) you’ll end up with a black photo. ( black + histogram)

This is exactly what we want, as it’ll provide us with a “blank” canvas to add our light to without having to worry about any other influences. It makes it a lot easier to see the adjustments you’ll be making, and will greatly improve your learning.

Then when you grasp the concepts and workings of light, you’ll have plenty of time later to add other lights or mix your flash with ambient light.

This way you’ll also don’t have to worry about any lights of different color spilling into your frame. (See the “about light” and “adding color lessons)

As long as you manage to keep this ambient light out, you won’t have to worry about any motion blur either. Since the duration of the flash is near-instantaneous and this is the only light in your scene, your sensor will only register your subject that 1/1000 second the flash goes off. Much faster than the 1/200 second shutter speed of your camera to freeze any motion.

Combining motion (from ambient light) and static elements (frozen by your flash) can lead to very interesting results. We’ll cover this later.

The 2 previous reasons are more than enough for me to always start a session with a test shot without any artificial lights. If the screen and histogram tell me no ambient light is getting into the camera, I can be sure that everything in my scene will be under my control.

I recommend you do the same. You’ll thank me later!

Aperture

(Controls flash and ambient light ⚡ + ⛅)

As you know, the smaller your aperture opening is (higher f) the less light gets in during a certain time frame.

Think of it like water. The smaller a hole is, the less water can get through. ?

So it makes sense to say that aperture controls your flash exposure, because the smaller your aperture opening is the less light can get in from that burst of light when you trigger your flash. And the larger your aperture opening is (lower f) the more of that light can get in of course.

But the exact same thing can be said of the ambient light.

The smaller your aperture opening is the less ambient light can get in while your shutter is open (shutter speed). And the larger your aperture opening is (lower f) the more of that light can get in.

That’s why aperture controls both flash and ambient light equally.

Of course, as long as your shutter speed is fast enough to keep the ambient light out (like we saw above) for your chosen aperture, the effect of changing your aperture only has a visible influence on your flash exposure.

Because of that, most people wrongly say that “aperture controls flash”.

But in reality, it has an effect on both.

The aperture also has an important effect on the power level required from your flash. The smaller your aperture opening (higher f), the harder your flash will have to work to get enough light through the opening and on to your sensor. This, in turn, has an effect on the maximum range and recycle time of your flash.

Lastly, don’t forget the artistic effect of your shutter speed.

Wanting a shallow depth of field or not is often a key choice you’ll have to make. This then will be a starting point for your other settings.

Luckily it’s easy to turn flash power up or down as long as you don’t run into the limitations of your equipment.

ISO

(Controls flash and ambient light ⚡ + ⛅)

Although ISO has less artistic influence on your image than aperture (except for grain), changing your ISO has the same effect as changing your aperture when talking about light.

The more sensitive your sensor (or analog film) is (higher ISO), the more light gets captured during a certain time frame. And the less sensitive (low ISO) the less light gets captured.

So what should my settings be?

The answer is, as always, it depends.

Like I told you in the first lesson, it makes far more sense to learn how to read and adjust your own lighting than to follow strict numbers.

This way you will be able to handle every situation.

So below is the checklist you should go through for setting up your camera and flash.

(You’ll also find this checklists and others in the Situation Checklists lesson)