Tuesday Photo Tip – Flash exposure & shutter speed

In this week’s Tuesday Photography Tips we’ll cover flash exposure, a trouble spot for most novice photographers. It’s easy to understand why flash photography causes so much consternation. Take everything you’ve learned about exposure and readjust your thinking for flash exposure. You need to learn a new exposure triangle and use it when appropriate. That’s all there is to learning flash photography and, quite frankly, it’s easy to do.

You’ve already learned that exposure is controlled by the three variables of the exposure triangle – aperture, shutter speed and ISO. This topic was covered at length in a series of posts starting with this one.

Orange County Wedding Photographer - BP90 (3)

Well – flash exposure is a bit different. Here’s why. Flash exposure does not depend on shutter speed. Yes, you heard that right and here’s an example of that perplexing statement.

Orange County Wedding Photographer - BP90 (2)

The two photos above were taken with vastly different shutter speeds. Granted these are busy shots that violates the composition mantra to keep it simple but they will help demonstrate the point of this post. That point is this – shutter speed does NOT impact flash exposure. Believe it – look at these two photos.

Specifically, look at the exposure on Jeremiah. Even though the shutter speeds varied by a factor of 4x (1/200 vs. 1/800) you see no significant difference in the exposure on his face. How does that work?

It works because the exposure triangle for flash photography consists of aperture, flash power and ISO. Notice shutter speed is not in the flash exposure triangle. We’ve traded flash power for shutter speed. Regardless of the shutter speed, your camera will automatically adjust the flash power to give the correct exposure for a given aperture.

So why doesn’t shutter speed enter into the flash exposure calculus? Think about this – the “flash” of light from your flash lasts a tiny fraction (~1/1,000) of a second. That’s why we call it flash!! Hence, the light on your flash subject is only available for that tiny fraction of a second. The light that illuminated Jeremiah was only on for 1/1,000 of a second – for a flash. So why would the shutter speed matter. It doesn’t except ….

…. let’s look a little closer at the background. Notice how much darker the background is in the right hand panel. Remember, the background is not significantly affected by the flash – it’s just too far away for the flash to illuminate it. The background exposure is subject to the standard triangle – aperture, shutter speed and ISO.

I wanted the background to go dark to reduce the visible clutter. Since the background exposure IS controlled by shutter speed I shot the right hand panel at 1/800 sec. Granted, that didn’t work so well but the two photos illustrate the point of this post – shutter speed does NOT affect the flash exposure. It’s simple, for flash photos you pick an aperture (more on this next week) and your camera will pick the appropriate amount of flash.

Here are some examples of what you can do when you master your flash exposure triangle and mix it with the background exposure to take complete control of the lighting – foreground and background.

These photos demonstrate that the (simple) key to flash photography is to understand there are two exposures to consider:  the background exposure – for those elements that aren’t illuminated with the flash and  the flash exposure  – for those elements in the frame that are close enough to be illuminated by the flash. Once you grasp the concept of two exposures you’ll become a master of flash exposures.

In next week’s post we’ll talk a bit more about this. We’ll tell you exactly what to do to nail your flash exposures. It really is easy once you “see the light”.

Stay tuned because next week we’ll give you practical tips on how to master your flash. Better yet – be updated automatically by “friending” our Facebook site.

PS – This is one of dozens of photo tips in our continuing Tuesday Photo Tips series of posts. There  are other resource articles on our site you may enjoy covering basic and more advanced photography topics. There are also tips covering topics such as preparing for family or infant/child portrait sessions. If you would like a topic covered just jot it down in a comment or send us a note.


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