Lighting for Photographers: Flash Exposure Fundamentals

Understanding why guide number math works


From:

Lighting for Photographers: Flash Exposure Fundamentals

with Brent Winebrenner

Start your free trial now, and begin learning software, business and creative skills—anytime, anywhere—with video instruction from recognized industry experts.

Start Your Free Trial Now

Video: Understanding why guide number math works

In this movie, I'm going to take a moment to review how the flash to subject distance and the aperture scales relate to one another. And then we're going to look at how we can use this information to maintain equivalent exposures when we're working with small strobes. Remember, the square root of 2 is an integral part of the Inverse Square Law, which we use to calculate the change in illumination when the flash to subject distance changes. It's also an integral part of calculating the size of an aperture. And an aperture is nothing more than a circle, and you may recall that the formula for the area of a circle is Pi R squared.
please wait ...
Watch the Online Video Course Lighting for Photographers: Flash Exposure Fundamentals
1h 27m Advanced Mar 04, 2013

Viewers: in countries Watching now:

Expand your lighting options and get the most out of your flash as photographer and teacher Brent Winebrenner takes a practical, hands-on look at the theory behind exposure, with a special emphasis on electronic flash exposure.

Even with today's automatic flash systems, there are good reasons to understand how flash exposure really works. Brent details these concepts in this course. The course describes how to calculate the true power of your flash and how to modify its output to match your needs, a technique that can extend battery life, reduce recycle time, and provide exposure control that is more predictable than fully automatic modes. The course concludes with several shooting scenarios during which Brent explores the creative use of gels, reflectors, and other light modifiers.

Topics include:
  • Understanding exposure basics
  • Reviewing aperture, ISO, and shutter speed
  • Adjusting strobe power and flash to subject distance
  • Finding the guide number of a strobe
  • Making calculations based on ISO
  • Understanding power ratios
Subject:
Photography
Author:
Brent Winebrenner

Understanding why guide number math works

In this movie, I'm going to take a moment to review how the flash to subject distance and the aperture scales relate to one another. And then we're going to look at how we can use this information to maintain equivalent exposures when we're working with small strobes. Remember, the square root of 2 is an integral part of the Inverse Square Law, which we use to calculate the change in illumination when the flash to subject distance changes. It's also an integral part of calculating the size of an aperture. And an aperture is nothing more than a circle, and you may recall that the formula for the area of a circle is Pi R squared.

Because Pi is a constant that doesn't change no matter how big or how small the circle, we can ignore it while determining the relative change in the size of the aperture. You probably already know that if we double the area of the circle, we'll create a one stop increase in the size of the aperture. And life would really be simple if the formula read area equals Pi times R because each time the radius is doubled, the area of the circle would double. In that world, a one-stop change from a 5 would be f/10 instead of 7.1.

Unfortunately, the formula demands that we use R squared, not R. This means that the area doubles when the radius increases by the square root of 2. If you look carefully, you'll notice that one-stop increments on the aperture scale are separated by a factor 1.4, which you'll recall from our previous discussion of the Inverse Square Law is equal to the square root of 2. As a result, 1.4 dominates the guide number scale, the flash to subject distance scale, and the aperture scale.

When we look at them together, their similarities become obvious and therefore useful. An understanding of how the square root of 2, or 1.4, is used to determine the change in both the aperture size and the flash to subject distance helps you to use the scales to solve problems more quickly, even when shooting in TTL mode or using big strobes. We alluded to this earlier, but now let's demonstrate the results of making offsetting adjustments to flash to subject distance and aperture.

Remember, the guide number for this flash on this camera is 90. So, I started shooting at f/18 from a distance of 5 feet. I opened up the lens in one-third stop increments, while compensating by backing up the flash in one-third stop increments until I got to a distance of 18 feet, and an aperture of f/5. Now, let's review the series of images. As expected, we see the depth of field changes. In addition because I move the flash, the position of the shadows changed and the level of illumination on the background changed.

However, if you focus on her face, you'll see that I was able to maintain equivalent exposures throughout the entire series. This proves that a change in aperture when offset by an equal change in the flash to subject distance results in an equivalent exposure. Once you've learned the scales, and you know your guide number, you'll know exactly where to place the flash to match your aperture. At first, this may sound like a lot of work in a TTL enabled world, but when you get the hang of it, you can really move fast in response to changing circumstances.

So, no matter what your shooting style, there are times when this level of manual control can really save the day.

There are currently no FAQs about Lighting for Photographers: Flash Exposure Fundamentals.

 
Share a link to this course

What are exercise files?

Exercise files are the same files the author uses in the course. Save time by downloading the author's files instead of setting up your own files, and learn by following along with the instructor.

Can I take this course without the exercise files?

Yes! If you decide you would like the exercise files later, you can upgrade to a premium account any time.

Become a member Download sample files See plans and pricing

Please wait... please wait ...
Upgrade to get access to exercise files.

Exercise files video

How to use exercise files.

Learn by watching, listening, and doing, Exercise files are the same files the author uses in the course, so you can download them and follow along Premium memberships include access to all exercise files in the library.


Exercise files

Exercise files video

How to use exercise files.

For additional information on downloading and using exercise files, watch our instructional video or read the instructions in the FAQ .

This course includes free exercise files, so you can practice while you watch the course. To access all the exercise files in our library, become a Premium Member.

Join now Already a member? Log in

* Estimated file size

Are you sure you want to mark all the videos in this course as unwatched?

This will not affect your course history, your reports, or your certificates of completion for this course.


Mark all as unwatched Cancel

Congratulations

You have completed Lighting for Photographers: Flash Exposure Fundamentals.

Return to your organization's learning portal to continue training, or close this page.


OK
Become a member to add this course to a playlist

Join today and get unlimited access to the entire library of video courses—and create as many playlists as you like.

Get started

Already a member ?

Exercise files

Learn by watching, listening, and doing! Exercise files are the same files the author uses in the course, so you can download them and follow along. Exercise files are available with all Premium memberships. Learn more

Get started

Already a Premium member?

Exercise files video

How to use exercise files.

Ask a question

Thanks for contacting us.
You’ll hear from our Customer Service team within 24 hours.

Please enter the text shown below:

The classic layout automatically defaults to the latest Flash Player.

To choose a different player, hold the cursor over your name at the top right of any lynda.com page and choose Site preferences from the dropdown menu.

Continue to classic layout Stay on new layout
Exercise files

Access exercise files from a button right under the course name.

Mark videos as unwatched

Remove icons showing you already watched videos if you want to start over.

Control your viewing experience

Make the video wide, narrow, full-screen, or pop the player out of the page into its own window.

Interactive transcripts

Click on text in the transcript to jump to that spot in the video. As the video plays, the relevant spot in the transcript will be highlighted.

Learn more, save more. Upgrade today!

Get our Annual Premium Membership at our best savings yet.

Upgrade to our Annual Premium Membership today and get even more value from your lynda.com subscription:

“In a way, I feel like you are rooting for me. Like you are really invested in my experience, and want me to get as much out of these courses as possible this is the best place to start on your journey to learning new material.”— Nadine H.

Thanks for signing up.

We’ll send you a confirmation email shortly.


Sign up and receive emails about lynda.com and our online training library:

Here’s our privacy policy with more details about how we handle your information.

Keep up with news, tips, and latest courses with emails from lynda.com.

Sign up and receive emails about lynda.com and our online training library:

Here’s our privacy policy with more details about how we handle your information.

   
submit Lightbox submit clicked
Terms and conditions of use

We've updated our terms and conditions (now called terms of service).Go
Review and accept our updated terms of service.