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Finding the guide number of a strobe

From: Lighting for Photographers: Flash Exposure Fundamentals

Video: Finding the guide number of a strobe

In this movie we're going to begin the hands-on process of exploring how to really master your flash. To do that, we'll start by explaining how you can find your flash's guide number and what to do with it once you have it. Small strobe manufacturers use the term guide number to rate the power of their pocket strobes. A guide number is a numerical expression of the ability of the flash to create at a normal exposure at any given ISO. When you leaf through your manual, you should find the table that looks something like this.

Finding the guide number of a strobe

In this movie we're going to begin the hands-on process of exploring how to really master your flash. To do that, we'll start by explaining how you can find your flash's guide number and what to do with it once you have it. Small strobe manufacturers use the term guide number to rate the power of their pocket strobes. A guide number is a numerical expression of the ability of the flash to create at a normal exposure at any given ISO. When you leaf through your manual, you should find the table that looks something like this.

Because a higher guide number means more power and more power means a higher value, manufacturers tend to inflate their published guide numbers. This means that if you're serious about getting consistently good manual flash exposures, you should perform your own guide number test. Another reason to perform your own guide number test is because a guide number is dependent upon the accuracy of the ISO rating of the camera sensor. Same flash unit when placed on two different cameras will have two different effective guide numbers if the camera's sensors are not calibrated.

You can determine your own guide number by identifying the best exposure from a series of portraits taken while maintaining a consistent flash to subject distance and bracketing the aperture. We want results that aren't contaminated by ambient light or too much light bouncing off of a white or pale colored walls and ceilings. So find a dimly lit room with some breathing room overhead and on either side of your subject. That's why we pulled the curtains here in the studio and covered part of the floor with the black duvatine.

Now is as good as time as any to get used to working with your flash off camera. So use a radio slave or a sync cord to allow you to get your flash out of the hot shoe and onto a light stand. The guide number test consists of a series of images beginning at f/22 and opening up in third stop increments all the way to f/56. Follow the recipe, and when you're finished, you'll have a series of 13 portraits, one of which will reveal your guide number. The PDF for this course will have the following guidelines included for reference.

Place the flash exactly 10 feet from your subject. This is going to make calculating your guide number easier. Place the flash in full manual mode, this is designated by an icon on the back of a display, reading 1/1. Now, it's important to know that most small strobes have reflectors and lenses that move to match the angle of the coverage of the lens. When the zoom head mechanism that's place in a telephoto setting, the resulting beam of light is more concentrated than one when the head is zoomed out to cover the field of view of a wide-angle lens.

You can hear the gearing mechanism that drives the lens move as we change the focal lengths. This is a good thing because it conserves energy by illuminating only what's seen by the chip. But it means that the same flash unit will have a different guide number at every zoom head setting. So for the sake of this test set the zoom head on the flash to 50 millimeters. Frame up a loose head and shoulder portrait. If you're shooting in a normal or a wide lens, this is going to require that you get between a flash and the subject.

So make sure your head doesn't contaminate your results by casting a shadow on your subject. Make sure the subject is wearing a white shirt, because we want to determine which exposure begins to lose detail in the texture or the fabric. Subject should be holding a gray card at an angle that prevents a hotspot from being reflected off of the card and onto the lens. Place the camera in manual mode and set the white balance on your camera to the flash setting. Set your camera at ISO 100. If your camera doesn't go that low and the table in your flash manual list the GNs at ISO 100, you will have to do simple conversion to compare your results to the guide number table.

But don't worry, we will take a look at how to make that conversion later in this chapter. Shoot at your camera's sync speed, and if you're using a radio slave then shoot at a third stop slower to account for the slight delay that sometimes occurs in the radio circuitry. In any event, the use of the highest possible shutter speed will illuminate ambient lighting conditions, provided you're shooting in a dimly lit room as previously called for. Finally, allow the flash to recycle completely between exposures.

After the beep or ready light, count to five and change your aperture before shooting again. This ensures consistent results by allowing the capacitor to fully recharge between exposures. Now it's your turn. Just a reminder, take your first shot at f/22 and bracket your exposures by opening up your aperture in third stop increments all the way down to f/56. Once you've generated your set of 13 images, it's time to analyze them, and we'll do that in the next movie.

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