PhotographyTips.com - the #1 guide to better conventional and digital photography Become a Member iPhone Posing GuideGuide to Posing the Female Model BookGuide to Posing the Model CD
Search
Login

Member Login

Find us on Facebook
Follow us on Twitter
Find us on Flickr
Connect with us on LinkedIn

SPONSORS

Sell Photos Online

FEATURED SITES


Aperture settings for flash

Flash distance determines aperture setting


Flash and aperture go hand-in-hand. The right aperture setting is determined by the distance from the flash to the subject.
Flash and aperture go hand-in-hand. The right aperture setting is determined by the distance from the flash to the subject.

APERTURE - THE CAMERA’S MAIN CONTROL FOR FLASH EXPOSURE

If you have already visited the section entitled Shutter speeds for flash, you will have seen that the shutter is unimportant in making an exposure with flash as the only light source, as long as the shutter is open and not blocking the light. In a completely-darkened room, the shutter can be left wide open before and after a flash is set off, and a correct exposure can be made.

So, if the shutter is not involved, what determines correct exposure for an image lit only by flash? The aperture, of course. The aperture must be opened to the right size to let in the right amount of light to correctly expose the film.

Your next question probably is, how do I know what size aperture (ƒ-stop) to use for flash photography? The answer lies in knowing the distance from the flash head to the subject. The size of the aperture is based on distance.

Flash was used to illuminate this group in a fairly dark room. The aperture was opened to the right size to let in the proper amount of light for correct exposure.<br> (About the picture: How would you look if you were being handed a juvenile crocodile?)
Flash was used to illuminate this group in a fairly dark room. The aperture was opened to the right size to let in the proper amount of light for correct exposure.
(About the picture: How would you look if you were being handed a juvenile crocodile?)

DISTANCE

Distance is everything in flash photography. Since light falls off rapidly over distance, its intensity four feet away from the flash is much greater than its intensity twenty-four feet away from the flash. This means your lens’ aperture must be smaller when the subject is closer to the flash, and larger when the subject is farther away from the flash.

Note that we said “closer to the flash,” not closer to the camera. The flash-to-subject distance is the important distance consideration for proper exposure. It doesn’t matter where your camera is located. It could be two feet, fifty feet or two hundred feet away from the subject, but as long as the flash remains the same distance from the subject, its illumination of the subject will remain the same, requiring no change in the camera’s exposure settings, no matter where the camera is in relation to the subject.

Light diminishes over distance in accordance with the inverse square law, which states that doubling the flash-to-subject distance reduces the light falling on the subject to one-quarter.

Use the exposure calculator attached to your manual flash unit to calculate the required aperture. Do not be intimidated by its looks. It's very simple, easy to understand and quick to use.
Use the exposure calculator attached to your manual flash unit to calculate the required aperture. Do not be intimidated by its looks. It's very simple, easy to understand and quick to use.

HOW DO I CALCULATE EXPOSURE FOR MY MANUAL FLASH?

The easiest method is to refer to the exposure calculator supplied on the top (or sometimes the back or side) of most electronic flash units.

  • The calculator must first be set for the speed of the film you are using.
  • This is usually a simple matter of turning a dial so that an arrow or triangle points to the ISO number for your film.
  • Then, you need only to know the distance from flash head to subject, which can be measured accurately enough by focusing your camera on your subject from the flash location, and looking at the distance index on the focusing ring.
  • The flash’s exposure calculator will show you what ƒ-stop (aperture setting) to use for that distance. The picture above shows that a subject 10 feet away (or 3 metres away) requires an aperture of ƒ/5.6 for proper exposure using flash.
  • Set your aperture for the indicated ƒ-stop, ensure your shutter speed dial is set at “X” or the appropriate shutter speed for electronic flash for your camera, and take your flash picture.

If you find after shooting a roll of film that there is consistent underexposure or overexposure in your images, odds are good that your flash’s exposure calculator may be providing you with slightly inaccurate settings, a point you should keep in mind for future flash photography. If consistent underexposure is the problem, set the film speed on the dial for a slightly lower speed than the film you are using. Rate ISO 100 film at ISO 64, for instance, and you should see an improvement. When you have consistent overexposure, set the film speed higher than the actual speed of the film you are using. ISO 100 film, for instance, could be set at ISO 125 on the dial. Trial-and-error will eventually provide you with a mental list of reliable variations in film speed settings for your particular flash unit.

You can also use guide numbers for determining the required aperture for correct manual flash exposure when you find your images are consistently over or under-exposed when you have been using the flashs exposure calculator, or when your flash does not have an exposure calculator.

Check your camera's manual for the effective range of its built-in flash, then be sure to stay within that range for flash photography.
Check your camera's manual for the effective range of its built-in flash, then be sure to stay within that range for flash photography.

DISTANCE WITH COMPACT CAMERAS

With a fully-automatic camera, you do not need to be concerned about aperture settings for flash photography because your camera measures the distance and does all the calculations automatically.

You do need to be aware, though, that you should not get too far away from or too close to your subject for your flash to do its job well. It works properly only within a certain range of distance. If you are too close, the brightness of the flash will greatly overexpose your subject, and you may see only a white glare where you expect your subject to be. (For an example, have a look at this picture in which the flash illuminated the photographer's thumb.) If you are too far from your subject, your flash will not be powerful enough to adequately illuminate it, and your subject will be underexposed, possibly just a dark shadow.

How close should you be? Most built-in, automatic flashes have an effective range of around 3 to 35 feet. Check your camera’s manual to verify its particular flash range, then be sure to keep your subject within its minimum and maximum boundaries so that the light from the flash will expose them properly.

 
Further information...
Flash guide number