Part 7 - The relationship between aperture, shutter speed, and ISO

 Using Flash - part 1

Kathryn in New Zealand - three photos comparing the use of flash.

It's obvious that the camera and lens are the most import parts of any camera system, but what makes it better?  Most would argue adding another lens, which certainly has its merits.  However, owning a good external flash can have a remarkable impact on your photos.

One of the keys to working with flash is to understand its relationship between itself and exposure.  How are flash, shutter speed, aperture, and ISO connected?

It is best to break down flash into the two kinds that exist for DSLRs and mirrorless cameras (compacts, point-and-shoots, and bridge cameras as well).  There is the built-in flash and the external flash.  Built-in flashes are there for convenience; they have relatively little power but are useful in certain situations.  External flashes attach to the camera via a hot-shoe (some models use a PC connection) and typically come with more power and a number of features.

A flash's guide number (GN) is measured either in feet or meters.  The aperture and ISO settings of the camera are important aspects of this value.  It is important to know that guide number is usually stated with an ISO of 100.  Built-in flashes often have a guide number of 40 (ft) or 13 (m).  At an ISO of 100, a built-in flash is effective to about 7 feet or 2.2 m.  If you look at the middle image above, a built-in flash was used to illuminate the foreground.  The flash itself made no noticeable impact, other than the reflection in the subject's glasses.  In this situation, the built-in unit just didn't have enough power.

External flashes have more power and can effectively illuminate things further away.  A common guide number for an external flash is 100 (ft) or 30 (m).  With an aperture of f/5.6 that same flash is effective up to 18 ft or 5.5 m.  This is an important consideration when shooting in brightly lit situations where you need to use a smaller aperture.  At f/16, the GN 40 flash is effective only to about 2.5 ft while the GN 100 unit works up to just over 6 feet.  The right image above was taken with an external flash; the extra power made a significant difference in the shot.

If you want your flash to go further you can increase ISO; it takes two full stops of change to double or half the flash distance.  A one-stop change alters the effectiveness up or down by 70%.  This is all well and good unless you are shooting in a bright environment, because another limiting factor of DSLRs and mirrorless cameras is something called the flash synch number.  This is the fastest shutter speed a camera can use and still make the flash work.  There are exceptions to this, but that becomes a much more technical issue.

Have a look at some of the pictures below that were taken with an external flash.

All photos taken by Eric Svendsen and are subject to copyright.

I will write part 2 another day and discuss shutter speed and flash in much more detail.

Thanks for reading.

Eric Svendsen     www.ericspix.com