TTL? Rear Curtain Sync? Guide Number? All the flash photography basics you need to know are right here, as we answer all the common questions to all the common problems photographers have about using flash, as well as share some of our best flash photography tips.
Sometimes my SLR’s flash pops up and sometimes it doesn’t. Why?
When you’re in fully automatic ‘green square’ mode, the small flash built into the majority of DSLRs will activate automatically when light levels are low.
However, in the advanced P, A, S, M modes you can pop the flash up manually whenever you like by pressing the button marked with a flash symbol.
This is useful for adding a little extra illumination, such as a brightening up an outdoor portrait. Bear in mind that you’ll have to stick with a shutter speed that isn’t faster than the maximum flash sync speed.
Maximum flash sync speed?
Despite the light from a flash being emitted very rapidly, you can’t freely choose the fastest shutter speeds your camera offers when you’re using a flash.
This is due to the way in which the two shutter ‘curtains’ inside a typical SLR work. These open and close to allow the imaging sensor to be exposed to light.
When you press the shutter-release button, the mirror flips out of the way and the first curtain opens to allow light to hit the sensor. This is followed by the second curtain, which closes across the sensor to end the exposure.
At very fast shutter speeds, the second curtain starts closing before the first curtain has finished opening – so the image is recorded through a rapidly moving gap rather than all in one go.
Because the burst of light from a flash is so brief, it would only be recorded on the narrow part of the sensor that’s clear of the curtains when the flash fires, and the exposure would be uneven across the picture.
The flash sync speed is the fastest shutter speed at which the shutter is fully open, ensuring that the exposure is even across the frame.
Historically, this has been 1/60 sec, but many current DSLRs take advantage of the fact that flash duration is extremely short and offer high sync speeds of 1/250 or 1/500 sec.
I’ve heard that I should use the built-in flash as a last resort. Is this true?
Yes and no. The trouble is that the small size of the flash unit and its position on the camera means that the light it produces is direct and harsh.
Other limitations include the risk of red-eye, due to the close proximity of the flash to the lens, and the fact that you need to be fairly close to the subject, as the Guide Number of a pop-up flash is comparatively low.
What’s the Guide Number?
The Guide Number (GN) is a measure of the maximum power of a built-in flash unit or a flashgun – the bigger the number, the more powerful it is.
The Guide Number = distance x f/stop at ISO100. We know – it all seems to be getting complicated again! Knowing the Guide Number is useful if you’re shooting with manual flash, because it helps you calculate the aperture you need to set, or the distance the flash needs to be from the subject in order to get a good exposure.
However, these days, exposure can be taken care of automatically by TTL flash metering.
TTL flash metering? Things really are getting complicated…
TTL – or Through the Lens – flash metering actually makes things easy! The latest ‘intelligent’ versions, such as iTTL on Nikon and E-TTL on Canon, fire one or more brief flash bursts or ‘pre flashes’ are fired before the main flash.
The camera registers the amount of light returning through the lens (hence the name), and then works out the right amount of light to pump out for the main flash exposure.
However, just like normal exposure metering it can be fooled by dark, light or reflective backgrounds, and you may need to adjust the flash exposure to compensate.
How do I alter the TTL flash exposure?
The camera will automatically control the flash exposure for consistent results in TTL mode, no matter what aperture or ISO or shutter speed (as long as it’s slower than the flash sync speed) you choose.
That is, as long as the flash has enough power to reach the subject. If you’re too far away, your flash-lit subject will appear too dark, and you’ll need to move the flash closer, choose a wider aperture and/or increase the ISO setting.
If you want to fine-tune the flash exposure, the easiest way is to use flash Exposure Compensation (FEC). Look for the button marked with a zig-zag flash symbol and ‘+/-’. See the previous page for alternative ways to control the brightness of your flash.