Digital Camera Flash
Why you can't rely on your pop-up flash

A digital camera flash external unit is a necessary piece of equipment for your start-up camera kit. If you are a novice, you may wonder why this is so, when most cameras are already equipped with a pop-up flash. If you are strictly an amateur, a pop-up flash may suffice. However in order to advance and become more proficient (which is why you are here) you will need to consider a separate digital camera flash unit. Flash units have a lot of specifications which I will try and take the mystery out in the course of this article.

Probably the most important reason to upgrade to a flash unit is because the pop-up flash has an insufficient guide number. The guide number is a term used since flash units were first manufactured and it measures the flash's power. The larger the guide number, the more powerful the flash.

The guide number is the product of the maximum flash-to-subject distance and the f-number of the aperture that will correctly expose an image at a specified ISO.

GN = distance × f-number

While all this may seem a little complicated it can be explained quite easily in the following example.

A guide number of 80 at ISO 100 means that a target 20 feet (6 m) away can be fully illuminated with an aperture of f/4 (80 = 20 × 4) and an ISO of 100. For the same guide number and an aperture of f/8, the light source should be 10 feet (3 m) from the subject (80 = 10 × 8).

To determine the f-stop needed, divide the GUIDE NUMBER by the desired distance. To determine the distance that can be illuminated, divide the GUIDE NUMBER by the selected f/stop.

A pop-up flash may have a guide number of 15 and a separate flash unit could be in the vicinity of 50 - 200. You can see why the investment of a separate flash unit is needed.

A basic understanding of the guide number can help assist you in purchasing a unit, and eventually taking better flash photography. You probably do not need to make calculations in the field, as your flash will have built-in sensors to aid you in correct camera-to-subject distance.

Digital camera flash units typically feature three standard modes: E-TTL, Auto and Manual. Just what do these mean and when do you know when to use them?

E-TTL or TTL

Evaluative through the lens metering means that as with ordinary TTL flash exposure systems, a sensor inside the camera reads the amount of light being reflected off of the camera sensor during the exposure. When the sensor determines that the exposure is sufficient, it terminates the flash pulse. I consider this mode to be the "no brainer." I use this mode most often outdoors as fill flash.

Fill flash helps fill in the outdoor shadows and adds a sparkle to the eyes. It is not utilized as the main light, which is the ambient light. I usually underexpose the flash by two stops so the flash does not appear too over-powering and I always use a diffuser (more on diffusers later). Fill flash can also freeze movement because you can use any shutter speed. Wildlife photographers utilize this mode a lot for that reason. Portrait photographers are split between using flash and reflectors or light modifiers outdoors. However,if you use your fill flash correctly outdoors the results are amazing. For more advanced flash users, an off camera flash will result in softer and more controlled lighting. ETTL or TTL can also be used indoors as the main light. Most modern flash units allow you to bounce flash off the ceiling or walls. This can provide a softer, studio type of lighting and prevent shadows on the wall caused by the subject(s). Using a bounce flash will result in a longer camera- to-subject distance so you will need to compensate for this by having a flash with a large guide number and or modifying your aperture or distance. The built-in features of your flash will help you determine this.

Most digital camera flash units come with this feature; it is the most common and most widely used. As with all photography you will need to take test shots, and lots of them, to see what system works best for you. The digital flash unit will have an instruction manual that I highly recommend referring to. More advanced flashes will feature two additional features: automatic flash mode and manual flash mode.

Automatic Flash Mode

The automatic flash mode had been an old stand-by for years. On the front of the flash unit is a sensor (make sure you never cover it up with your hands). This sensor determines when the flash should shut down. This occurs when the correct exposure is reached. This is different from TTL flash which is determined by the camera's through-the-lens metering. The amount of light that is reflected back to the flash's sensor determines the length of the flash. You will need to set your aperture within the range of your flash and guide number so you will have adequate coverage. (refer to the guide number illustration above).

Automatic flash units typically are independent of camera models and can be purchased for less than a flash unit featuring ETTL and manual modes. Beware of older flash versions if using it on a digital SLR camera. You should check with the camera manufacturer to make sure they are compatible.

Automatic flash can provide favorable results in most circumstances. However if you are photographing a bride in a white gown and a groom in a black tuxedo, the flash will be fooled. If you focus on the bride the white gown will cause a short flash duration because white is being reflected back to the sensor while if you focus on the groom it will have a longer flash duration. As you can see, neither will be properly exposed. It is best to avoid auto flash in these situations.

Manual Flash Mode

Manual modes offer the most consistent results. However, they require a little bit of work on your part. It is the opposite of the "no brainer" TTL. With manual flash you will need to determine the correct aperture, flash to subject distance and power ratio of the flash. The further away you are from the subject, the larger aperture you will need due to light fall-off. (or if using bounce flash). The shutter speed cannot exceed the sync speed of the flash which is typically 1/250 sec. Manual flash can be a little complicated if you are working with moving subjects, when the distance will vary. If your subject is still or if you are working in a studio setting however, manual flash will give you very predictable results.

You will need to determine which flash mode is most appropriate for your situation. With the ease of digital photography you can see your results instantly. There is no cost to delete digital images. If you do decide to turn your hobby into a full or part-time job then determining which flash mode is most reliable for you in certain conditions is of utmost importance. Wedding photographers will switch back and forth from manual to automatic in the church and to TTL for the reception. As a professional you cannot afford to waste time and images. If you are not a professional, and can afford to play around a little bit more with the flash modes.

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