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Simple tips to
help you take better pictures
Tips for Better Pictures

This is the first of several
e-tips
on how to take
better flash pictures

Today's e-tip:

Getting the most out of your
Electronic Flash




 
Electronic Flash is an extremely versatile technology. At first, they can seem overwhelmingly confusing, but in reality, they aren't. This series of e-tips is designed to help you understand the capabilities of your flash and how to best use it.

Part 1: Understanding your Flash

Better Flash Pictures
What’s the purpose of an electronic flash? Well, the answer might sound obvious, but is it??

The obvious answer is “to produce enough light to take a picture”. Indoors, it would be difficult to take a decent picture if you couldn’t “add” to the ambient light. But, flash also serves another very useful purpose: to fill in shadows. I can’t imagine taking “people” pictures outdoors on a sunny day without flash. Bright sun causes deep shadows. Photography (film or digital) can’t capture the range of tonal values that our eyes can, so you don’t EVER get what you see. Bright and shadow areas in a scene can’t both be captured well, thus, shadows on a face can become a serious issue.

What’s in a name?
Do you have a flash, electronic flash, speedlight or speedlite? Unless you’re still shooting with flash bulbs (believe it or not, you can still buy flash bulbs), they’re all the same – it’s just a matter of who makes it - different manufacturers use different names, but they basically have the same purpose - some just have more light and/or features.

Understanding Guide Numbers
An important thing to understand about the light from your flash is that when you double the distance between the flash and your subject, only one quarter of the light reaches your subject because the light is spread over a much larger 3-dimensional area.

An industry-wide standard for rating the output of a flash unit is called the “Guide Number”, which we’ll abbreviate with “GN”. The guide number is a measure of the distance at which the flash can illuminate a subject.

Guide numbers are generally rated at a specific ISO speed and the distance is expressd in either feet or meters. For our examples, we’ll use ISO 100 (the de-facto standard for Guide Numbers) and feet. Here's the simple formula for determining the correct flash exposure:

        Aperture (f-number) = Guide number (GN) divided by the distance

Guide numbers can be expressed in FEET or METERS - you just have to divide the GN by the correct distance (in FEET or METERS). For example, the Guide Number for the Nikon SB-800 flash is listed as 38/125 (m/ft). This means that the guide number is 38 when using "meters" and 125 when using "feet". (note that they're both exactly the same - for example, 10 feet is about 3 meters - divide 38 by 3 or 125 by 10, and you get the same aperture - about f12.5).

So, let’s say your flash has a GN of 110 (in FEET) and you are using your flash in MANUAL mode (granted you probably won’t be using it in MANUAL mode, but it makes it easier to explain!). That means that at 5 feet, you would use an f-stop of f11 (GN of 110 divided by 10 feet = 11). At 20 feet, you would use an f-stop of f5.6 (110 / 20 = 5.5), and so forth.

There are two practical effects of this:
  1. The higher the GN, the more light output. Double the GN and you quadruple the light output. If one flash has a GN of 110 and another has a GN of 55, it means that the first flash outputs 4 times the light of the second flash.
  2. Light falls off VERY fast from a flash. When you double the distance between the flash and your subject, you get one-quarter the light!

    With a GN of 110, we use f11 at 10 feet and f5.6 at 20 feet. We doubled the distance from 10 to 20 feet, but got 1/4 the light (f11 to f8 is 1/2 the light; f8 to f5.6 is 1/2 the light, so f11 to f5.6 is 1/4 the light). Double the distance to 40 feet and you get 1/16 the light!
Flash Guide Number Effect

Unless you want to shoot in MANUAL flash mode, which you probably don't want to do, Guide Numbers are most important when evaluating which flash to purchase. However understanding the nature of the light produced by an electronic flash and how Guide Numbers relate, is important in understanding the use of your flash.
Consider this:
Let’s say you’re photographing a person indoors where the only ambient lighting is an incandescent lamp in the room. If you were to photograph the room (without the person in it) with just the ambient light, your exposure might require an f-stop of f2.8. But if you take a picture of a person in that room with flash, you might use an f-stop of f11. In that case the person would be well lit by the flash, but the background would get very little light from the flash and would come out very dark because it required a much larger f-stop (f2.8 in this case). To light the background, you may need a second (or third) flash.

Fill-Flash
Almost all flash units and cameras today are pre-programmed by default to do what’s called “fill flash” – what they try to do, assuming there’s enough ambient light, is match the flash output to the ambient light. This doesn’t do much for us in the above example of shooting indoors with just an incandescent light bulb (just not enough ambient light), but consider outdoors on a sunny day. Let’s assume the correct f-stop on this sunny day is f11. What the flash will try to do is match its output with that of the ambient sun light. It will reduce its output so the correct f-stop for the light coming from the flash and hitting your subject will be f11. Thus the exposure for the flash and the background will both be f11. This means that nothing becomes darker or brighter, and the effect is to just fill-in the shadow areas – perfect for people pictures. Just about every current major manufacturer's flash works this way. You just want to use the flash in TTL mode, which uses the camera's metering system to determine the correct exposure for the ambient light and the flash.


Built-in vs. External Flash
Your DLSR or point-and-shoot camera probably has a built-in flash. It probably also has a “hot-shoe” that is designed to connect to an external flash – one you’d have to buy. You may wonder, why would you want to spend between $200 and $500 for an external flash when you already have a built-in flash (of course, if your DSLR doesn’t have a built-in flash, you don’t have a choice if you want to use flash).

Well, there are several important differences between built-in and external flash units:
  1. Built-in flash units provide a minimal amount of light. External flash units provide much more light. For example, most DSLR cameras with built-in flash provide a GN of about 40 whereas many external units can provide GNs of over 130. That’s a difference of f4 to f13 at 10 feet – which is a HUGE difference. This also means that the max distance of the built-in flash is about 10 feet, whereas the max distance of the external flash is about 32 feet (assuming an aperture of f4).
  2. Built-in flash units can only point in one direction – straight ahead. External units can “bounce” or reflect their light off of ceilings, walls or other objects – a very useful technique as we’ll describe below.
  3. The occurrence of “Red-Eye” is much greater with built-in flash units, since they are closer to the lens than external units.
  4. There are many “accessories” sold for external flash units that just aren’t available for built-in flash units.
Shooting Indoors
When camera manufacturers provide a GN for their flash units, as with cars and MPG estimates, their ratings are always based on the “ideal situation”. What’s an ideal situation with flash? It’s indoors with a low white ceiling and close white walls. That’s because the exposure isn’t only based on the light that directly hits your subject from the flash, but also the light that bounces off the walls and ceiling, which can be considerable, if the walls and ceiling are white! In a banquet hall with a high ceiling and distant walls, almost NO light will be reflected onto your subject, and the effective GN will be lower. So, the 10 feet max distance for your built-in flash, is more like 6 feet in a banquet hall or outdoors.

Ok, so fill flash works well outdoors – but indoors, it doesn’t do much for backgrounds. Let’s return to our example above where we are shooting indoors with just one light bulb for light. How do we get almost equal lighting for the background and foreground? The answer is to “bounce” the flash - a topic we'll cover in the next installment of PhotoBert e-Tips.




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