Wednesday, January 18, 2012

Tools of the Trade: Flash Part II

Greetings, volks. Welcome to Hohenfels Volks, THE place for our place! Today’s post is going to deal with using your flash in manual mode.

For some Volks, manual mode and photography don’t seem to go together. They leave everything on automatic, letting the camera tell them when to click the shutter, which seems a bit too much like serving the camera, not the camera serving us. By now, most of you have been shooting manual for a while. Chances are, though, that your flash is doing the majority of your work when you use it. I hope that this post will change that some!

First, we’re going to learn some terms and concepts. Flashes are rated by something called the GN, or Guide Number. It’s a measure of a flash's output. You have to know this number. The YN-560 that I have is rated by the manufacturer for 58 meters at 105mm, ISO 100. The actual rating is more like 35 meters at 35mm, ISO 100, which should give us about 50-55 meters at 105mm. I haven’t formally tested it in a controlled manner, but that seems about right. What that means is shooting at ISO 100, 105mm, and with the flash at full power, my max range for the flash is about 58 meters. At that distance, the light has fallen drastically in intensity, but spread out in coverage. The next thing we need to understand is aperture, or the size of the opening in our lens. A larger aperture means more light and conversely a smaller aperture means less. Aperture is measured as f/stop, a ratio of your aperture’s diameter to the level of light it allows. That’s oversimplified, but you get the idea. You already know about aperture, but will need to understand it for flash work. The last thing we’re going to bring up is distance between the flash and the subject. These 3 things are how you determine a manual flash’s power and set your exposure correctly. A flashes GN is the rating assigned to it by the manufacturer, so you have to be careful to read the specs before buying one. Some makers will call their flash GN58, but if you look at the tiny print, it’s at ISO 200 or even 400. That makes it more like a GN20 at ISO 100. Also, make sure the units your GN is listed in, feet or meters, or both are consistent throughout your calculations. Also remember that we're dealing with light, so sometimes it doesn't seem to make sense due the inverse square law. That's why we use GN!

The 3 things listed above are what you need to know to expose properly for manual flash. We’re going to start this part by giving you the equations to use, then giving some examples.

The first calculation is to determine your GN. First, set your flash to manual mode, full power, and if it has it, full zoom. Then place your flash a measured distance to your subject, say 5 meters. Then shoot a series of shots at successive f/stops. Note the aperture that gives you a proper exposure. For this we’re going to say f/8. Now multiply the distance by the f/stop. For this we’re going to multiply 5 meters by f/8, which means 5x8=40, or a GN of 40 at ISO 100, full power, max zoom, or about 20 at half power.

The next calculation is to determine your f/stop for that GN. If you have a GN 20 at ISO 100 flash, what f/stop do you use for 4.5 meters? The calculation for that is GN divided by distance. For GN 20, that would be 20/4.5=4.5 or f/4.5. Set up the above flash, GN 40 at half power, 4.5 meters and shoot using f/4.5, you should be almost dead on every time!

Last, we have figuring out your distance for a GN with a desired aperture. That is calculated by dividing GN/f/stop. As above, GN 20 at half power, we’ll say f/8. To get that we divide 20 by 8, or 20/8= 2.5, or 2.5 meters.

Here are the calculations listed:

Your GN=f/stop x distance to subject
F/stop= GN/distance to subject
Distance to subject= GN/f/stop

Here are some things to note. The distance is ALWAYS the distance from the flash head to the subject, not the camera to subject or flash to camera. Light modifiers like a Sto-Fen or umbrella lower your flash power by about 1-2.5 stops. Measure with them in place for more accurate readings. The same applies to bounce flash, count your distance as being from the flash to the reflector + the reflector to the subject, or adjust your exposure to compensate. A few times doing this with your calculator and flash manual will give you a great starting point for your flash shots, then before you know it, you’ll be nailing them every time!

Now that you know all that, use your camera's built in spot metering mode, go to AV or aperture priority, set your required f/stop, and meter for the rest of the scene. That shutter speed will allow you to balance your flash with the ambient light to create a nearly perfect exposure. Tinker with it, and you'll make that magic of having your image match your vision that much easier!

Working with your flash off camera in slave mode can also be a trial. The secret to that is point your optical sensor at the camera and rotate your flash head toward the subject. The sensor detects the flash from your camera and triggers the flash. It also has to be where it can see the flash from on camera. For some flashes, this will trigger it before it should go. The Yongnuo flashes have a mode called S1 that trips the flash at the very first blip of flash, and S2 that only triggers when the actual flash goes for the shot. Use S2 to ignore the red eye and pre-flashes your camera sends out. Also, turn of red eye reduction or pre-flash. Having your flash off camera will help prevent this phenomenon from occurring. Of course, you can also buy a wireless RF trigger and receiver from anywhere online for about $30 and avoid slave mode altogether.

A great resource for all things flash related is Strobist. We have a link to them in our links and they can really up your knowledge levels on flash. They put out some good tutorials and lessons in Lighting 101. Give them a read!

You’re probably bored after all that math and calculating. Get out there and shoot something, it’ll wake you back up! While you’re out shooting, don’t forget the milk, I mean your shots for this week’s theme! Complimentary colors with a contrasting subject. Should be something great in there and I can’t wait to see your work!

Don’t forget to post any of your images you’d like to see here at the Hohenfels Volks Facebook page. Of course, commenting on both Facebook and here is always appreciated, too!

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