An Introduction to External Flash Units

The camera manufacturers have a remarkable staff of engineers and those engineers, based on input or suggestions from actual users throughout the world continue to develop some amazing gear. The equipment continues to help produce outstanding images and it makes   our job easier, makes us better or both. One of the areas of remarkable and accelerated achievement is in external hot-shoe mounted flash units. 

This article will focus on the dedicated flash units by the primary manufacturers such as Canon.

Most of the latest units are given the acronym E-TTL which means “Evaluative-Through-The-Lens” operation. There are several other ways to use these newest flashes including manual operation where you decide how much flash it should put out. Even 3rd party companies such as Sigma and Metz make E-TTL flash units that are specific to Canon, Nikon and other name brands.

A Common Misconception about Flash Output
The first thing that needs to be addressed is the very common misconception flash output is determined by the how bright the flash gets when you fire a unit. That is not correct. Flash output is determined by the amount of time or duration the flash unit emits light from the strobe unit. So what determines the duration of the flash? That is a complicated algorithm but in simple terms, it is the combination of your aperture, shutter speed and amount of light needed as determined by your camera.
If it is being used outdoors as a fill light, the duration is measured in milliseconds. If it is being used in very dark conditions, the duration will still be in milliseconds but much longer than fill light. So, make sure you understand this… the amount of brightness a flash unit puts out stays constant. What determines you getting the correct exposure is the duration of that flash.

A Word of Caution About Using Flash Units
Use caution at all times using flash units. Do not ever place the unit close to you or any person look directly at the business end of the unit and fire the flash. The light is very intense and will wreak havoc on your eyesight for a while. Point it away from you when testing it at all times. Also, the voltage inside a charged speedlight is extremely high and dangerous. Do not curiously open a unit to see how it works.

How External Flash Units Work
The latest offerings of flash units that mount on your hot-shoe are ingenious devices. They are as automatic as one would ever think could be developed. In fact, if you are accustomed to point and shoot cameras, using one of these flashes on your hot-shoe is as easy as pushing shutter release. What happens is an amazing technological achievement. When you have the unit turned on to take a photo and press the shutter release, the flash puts out a pre-flash to the subject you are focused on and sends that information back to the camera. The camera processes the pre-flash data then fires again to take the picture. Mind you, the subjects are totally unaware that there were two flashes firing in their eyes, a pre-flash and then the main flash. All this happens in what is measured not in milliseconds, but in microseconds!

In the pre-flash, the flash unit and camera are working together to determine the ambient light, measured by the sensor in the camera. The sensor compares the ambient light values with the light reflected by the subject. Canon’s E-TTL II even measures the distance to the main subject and that also becomes part of the evaluation taking place. The pre-flash determines how much flash duration will be needed, sends that information to the camera and you are the recipient of a perfectly exposed subject and background. 

Camera Settings and Different Kinds of Flash Photos
Now let us examine the setting on the camera and how that determines what kind of flash photo you will receive. Every modern DSLR has the following settings on the camera: there is “P”, then “TV”, “AV” and “M”. Yes, then there are the totally auto modes on the same control….those icons that mean portrait, landscape, close-up, sports and night portrait. For now, focus on pretending those do not exist so that you can study the use of the P, TV, AV and M modes. These are the modes that once you learn how to use them, you will rarely go back to the “auto modes”.

The “P” mode is in reality a semi auto mode, that is the P is the first in the line of programmed modes, but in practical use, the camera still continues to do auto exposure. That is what Canon calls Program AE. There is a huge difference in the P mode as compared to the “fully automatic” mode indicated by a rectangle usually colored green on the camera. In P mode, you will be allowed to change shutter speeds. In the fully automatic mode, you cannot change anything.

Try it. Set your camera to P mode, and then point it at a subject in some nice daylight. Notice that perhaps your camera will select, on its own, a shutter speed of 1/125 with an aperture of F11. Then using the wheel selector of top of the camera, you can actually change the shutter speed to 1/250, 1/500 etc but at the same time as you change to faster shutter speeds your camera will open the lens up accordingly to allow the same amount of light to enter.

Moreover, you will notice that if the camera chooses 1/125 shutter speed and F8 as the aperture and you decide to increase shutter speed using that wheel selector and change it to 1/250 you will notice that the camera automatically changes the aperture to F4. That tells you immediately, once you think about it, that 1/125 @ F8 is really the same exposure as 1/250 @F4! The trade off is that F4 will give you a much more narrow depth-of-field compared to F8. On the other hand 1/250 shutter speed might make the difference between a blurred shot and a clean shot since the lower the shutter speed the more difficult it is to hand-hold a camera.

Now go back to flash and how the “P” setting affects the images you take. The following will assume that you are using the center focus point on your camera. Most DSLR’s have many focus points. Set yours to use only the center focus point for now.

Remember above how the flash sends out a pre-flash? In the “P” mode, the camera and flash assume you want ONLY to expose the subject and nothing else around it including the background. This is very important because take a situation where you are in the wedding reception hall, the lights are turned down, you take a photo of the bride and groom dancing and you are in “P” mode. You will get a great photo of the bride and groom but the background will be dark. Not that this is bad understand but what if you really wanted to light the background as well because about ten feet in back the bride and groom is the brides Mom and Dad? They will not be seen.

At this point, you can “fix” this problem two ways. One is to go back to the icons mentioned above. Go to the icon that means “night portrait”. Take the photo again and suddenly the background is exposed. But there is a price to pay for that. The shutter speed is drastically reduced by the camera in that position. In fact, it may be so slow that the subjects will be blurred if they move. Shutter speeds may go as low as 1/10 second or perhaps 1/20 second…much to slow without having your subject remain very still while you focus and take the shot. But you will light the background!

Another Way – Shooting in Manual (M) Mode
There is a better way. Every pro knows this. Learn this. You simply put your camera on “M” for manual. Now, with the camera on “M”, you can set the shutter speed and lens opening (aperture) to wherever you desire. The desired and most widely used pro settings are a shutter speed of 1/60 and an aperture of F5.6. I use that one. You might also try 1/60 and aperture of F8. Suddenly, a whole new world of flash photography opens up to you. First, you will not be concerned if someone moves and secondly you will light the background.

This is because in any programmed mode such as TV, AV or M, the camera takes good direction from you and assumes you want the background lighted as well. This is built in by the bright engineers at the factory we talked about. But please do only try this in the manual mode. Once you enter the reception hall, automatically set up your camera to “M”, 1/60 @ F5.6 and fire away all night. If you do want a shot with the background dark simply move the dial back to “P” mode and fire away. Nothing else needs to be adjusted.

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