FLASH PHOTOGRAPHY 101 – A BEGINNER’S GUIDE

Nice post, thats a good explanation photo explaining f-stops. Its also helpful to explain it linearly - like with the flash on the wall, where you can see the falloff as brightness.

I wrote a little tutorial on my site about advanced flash lighting techniques, once you've graduated from these starters.

What an awsome resource! I have a question after reading it:

Fact #4 (see quote below) says the camera can't and doesn't try to predict how much light it will get from the flash and just goes on ambient light. OK, that makes sense. What I don't understand is how it then keeps from having the flash totally blow out the shot. If it adjusts just to ambient light without making any compensation for flash I would assume it would end up with such a slow shutter speed and/or wide aperture that the flash would not be necessary in terms of exposure (though clearly needed to achieve what you are aiming for, like no blur). Does it just assume a set amount of light will come from the flash, and then adjust the flash as it fires (or use the flash settings you give it)? Seems like it has to make allowences of some sort for the flash. Hope I wrote this in a way that makes sense.

Thank you,
Fitzhugh

Curtis N wrote in post #1527539
Flash fact #4: Your camera measures ambient light and flash illumination separately. In Av, Tv or P modes, it will attempt to expose properly for the ambient light by adjusting either the shutter speed, aperture, or both. The fact that you have your flash turned on has no effect on this** ( one exception is that in P mode it will not use a shutter speed slower than 1/60 with flash). The camera’s metering system cannot predict how much illumination will be gained by the flash, so it doesn’t try. In manual mode, the meter in the viewfinder measures only ambient light, because that’s all it has to measure.

Fact 5 refers to any form of automatic flash metering, including older “auto thyristor” flash units, TTL film cameras, and E-TTL or E-TTL II digital cameras.
Flash fact #5: With automatic flash metering, the flash illumination is measured after the shutter button is pressed, and the flash output is adjusted accordingly. There are technical differences between the various types of flash metering, but all of them operate independently from the camera’s metering of ambient light, and all of them work by adjusting the output of the flash, not by changing the camera’s exposure settings.

Fitzhugh,

First, I'll try to explain how automatic flashes used to work. Imagine lying on a tanning bed until your skin turned a certain shade of brown. At this point you turned off the switch. This is sort of the way flash metering used to work. Automatic flash units had a light sensor on the front that measured the light reflecting back from the subject and turned themselves off when sufficient exposure had been obtained. This system worked well, as long as your camera's aperture was set to match the flash. Then along came TTL metering, where the light sensor was inside the camera, measuring light coming Through the Lens and reflecting off the film. This was really cool because it automatically compensated for different apertures and would also compensate for the film speed if your camera was appropriately set.

Then digital cameras came along, and for some reason, this TTL method didn't work with the digital sensors so camera makers had to come up with a new system. E-TTL is Canon's version. It works by firing a preflash, measuring its light through the lens and comparing it to the ambient metering, and from this, the camera would tell the flash how much light to produce after the shutter opened. It happens so quickly that some people don't even notice two flashes.

When you use flash in Av or Tv mode, it is certainly possible to get overexposed images. When using fill flash it's important to watch your histogram and adjust EC, FEC, or both, to get the desired exposure and the desired balance.

neilcowley wrote in post #2162871
I wrote a little tutorial on my site about advanced flash lighting techniques, once you've graduated from these starters.

Thanks, Neil!
I'll check it out when I get a chance.

Ah, thanks!

Curtis N wrote in post #2165598
When you use flash in Av or Tv mode, it is certainly possible to get overexposed images. When using fill flash it's important to watch your histogram and adjust EC, FEC, or both, to get the desired exposure and the desired balance.

Ok, this reminds me of a mostly unrelated question: on my a620 I only see a histogram after shooting a shot, can't see one before. I had been thinking that this was because it was a lower-end camera, but now that I know a little more about the difficulties in predicting the resulting exposure I suspect that is normal... Do digital SLRs and such show them before the shot? Or do you still have to shoot a picture to see the histogram (at least there is no film wasted nowdays)? Also, do EC and FEC stand for Exposure Control and Flash Exposure Control? Just a guess. Either way, what do they do, exactly? I know I can change the flash output level, and in some modes I can adjust exposure as well, but I don't know what that really does... does it change shutter speed and/or aperture?

My pictures are improving a bit, but more importantly, I'm enjoying putting what I learn here to use - makes it just all the more fun... thanks!

There are some cameras nowadays that show you a "live" histogram on the LCD, but Canon DSLRs have a mirror and mechanical shutter which prevent the sensor from "seeing" the scene until the shot is taken. Obviously a live histogram will only show the ambient light contribution.

EC = Exposure Compensation (i.e. ambient light)
FEC = Flash Exposure Compensation.
If you're in Av mode, EC will alter the shutter speed. In Tv mode, it will alter the aperture. FEC will change the output of the flash.

Curtis, once again you make it clear.

Spent much of the day reading the forum, finding a huge amount to learn as I go. Did stop to eat, and take hundreds of test shots. The responses I get to my questions here are an incredible help!

Thanks.

This is a bad ass bible. do you teach new photographers? lol

Curtis N wrote in post #1527539
FLASH PHOTOGRAPHY 101 – A BEGINNER’S GUIDE
Just about everything here is covered in The EOS Flash Bible, but that’s a pretty long document and I thought this forum could benefit from having a shorter guide to use as a reference.

FLASH PHOTOGRAPHY CAN BE COMPLICATED.
If you find using flash to be a frustrating experience, don’t despair. Flash photography is at least three times more complicated than ambient light photography. But when you come to understand all that happens during those few milliseconds after you press the shutter button, you’re well on your way to taking consistent flash photographs with predictable results.

THE BASICS
Before you venture into the world of flash shooting, you need to first understand the basics of exposure. This guide assumes that you understand how shutter speed affects exposure and motion blur, how aperture affects exposure and depth-of-field, and how the ISO setting affects exposure and digital noise. If you don’t yet have at least a theoretical grasp of these concepts, then it’s best to learn about them before venturing into the flash world.

LESSON 1: FACTS THAT EVERY FLASH SHOOTER MUST UNDERSTAND

The first four facts are universal, whether you’re using the camera’s built-in flash, a hotshoe-mounted flash unit, or studio strobes.
Flash fact #1: Every flash photograph is two exposures in one – an ambient light exposure and a flash exposure. This is a critical fact to remember. The shutter opens, the flash fires, the shutter closes. During this time, both ambient light and flash will contribute to the recorded image. Flash photography requires managing both exposures.

Flash fact #2*: Flash exposure is not affected by shutter speed. The entire burst of light from the flash begins and ends while the shutter is open, so keeping the shutter open longer won’t help with flash illumination. The flash exposure and the effective range of your flash unit will be affected by aperture and ISO settings, but not the shutter. Of course, the ambient light component in a flash photograph is affected by shutter speed. So changing the shutter speed is one way to manage the amount of ambient light that contributes to a flash photograph.

Flash fact #3: Flash illumination is dramatically affected by distance. This is known as the inverse square law. Think of it this way: Suppose you’re using a lens that gives you a 4 x 6 ft. field of view at a distance of 10 feet. That same lens will give an 8 x 12 ft. field of view at a distance of 20 feet. So when you double the distance, the same light is covering an area four times larger (96 square feet vs. 24 square feet)! So you need four times as much light to get the same illumination. This phenomenon, sometimes referred to as “flash falloff”, will affect any image with more than one subject at different distances. Whenever your subject distance increases by a factor of roughly 1.4 (the square root of 2), the flash illumination will be cut in half. Suppose you’re taking a large group portrait. The people in the first row are 10 feet away, and the people in the back row are 14 feet away. With on-camera flash as the primary light source, the front row will be a full stop brighter than the back row!

In the image below, each cup is one stop brighter than the one behind it, and one stop darker than the one in front of it. It would take 16 times as much light to properly expose the cup at 11 feet verses the cup at 2.8 feet. Do those distance numbers look familar? They're the same as standard f/ stops for aperture settings, and the relationship is identical. This thread from PhotosGuy gives an example of how to use this relationship in the field.

Flash fact #4: Your camera measures ambient light and flash illumination separately. In Av, Tv or P modes, it will attempt to expose properly for the ambient light by adjusting either the shutter speed, aperture, or both. The fact that you have your flash turned on has no effect on this** ( one exception is that in P mode it will not use a shutter speed slower than 1/60 with flash). The camera’s metering system cannot predict how much illumination will be gained by the flash, so it doesn’t try. In manual mode, the meter in the viewfinder measures only ambient light, because that’s all it has to measure.

Fact 5 refers to any form of automatic flash metering, including older “auto thyristor” flash units, TTL film cameras, and E-TTL or E-TTL II digital cameras.
Flash fact #5: With automatic flash metering, the flash illumination is measured after the shutter button is pressed, and the flash output is adjusted accordingly. There are technical differences between the various types of flash metering, but all of them operate independently from the camera’s metering of ambient light, and all of them work by adjusting the output of the flash, not by changing the camera’s exposure settings.

Facts 6 and 7 apply to any camera with a focal plane shutter (all SLR cameras with a mechanical shutter).
Flash fact #6*: Every SLR camera with a mechanical shutter has a maximum flash sync shutter speed (1/200 or 1/250 on current Canon DSLRs). This has to do with the way focal plane shutters work. At slower shutter speeds, the first curtain opens, the flash fires, and after the specified time duration, the second curtain closes behind it. At shutter speeds faster than flash sync, the second curtain begins to close before the first curtain is completely open. The second curtain follows the first across the frame, exposing only a slice of the image at any given moment. Firing a flash during this process would illuminate only part of the image.

Flash fact #7*: (Applicable to modern electronic cameras only) If you set your shutter speed faster than flash sync, or use Av mode with an aperture setting that requires a shutter speed faster than flash sync for proper exposure, the camera will automatically revert to flash sync speed when the shot is taken if a built-in or hotshoe-mounted flash is turned on. Usually this results in overexposure (unless you have a “safety shift” custom function enabled). If you’re getting overexposed images when using flash outdoors, this is probably the reason. The image is not overexposed because of light from the flash. It’s overexposed from ambient light because the shutter speed was too slow. If you’re using flash for fill in bright situations, it’s necessary to stop down the aperture or lower the ISO setting to get the shutter speed below flash sync.

* The exception to facts 2, 6 and 7 is FP Flash, sometimes referred to as “high-speed sync.” That topic is covered in Chapter 4.
**With some Canon cameras there is a poorly-documented phenomenon called NEVEC (negative evaluative exposure compensation) which will adjust the ambient exposure by up to a full stop when the flash is turned on, but that’s also a topic for another chapter.

Chapter 2 - (WHY) SHOULD I GET A FLASH UNIT FOR MY CAMERA?
Chapter 3 - A SYSTEMATIC APPROACH TO BOUNCED FLASH
Chapter 4 - Guide Numbers and High Speed Sync

Curtis N wrote in post #2171432
EC = Exposure Compensation (i.e. ambient light)
FEC = Flash Exposure Compensation.
If you're in Av mode, EC will alter the shutter speed. In Tv mode, it will alter the aperture. FEC will change the output of the flash.

Curtis,

I am a little confused. If I am just using my Canon 30D, I can set EC by pushing 1/2 way on the shutter, then rotating the main dial. Do that all the time when shooting sports in bright sun light.

If I have my 430 EX flash attached in E-TTL and I take the same action, (1/2 shutter main dial), am I not setting the FEC?

How can you set both EC and FEC? Why would you want to? What types of shots would benefit?

Thanks for helping.

Tim

ust82gopher wrote in post #3059693
If I have my 430 EX flash attached in E-TTL and I take the same action, (1/2 shutter main dial), am I not setting the FEC? How can you set both EC and FEC? Why would you want to? What types of shots would benefit?

EC affects the ambient light exposure.
FEC affects the flash exposure.
(Remember flash fact #1: There are two exposures and you need to manage both.)

It's about getting the balance of the different light sources that you want. There's no single correct setting.

To set FEC with the 30D, you need to press the button on the top of the camera behind the main dial, then rotate the quick control dial. Review page 106 of the instruction manual for details.

One other good source of flash (Canon specific) info:

http://super.nova.org/​DPR/

If the camera adjusts the flash output...on my xti ,what level do I put the flash exposure compensation? This might sound like a ridiculous question but I am new to SLR cameras and a real amature to photography. Thanks!!

cappy1927 wrote in post #3217502
what level do I put the flash exposure compensation?

In theory, exposure compensation (and flash exposure compensation) is used to compensate for scenes that aren't "average" in luminance.

Since the camera doesn't know the brightness of the colors it's looking at, it can only shoot for average and expose the scene to medium gray. It will tend to underexpose bright scenes and overexpose dark scenes. So the idea is to use +EC if you're shooting a polar bear in the snow, and -EC if you're shooting a black cat in a coal bin. FEC is used the same way.

In theory.

Now, for the reality. Most of us who use Canon's E-TTL flash metering have found it to be a bit unpredictable, and it tends to underexpose "average" scenes. So in reality, most of us start with FEC around +2/3 if we're using flash as the main light source. If I'm at a formal dinner, shooting tables of people where the tablecloths are white, I'll push it to +1 or maybe even higher. If it's a bunch of people in black dresses and tuxedos, I'll crank it down.

Most importantly, check your histogram often and adjust accordingly.

How come this was removed as a Sticky? I like having it up on top and viewed it frequently. Have you overstayed your stickyness and become unstuck...that would suck...he he. No seriously, what happened?


JA

The mods don't like me anymore.

No, seriously, there's only room for so many stickies. But one if you click on ** IMPORTANT LINKS: EOS Flash ** (READ FIRST), you'll find it listed.