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Old 19th of May 2006 (Fri)   #1
Curtis N
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Default FLASH PHOTOGRAPHY 101 – A BEGINNER’S GUIDE

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
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Last edited by Curtis N : 27th of December 2006 (Wed) at 14:31.
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Old 19th of May 2006 (Fri)   #2
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Default Re: FLASH PHOTOGRAPHY 101 – A BEGINNER’S GUIDE

Why couldn't you have done this a week ago? I just printed all 90+ pages of the EOS bible the other day.
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Old 19th of May 2006 (Fri)   #3
Curtis N
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Default Re: FLASH PHOTOGRAPHY 101 – A BEGINNER’S GUIDE

[Sunday School teacher voice]Read your Bible![/Sunday School teacher voice]
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Old 19th of May 2006 (Fri)   #4
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Default Re: FLASH PHOTOGRAPHY 101 – A BEGINNER’S GUIDE

Nice work Curtis, I look forward to your follow up on High speed sync.
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Old 19th of May 2006 (Fri)   #5
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Default Re: FLASH PHOTOGRAPHY 101 – A BEGINNER’S GUIDE

Thankyou, Curtis.
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Old 19th of May 2006 (Fri)   #6
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Default Re: FLASH PHOTOGRAPHY 101 – A BEGINNER’S GUIDE

That is awsome. I too just printed out the Bible- right before I clicked on this link. Which I will read as well. The internet is great, but sometimes one gets overloaded with info, or contradictory info. Thaks for laying it out so well.
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Old 19th of May 2006 (Fri)   #7
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Default Re: FLASH PHOTOGRAPHY 101 – A BEGINNER’S GUIDE

Excellent job, Curtis. I'm sure a lot of people will benefit from this thread. I'm going to include a link to this thread in the EOS Flash "stickied" thread.
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Old 19th of May 2006 (Fri)   #8
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Default Re: FLASH PHOTOGRAPHY 101 – A BEGINNER’S GUIDE

Thank you for this. My flash just came today so this is excellent.
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Old 19th of May 2006 (Fri)   #9
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Default Re: FLASH PHOTOGRAPHY 101 – A BEGINNER’S GUIDE

Ah,Ah!!!(Hand raised sheepishly) Mr. Curtis will there be a test on this tomorrow
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Old 19th of May 2006 (Fri)   #10
Curtis N
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Default Re: FLASH PHOTOGRAPHY 101 – A BEGINNER’S GUIDE

Quote:
Originally Posted by lostdoggy
will there be a test on this tomorrow?
Yes. You must submit three flash photos in the critique section. If they aren't as good as Schmoelzel's, you fail.
(Insert diabolical laugh here)
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Old 19th of May 2006 (Fri)   #11
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Default Re: FLASH PHOTOGRAPHY 101 – A BEGINNER’S GUIDE

whimper!!! Tears!!!
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Old 19th of May 2006 (Fri)   #12
Curtis N
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Default Re: FLASH PHOTOGRAPHY 101 – A BEGINNER’S GUIDE

Quote:
Originally Posted by PacAce
I'm going to include a link to this thread in the EOS Flash "stickied" thread.
Thanks, Leo. Being "stickied" would put me in some pretty good company.

The truth is, almost everything I know about flash, I learned in this forum, from the likes of you, Scott Berger and other notables. The afforementioned "bible" is a great resource, too.

Chapter Two will include some sample images, if I can ever find a willing "victim."
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Old 20th of May 2006 (Sat)   #13
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Default Re: FLASH PHOTOGRAPHY 101 – A BEGINNER’S GUIDE

Quote:
Originally Posted by Curtis N
Thanks, Leo. Being "stickied" would put me in some pretty good company.

The truth is, almost everything I know about flash, I learned in this forum, from the likes of you, Scott Berger and other notables. The afforementioned "bible" is a great resource, too.

Chapter Two will include some sample images, if I can ever find a willing "victim."
How about that guy in your avatar.

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Old 20th of May 2006 (Sat)   #14
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Default Re: FLASH PHOTOGRAPHY 101 – A BEGINNER’S GUIDE

Booky Marky.
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Old 5th of June 2006 (Mon)   #15
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Default Re: FLASH PHOTOGRAPHY 101 – A BEGINNER’S GUIDE

Need high sync flash help. How do I utilize a 1000 shutter speed on my 20D AND a 430 hotshoe flah for fill in thanks
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