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May 17, 2008 05:55 | #23

From the shot on the left:

Lens (mm): 45
ISO: 500
Aperture: 4.5
Shutter: 1/60

First of all, never, and I mean never, do a portrait with a reflective "thing" in the background, especially when it is RIGHT behind the subject. Secondly, this image is OOF and the one on the right has red eye. Did you use any kind of flash bracket to elevate the flash? Thirdly, f/4.5 is pretty wide for portraits taken at a 45mm focal length. I'm guessing you were maybe 10 feet from the subject? At that distance, your depth of field would have been just over a foot in front of and behind the subject. Do you have maybe an 85mm lens you can use, or a 70-200?

A standard portrait shot taken with on-camera flash in ETTL might have ISO400 (see below), 1/125s shutter speed (to reduce the amount of ambient light, make it faster; to increase ambient, make it slower, but don't go below ss=[1/(focal length)] and f/5.6, or even f/8 or f/11 for large groups. If you put your flash in ETTL mode, it'll figure out the appropriate flash levels for your exposure settings and distance to the subject.

To make portraits look better than people can take themselves with their P&S requires some skill and work, but it's not rocket science.

And, if you're gonna use a flash diffuser, use one that makes the effective size of the flash surface larger, not keeps it the same (like the tissue mentioned earlier, or an Omnibounce shot straight on). The only real way to make the light softer is to increase the size of the flash relative to the subject.

Here is a web site with a TON of great information on using on-camera flash (and even more on using strobes - I took a course at their school and it was excellent):

http://www.shootsmarter.com/ (they'll ask for an email registration, but in three years I've never been spammed by them, so don't worry - it's worth it to register just to browse the "smarticles")

And here's some of what it said regarding ETTL and on-camera flash for portraits:

Indoor TTL works best at ISO 400 and higher because it allows the TTL system a greater range of control and allows your flash power to cover a longer distance - particularly if you bounce of use a pro grade diffusion type light modifier. Remember, dialing your ISO up from ISO 200 to ISO 400 is a one stop boost, which means you have just DOUBLED your flash output power. Dialing up from 400 to 800 DOUBLES IT AGAIN.
Also, high ISO shooting means you use less battery power in your flash so you can shoot more and faster too.
Shooting at ISO 800 means you have the ability to bounce your SB800 or 580EX flash off of the ceiling in the church and shoot at f 2.8 and still have the FLASH DOMNANT lighting your TTL system needs.

Outdoors, in order to make the required FLASH DOMINANT flash-to-ambient light ratio work, you will be at a setting LESS than 400 which means you will need some "real" flash power to stay as flexible with your shooting option outside as you are inside.
Outdoor shooting is what separates the rubber from the road when it comes to flash choices. If you want to shoot TTL outdoors, you will need to:
- use a short focal length lens to keep your camera-to-subject distance under about 9 feet (past 9 feet is where your TTL system will run out of gas)

- go without a nice smooth light quality flash modifier,

- add a little more beef in that samich. ; ) - in other words, a more powerful on-camera flash

Fill Flash vs. Flash Dominant
If your flash is brighter than the ambient roomlight, then that is referred to as a "flash dominant" shot and this is where TTL works best. If the flash power is set so the ambient light brighter than your flash - that is referred to as "fill-flash" and TTL stinks at fill flash.
If you want to use TTL for fill flash - don't. : ) If you do, you will be forced to shoot color neg film (with GREAT results!) or will be forced to shoot in RAW mode and patch exposures later. Life is too short for that nonsense on most shots. So if you are looking for fill flash techniques, turn off the TTL and turn on AUTO or MANUAL flash exposure modes? Test it yourself and see.

Only use Flash Dominant lighting for TTL
Let's aim to have the flash to be 1 to 1 /12 stops brighter than the ambient roomlight when we shoot TTL. That can also be thought of as 2/3 flash + 1/3 ambient = TTL success.
BTW, 2/3rds flash and 1/3 ambient is the same as the flash being (roughly) one STOP brighter than the ambient.

How do we manage that?
The easiest way to keep track of this is to use a light meter. I know, I know, you cannot measure the TTL flash with a flashmeter (easily) because of the preflash, but all we need to measure is the ambient light. So, I have a new best friend...the Gossen Digiflash.

Camera Exposure Modes:
There are a wide variety of automated exposure modes on your camera, but only a few will help you nail down TTL exposure control. Forget about using the fully automated Program modes. They are the worst for TTL flash control. Sure you can use the Aperture priority auto exposure modes for some jobs, but gaining professional control over TTL means working in the MANUAL exposure mode on your camera, and the TTL mode on your flash. This will reduce the amount of incorrect guessing from your camera, speed up the shutter delay, and most importantly allow you more input to decide what is properly exposed and what is not - like your backgrounds being the same exposure level as the subject, darker than the subject, or brighter that the subject in "fill flash" photography.
I do get asked in my live programs if TTL behaves the same way when the camera is set to MANUAL exposure mode as it does when set to PROGRAM mode and the answer is yes. Same thing on all pro and semi-pro (including the D40, D70, Digital Rebel folks too) DSLRs. So don't think you "turn off" any TTL functions by shooting the camera in MANUAL. : )

Camera Focus Modes:
Proper focus is CRUCIAL to TTL exposures and most of us are actually less than "good enough" at applying correct autofocus using our cameras. More on this topic coming up in future TTL smArticles. We need to provide correct "camera to subject" distance data to our TTL system by precise focus - the more accurate the better. To do that, we will be taking control of our focus - and our lack of precise focus too - by setting up our cameras' focusing modes this way for TTL:

For the Nikon and Fuji camera shooters:
These cameras offer three basic focus modes to choose from: There's "S" which stands for Single Servo AF mode, and there's the "C" or Continuous Servo AF mode, and of course there's "M" for Manual focusing mode. Let's learn what these can do for us...

S (Single Servo AF) mode:
When you use the S mode, your lens focuses on whatever you choose as long as you keep the shutter button pressed at least half way down. This is typically referred to as "focus-priority" AF mode, and means the shutter won't fire until the camera confirms that focus is locked and this is the best mode to use for TTL flash systems.

C (Continuous Servo AF) mode:
The C mode will permit the shutter to fire without achieving confirmed focus lock so you can shoot faster and "predict" focus areas better and faster. This will cause major troubles for TTL - please don't use it for any TTL shots.

M (Manual) Focusing mode:
Manual mode will also work just fine for TTL and may allow you to shoot a little faster in some situations, but my over 40 year old eyes need a little help with autofocus and so do most of the shooters I know, so select single AF mode and lets work on our autofocus skills.

For the Canon camera shooters:
Most of the Canon cameras used by the ShootSmarter readers have 3 basic choices for autofocus settings. There's One-Shot autofocus, AI SERVO Autofocus, and AI Focus too. Here's a closer look at what they can do for us...

The AI Servo mode:
This mode is great for subjects that are moving in bright light and it will continually focus while you have the shutter button pressed halfway down. Although this mode works well for sports and news coverage, it's not the best for TTL.

The OneShot mode:
Here is the winner for portrait, wedding or event coverage when your subject moves only a little. This will permit us to lock focus very well using only one autofocus sensor area when we push the shutter release halfway down or setup the back button focusing function (more on backbutton Focusing coming in later smArticles), then fire the shutter only when focus is locked. This sends the best camera to subject distance info to the TTL system and is what we recommend.

The AI Focus mode:
This is a super-high tech system that acts like the one-shot mode until it senses movement, then switches into the AI Servo mode. Even though this is pretty cool (and under some conditions works amazingly well), and you think it may be the smart for TTL, our testing reveals that the one-shot mode is the way to go for all Canon TTL shoots on wedding, portrait, seniors, corporate event subjects.

The lens you choose for shooting TTL jobs is more important than you may think. You will get more accurate and consistent TTL exposures if you shoot with an "honest" f 2.8 zoom or a prime (non-zoom) that is f 2.8 or faster. Here's the facts...

Let's open up the can of worms first:
What do we mean when we use the word "honest" to describe an f 2.8 zoom?
An honest zoom is one that does not change or shift it's exposure more than 1/10th of a stop through the range of it's zoom. If a lens that is labelled "f 2.8" or "f 4.0" or marketed as "constant" we expect it to deliver light back to the camera at the aperture we set right? Unfortunately, that's a lot to expect. There are a LOT of zooms that are advertised as non-shifting, or as "constant" that do shift 2/10ths or 3/10ths of a stop between 2 different focal lengths. Some of the expensive Canon, Nikon, Tamron, Sigma and other lenses that are "f 2.8" lenses simply will not hold any aperture setting (f 2.8, f4, f5.6, etc) within a 1/10th stop tolerance. Others do hold their aperture setting and of course all prime lenses are very accurate in their aperture values and have no shifting issues.

Why does exposure shifting matter?
Your TTL system uses a small pre-flash before it fires the primary flash in order to "look" at the exposure of your scene. This pre-flash is extremely important and works by first reading the exposure setting you dial into the camera, let's say it's f 4.0 and a 1/60th second ISO 400, then the flash sends out the small preflash that is not more than 10% of your flash's total output capacity. Then it reads the light reflected back to your camera - through the lens (TTL stands for Through The Lens) highlighting the area that you have critical focus on and compares the preflash exposure to what you have your camera set to. So if you are set to f 4.0 and the light from the preflash is guessed by the camera to be underexposed by 2/3 stop, the camera will send out the big primary flash at 2/3 over power. Sounds cool right? Well, no.

Guessing is bad.
We see accurate and consistant exposures when the camera does not have to guess - or at least guess much. The greater the difference between your exposure settings on the camera and the TTL pre-flashes exposure report - the further off the exposures will be. This is where the shifting lens issue comes in. If we have a lens that shifts it's exposure values when we zoom, we may set the camera to f 4.0 and while we shoot in the middle of the zoom range it will most likely deliver f 4.0 worth of light. But if we zoom out to a wide shot with that lens - with the camera still set to f 4.0, the lens shifts our exposure to 2/10ths or 3/10ths of a stop OVER exposed without the camera knowing it. So the preflash fires - thinking you are at f 4.0 but you are really at 2 or 3 tenths brighter than f 4.0 - and it sees too much light. It guesses that it needs 1/2 stop less light (for example) and your resulting photo is now dark because the camera guessed wrong. This is one reason you can shoot from the same darned spot - only changing the zoom - and see varying exposures. BUT... if we would have used a lens that does not shift, or if we were shooting with a prime lens like a 50mm f 2.8 set at f 4.0 - bingo! The TTL preflash is compared to your camera settings and it matches - life is good!.

What is the Smart Choice for Lenses with TTL?
There's no way I can test all the possible lenses in the pro world, but I do see a few patterns in the results that I trust. All zooms that are shorter than 28mm will shift - some a lot and some a little - even the REALLY expensive ones.
The Canon 24-70mm f2.8L lens, the Nikon 28-70mm f2.8 IF-ED and the Tamron 28-75mm f2.8 lenses all live up to their claims in holding a constant aperture setting within a 1/10th stop margin and are great choices for TTL. Lenses shorter than that all suffer from an exposure shift greater than 1/10th, so I choose to go with prime lenses for wide work using TTL. I personally LOVE the Tamron / Nikon 14mm f 2.8 lenses for flash work! And if I am in a really dark environment and need solid TTL and autofocus capability, I will use a Nikon 50mm f 1.4 or an 85mm f 1.4 lens and am amazed at how accurate my TTL exposures are.

http://www.denniswierzbicki.com
http://www.sportsshooter.com/dmwierz

Dennis "Yeah, well, sometimes nothin' can be a real cool hand."

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