Aperture Priority (Av), The ST-E2, and a Slave

(Photograph outside the box...;-)

by TroyB

Let's start by looking at one of the most frequently asked questions in all of "SLR-dom", and a question that invariably indicates that the person asking the question does not understand the purpose of Av mode when used in conjunction with flash:

"I have my camera set to Av mode (aperture priority), and my flash was on but all my pictures came out blurry... what happened???" Or,  "I have my camera set to Av mode (aperture priority), and my flash was on but the shutter speed was very low, why???".

Back in the old days, say the early 90's, Canon manuals were a bit like "Japanese Engrish" (you may want to click that link later for a laugh) and admittedly it could be difficult to figure out what anything was supposed to do. Things have improved, and these days this question is answered in both the Canon camera body, and the Canon dedicated flash user manuals, albeit briefly, and I think they could still make a few improvements on this particular topic.

First off, the camera is working fine. It was designed to do exactly that, and in a moment, we will see some examples of just how desirable this initially puzzling characteristic can be. Av mode used in conjunction with a flash is designed to produce a picture where the subject and the background exposure are in perfect balance. This would probably be a good time to point out that Av mode is not particularly useful when there is very little, or NO ambient light, since it would be pretty hard to balance total darkness with anything!

The highly sophisticated multi-cell metering system on Canon cameras (yes, other manufacturers have this too) breaks the scene down onto areas, and tries to balance the metered exposure of the cells around the edge of the frame with the metered exposure of the cells in the "center" of the frame. Depending on the camera model and various configuration parameters, the "center" of the frame could literally be the center of the frame, OR it could be the cells clustered around the active focusing point.

Note that the camera is making a couple of assumptions here:

1) The background is going to be around the edge of the frame.

2) The subject will be in the center of the frame, or under the active focus point.

In portraiture, it would be odd that these two assumptions were not true, but it is worth keeping them in mind anyway, since it greatly aids in understanding of the use of flash in Av mode. It also helps understand why it is CRITICAL that you do NOT recompose the shot after you have pressed the shutter button half way to meter the scene, UNLESS you use Flash Exposure Lock (FEL).

In order to successfully achieve the Av mode objective of balancing subject and background exposure while using flash, the camera is going to select a shutter speed based on the data it gets from the cells around the edge of the frame. If there is a lack of ambient light, the selected / metered shutter speed could be quite low, which answers the opening question. To complete the equation, and deliver a picture with a nice, balanced exposure, press the shutter button fully. At the shutter button is depressed fully, the camera fires a "test" flash (technically, the "pre-flash"), calculates the proper exposure, and then fires the flash at the calculated value while tripping the shutter to expose the image sensor (or film).

The time between the pre-flash and the main flash is so small that it is for all intents and purposes un-noticeable. HOWEVER, if you set the flash for "second curtain", or in other words, to trip just before the shutter closes as opposed to the moment it is fully open, there is a slight additional time delay between the two bursts of light. There are a few people (rare, but non the less, they exist) who can actually react so quickly that the main flash will catch them with their eyes closed. So if you are photographing someone, and they always seem to have their eyes closed, make sure you don't have the camera / flash set for second curtain sync (second curtain is usually used to leave those cool trailing motion streaks in night time motion flash photography).

It is also worth noting that there is a custom function in all the Canon D-SLR bodies, and some film bodies called "Auto Reduction of Fill Flash". In theory, this feature will reduce the amount of light the flash produces so as to create a pleasantly blended daylight fill flash, which is what the rest of this article discusses.  Since Canon refuses to publish a spec on how this algorithm calculates the amount of flash underexposure that will be used in a given situation, most experts agree that it is better to disable this feature, and dial in some flash underexposure as necessary in any particular situation. (CF.14 = "disable" on Dxx bodies)

That covers the basics of using flash in Av mode, so now let's expand on Q8 from the Wireless Flash FAQ: Can I use the ST-E2 with just one slave, and why would I do this?

If you like taking portraits, and have ever asked yourself "what is the one thing that I could do / buy that would really make a difference in the quality of my portraits (especially outdoors)", my answer would be "the ST-E2".

I realize this is a pretty bold statement, but let's examine photography from an artistic point of view for a moment. Everybody knows that it is highly unlikely that the sun is shining from out from a hole in your forehead, but this is exactly what you are asking people to believe when you use a flash that is attached to your camera. If you examine the world around you, you will find that it is pretty rare to find lighting similar to that of a flash attached to your camera. The sun rarely shines from that hypothetical hole in your forehead when it lights an object. So if we want to create a photograph with truly believable lighting, we need to get that flash OFF the camera.

This is where the ST-E2, and a Canon 420EX or 550EX come in handy. Let's take a look:

Av Mode with Offboard Flash	Av mode with NO Flash	More Fun with Offboard Flash
Key EXIF Data: Mode: Av ISO: 200 Shutter: 1/100 Aperture: 5.0 Metering: Evaluative Focus Point: Left (top - I hold the camera opposite of most people) Flash: on Exposure Comp: 0 Flash Exposure Comp: - 2/3 Auto Reduction of Fill Flash: off	Key EXIF Data: Mode: Av ISO: 200 Shutter: 1/100 Aperture: 5.0 Metering: Evaluative Focus Point: Left Flash: off Exposure Comp: 0	Key EXIF Data: Mode: Av ISO: 200 Shutter: 1/100 Aperture: 2.8 Metering: Evaluative Focus Point: Left (top - I hold the camera opposite of most people) Flash: on Exposure Comp: 0 Flash Exposure Comp: - 2/3 Auto Reduction of Fill Flash: off

Equipment: Canon EOS D60, Canon 28-70 f2.8 L, Canon ST-E2, Canon 550EX in slave mode.

Please note that all these shots are UN adjusted, except for cropping, so as to give you the best example possible of what to expect from the camera / flash combo.

The first shot is really the epitome of Av mode. The average person who sees this shot has no idea that flash was even involved. Of course, with a bit of knowledge about photography we can easily see that there is a "catch light" in Shakira's eyes, and there is a shadow on her arm that does not jive with the overcast conditions we can see in the background. So as photographers, we know that a flash was used. But thanks to Canon's Av mode balanced exposure algorithm, we end up with the subject and the background exposed in such a way that it is easy for our mind to believe that this is just an exceptionally good natural light photograph.

Also note from the EXIF data that some negative flash exposure compensation was used. This is critical in making the lighting look natural. The amount of negative comp required will vary, but -3/4 stop is a good place to start, and then work your way down if the flash is too obvious. If you have not disabled "Auto Reduction of Fill Flash", you will have no idea how much additional negative comp the camera applied automatically, so it is wise to disable this custom function so as to obtain consistent results.

The second frame shows us what it would look like if no flash was used at all. Most non-photographers would consider this to be a perfectly acceptable shot. Indeed, our intrepid model Shakira and her friends liked this shot a lot. But for me, this shot is entirely unacceptable... the eyes are the window to the soul, so if there is no sparkle in the eyes, then someone must have stolen the soul. Not good. We always want our models to have soul. So the trick is to fire the flash, but to deliberately underexpose the flash a bit so as to ensure that the flash is just bringing out detail in the shadows, and not acting as the main source of light.

Note that the exposure settings are identical in the first two shots, regardless of whether the flash is on or not. Using Av mode, I set the aperture to 5.0, and the camera picked a shutter speed of 1/100 as giving the best subject / background ratio. But what would happen if it was a bit darker, or I had set the aperture to f11? Well, REGARDLESS of whether the flash was on, or off, the shutter speed would have dropped considerably. This is where newbies invariably go wrong, and pose the dreaded "why is my picture blurry" question. Remember, in Av mode, we are telling the camera to expose the background PROPERLY without flash (it would be impossible to light up the background with the flash anyway), then fire the flash, and quench it as soon as the foreground subject is properly exposed. If it is overcast, or near sunrise / sunset, exposing the background properly could require a considerably slow shutter speed, thus resulting in your foreground subject being quite blurry (or the whole picture for that matter), despite the flash.

Note the wide f2.8 aperture in the third frame. Being overcast, it was even darker under the tree. So rather than risking blurry shots due to an impatient young model, and hand holding the camera at 1/30 of a second, I chose to open up the aperture more to keep the 1/100 shutter speed. And I wanted Av mode, because I did not want it to appear the picture was taken at night, which is what happens when you don't balance background and subject exposure on a very overcast day, or when shooting early morning or late evening.

In the two examples above where flash was used, a single 550EX was placed 40-80 degrees from the camera. For me, this is the real magic, the one thing that keeps these shots and any others done using this technique from being "just another snapshot". As viewers, our minds are numb from the 1000's and 1000's of snapshots that we have seen in our lifetimes. We expect eyes that look like "Coal Holes" if it was sunny, and no flash was used (the "Coal Hole" is a pub in London, England, but the name says it all, doesn't it?). Or we expect the red eyes and "sun shining from a hole in our forehead" effect of the ubiquitous pocket camera when used indoors, or at night. Every time you see a shot with one of these characteristics, you end up saying "Ho-Hum, yawn, another snapshot".

So if you own a ST-E2 and even a single 550/420EX, get that flash off the camera, and start taking some truly memorable pictures. Grab your favorite victim... er, I mean model (even if it is just the cat ;-), and practice moving the flash away from the camera. Try positions above and below the model's plane, and move the flash around a radius formed by using the model as the center of the circle. Just a few hours of practice, and you can be taking shots where your viewers will have too look twice because the image is so different, so memorable and so natural. Everyone has heard the expression "think outside the box", well, why not take pictures outside the box? It is for this reason that I consider the ST-E2 to be the single best addition that those who are interested in portrait photography could possibly add to their camera bags.

Sounds great doesn't it? But before you all go run and grab your cameras, let's look at a little gottcha. What if it is a bright sunny day, and our subject is out in the sun, but we want to use this same exposure balancing technique to ensure that the eyes are not buried in deep shadow, as is often the case? And because we are doing a portrait, we want to shoot with an aperture around 5.6, or maybe a little less, so as to get a nice blurred out background.

This scenario is likely going to result in a fairly high shutter speed, perhaps 1/800 or higher. You have everything set up as described above, but the camera doesn't like it for some reason, why? When trying to use Av mode in bright sun with the flash, we are going to exceed the "X-sync" speed of the camera body which is 1/200 for all Dxx bodies, 1/250 for the 1Ds, and an astounding 1/500 for the 1D. A discussion of "X-sync" speed is beyond the scope of this document, but suffice to say that it is the MAXIMUM shutter speed that the camera body can use when the flash is operating in a "normal" way, ie, a single burst of light.

Fortunately, there is a solution. The ST-E2, and many of Canon's late model flash units support HIGH SPEED SYNC, which is activated by the button that has an H with a lightning bolt beside it. Note that high speed sync is NOT available on ANY built in flash on ANY Canon body, digital or film. In high speed sync mode, the flash sends out a series of pulses of light so quickly that they appear as continuous light to the eye. This allows the camera to be used with the flash at ANY shutter speed, even the very high shutter speeds required to use big apertures in bright sunlight. So to use Av mode and flash on a sunny day, just push the "H lighting bolt" button on the ST-E2, and you are ready to go.

And that wraps up my guide to Av mode, and Canon wireless flash. If you have not already looked at my "Canon Wireless Flash FAQ", or my "Using the ST-E2 as an (Expensive) Focus Assist Lamp" articles, you may wish to do so. Click the corresponding article title to jump to those pages. To see the list of all the equipment I use as my "Portable Studio", and the logic behind those decisions, click HERE.

The Canon ST-E2 and Av mode - Photograph outside the box!