What gives these lenses their potential is that they have a larger image circle than regular lenses. Instead of simply being large enough to cover the sensor, their image circle extends far beyond and can therefore be moved around without introducing vignetting (not to be confused with illumination fall-off).

Geometric Distortion

The first set of adjustments are made to control convergence.

For example, this is what happens when the top part of a building appears smaller than the bottom part, because of a viewpoint that forces the camera to be tilted back. You cannot adjust the perspective with lens movements (only physically moving to a different position can change the relative distance the camera stands from the bottom and the top part of the building), but you can do something about the converging lines.

The basic idea is to position the camera so that the sensor plane is parallel to the surface you want to keep straight and simply slide the lens in order to place the part of the image circle that contains the subject where you want it. The amount of correction is therefore limited by how large the image circle is.

The sliding of the lens can be made in any direction allowed by the lens, to correct for lines converging in various directions (convergence doesn't necessarily only happen when looking up!) You can also create high-quality panoramic images by sliding the lens between exposures (never actually panning the camera), to produce (almost) seamless stitches.

If the lens is slid upwards, the movement is called a "rise". A downwards slide is called a "fall". Sideways, it is called a "shift" (left or right).

Focus Plane

The second set of adjustments are made to control the focus plane.

With regular lenses, the sensor plane and the lens plane are parallel, resulting in a parallel focus plane (depth of field extends front to back, parallel to the sensor). By changing the angle of the lens, the Scheimpflug principle explains that the plane of focus will end up at an angle as well, with the depth of field extending like a cone around that plane (see article for a complete explanation).

You don't need to bother with all that math. In simple terms, what it means is that because you can change the angle of the plane of focus, you can exert more control over which part of the image will be sharp. You could, for example, achieve a sharp image from right in front of the lens to the infinity, without requiring an overly small aperture that would be at worse impossible with regular lenses, at best detrimental to image quality because of diffraction.

By exploiting this optical phenomenon in the "wrong" direction, you can create a very slim section of sharpness in the image (almost perpendicular to the sensor plane), with the rest falling sideways out of focus — the trick behind the "fake miniature" effect.

When the lens is angled up or down, the movement is called a "tilt". When the lens is angled left or right, the movement is called a "swing".

Because the lens can be rotated, a combination of rise/fall/shift/tilt/swing can be achieved (depending on the capabilities of the lens — recent designs allow the slide movements to be rotated independently from the tilt/swing movements).

What is TTL flash metering?

At first there was manual flash exposure, which meant that the photographer had to know the guide number of his unit and calculate effective distances against apertures and ISO numbers using the inverse-square law bla bla bla... For example, a flash unit with an (imperial) guide number of 100 would correctly expose a subject 25 feet away with an aperture of f/4 (because 100 = 25 x 4). And, oh, that's for ISO 100, and no, you can't see if what you just did was correct, because you're shooting film. Now calculate equivalences along the way, as you're shooting your event. Lots of maths, trial and error (but mostly error). Thank goodness, this era is long gone.

Then there was automatic flash exposure, which was calculated by the flash unit itself using a little photo cell. The flash would emit light and judge if enough had been emitted by the general amount of light reflected back. Needless to say, this was not very precise and could only hope to work when the flash was used on the camera's hot shoe. (Later versions offered a way to separate the photo cell from the flash unit so the flash could indeed be used off-camera, but this only solved part of the problem.)

Then, at last, came TTL flash metering. (TTL metering had appeared long before for ambient light metering, but was now also available for flash exposure.) Instead of letting the flash unit calculate if the proper amount of light had been emitted, it left that job to the camera, using exactly what it had seen "Through The Lens". The thing to remember, though, is that the core addition to TTL flash is not so much the fact that the camera and flash unit now talk to each other using more pins on the shoe (which it obviously has to, to achieve the result), but the fact that the metering is done by the camera, through its lens! The more the camera and flash can talk the better to automate the process and achieve more accurate results, but that doesn't change the core fact of who's doing the metering.

What's the focal length got to do with the flash?

The flash doesn't have to know what focal length is used to work properly, but by concentrating its beam on an area that is not wider than the area covered by the focal length, it can preserve its power that would otherwise be wasted to light areas not even seen by the camera. By doing so, it can actually improve its effective reach and illuminate more distanced subjects. This used to be performed manually with Fresnel adapters installed on the end of flash units, but is now fully automated — insofar as the unit is able to. (As a matter of fact, if you're using very long lenses (in wildlife photography, for example), you might be interested by accessories such as the Better Beamer.)

The image area, when using a longer focal length, is increasingly smaller than the area covered by a flash that doesn't concentrate its beam

The flash's zoom allows it to concentrate its beam to the meaningful area, improving its reach/conserving its power

That's all there is to it! The fact that each flash unit can cover different "zoom" ranges doesn't mean it stops working properly if the lens is set to a longer focal length, it only means it won't be able to concentrate further its beam, therefore not improving its effective range further. There is nothing you can do about it (apart from hooking an accessory to the end of the flash).

Why does the flash ignore the lens' focal length when aiming its beam anywhere but forward?

Well, this should be rather obvious: if you're not beaming in front of you with the flash, then whatever focal length you are using is meaningless for the flash, since its beam is no longer aimed in the same direction.

When you aim the beam to the ceiling, for example, what you're hoping to do is to bounce the light so that the (main) source of light now becomes the ceiling — at this point, changing the flash's zoom would mean beaming a wider or narrower spot on the ceiling, which has absolutely nothing to do with the lens' focal length. Setting the orientation angle of the flash head likely has a much more significant impact on the reach/effect of the bounced light than how wide the beam is on the ceiling, depending on how far it is, etc. Since the camera has no way of knowing how far the ceiling is (or if it is flat, etc.), there is no way it can decide how wide the beam should be. Therefore, it defaults to an average value — not the longest, because a small beam on the ceiling would mean harsher light, which is exactly what we are usually trying to avoid.

You can always change the flash's zoom value manually, if you so wish, but there is no question that linking the focal length to the flash's zoom when beaming anywhere but in front doesn't make any sense.

Additionally, with Canon Speedlites (I cannot comment on other brands, which I don't know as well), when using the included diffuser (the one that retracts inside the flash head), the flash's zoom automatically goes to its widest value — because the point of this diffuser is to achieve an extra-wide angle of 14mm (14mm being the widest lens available in the Canon lineup). This also means that the flash won't be as powerful, since it is spreading its beam much more.

Is the flash metering related to the focus point?

Since I am more familiar with Canon equipment, I cannot confirm for every other brand (check your equipment's documentation), but I would think that the behavior is very similar. With Canon cameras, ever since E-TTL II came along (that's a long time ago, circa 2004), flash metering is no longer linked to the autofocus point selected. This means that you can use the "focus-recompose-shoot" technique freely without worrying that the flash metering will be thrown off.

The metering will also use information from the focus distance (when available — this is a per-lens capability) to calculate its power. This means that if the focus distance is on a subject, it will likely expose the subject properly and pretty much ignore the background (which means potentially leaving it dark). If the focus distance is on a background, it will likely expose the background properly and pretty much ignore the subject (which means potentially overexposing it). Exactly what we would expect, considering that the in-focus area is likely the most important part to light properly.

What is the FEL button doing?

First, we have to understand that regardless of our usage of the Flash Exposure Lock (FEL) function, there is always a pre-flash when using TTL metering. Since the pre-flash occurs right before the actual exposure begins, it is hardly noticeable. (When using rear-curtain synchronization, the two separate flashes will be very obvious, because they will occur before the exposure and right before the end of the exposure, respectively, giving you plenty of time to see the two bursts.)

So the goal of the FEL is not to generate a pre-flash (that, we always get). The point is to lock the flash metering — exactly the way the auto-exposure lock (AEL) works with ambient light, but with flash. Why would you do that? Well, if you don't want the flash to be metered based on the final scene (because there might be something you anticipate will throw off the metering, such as a bright white background or, conversely, lots of black tuxedos in the frame), you can trigger the calculation on a different scene or on a specific area of the scene (by zooming in on a subject's face, or by using a different metering zone pattern, for example) and then recomposing-shooting, which will use the flash metering that was calculated before.

If you're systematically using the FEL before each time you shoot, ad nauseum, without significantly changing the frame, you are wasting your time (and battery power). I hardly ever use that function, but it can be useful in difficult situations — just as much as AEL is with ambient light (which I never use). Instead, I tend to prefer using exposure compensation, which I find more predictable.

Note that here, Canons and Nikons work rather differently. Canon's FEL locks the flash metering only as long as you hold the shutter release half-way (otherwise it forgets the flash metering after about 16 seconds, or very quickly after a shot). A little star (*) appears in the viewfinder as long as the FEL is active, and a new FEL will have to be be performed each following time you need it. Nikon's FEL (at least, the way I was told it worked) locks the flash metering for as long as you don't press the FEL button again — the flash exposure will be the same for all the following frames until you unlock it.

What about exposure compensation?

Like I was saying, I tend to prefer exposure compensation over FEL, because I can better judge how to tweak the exposure than by trying to aim at something that I think would be closer to what I want (good luck!)

There are two ways you can apply flash exposure compensation (FEC). You can either apply FEC on the camera, or on the flash unit itself. With Canon (again, I cannot confirm for other brands), if you apply a FEC on the flash unit, it overrides whatever setting was in the camera. I personally prefer applying FEC with the camera, because I can reach the button right next to the shutter release without even having to move my eye away from the viewfinder, and I see what I'm doing from the display at the bottom of the viewfinder.

To wrap up...

One thing is for sure, working with flash requires practice, because it is much less predictable (especially when bouncing off various surfaces). It gets particularly tricky when mixing flash with ambient light, because now you have two independent exposures to oversee, plus color balancing with gels, etc.

Get to work!

Well, in a way, I would be forced to answer: yes, it does—at least, that's what I have observed with my Canon cameras (I can't comment for other brands). It's not so much that the cameras consume more battery power, it's that once the camera is plugged in and recognized by the EOS Utility (in the case of my Canon cameras), it will never go to sleep!

You know how after a while (say, a minute), the cameras will go in "stand by" mode (you can tell, because its top display turns off), consuming very, very little power? (So little, in fact, that I sometimes don't even bother turning them off.) Well, they never do that when they are hooked to the computer, to maintain the communication alive.

You'll notice that if you hook your camera to your computer and never fire a single frame, a couple of hours later the battery will be dead anyway! If you're going to shoot for a full day tethered, I recommend you bring a second battery with you—or, at least, recharge your battery during lunch time...

Various reading display settings on the Sekonic Flash Master L-358

For those who have learned their f-stops in the digital era, it usually makes more sense to talk of thirds of stops. If you want your Sekonic Flash Master L-358 to display direct readings, enable the DIP switch #3. Switch #4 determines if the reading will be in half or thirds of a stop.

Here's how my unit is normally setup:

DIP Switches on my unit

• Never use "Dust-Off" or other regular compressed air products. These products are made to clean keyboards, computer cases and such, but not digital camera sensors. They are liquid-based and may spray harmful particles on your sensor. (If you're into compressed air, specialized compressed CO2 products are available that do not risk blowing harmful particles onto the sensor.)

• Never expose the sensor using the "bulb" exposure mode. There is a dedicated feature in your camera that reveals the sensor. First off, the "bulb" mode would not prevent you from using it when the battery is near depletion, which could surprise you by slapping the mirror back down while you're doing your thing, risking damage. Secondly, the dedicated sensor-cleaning feature reveals the sensor, but does not actually record an image. Unlike the "bulb" mode, the sensor is not electrically charged, which means that it won't attract more dust. And finally, using the sensor cleaning feature frees your two hands!

So then, which methods are adequate? Many, but it depends on the severity of the problem.

1. Blowing air onto the sensor usually removes the most obvious particles. This is the less intrusive method, as nothing but air touches the sensor. As said previously, do not use regular compressed air products — a simple blower will do. Be careful though: some cameras are constructed in such a way that blowing air into the mirror box might cause some dust to lurk into the prism assembly, which will require servicing to get removed!
2. For a thorough removal of dust specks, a static-charged brush (such as one made by VisibleDust) will do a very good job. This method requires touching the sensor, but there is no real danger, as these brushes are very delicate. Be careful though: the brush needs to remain immaculate and cannot be cleaned! When locating dust specks, remember that the image you see on your LCD was actually upside-down on the sensor!
3. For stains that are not simple dust — such as moisture or other liquids — you'll need to use sensor swabs (of a size appropriate for your camera's sensor) with a special cleaning fluid (of a nature appropriate for your camera's sensor), such as Eclipse. This is the most intrusive method, but is sometimes required for tough stains.