4:41 pmOff
Pushing Exposures
It's been suggested that raising the exposure in the raw processor would generate a cleaner file than raising the ISO by the same amount, since the camera doesn't have internal software as sophisticated as the raw processors you run on your computer.
Nonsense.
For one thing, when shooting raw files, the software in the camera has absolutely nothing to do with the resulting image. That's what shooting raw means: the raw data from the sensor, once digitized by the A/D converter, is not processed further and is dumped as is on the memory card. Understand that when the ISO is changed, it doesn't simply mean that the camera will "push process" the file with its internal software—that would be a completely pointless enterprise which would render even the concept of selecting an ISO meaningless. Why would you bother changing the ISO at capture time if you could simply push a slider farther right in your raw processing software in post?
Raising the ISO is not a matter of software, just as raising the gain on any signal processor isn't:
In digital camera systems, an arbitrary relationship between exposure and sensor data values can be achieved by setting the signal gain of the sensor.
This cannot be relegated to post production, as by then it is too late. If you have underexposed an image, no amount of pushing will recover lost information—just like it was with negative film. Having raised the signal gain prior to digitization (by setting a higher ISO) gives you additional information (especially noticeably in the shadow area), albeit at a cost of higher noise.
Back in the film days, it was a well known fact that a "push" processing was detrimental to the quality of the image (unless, of course, that particular "look" was what you wanted, for aesthetic reasons):
Push processing allows relatively insensitive films to be used under lighting conditions that would ordinarily be too low for adequate exposure at the required shutter speed and aperture combination. This technique alters the visual characteristics of the film, such as higher contrast, increased grain and lower resolution. Saturated and distorted colours are often visible on film that has been push processed.
If you were shooting slides (positive film, which is closer to the behavior of digital exposure), you could push an exposure by a larger margin than if you were shooting negative film, and the opposite was true of "pulling" (which is the opposite of pushing).
This was, of course, the film days, and there might be various reasons for doing so (such as the sudden need for a different ISO when the film had already been loaded). I am not a avid film shooter, so I cannot discuss in detail about the many motivations behind the practice, but the fact remains that pushing definitely didn't have the same effect as using a higher ISO (i.e.: not just an equivalent increase in grain).
In digital photography, pushing a file not only increases the noise (which is okay—raising the ISO does so as well), but more importantly, it also compresses the dynamic range of the file in a way that raising the ISO doesn't. That, you want to avoid.
Same aperture and shutter speed, ISO 800, 400 and 200.
Here is the boring picture of a camera shot with various ISO speeds. As you can see, the histogram reveals that tonalities are not just generally farther to the left, they are also more compressed to the left—bringing the exposures on a par in post will unavoidably be harder on the underexposed frames.
But those are raw files, and we're working with very powerful, sophisticated raw processors, aren't we? Let's see what happens when we bring the three exposures even (moving the white point and black point):
Pushing the exposures
Well, well! It seems the file with a higher ISO turned out cleaner than the files pushed in post! The more you push, not only do you get more noise everywhere (especially in the shadows), but the files get muddier, and tonal gradations are less smooth. Why is that? Because there is more information in the highlights than in the shadows (hence "expose to the right").
The morale of the story is that you are better off raising the ISO to get a proper exposure than compensating for underexposure in post.
7:56 amOff
X Sync Limit
When photographing on location, you might want to use flash to complement the available light, allowing you to have greater control over the exposure of the subject and its environment.
When the ambient light is rather dim, you might need to "drag the shutter" (use a slow shutter speed to allow enough of the ambient light to register on the sensor), while freezing the subject with flash—the flash lasting only a very, very brief moment. In this situation, your only potential problem, in terms of exposure, is having a flash that is too powerful, forcing you to figure out a way to limit its output (by using neutral density gels, for example).
Conversely, when the ambient light is very bright, such as when shooting in broad daylight, you might encounter many challenges. First, you will need to use more powerful flashes, otherwise their light won't be bright enough to be noticeable, in comparison to the available light. Second, you will need to use very fast shutter speeds, otherwise the ambient light will be too powerful—the shutter speed, in this case, will allow you to control the ambient light without affecting the flash exposure, since the flash is so fast as to be unaffected by the shutter speed.
1/60 second X sync (denoted by lightning bolt) on the shutter speed dial of a Canon AE-1 Program camera using a curtain-type focal-plane shutter
But there lies the problem: when using flash, you won't be able to exceed the "X sync" of your camera's shutter—a speed that is very often much too slow to control bright sunlight when using larger apertures.
So where is that X sync limitation coming from? It comes from the simple fact that for flash to register on the sensor, its burst must occur at the same time as the sensor is fully exposed to it. And what controls the exposure of the sensor? The shutter. Whatever type of shutter your camera is using, there are unavoidable physical limits to the speed at which the mechanical parts of the shutter can move. Since the flash exposure is usually much briefer than the speed at which the shutter can move, the limit in the fastest usable speed is determined by the speed at which the shutter can fully expose the sensor.
Most cameras today employ a focal-plane shutter, a shutter that is located just in front of the sensor and can fully expose the shutter no faster than speeds around 1/250 second, often slower. You might wonder why the X sync is so slow while it is usually possible to use exposure times as fast as 1/8000 second...
Rather than explaining it myself, I will let Ansel Adams do it, with an excerpt from one of his famous books:
Current focal-plane shutters usually consist of two separate curtains. As the first one travels across the focal plane it uncovers the film to begin the exposure, and the second curtain follows after a controlled interval to terminate the exposure.
At longer exposures the first curtain will open completely, and, after the measured delay, the second curtain then closes. As the shutter speeds become faster, however, the second curtain begins closing before the first has fully uncovered the film, thus following the first curtain across the film. The exposure is made through the slit formed by the two curtains, and very fast shutter times are possible. [...]
With electronic flash the pulse of light is very brief, in the range of 1/500 to as little as 1/50,000 second, so the flash must be triggered at the moment the shutter is fully open. This requirement presents no problem with a leaf shutter, since there is always a moment when the shutter is fully open, even at the fastest speeds. With a focal-plane shutter, however, the maximum speed that can be synchronized with electronic flash is the fastest at which the entire film surface is exposed at the same moment, usually 1/60 to 1/90 second with the curtain-type shutter and 1/125 second with the metal blade-type. Using electronic flash with a higher speed means that part of the film will be covered by one or both curtains when the flash fires, and only a section of the film will be exposed.
Ansel Adams, "The Camera", pp. 84, 86, Little, Brown and Company.
There's a reason some photographers pay a hell of a lot more for camera systems that allow the use of central shutters. Apart from focal-plane shutters having achieved faster speeds than the ones Adams was referring to in 1980, this basic principle has remained true to this day, no matter what you might have understood from David Hobby or Joe McNally's insights. If all it took to circumvent the X sync was to use low-powered flashes, a whole sweep of the photographic educational material would become meaningless.
Take a look at this video of a Nikon D3 in slow motion, where you can clearly see the rear curtain following the first without ever fully exposing the sensor (which is to be expected, at 1/4000 second). This video is also interesting for it demonstrates the full process of the aperture blades stopping down and the mirror (and sub-mirror) raising, the shutter curtains moving down, followed by the cocking of the shutter while the mirrors and aperture blades go back to their original position.
No, what David Hobby and Joe McNally are referring to when they talk about a way to use flash at faster-than-X sync speeds is, for example, Nikon's "FP" or Canon's "High Speed Sync" modes. The basic idea is that instead of sending a single flash, these modes send a rapid succession of flashes during the whole exposure so as to allow the sensor to be exposed to the flash throughout, even if only a small portion of the sensor is exposed at any point due to the use of a faster shutter speed.
Of course, because the flashes have to send multiple bursts of light, they cannot be used at their full power—there must be enough juice to flash during the whole exposure. As a consequence, they can only be used at a lower power, hence the frequent use of a multitude of flashes at the same time to compensate for the loss of power.
The key issue here is the fact that multiple bursts of light are emitted because the sensor is not fully exposed at any point, not the fact that the flashes are used at lower power. To be very clear: the power of the flash has nothing to do with the ability to use fast shutter speeds, but is only a drawback to be able to use this trick. Moreover, these special modes can only be used with proprietary systems, when the flash can speak to the camera—it is definitely not a trick you can achieve with any combination of camera and flash, and definitely not when using studio strobes.
Here is Joe McNally on the subject:
On a bright, sunny day, your ISO becomes (roughly) your shutter speed at f/16. Thus, ISO 200 translates to a 1/250th of a second shutter speed at f/16. Right there, if you notice, you are at the top end of your flash sync speed, with a small flash powered by four AA batteries. [...]
Auto FP high-speed sync is an interesting and valid alternative to having the flash launch a missile's worth of light in one pop. In this mode, you are asking it to make many, many small pops of light. It syncs up with the focal plane shutter (hence the FP) to burst tiny bits of light through the blades of the shutter as it exposes the scene. Effectively, the light stays on for the whole exposure, which is a very short amount of time. This bursting capacity enables the flash to stick with the shutter all the way to a speed of 1/8000th of a second, depending on the camera.
Somethin's gotta give, right? You bet. The power of your flash falls victim to the speed and repetition of all those little pops. [...] A way around this is to use a bunch of Speedlights [...].
Joe McNally, "Hot Shoe Diaries", pp. 256-257, New Riders.
Here is Kirk Tuck on the subject:
You're probably aware that most digital camera shutters can only synchronize with flash at shutter speeds of up to 1/250 second. [...] If you are using one of the more sophisticated flash and camera systems, you should also know about a nifty feature that can be helpful for shooting in sunlight with your flash. It's called "FP" flash, and it works like this: With your camera and flash set to handle FP [...], choose a shutter speed and aperture combination that works with the ambient light and the flash will actually pulse consistently enough to light the camera sensor evenly as the shutter opening travels from side to side or from top to bottom. This technique is best used to supply a bit of fill flash when you are using a fast lens near its widest aperture to blur a background. Since the flash must pulse instead of unloading one big burst of light the output is much lower.
Kirk Tuck, "Minimalist Lighting", pp. 68-70, Amherst Media.
And here's a bunch of Wikipedia articles, all speaking with the same voice:
Today, certain modern xenon flash units have the ability to produce a longer-duration flash to permit X-synchronization at shorter shutter speeds. Instead of delivering one burst of light, the units deliver several smaller bursts [...]. This allows light to be delivered to the entire area of the film or image sensor even though the shutter is never fully open at any moment. The downside is that the flash is of less effective intensity since the individual bursts are lower powered than the normal capability of the flash unit. Only certain camera and flash combinations support this feature, and the camera-flash pairings are almost exclusively from the same manufacturer [...]
When using a focal-plane shutter with a flash, a photographer will typically operate the shutter at its X-sync speed or slower; however, some electronic flashes can produce a steady pulse compatible with a focal-plane shutter operated at much faster shutter speeds.
Focal-plane shutter — Breaking the X-sync barrier:
Electronics are also responsible for pushing the focal-plane shutter's X-sync speed beyond its mechanical limits. [...] At higher speeds, a normal 1 millisecond electronic flash burst would only partially expose the film – the part open to the slit. [...] In 1986, the Olympus OM-4T introduced a system that could synchronize a specially dedicated accessory Olympus F280 Full Synchro electronic flash to pulse its light at a 20 kilohertz rate for up to 40 ms, to illuminate its horizontal FP shutter's slit as it crossed the entire film gate – in effect, simulating long-burn FP flashbulbs – allowing flash exposure at shutter speeds as fast as 1/2000 sec. This allowed daylight plus fill-flash use in almost any situation. However, there is a concomitant loss of flash range.
6:31 pmOff
Lens Movements
There is quite a bit of confusion—at least in the terminology—regarding the lens adjustments made possible by perspective control lenses (also called "tilt/shift" lenses). The best, thoroughest, clearest explanation I've come across for this part of the photography technique comes from chapter 10, "View-Camera Adjustments", in Ansel Adams' famous "The Camera" book. I recommend you look it up for a more extensive explanation complete with illustrations and examples.
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).
2:25 amOff
Gel That Flash
When you want to harmonize the color of your flash with the ambient light, you first have to know which color the ambient light is, obviously. You might try to go at it by eye, or even by tradition, but you might not get the result you expected (for example, fluorescent light used to be very green, but this is not necessarily a reliable presumption anymore, since they now come in many colors, with arbitrary names like "cool white").
Since we don't walk around with colorimeters or computers to analyze our images (and because we just don't have the time to fool around anyway), there is an easy trick to reliably figure out which color the ambient light actually is. Your camera has an RGB histogram — all you need to do is to fill the frame with a neutral reference and you'll know!
Remember that since you only want to read the ambient light for this test, you need to turn off your flash. Also, it is important to use the "daylight" (neutral) white balance to do this test, otherwise you will not get a true assessment of the color (you will get something that was corrected in one way or another). The neutral reference could be a grey card (ideally), but it could also just be a white wall or piece of paper — as long as it fills the frame and is "close enough" to neutral you'll be alright, since you won't be able to surgically match your flash to that color anyway (there's only so much you can do with a couple of gels!)
I tested this in my living room, where the lamps use "compact fluorescent" bulbs — those are hard to guess. When the image appears on your LCD, display the RGB histogram, and you will see the color dominance right there:
In this case, we can see that there is far more red and green than blue. We can also see that although this is fluorescent, this is definitely not just green. Remember that with light, red+green=yellow! So in this case we have some kind of orange, because the red component is a bit stronger than the green.
Now all you have to do is pick gels that will add the same cast to your flash. If I had to act quickly, I would pick a strong orange and I know that I would get a pretty decent result. With just this one quick test, I am certain not to make a huge mistake and use a color that is way off.
To illustrate the effect the different gels produce on your flash, you can do a similar test: shoot a neutral reference, in "daylight" white balance, but this time with only the flash exposure (use a fast shutter speed and low ISO to remove any ambient light).
Using a strong green gel, we get this result:
Definitely not! Notice how the histogram doesn't look at all like the one I had with the ambient light only. Let's try a strong orange gel:
Well, this is much better. I could certainly shoot knowing that the color of the ambient light and the color of my flash are "close enough". If you want to match the ambient light even more closely, it's possible — at some point you'll get something like this:
A-ha! Well this is very, very close. But I had to use three different gels (from what I had in my kit), so this is getting a bit crazy. Also remember that each gel you add in front of your flash cuts some light — the more gels you use, the less powerful your flash becomes.
In practice, you might only have 2 or 3 degrees of orange and green in a basic kit. That's fine — most discrepancies can be removed with just these, even if they are not "perfect". The more you do this, the more you will be able to pick the appropriate gel (or combination of gels), since you will know what histogram they produce.
6:55 pmOff
Mixing Flash with Ambient
Setting the shutter speed in the light meter
When using a Sekonic L-358 light meter to read a flash exposure (the same applies to other models, I am simply not familiar with them), one of the nice features it has is its ability to indicate which percentage of the total exposure came from the flash — the rest of the exposure being from the ambient light available in the location where the metering was done.
Here is a figure taken from the L-358's user manual:
Metering flash exposure
As you can see in this example, 70% — "Percentage of flash in total exposure" — means that the flash was responsible for 70% of the total light, while 30% came from the ambient light available. The total amount of light ended up requiring an aperture of f/5.6 and 3/10th (let's call this f/6.3).
If 30% of the total light gathered by the light meter during a reading comes from the ambient light, if you change the shutter speed (without changing the flash power), you should expect that percentage to change: the longer you expose, the more the ambient light will have an impact on the total light, since you will gather more of it (while still gathering the same amount of flash). On the other hand, if you expose for a very brief moment, you will gather very little of the ambient light (while still gathering the same amount of flash), so the ambient light won't have much of an impact in the total light.
You might very well end up in a situation where the flash is only responsible for a small fraction of the total light (say, 10%) or, at the other extreme, end up in a situation where the ambient light has no influence on the exposure (which would give a 100% flash exposure). For example, when working in a studio environment, we usually don't mind leaving the modeling lights on when shooting, because we know that the light they produce is insignificant in the total exposure compared to the flash power (we get a 100% flash exposure all the time regardless).
Now, coming back to a location shoot where we mix some ambient light with flash, we should often expect situations where the percentage of the flash in the total exposure will not be 100%. In the situation of the figure above, where 70% of the total light came from the flash, suppose we were to expose for 1/30 instead of 1/125 — that is two stops more ambient, giving us a brighter background. In this case, the flash would now only account for ~40% of the total exposure. What's more, since we're adding ambient light to the previous exposure (flash remained the same), we now have more light overall, so we must use a smaller aperture — we go from f/6.3 to ~f/9.
This makes the shutter speed a crucial parameter to provide to the light meter, otherwise the obtained value will be wrong.
The only situation in which we could ignore the shutter speed would be in a pitch-black room where the only light is the flash — in this case, the ambient light (or lack thereof) would have no impact in the reading whatsoever.
Changing our perception of flash color gels
If you're mixing flash with ambient light, you know that you can (must?) correct the color of the flash so that its light color is well harmonized with the ambient light color, lest you get a quirky result.
But you can also use color gels not necessarily because you want to make the two light sources the same, but because you want to add an effect, such as warming up the subject so that it contrasts more with a colder background, for example.
In the image below, both the foreground and the background lights are of the same neutral color:
Foreground and background light have same color
Using a CTO gel (orange) on the flash aimed at the subject, you can warm up the subject, which will further isolate it from the background (which will now be of a relatively colder color than subject, even if it is the same as in the first shot):
Foreground light is warmer than background light (note: a strong color was used to make the illustration obvious)
Indeed, while the background remained the same, it can be said that the subject is warmer.
You will notice that I also (conveniently) included a gray card in the frame. If we take the picture above (the same exact file), but that we color correct using the gray card, here's what we get:
Image above, but white balance corrected with the gray card
You will notice that the subject is now perfectly neutral — exactly back to what we had in the first frame — but that the background turned blue (colder)! In this case, can it be said that the subject is warmer than in the first shot? Absolutely not — in fact, they are exactly the same, after correction, even if a "warming" gel was used.
If you're going to include a gray card (or GretagMacbeth chart) in your frame, you might want to do it while you're not using a gel on the subject, otherwise, balancing the color with a neutral reference will remove any color you might have thrown on the subject and, rather, affect the background. If you're going to include a gray card in your frame while you are using a gel, the way you should look at it is that you are in fact changing the color of the background in the opposite direction than that of the color gel (e.g.: if you're using a warming color on the subject, you're actually not changing the subject, but rendering the background colder!)
The fact that we are using a "warming" gel has no bearing on the final look of the image, because it depends on the white balance selected. A "warming" gel might therefore be more meaningfully seen as a "gel that makes the background colder"! That is why, when you work indoors under tungsten lights, you use a warming gel: not to warm up the subject (you still want it neutral), but rather to make the background colder.