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(Part 2 will have to wait until after the weekend.)
I knew it was going to be nice. I knew that, theoretically at least, the autofocus was going to be faster, quieter, and more accurate.
Digression the first: Focal lengths, f-stops, photon density, and autofocus.
The focal length of a lens, almost always measured in millimeters these days*, measures the distance an objective lens must be from the film or sensor to provide the given magnification. This is from the days when a camera used only a single lens, and that lens could be adjusted closer or farther from the film to adjust the magnification. So when we say lens A is "longer" than lens B, we mean lens A's focal length is greater than lens B's, and thus lens A provides greater magnification than lens B. The difference in magnification is directly proportional to the difference in focal length, so a 200mm lens magnifies your image twice as much as a 100mm lens. Put another way, if you stand 30 feet away from your subject with a 100mm lens, you would need to stand 60 feet away to get exactly the same image with a 200mm lens.
Modern SLR lenses are no longer just a single lens, and can have upwards of fifteen actual lenses (each called a "lens element") within one body, allowing an effective focal length that is different than the distance between the frontmost element and the film/sensor. Technically, a lens that gives an effective focal length greater than the distance from the frontmost lens to the film/sensor is called "telephoto". In practice, any lens longer than about 70mm is called telephoto, and the technical definition is rarely if ever used.
A "prime" lens is one that has only one focal length. A "zoom" lens is one that can be adjusted through a range of different focal lengths. Historically, prime lenses were of significantly higher quality than zooms, but quality modern zoom lenses are now close enough to good prime lenses that the convenience of having several focal lengths available without changing lenses far outweighs the marginal image quality differences.
My new lens is a zoom, giving me effective focal lengths of 70mm to 200mm. The lens is 197mm long, which means that while we refer to it casually as a telephoto lens, it does not quite technically qualify as such. (There are a few mm between the rear of the lens and the film/sensor, which push the entire distance from frontmost element to the film/sensor to or past the 200mm effective focal length.)
So, yeah. That's focal length. Now "f-stops".
The "f" in "f-stop" is the effective focal length of the lens. The numerical portion of the f-stop is a divisor. When you divide the focal length of the lens by the number of the f-stop, you arrive at the diameter of the aperture, or the hole through which light passes on its way to the film/sensor. The f-stop listed as part of a lens' attributes is its "maximum aperture"--the widest hole it can make to allow light through. So a 100mm lens with an f/2.8 maximum aperture gives you a 35.7mm hole. (There is an obvious question here. Don't worry, I'll get to it.) Light travels at a constant speed. (It's famous for it.) Film and sensors react to light at a constant** rate. The more light you dump onto the film/sensor, the faster it is able to record the image. There are two ways to control how much light reaches your sensor--first by controlling the amount of time the shutter is open, second by controlling your aperture. All else being equal, a longer-duration shutter opening allows more light to get to your film/sensor, and by the same token, a larger aperture (which requires a *smaller* f-stop number!) also allows more light through. Thanks to reciprocity, you can adjust shutter speed and aperture in tandem for a variety of creative effects, but that is several other major posts, only one of which I have yet written (http://georgmi.livejournal.com/58990.html). The important thing at the moment is that, when you double the aperture (by halving the f-stop number), you quadruple the amount of light that gets to your film/sensor in any given amount of time, for the area of a circle varies with the square of the diameter.
This is what we mean when we speak of the "speed" of a lens--how fast does it let light get through to the film/sensor? The smaller the f-stop listed on the barrel of the lens, the larger the maximum aperture, and the faster the film/sensor can record an image. An f/2.8 lens is pretty darn fast, and an f/8 lens is pretty darn slow. Though shorter lenses can easily have maximum apertures as wide as f/1.4 for even f/1, and really long lenses usually max out at f/5.6 or even f/8. At a given focal length, the larger the maximum aperture, the more glass is required to accommodate it, which leads directly to greater costs of manufacture and much greater weight. (If you want to plunk down $9500, you can get a *very* nice Nikon 600mm f/4 lens, but at 11.2 lbs for just the lens, you'll need an assistant to huck the thing around.)
Maximum apertures, like focal lengths, also come in fixed or variable values, though for a zoom lens, even a "fixed" maximum aperture represents a variable diameter of maximum aperture. My 28-300mm zoom lens' maximum aperture varies from f/3.5 at the 28mm end of the range (for an aperture diameter of 8mm) to f/6.3 at 300mm (47.6mm). My new lens has a fixed maximum aperture of f/2.8, which means 25mm diameter at 70mm focal length and 71.4mm at 200mm.
"Wait, wait!" you say. That f/2.8 thing is just confusing! If it's the diameter of the aperture that's important, why not just list the aperture's diameter and be done with it?
Yes, that's the obvious question to which I previously referred. I am *so* glad you asked me that. :) We measure aperture as a fraction of focal length because doing so automatically corrects for photon density.
Photons bounce off your subject. Your camera captures those photons and translates them into an image. We've seen above that the more photons your camera captures in a given timeframe, the faster your image is rendered on the film or sensor. But photons spread spherically off your subject, which means that all else being equal, the further you stand from your subject, the fewer photons are available for you to capture. This photon density varies (conveniently for us) in inverse proportion to the square of the distance to the subject. In other words, if you stand twice as far away from your subject, you have only a quarter as many photons entering your camera, which means you need a hole twice as big to capture the same number of photons as when you were in the original spot.
Let's do the math, shall we? We'll start with theoretical 100mm and 200mm lenses. We'll set the 100mm lens up 30 feet from our subject, and the 200mm lens 60 feet away. (We saw above that this gives us the same image on the film/sensor for each camera.) We'll arbitrarily set the aperture on our 100mm lens at 25mm (ignoring f-stops for now), and shutter speed at 1 second. Photon density at 30 feet from our subject is X photons per square mm per second (p/mm^2/sec (where the ^ operator means "to the power of", and the * means "times", for those of y'all unfamiliar with computer math symbols), but don't worry, mm^2 and seconds will factor out at the end).
This gives us the equation Pt = X * (12.5mm^2*pi) * (1 sec)
Or Pt = Xp/mm^2/sec * ((156.25*pi)mm^2)sec
And because mm^2 and seconds cancel each other out between X and the rest, we are left with:
Pt = 156.25*pi*X photons
Where:
Pt is total photons touching the sensor
X is our photon density (p/mm^2/sec)
(12.5mm^2*pi) is the area of an aperture of diameter 25mm
1 sec is the time the shutter is open
In order to get the same image with our 200mm lens, we need to capture the same number of photons. We will hold shutter speed constant, allow for the reduced photon density because of the greater camera-to-subject distance (X/4, as it happens), and solve for aperture (A):
156.25*pi*Xp = .25Xp/mm^2/sec * ((A/2)^2*pi) * (1 sec)
We can cancel out the mm^2 and seconds to give us:
156.25*pi*Xp = .25Xp * ((A^2)/4)*pi)
Multiply the whole thing by 16:
2500*pi*Xp = Xp*(A^2)*pi
Divide the whole thing by pi:
2500*Xp = Xp*A^2
Divide by X pixels:
2500 = A^2
Solve for A:
50=A
So to capture the same image with the 200mm lens from twice as far away, we need an aperture of 50mm diameter, compared to 25mm for the 100mm lens. Ah-ha! Please note that a focal length of 100mm divided by an aperture diameter of 25mm gives an f-stop number of f/4. And note that a focal length of 200mm divided by an aperture diameter of 50mm *also* gives f/4. *This* is why lenses are calibrated in f-stops instead of raw aperture diameters--it saves the photographer from having to do the above math every time I want to take a picture. And this is also why, if you asked 100 professional photographers about photon density, 90 of them will most likely respond, "Huh?" :)
Now, we're (finally!) ready to talk about autofocus. AF, in my camera at least, works by identifying edges within the field of view, and adjusting the lens' focus until the contrast between the two sides of the edge is maximized. (This begs the questions of what constitutes an edge, and what is contrast, but...well, yeah. We're going to leave those alone. I know you're relieved.) What you need to know is that the appearance of contrast is directly related to the amount of light passing through the lens. And you'll recall that the larger the maximum aperture, the more light is getting through. So a lens with a larger maximum aperture makes it easier for the camera to focus, by increasing the camera's ability to detect edge contrast. This reduces the incidence of "hunting", which is where your camera focuses back and forth several times, trying to identify the point of sharp focus, which means you spend less time waiting for your camera to focus, and more time capturing photons.
Recall that my 28-300 lens is f/3.5-6.3. At 200mm, its maximum aperture is around f/5.6. The new 70-200 is fixed at f/2.8. At 200mm, the new lens is letting *four times as much light* through to the autofocus system.
Add the fact that the new lens has an internal, specifically-tuned--and thus both faster and quieter--focus drive motor and does not need to be driven by the camera body's external motor, and the comparison test I did between AF on the two lenses in my kitchen last night can be seen to have had predictable results.
With the old lens (200mm at f/5.6) aimed at the cabinets across the room, the camera went through the entire focus range four times, spending a good two or three noisy ("wheeeet, hummmmm, wheeeet, hummmmmm") seconds, before it gave up and refused to take a picture at all.
But the new lens, at f/2.8, went straight to the point of sharp focus and locked in. A small fraction of a second, and almost silent.
It's important to note that it is the camera in both cases deciding on the focus point. The new lens does not employ a different algorithm than the old one.
So as I said, I knew there would be a difference with the new lens. I had *no idea* the difference would be so profound and obvious. Adding the teleconverters (accessory lenses which multiply the lens' focal length by 1.4x and 2x respectively, while reducing the maximum aperture by the same factor) does introduce a little hunting, but it's still quicker and quieter than the old lens, and effectively gives me four lenses for the price of one-and-a-half:
70-200mm f/2.8 (standalone)
98-280mm f/4 (with 1.4x teleconverter)
140-400mm f/5.6 (with 2.x teleconverter)
196-560mm f/8 (with both teleconverters in series)
Though I would have to be desperate or insane to try and use that last one. Because for only 6 more thousand dollars, I could get a 300mm f/2.8, and teleconvert that to 600mm f/5.6, or 840mm f/8! Mwahahahahaha!
Ahem.
Anyway, I likes me my new lens.
And I haven't even taken any pictures yet!
*I understand that some large-format cameras measure the lens-film distance in inches, but you nor I ever likely need to worry about that.
**This rate is *adjustable* by choosing films with, or changing your camera settings to, different ISO ratings, and it turns out that film's reaction to light is not constant at extremely low light levels, but for photography under normal conditions, and with settings held the same, "constant" is a useful approximation.
The short story: Oh. My. God. What an *awesome* lens.
Related topics (will add links when associated articles are written, or you can search on my “photography” tag):
Exposure
Focal lengths, f-stops, photon density, and autofocus
What the hell is "bokeh"?
Depth of Field
Camera shake and motion blur
Using the flash (Yeah, right! As soon as I figure it out myself…)
Film: Reciprocity failure
Expanding depth of field with Photoshop
Digital: The histogram is your friend
I knew it was going to be nice. I knew that, theoretically at least, the autofocus was going to be faster, quieter, and more accurate.
Digression the first: Focal lengths, f-stops, photon density, and autofocus.
The focal length of a lens, almost always measured in millimeters these days*, measures the distance an objective lens must be from the film or sensor to provide the given magnification. This is from the days when a camera used only a single lens, and that lens could be adjusted closer or farther from the film to adjust the magnification. So when we say lens A is "longer" than lens B, we mean lens A's focal length is greater than lens B's, and thus lens A provides greater magnification than lens B. The difference in magnification is directly proportional to the difference in focal length, so a 200mm lens magnifies your image twice as much as a 100mm lens. Put another way, if you stand 30 feet away from your subject with a 100mm lens, you would need to stand 60 feet away to get exactly the same image with a 200mm lens.
Modern SLR lenses are no longer just a single lens, and can have upwards of fifteen actual lenses (each called a "lens element") within one body, allowing an effective focal length that is different than the distance between the frontmost element and the film/sensor. Technically, a lens that gives an effective focal length greater than the distance from the frontmost lens to the film/sensor is called "telephoto". In practice, any lens longer than about 70mm is called telephoto, and the technical definition is rarely if ever used.
A "prime" lens is one that has only one focal length. A "zoom" lens is one that can be adjusted through a range of different focal lengths. Historically, prime lenses were of significantly higher quality than zooms, but quality modern zoom lenses are now close enough to good prime lenses that the convenience of having several focal lengths available without changing lenses far outweighs the marginal image quality differences.
My new lens is a zoom, giving me effective focal lengths of 70mm to 200mm. The lens is 197mm long, which means that while we refer to it casually as a telephoto lens, it does not quite technically qualify as such. (There are a few mm between the rear of the lens and the film/sensor, which push the entire distance from frontmost element to the film/sensor to or past the 200mm effective focal length.)
So, yeah. That's focal length. Now "f-stops".
The "f" in "f-stop" is the effective focal length of the lens. The numerical portion of the f-stop is a divisor. When you divide the focal length of the lens by the number of the f-stop, you arrive at the diameter of the aperture, or the hole through which light passes on its way to the film/sensor. The f-stop listed as part of a lens' attributes is its "maximum aperture"--the widest hole it can make to allow light through. So a 100mm lens with an f/2.8 maximum aperture gives you a 35.7mm hole. (There is an obvious question here. Don't worry, I'll get to it.) Light travels at a constant speed. (It's famous for it.) Film and sensors react to light at a constant** rate. The more light you dump onto the film/sensor, the faster it is able to record the image. There are two ways to control how much light reaches your sensor--first by controlling the amount of time the shutter is open, second by controlling your aperture. All else being equal, a longer-duration shutter opening allows more light to get to your film/sensor, and by the same token, a larger aperture (which requires a *smaller* f-stop number!) also allows more light through. Thanks to reciprocity, you can adjust shutter speed and aperture in tandem for a variety of creative effects, but that is several other major posts, only one of which I have yet written (http://georgmi.livejournal.com/58990.html). The important thing at the moment is that, when you double the aperture (by halving the f-stop number), you quadruple the amount of light that gets to your film/sensor in any given amount of time, for the area of a circle varies with the square of the diameter.
This is what we mean when we speak of the "speed" of a lens--how fast does it let light get through to the film/sensor? The smaller the f-stop listed on the barrel of the lens, the larger the maximum aperture, and the faster the film/sensor can record an image. An f/2.8 lens is pretty darn fast, and an f/8 lens is pretty darn slow. Though shorter lenses can easily have maximum apertures as wide as f/1.4 for even f/1, and really long lenses usually max out at f/5.6 or even f/8. At a given focal length, the larger the maximum aperture, the more glass is required to accommodate it, which leads directly to greater costs of manufacture and much greater weight. (If you want to plunk down $9500, you can get a *very* nice Nikon 600mm f/4 lens, but at 11.2 lbs for just the lens, you'll need an assistant to huck the thing around.)
Maximum apertures, like focal lengths, also come in fixed or variable values, though for a zoom lens, even a "fixed" maximum aperture represents a variable diameter of maximum aperture. My 28-300mm zoom lens' maximum aperture varies from f/3.5 at the 28mm end of the range (for an aperture diameter of 8mm) to f/6.3 at 300mm (47.6mm). My new lens has a fixed maximum aperture of f/2.8, which means 25mm diameter at 70mm focal length and 71.4mm at 200mm.
"Wait, wait!" you say. That f/2.8 thing is just confusing! If it's the diameter of the aperture that's important, why not just list the aperture's diameter and be done with it?
Yes, that's the obvious question to which I previously referred. I am *so* glad you asked me that. :) We measure aperture as a fraction of focal length because doing so automatically corrects for photon density.
Photons bounce off your subject. Your camera captures those photons and translates them into an image. We've seen above that the more photons your camera captures in a given timeframe, the faster your image is rendered on the film or sensor. But photons spread spherically off your subject, which means that all else being equal, the further you stand from your subject, the fewer photons are available for you to capture. This photon density varies (conveniently for us) in inverse proportion to the square of the distance to the subject. In other words, if you stand twice as far away from your subject, you have only a quarter as many photons entering your camera, which means you need a hole twice as big to capture the same number of photons as when you were in the original spot.
Let's do the math, shall we? We'll start with theoretical 100mm and 200mm lenses. We'll set the 100mm lens up 30 feet from our subject, and the 200mm lens 60 feet away. (We saw above that this gives us the same image on the film/sensor for each camera.) We'll arbitrarily set the aperture on our 100mm lens at 25mm (ignoring f-stops for now), and shutter speed at 1 second. Photon density at 30 feet from our subject is X photons per square mm per second (p/mm^2/sec (where the ^ operator means "to the power of", and the * means "times", for those of y'all unfamiliar with computer math symbols), but don't worry, mm^2 and seconds will factor out at the end).
This gives us the equation Pt = X * (12.5mm^2*pi) * (1 sec)
Or Pt = Xp/mm^2/sec * ((156.25*pi)mm^2)sec
And because mm^2 and seconds cancel each other out between X and the rest, we are left with:
Pt = 156.25*pi*X photons
Where:
Pt is total photons touching the sensor
X is our photon density (p/mm^2/sec)
(12.5mm^2*pi) is the area of an aperture of diameter 25mm
1 sec is the time the shutter is open
In order to get the same image with our 200mm lens, we need to capture the same number of photons. We will hold shutter speed constant, allow for the reduced photon density because of the greater camera-to-subject distance (X/4, as it happens), and solve for aperture (A):
156.25*pi*Xp = .25Xp/mm^2/sec * ((A/2)^2*pi) * (1 sec)
We can cancel out the mm^2 and seconds to give us:
156.25*pi*Xp = .25Xp * ((A^2)/4)*pi)
Multiply the whole thing by 16:
2500*pi*Xp = Xp*(A^2)*pi
Divide the whole thing by pi:
2500*Xp = Xp*A^2
Divide by X pixels:
2500 = A^2
Solve for A:
50=A
So to capture the same image with the 200mm lens from twice as far away, we need an aperture of 50mm diameter, compared to 25mm for the 100mm lens. Ah-ha! Please note that a focal length of 100mm divided by an aperture diameter of 25mm gives an f-stop number of f/4. And note that a focal length of 200mm divided by an aperture diameter of 50mm *also* gives f/4. *This* is why lenses are calibrated in f-stops instead of raw aperture diameters--it saves the photographer from having to do the above math every time I want to take a picture. And this is also why, if you asked 100 professional photographers about photon density, 90 of them will most likely respond, "Huh?" :)
Now, we're (finally!) ready to talk about autofocus. AF, in my camera at least, works by identifying edges within the field of view, and adjusting the lens' focus until the contrast between the two sides of the edge is maximized. (This begs the questions of what constitutes an edge, and what is contrast, but...well, yeah. We're going to leave those alone. I know you're relieved.) What you need to know is that the appearance of contrast is directly related to the amount of light passing through the lens. And you'll recall that the larger the maximum aperture, the more light is getting through. So a lens with a larger maximum aperture makes it easier for the camera to focus, by increasing the camera's ability to detect edge contrast. This reduces the incidence of "hunting", which is where your camera focuses back and forth several times, trying to identify the point of sharp focus, which means you spend less time waiting for your camera to focus, and more time capturing photons.
Recall that my 28-300 lens is f/3.5-6.3. At 200mm, its maximum aperture is around f/5.6. The new 70-200 is fixed at f/2.8. At 200mm, the new lens is letting *four times as much light* through to the autofocus system.
Add the fact that the new lens has an internal, specifically-tuned--and thus both faster and quieter--focus drive motor and does not need to be driven by the camera body's external motor, and the comparison test I did between AF on the two lenses in my kitchen last night can be seen to have had predictable results.
With the old lens (200mm at f/5.6) aimed at the cabinets across the room, the camera went through the entire focus range four times, spending a good two or three noisy ("wheeeet, hummmmm, wheeeet, hummmmmm") seconds, before it gave up and refused to take a picture at all.
But the new lens, at f/2.8, went straight to the point of sharp focus and locked in. A small fraction of a second, and almost silent.
It's important to note that it is the camera in both cases deciding on the focus point. The new lens does not employ a different algorithm than the old one.
So as I said, I knew there would be a difference with the new lens. I had *no idea* the difference would be so profound and obvious. Adding the teleconverters (accessory lenses which multiply the lens' focal length by 1.4x and 2x respectively, while reducing the maximum aperture by the same factor) does introduce a little hunting, but it's still quicker and quieter than the old lens, and effectively gives me four lenses for the price of one-and-a-half:
70-200mm f/2.8 (standalone)
98-280mm f/4 (with 1.4x teleconverter)
140-400mm f/5.6 (with 2.x teleconverter)
196-560mm f/8 (with both teleconverters in series)
Though I would have to be desperate or insane to try and use that last one. Because for only 6 more thousand dollars, I could get a 300mm f/2.8, and teleconvert that to 600mm f/5.6, or 840mm f/8! Mwahahahahaha!
Ahem.
Anyway, I likes me my new lens.
And I haven't even taken any pictures yet!
*I understand that some large-format cameras measure the lens-film distance in inches, but you nor I ever likely need to worry about that.
**This rate is *adjustable* by choosing films with, or changing your camera settings to, different ISO ratings, and it turns out that film's reaction to light is not constant at extremely low light levels, but for photography under normal conditions, and with settings held the same, "constant" is a useful approximation.
The short story: Oh. My. God. What an *awesome* lens.
Related topics (will add links when associated articles are written, or you can search on my “photography” tag):
Exposure
Focal lengths, f-stops, photon density, and autofocus
What the hell is "bokeh"?
Depth of Field
Camera shake and motion blur
Using the flash (Yeah, right! As soon as I figure it out myself…)
Film: Reciprocity failure
Expanding depth of field with Photoshop
Digital: The histogram is your friend
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