We've arrived at the last and least important chapter in the text. As
noted on the cover page, this is where most
photography textbooks start. Your choice of camera will not
have much effect on the final image. If you're going to be a working
photographer, however, you should know what tools are available.
Here are the factors that go into the choice of a camera for a project:
- What is the required final image quality?
- At what magnification will the image be viewed?
- How much weight can you carry to the subject?
- How much time do you have to take the picture?
Suppose that your project demands high image quality and high
magnification. For example, you are going to make a 40 x 60 inch
enlargement and display it in a corridor where people can walk right up
to it to check out fine detail. This requirement pushes you toward
using high resolution high quality sensor for the original exposure.
That high-res, high-quality sensor may come wrapped in a large
relatively heavy camera, which gets us into Factor 3: "How much weight
can you carry to the subject?"
Different cameras work at different speeds. With the latest Canon or
Nikon autofocus systems, you might be able to capture an unanticipated
event on a soccer field. A photographer using a
view camera and digital back will not be able to do this; he will still
be setting up the tripod, focusing, stopping the lens down to taking
aperture, closing the shutter, cocking the shutter, attaching a laptop
computer, etc. If your subject is a big mountain, you can probably
afford to take your time making the image.
Now that we have the factors in mind, let's dive into the types of
cameras available:
- point and shoot (subject of a separate article)
- single-lens reflex (SLR) cameras; the standard mass-market tool
- lens-shutter and rangefinder cameras; great values, terrible values, and nothing in between
- digital backs for medium-format and view cameras
- panoramic cameras
Sensor Sizes
Larger sensors offer lower noise at high ISO settings and are therefore
essential for taking pictures in low light conditions. The best light
for photography is typically fairly dim and therefore using a larger
sensor is highly desirable. Unfortunately, the cost of manufacturing a
sensor goes up exponentially with size and the largest sensors can cost
more than a car.
Here are the standard sizes:
- 1/1.8": 5x7mm, common size for point and shoot cameras
- 4/3": 13.5x18mm, the Olympus Four Thirds system
- APS-C: 15.7x23.7mm, the standard for "small sensor" digital SLRs
such as the Nikon D200 (review) and Canon Digital Rebel XTi (Black) (review)
- 35mm film: 24x36mm, Canon's better digital SLRs, starting with the
Canon EOS 5D (review)
- 48x36mm, Hasselblad H3D and digital backs from companies such as
Leaf, MegaVision, Phase One, Sinar
- 60mm line, scanning backs from Seitz
- 72x96mm, scanning backs from BetterLight
The scanning back, a line of CCD elements that are swept
mechanically behind the lens, is a great idea, but it only works for
static subjects since the minimum scan time is about one second.
What about megapixels?
Photo quality in a final print is 200 pixels per inch (the "300dpi"
figure that you hear sometimes relates to commercial printing presses
and isn't meaningful for digital cameras and digital lab printers).
That said, 200 pixels per inch is no guarantee of high quality. The
10-megapixel point and shoot camera may have low contrast and sharpness
from the cheap lens plus high noise in shadow areas from the small
sensor. You would probably get a better print from an old 6-megapixel
digital SLR.
- 2000x3000 pixels (6 megapixels); good for prints up to 10x15" in size
- 2700x3600 pixels (10 megapixels, average digital SLR); good for prints up to 13x18" in size
- 2900x4400 pixels (13 megapixels, Canon 5D); good for prints up to
15x22" in size
- 3300x5000 pixels (16.6 megapixels, Canon 1 Ds Mark II); prints up to 17x25"
- 4080x5440 pixels (22 megapixels; medium format backs); prints to 20x27"
- 5400x7200 pixels (39 megapixels; medium format backs); prints to 27x36"
- 10000x14000 pixels (140 megapixels; large format scanning backs); prints to 50x70"
Note that the "print size" is the maximum at which you'll get the kind
of print quality that one would have gotten with the best film
equipment, enlarged no more than about 10x. By this standard, the
largest that you could have enlarged the typical 35mm negative before
a noticeable reduction in quality would be 10x15", the same as the
6-megapixel digital SLRs. A 6x7cm medium format negative at 10x will
enlarge to 24x28". A 4x5" sheet of film could enlarge to 40x50" and
withstand close inspection.
What about dynamic range?
A glossy photographic print has a dynamic range of about 100:1, i.e.,
the brightest highlight reflects about 100 times more light than the
darkest shadow. This is a 6.6 f-stop range, 2 raised to the 6.6 power
being close to 100. Ultimately our goal is to represent a real-world
scene within this 100:1 ratio. How tough should that be? Things in
the real world are either white like the blank photographic paper,
black like a shadow printed on the photographic paper, or somewhere in
between. You'd therefore naively imagine that a typical real-world
scene would have about the same dynamic range as a photographic print.
Materials in the real world, however, are more varied than the
photographic paper. Snow is extremely reflective while black velvet
or matte black paint have textures that absorb light. Differences in
surface properties can push the dynamic range of a real-world scene up
over 200:1, which is all the dynamic range you'd need if not for
shadows. Consider a granite cliff face with bright white highlights.
Add a cave. The interior of the cave will be dark, despite the fact
that it is made of the same rock as the cliff. The fact that the cave
interior is in shadow and receiving a very different amount of light
than the rocks and trees outside adds a huge amount of contrast. Now
put a black bear inside that cave and try to take a photo that
captures detail in the shadowed-by-the-cave bear's face and the
lit-by-the-sun cliff face. You're struggling with as much as 16
f-stops of dynamic range or 64,000:1. For practical purposes, a
high-contrast real-world scene is usually limited to 1000:1, or 10
f-stops.
Thus the digital photographer is presented with two challenges: (1)
capturing the full range of tones that are present in a scene, (2)
figuring out how to map those tones into the more limited range that
is representable in a finished print.
How many bits are necessary to capture our 1000:1 scene? As 2 raised
to the 10th power is 1024, one would naively suppose 10 bits, but,
since the RAW files are encoded linearly, that would leave only two
levels in the dark shadows: on and off. The result would be banding
in the shadows. We will have to add a couple of extra bits to ensure
the same number of levels that the human eye can distinguish. We
therefore need at least 12 bits per color, which is coincidentally
what the mid-range digital camera RAW formats provide. That will be
sufficient to capture a high-contrast scene, assuming the exposure
setting is perfect. The expensive digital camera backs, some of which
incorporate electronic cooling to reduce sensor noise in the shadows,
offer a 12 f-stop dynamic range and output 16 bits.
What if you capture JPEG files, in effect asking the camera to do the
RAW to JPEG conversion? A standard JPEG encodes 8 bits per color or
256 levels and incorporates a gamma factor so that the numbers do not
linearly correspond to luminance. The human eye can distinguish
roughly 200 levels with the 6.6 f-stop range of a final print.
Therefore a perfectly exposed and converted JPEG ought to be adequate
for making the best possible final print. Unfortunately, in practice,
the exposure won't be perfect and the computer in the camera won't
make the same same decision that you or a professional darkroom
technician would about how highlights and shadows ought to be mapped
into the print tones.
Unless you're going to do all of your photography in a studio with
controlled lighting and a calibrated camera-to-printer setup, you must
have a camera that outputs RAW files with at least 12 bits per color.
This rules out nearly every point and shoot-style camera.
Point and Shoot
Well, they're compact, but due to the small sensor and small lens, you
can almost always get substantially higher image quality if you're
willing to carry a larger camera. The megapixel count will
undoubtably be high, but the contrast and sharpness will be low and
the shadow noise high. Very few P&S cameras provide a RAW capture
option, so the number of luminance levels is limited to 256, only
1/16th as many as you'd get with a 12-bit RAW from a low-end digital
SLR. We cover point and shoot cameras in a separate article.
Single lens reflex (SLR)
A single lens reflex (SLR) is a camera in which the same lens is used
for viewing and taking pictures. A mirror in the body directs the light
from the lens up into a prism for viewing, then flips up out of the way
just before an exposure is made. Note that this is not an exotic
technology; the standard Nikon or Canon camera body (photo at right) is
an SLR.
Suppose that the photographer has chosen an exposure of f/8 and 1/125th
of a second. Here is how most SLRs work during exposure:
- lens is kept open to maximum aperture (e.g., f/2.8) for ease of
viewing and metering
- when the photographer presses the shutter release, the lens aperture is
stopped down to the taking aperture of f/8. On old-style camera/lens
interfaces (e.g., Nikon, Hasselblad), this is accomplished by moving a
lever. With camera/lens interfaces designed in the 1980s (e.g., Canon,
Rollei), this is accomplished by sending an electrical signal to a
solenoid in the lens.
- the mirror is flipped up out of the way of the light (and parked
flat up against the prism)
- now that the lens is stopped down and the mirror is up, the
shutter opens and light begins to strike the CCD or CMOS sensor
- as soon as the shutter is fully open, the camera signals an
electronic flash, if attached to fire
- when 1/125th of a second has elapsed, the shutter is closed
- the mirror is pushed back down to viewing position
- the lens aperture is reopened to its widest setting
SLR manufacturers generally provide a range of interchangeable lenses.
This works out nicely because changing the lens simultaneously changes
the scene magnification on film and in the viewfinder. It is tough to
mix and match brands. Camera bodies and lenses are coupled
mechanically and electronically in non-standard ways. A lens for a
Canon EOS body won't fit a Nikon body and vice versa.
The best thing about an SLR is that what-you-see-is-what-you-get. If
you've left the lens cap on, fitted a really long telephoto, attached a
strange filter, you can see the effect in the viewfinder.
One obvious problem with an SLR is weight. The prism on top of the
body that lets you see a properly-oriented image is heavy.
Another problem with the SLR is noise. The mirror is light but it has
to be flipped up as fast as possible, which is necessarily noisy.
Photographers who work during live theater or concerts often
surround the camera in a "blimp" to muffle the noise.
A final problem with an SLR is exposure latency. If you wait for the
decisive moment and press the shutter, the camera doesn't take a
picture until it has stopped down the lens and flipped up the mirror.
This takes between 50 and 100 milliseconds for the average SLR, which
can be reduced to about 40 milliseconds by using the mirror lock-up
custom function. A standard digital camera uses the final 40
milliseconds to register dark current levels from the image
sensor. These levels vary based on temperature and other conditions,
and must therefore be updated for every picture or sequence of
exposures.
[Do not confuse an electronic viewfinder (EVF) point and shoot camera
with a true mirror-and-optics SLR. The EVF camera is sending light
continuously to the sensor and feeding the sensor output to a little
TV screen on top of the camera. Physically the format is very similar
to a true SLR, but current TV screen technology isn't nearly as good
as current optics.]
More on this kind of camera:
"Building a Digital SLR System".
Rangefinder and lens-shutter cameras
The simplest camera would include the following components:
- a lens in front, with diaphragm for aperture control
- a focus ring or bellows to move the lens back and forth
- a shutter in the middle of the lens
- a light-sensitive medium, film or digital sensor, at the back
Such cameras have been common since the invention of photography and
are known as lens-shutter cameras.
With a lens-shutter or rangefinder camera, you can't look through the
lens. You view the image through a separate optical viewfinder. The
image that you take home will be a bit different than what you viewed due
to parallax: the viewfinder isn't exactly aligned with the
lens.
It turns out that people aren't very good at estimating distance
precisely. So companies began putting military rangefinders into
lens-shutter cameras, coupled to the lens and the viewfinder. The
photographer turns a ring on the lens until two superimposed images
are aligned in the viewfinder. The only digital cameras that include
traditional optical/manual rangefinders are the Epson R-D1 and the
Leica M8. Both accept lenses from the Leica M film camera system,
which are designed for the 24x36mm frame of 35mm film. Both the Epson
and Leica digital rangefinders incorporate a small sensor, thus
multiplying the effective focal length of the old lenses. These
cameras accept a large range of lenses and therefore use a shutter
just in front of the sensor, i.e., a focal-plane shutter.
If you put an autofocus and autoexposure mechanism into a traditional
lens-shutter camera, what do you have? A consumer's point and shoot
camera.
Without the mirror and prism, lens-shutter cameras can be much lighter
and more compact than an SLR using the same sensor. With no mirror to
slap, lens-shutter cameras are also quieter than SLRs.
There is no mass market for high quality digital rangefinder cameras
and therefore the cameras that are available are much more expensive
that SLRs that produce the same image quality. The image quality of
the best SLRs is not available at any price from a digital rangefinder.
Digital Backs for Medium-format and View Cameras
If you're not happy about paying Canon $2500 for a 24x36mm sensor
wrapped in a fancy electronic body
(Canon EOS 5D),
perhaps you'd prefer to pay $30,000 for a 48x36mm sensor, camera not
included. What do you get when you pay 10 times as much?
- a sensor that is double the size, offering superior low light performance
- the requirement that you buy lenses designed for medium-format
(120/220) film or this new digital format; these are much larger,
heavier, and more expensive than lenses designed for small-sensor and
film SLRs
- an additional f-stop or two of dynamic range and 4 extra bits (16
extra levels) of information per color; the typical digital back puts
out 16-bit RAWs
- higher resolution, enabling printing up to about 2x the linear dimensions
- the flexibility to attach the back to a variety of different
camera systems, all of which are much more cumbersome to use than a
Canon or Nikon body
To what kinds of cameras do these digital backs attach? View cameras and medium format SLRs.
A medium format SLR is easy to understand. It has all of the same
components as the familiar Canon and Nikon SLRs, but everything has
been scaled up. Hasselblad, Mamiya, and Rollei were the leading
manufacturers of these cameras in the film days. They usually
engineered the camera body and the film back in separate pieces. In
the studio you'd attach the Polaroid back to check the lighting with a
few test exposures ($3 each, kids!). Then you'd attach a back loaded
with slide film for a few photos. If you wanted to be 100 percent
sure that you had something usable, you'd attach a second back loaded
with more forgiving color negative film for insurance. Standard frame
sizes included 45x60mm, 60x60mm, and 60x70mm. When digital came
along, it was only natural that people would engineer a back to fit
the existing inventory of camera bodies and high quality lenses.
How well do these systems work? The most integrated is the
Hasselblad H3D. The manufacturer claims "the ease of use of the
best 35mm DSLRs", which means "not as fast or easy to use as a Canon
or Nikon, but the fundamentals of viewing and photographing will
probably work better."
What is the fundamental limitation with a medium-format SLR? The lens
and the film are fixed parallel to each other. We can remove that, at
the expense of ease of use, by attaching the digital back to a view
camera.
View cameras are the most flexible cameras, usually made from a basic
design that has not changed for over 100 years. A view camera is a
light-tight bellows with a bracket at one end for a lens and a bracket
at the other for the digital back. You compose and focus your image
on a groundglass, then displace the glass with a sheet of film or a
digital back. Adding to the challenge is the lack of a mirror or
prism, which means that you view the image upside down and reversed
left-to-right.
Why work this hard to take a photo? The flexibility of arbitrarily
positioning lens and sensor opens up a huge range of creative
opportunities that are unavailable to most photographers. For
example, if you want to take a photo of a building, the obvious thing
to do is point the camera up towards the center of the structure.
However, this results in projecting the vertical exterior of the
building onto the angled sensor surface. The lines of the building
will converge towards the top of the frame. A view camera allows you
to keep the camera level with the ground and either shift the
lens up or the digital back down. The sensor is now "looking up" at
the building through the lens, but the sensor is still parallel to the
building exterior so lines don't converge.
If you're taking a picture of rocks in a stream with a view camera,
you can achieve sharper focus by tilting the lens forward a
bit. This will get the Scheimpflug Rule working for you: the planes
of the subject, the lens, and the sensor should all intersect in a
line. You can achieve the same result by leaving the lens fixed and
tilting the sensor back a bit. This will improve the focus and also
increase the relative prominence of nearby rocks since they will be
stretched out onto the film.
A complete view camera system consists of the camera, a couple of
lenses, the digital back, a dark cloth, a focusing loupe, a tripod,
etc. Complete with protective case, it will weigh at least 30
lbs. and you will agree with the wise 8x10" photographer who said "If
it is more than 100 yards from the car, it's not photogenic."
Companies that make digital backs include Hasselblad, Leaf, MegaVision, Phase One, and Sinar.
Panoramic Cameras
Any camera can be a panoramic camera. You need only take a digital
file to a professional laboratory and say "make me a long skinny print
from a portion of the file". Or take a file to any lab and say "make
me a big print from this file". Once you get home, use a pair of
scissors to trim the big print until it is long and skinny and
contains the subject matter of interest. If your subject isn't moving
and you are equipped with a
tripod, you can improve the image quality dramatically by taking
four or five overlapping images and stitching them together into one
big long file.
If you want the maximum in convenience and image quality, there are
purpose-built wide-angle and 360-degree panoramic cameras that
incorporate a line of CCD sensors and a motor to scan the sensors
behind the lens. The folks at Roundshot will be happy to sell
you one for about $30,000. Keep in mind that, with a scanning back,
different portions of the image will be captured as much as a few
seconds apart. Files range in size up to 470 megapixels.
More
This chapter is meant to provide background, not purchasing advice. We
try to keep some current practical advice in the following articles:
