(after last publication)
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.
to be continued………………
No comments:
Post a Comment