How Cameras Work

Transcript

HOW CAMERAS WORK A CAMERA IS A LIGHT TIGHT BOX… Pinhole Principle All contemporary cameras have the same basic features • A light-tight box to hold the camera parts and recording material • A viewing system to allow you to compose images • A lens and mechanism to focus the image * • A way to store the image * And the lens isn’t really necessary EXPOSURE Exposure THREE THINGS DETERMINE EXPOSURE… ISO The lower the number the less sensitive to light. Select an ISO based on available light. Remember, a higher ISO can cause noise in your image. Film Speed/ISO With film stocks, the lower ISO rating also meant that the photosensitive grains of salt on the film acetate were very fine, thus producing a smoother, cleaner image. A higher ISO had larger, jagged grains of salt, thus producing “rougher” or grainier images. Film Speed/ISO Film Speed/ISO In digital photography, the same logic applies… the lower the ISO rating, the less sensitive the image sensor is to light and therefore the smoother the image, because there is less digital noise in the image. The higher the ISO rating (more sensitive) the harder the image sensor has to work to establish an effective image, which thereby produces more digital noise (those multicolored speckles in the shadows and in the midtones). Noise • Unwanted information (artifacts) that belongs to the process rather than the image • More likely at higher ISO’s or longer shutter speeds ISO/Noise Digital Noise Aperture The f-stop refers to the diaphragm in your camera lens that controls the amount of light that enters the camera. It is figured by dividing the focal length of the lens by the diaphragm opening. The smaller the number, the larger the opening or The bigger the number, the bigger the depth of field in your image. Aperture F number = N Focal Length = f Diameter = D Light Anything with a small hole can cast an image The smaller the hole, the smaller the overlapping circles and the sharper the image appears. But nothing is sharply focused The small circles cast by the small hole are called circles of confusion. A large aperture with large overlapping circles of confusion. A small aperture with small, overlapping circles of confusion. A focused lens will cast an image with a single sharp plane. An aperture, when used with a lens, makes things appear sharp in front of and behind the plane of critical focus Depth of field = Range of acceptable sharpness It’s always 1/3 in front and 2/3 behind the plane of critical focus. How aperture affects depth of field How distance affects depth of field How focal length affects depth of field Aperture TWICE as much light as F4 F2.8 lets in _______ F11 lets in _______ HALF as much light as F8 Shutter Speed Expressed in fractions of a second 1/x The higher the number the faster the shutter speed. Shutter Speeds 1/8 1/15 1/30 1/60 1/125 1/250 1/500 Fast – Medium – Slow Shutter Speeds HALF as much light as 1/30 1/60 lets in _______ TWICE as much light as 1/250 1/125 lets in _______ Equivalent Exposure Equivalent Exposure All three = exposure “Stops” & Equivalent Exposure Settings based on available light Film Speed 100 200 400 800 Less Sensitive 1600 3200 More Sensitive Aperture 2 2.8 4 5.6 8 11 More Light 16 22 Less Light Shutter Speed 30 More Light 60 125 250 500 1000 Less Light Histograms Good Exposure Over Exposed Under Exposed Histograms Histograms A graph that shows the distribution of tones or colors in an image. Histograms In digital imaging, black is 0 and white is 255 for 256 tones in each color channel. Camera Parts Data Panel Command Dial Shutter Release Hot Shoe Viewfinder Prism Mode Dial Sensor Shutter Mirror Cable Connections Body Inside: Lens – multiple elements Lens Parts Diaphragm- Circle of overlapping leaves to adjust the size of the aperture Focus Selector Focusing Ring Zoom Adjustment Digital Single Lens Reflex LENSES Lens Focal Length Lens Focal Length The effect of increasing focal length while keeping the same distance is an increase in magnification and a decrease in angle of view. 29mm from 1 foot away to 105mm from 12 feet away. Inside the lens – multiple elements Diaphragm- Circle of overlapping leaves to adjust the size of the aperture Zoom Adjustment Focusing Ring Lens Markings Lens speed isn’t the same as shutter speed. A “fast” lens has a very large aperture and can be used in lower light. Ø __ mm – Filter size Filters • UV filter – a good investment to protect your lens. Better to buy a new filter than to replace a scratched lens. • Polarizing filter – reduces glare on reflective surfaces, reduces haze and makes colors appear richer. Zone Focusing Some lenses include a focusing scale for “zone focusing.” You can focus your camera for a fixed distance and depending on your aperture, anything on that plane and 1/3 in front and 2/3 behind will have acceptable sharpness. Good for sports or if you need to be discrete in focusing your camera. Normal Lenses Most like regular human vision For 35mm film - 50 mm lens Larger camera formats have larger normal focal lengths A lens with a fixed focal length is called a “prime” lens. Telephoto Lenses Decreases angle of view, flattens the image and is usually heavy. A general rule of thumb for using a long lens: the slowest shutter speed for handholding the camera is approximately the same as the focal length – a 200 mm lens & 1/250 of a second Zoom Lenses Contain a range of focal lengths (i.e.18-55 mm) More expensive than fixed length lenses, but usually more practical Macro Lenses Designed for close-up photography Lens elements correct the aberrations that can happen in short focusing distances. There are combination Macro/Zoom lenses on market, though they don’t usually get as close as a dedicated macro lens. Wide Angle Lenses A short focal length Includes a lot of the scene Can distort perspective (serious foreshortening) Fish-eye Lenses Up to 180 ° field of view Exaggerate differences in size of objects Usually have a great depth of field Lens Markings 1. Lens Focal Length 2. Maximum Aperture 3. Manual/Auto Focus 4. Distance Scale 5. Filter Size Automatic Focus Auto-focus can be handy, but sometimes won’t cooperate. If your camera won’t focus on your subject, try centering the subject in the frame and pressing the shutter release half way. This should bring your subject into focus. Without releasing the shutter button, re-compose your image and fully depress the shutter. DIGITAL IMAGING TERMINOLOGY Image quality depends on 4 things film or digital • How much detail it contains – Resolution • The range of brightness it can record – Dynamic Range • How true the colors are – Bit Depth • Level of imperfections – Noise Resolution • The measure of detail; depends on the number of pixels in a given are of the image • Most often expressed per inch – Pixels per inch (ppi) – Lines per inch (lpi) – Dots per inch (dpi) Resolution in digital cameras • Optical Resolution – The measure of the actual information focused on the sensor by the lens • Interpolation – Digital manipulation to fill in the gaps between pixels to create more information – Sometimes good, sometimes not… – Digital Zoom is actually interpolation. (not good) Interpolation • The receptors on the CCD are individually filtered red blue and green in groups, then the missing information is supplied or interpolated from the nearby pixels. • Interpolation can help overcome all sorts of problems, but it is processed data (a guess) and not actual data. Dynamic Range • Dynamic range is the breadth of information from highlights to shadows • DMin – the brightest level at which a sensor or film can detect detail • DMax – the darkest level at which a sensor can detect detail Bit Depth I was told there’d be no math • The measure of how many different colors a pixel can display • 256 is usually sufficient and is used by most software • Following the mathematical expression for large numbers, bit depth is usually expressed as 2 raised to whatever power, 256=28 • Some scanners and cameras work at 210 (1,024) and 212 (4,096) ADDITIVE COLOR THEORY Gamut • The range of color a device can produce, or the range of color a color model can represent. The gray area is the entire range of possible colors. The colored area is the range from a CRT monitor. Each corner of the triangle represents a additive primary color. SENSOR FORMATS Color Filter Array - CFA • The sensor array is covered with a CFA, a mosaic of red, blue and green, with one color covering one sensor. • The most popular CFA pattern is the Bayer pattern What your sensor records… The final result… Charge Coupled Device CCD • Accumulates a charge proportionally to the light intensity • Can only record monochrome light. Complementary Metal-Oxide Semiconductor - CMOS • Works on the same principle as CCD, but is manufactured by the same process as computer memory chips • Each cell is a memory chip with a light sensor DIGITAL TERMINOLOGY Digital Components • Bit – short for binary digit, the basic unit of computing and has two states, on or off • Byte – eight bits, one byte has 256 (28) combinations • Megabyte – MB; approx. 1 million bytes (1,048,576) • Pixel – smallest unit of a digital image, short for picture element • Megapixel – approximately 1 million pixels Lossy vs. Lossless Lossless file formats allows you to recreate the file exactly as it was when you saved it. Lossy file formats compress the data to make the file smaller, but have to simplify some of the data to do this. Once the data has been compressed, it can’t be restored to it’s original state. File Formats • JPEG – compressed image file named from the International Standards Organization’s Joint Photographic Experts Group (lossy) • TIFF – tagged image file format; a file format for bitmapped images. It supports CMYK, grayscale and RGB files. (lossless) • PSD – The proprietary file format for Adobe Photoshop files. (lossless) File Formats • RAW – A camera file format that preserves the data capture and reduces in-camera interpolation. Interpolation happens during image editing. Each camera manufacturer has it’s own proprietary file designation. Nikon – NEF, Canon – CR2, Sony – ARW • XMP – Called a “sidecar” file, it is a text file that contains the adjustments made to a Camera RAW file. • DNG – Digital Negative – A cross platform RAW file format. You can convert your camera’s RAW file to DNG and then your are no longer dependent on your camera’s manufacturer for access to the file. It’s also smaller than most Camera RAW files. WHITE BALANCE Gamma Rays <10 picometers X-Rays 1 nanometer 400 nanometers 500 nanometers Ultraviolet Visible Spectrum Infrared 600 nanometers Microwaves 1 centimeter 1 meter Radio 700 nanometers Electromagnetic Spectrum Dusk to night 6500 ° K - on camera flash Open shade at noon Hazy to overcast Average clear day 10am – 3pm Morning & afternoon light 5500 ° K – average strobe system Daylight balanced film 3200 ° K – Photofloods Tungsten balanced film 2700 ° K 60 watt bulb Sunset 1800 ° K - candle Black-Body Radiation