Cameras are nothing more than light-tight boxes and the technical side of photography is all about controlling the light entering the box. This is accomplished through two things: the hole that the light is entering the camera through, called the aperture, and the amount of time that the light is allowed to enter the camera which is controlled by the shutter.
In photography, each stop is the doubling or halving of the amount of light let into the camera.
The aperture is the hole that light enters the camera through. A fast lens is so called because it has a large aperture relative to its focal length meaning that it is able to let a large amount of light in allowing for a faster shutter speed.
Generally, a lens is considered fast if its maximum aperture is f/2.8 or greater. By greater, a smaller number is meant so f/2 is greater than f/2.8 which is greater than f/4. f/4 to f/2.8 is a one stop increase, so it is doubling the amount of light let into the camera. f/2.8 to f/2 is another doubling of the amount of light let into the camera. f/1.4 to f/2 is halving the amount of light let into the camera.
Typically, a lens does not perform its best at its maximum aperture. As the lens is stopped down, its contrast increases and maximum sharpness covers a greater area until it reaches between f/5.6 and f/8. As the aperture decreases, lenses lose sharpness due to diffraction. Diffraction is the loss of sharpness caused by light waves having to pass through too small of an aperture. Some lenses do actually perform their best at their widest aperture, but this is atypical.
Many zoom lenses have variable apertures. For example, an 18-55mm lens may have a maximum aperture of f/3.5 at 18mm and a maximum aperture of f/5.6 at 55mm.
The smallest apertures are usually pinholes.
The shutter speed controls the length of time that light will expose the digital sensor. Logically, doubling the shutter speed lets twice as much light in. So, a shutter speed of 1 second lets in half as much light as does 2 seconds and 1/1000th sec. lets in half as much light as 1/500th sec. If doubling the shutter speed then, to maintain the same exposure, one must decrease the aperture by one stop. Or, to maintain the same aperture, ISO could be decreased by half. Each time the shutter speed is doubled, it is increased by one stop and, each time it is halved, it is decreased by one stop.
Use the shutter speed calculator.
ISO works similar to shutter speed. It is a rating that explains how sensitive the sensor is to light. If using a digital camera, ISO can be changed between photographs. An ISO of 400 is twice as sensitive to light as an ISO of 200, so if one was to double the shutter speed then the ISO would have to be decreased by half in order to maintain the same exposure. As ISO increases, image quality decreases seen as an increase in noise/grain. There is no free lunch here.
ISO can be increased to allow the shutter speed to be increased thereby preventing blur in photographs.
Regardless of the claims of manufacturers, sensors have not become more sensitive to light for a long time. Image processors have just improved.
Depth of Field (Calculator)
Aperture is very important not just in controlling how much light enters the camera but also in controlling depth of field. Neither ISO nor shutter speed affect the depth of field. Depth of field is defined as the distance between the nearest and the furthest objects that give an image judged to be in focus in a camera . This is controlled by the aperture. The larger the aperture, the smaller the depth of field is. The smaller the aperture, the larger the depth of field is. As focal length increases, depth of field becomes smaller. Likewise, as the distance from the camera decreases, depth of field becomes smaller, so if the lens is focused on an object one foot away then the depth of field is smaller than if it was focused on an object one thousand feet away. At 1000 ft, depth of field is very large with all but the longest lenses (telephoto lenses). Focused very closely, like in macro photography, depth of field is very small.
Read more about depth of field in the Circles of Confusion article.
Most digital cameras have aperture priority, shutter priority and program exposure modes.
Aperture priority means the photographer chooses the aperture and the camera will match the shutter speed to make a perfect exposure. This is good to control depth of field but the photographer must be mindful that the chosen shutter speed does not require a tripod to prevent blurring by a handheld shot.
Shutter priority means the photographer chooses the shutter speed and the camera chooses the aperture but the photographer must be mindful of his depth of field.
Program mode is used to let the camera decide what is best based on a different program such as action or portrait.
Manual Exposure Mode
With the exception of point-and-shoot cameras, all cameras have a manual exposure mode which means the photographer chooses the shutter speed and aperture to expose the sensor. Also, Auto-ISO works with this exposure mode to make things like sports and action photography easy. In Auto-ISO mode, the camera will choose the appropriate ISO for the camera settings — shutter and aperture — that the photographer has chosen.
Digital cameras usually have several metering modes to choose from. The most common are spot metering, center-weighted metering and whole subject evaluative, a.k.a. matrix metering.
Matrix metering evaluates every tiny bit of the subject and tries to expose it all without washing out details on the highlights and shadows by choosing a middle ground. It's not perfect, though, and often misses the mark. This is why photographers bracket. Bracketed shots usually vary by Exposure Value (EV) but can vary by other things such as the metering mode.
Spot metering evaluates just one small spot on the image, and center-weighted evaluates all the image but is more concentrated on the center.
Another, newer, method of metering a subject is to perfectly expose just the highlights, a.k.a. highlight-weighted metering. This is because the underexposed shadows can be recovered without much picture degradation but the highlights cannot be recovered when they are overexposed.
Generally, lenses come in two varieties: Zoom and Prime. Prime lenses have just one focal length. Zoom lenses have an infinite number of focal lengths. Zoom lenses that change their focus when zooming are known as a varifocal zoom lens. Most consumer lenses are varifocal. True zoom lenses maintain their focus and are called parfocal zoom lenses.
Perspective is very important and simple to understand. Basically, perspective changes by increasing or decreasing the distance between the camera and the subject. It is important to understand that changing the focal length of the lens alone does not change the perspective. For portraiture, do not get close to the subject with a short focal length lens because, for example, the nose will be exaggerated. Have some distance between the photographer and subject, and use a long lens. It's not unusual to use a 300mm lens for a tight head shot.
A telephoto lens reduces the distance between the back of the lens and the image plane, or sensor, to a figure shorter than the focal length of the lens. This is known as an inverted telephoto design and is common of all telephoto lenses. Wide-angle lenses reverse this principle which means that they can be farther from the sensor (image plane) than their focal length would suggest. This makes room for the mirror in DSLR's and reduces vignetting. This wide-angle lens design is known as a retrofocus lens.
Wide angle lenses have a very long hyperfocal distance while telephoto lenses have a very short one.
Usually, prime lenses have greater contrast and let more light into the camera than zoom lenses. This is because zoom lenses have more glass/elements in them. This also means zooms have a lower t-stop rating than primes. Sharpness does not necessarily suffer because of this, but zooms do tend to have distortion. Distortion of 1% or greater is considered high.
Compared with older designs, flaring, or ghosting, is not really a problem given today's lens coatings. This includes washing out of dark areas or low contrast when shooting very bright subjects.
Focus or Lens Breathing
Some lens designs do something called breathing. This is when the focal length shortens as the lens is focused closely, or when the field of view widens as the lens is focused closely. This is common to zooms but not limited to them.
To determing if a lens exhibits breathing, install an extension tube and focus the lens to infinity. Then focus the lens without the extension tube to the same distance and see if there is a difference between the size of the subjects. This is really only a problem for the times when needing to focus the lens to its closest focusing point. For distant subjects, it should not matter. This also presents problems for cameras that can have their focus point on the edge of the frame because the subject can move out of the frame as the lens breaths.
Teleconverters take the center of the image and expand it to fit the whole frame. There is no magic here. Teleconverters cannot make a lens resolve more detail than it already does. If, after installing a teleconverter, there is more detail in the image then it is because the camera's sensor is not able to resolve all the detail that the lens has to offer.
Because digital camera sensors are sensitive to ultra-violet and infrared light, they have an ultra-violet/infrared filter built-in so there is no need to use an ultra-violet one on the lens unless for protection of the front lens element. There are clear filters that are made for this purpose.
Neutral Density (ND)
ND filters limit the light reaching the camera's sensor. They allow the photographer to leave the camera at a greater aperture with a slower shutter speed. This is useful for photographing waterfalls, for example, letting the water blur.
ND filters are gray and are not supposed to modify the colors in an image.
By understanding the decibel (dB), it's easy to understand the ratings of ND filters. A rating of ND0.3 is 3dB so it is a simple matter of multiplying by ten. ND0.6, ND0.9, ND1.8 and ND3.0 is 6dB, 9dB, 18dB and 30dB, respectively. Now, just divide the dB value by 3 to determine the number of stops. Every 3dB is a halving/doubling of the amount of light. In other words, a stop.
A polarizer is a type of filter. It lets light waves of a particular polarization pass through it while blocking lightwaves of other polarizations. A polarizing filter is usually placed in the front of a lens so as to manage reflections like on glass and water. It can also be adjusted to darken the sky. Notice that the polarized image below has less reflections on the leaves of the tree.
A polarizing filter blocks a lot of the light and can function as a neutral density filter, too, though with some modification of the light. Infact, variable neutral density filters are simply a linear and circular polarizer in one filter.
How the Camera Shutter Works
There are two curtains, top and bottom, that make up the shutter. At slower shutter speeds, the camera will open the top curtain and it will reach the top, exposing the sensor, before the bottom curtain begins to rise, closing the shutter and hiding the sensor from the image and light. At faster shutter speeds and before the top curtain has reached the top of the sensor, the bottom curtain begins to rise. This means the sensor is never exposed in totality at higher shutter speeds. At very high shutter speeds, like 1/8000th sec., the bottom curtain begins to close as soon as the top has begun to open. This results in a slither of an opening that moves from the bottom to the top of the sensor, and with moving subjects — like that that is in a moving car — the image will appear slanted instead of straight at these super high shutter speeds. The same thing happens in most mirrorless cameras with electronic shutter because the sensor is read one line at a time.
Sensor Crop Factor
The crop factor must be applied to the focal length AND aperture of the lens. When using a Nikon DX format camera (or other APS-C sensor camera), which has a crop factor of roughly 1.5x, with a 200mm/2.8 lens attached, the lens has the same reach as a 300mm lens, but the depth of field is still equivalent to that of a 200mm/2.8 lens, not a 300mm/2.8 lens. Do not forget to multiply the aperture by the crop factor. The 200mm/2.8 lens acts like a 300mm/4.2 lens. The lens' ability to gather light is unchanged with the addition of a crop.
Infrared and Full Spectrum
Infrared photography requires the use of an infrared filter either on the lens or installed on the sensor.
Full spectrum photography simply means there is no filter of any kind installed allowing all wavelengths of energy to be recorded. Typically, a full spectrum camera will have a filter installed on the lens to allowing recording only the infrared spectrum or UV spectrum. So called ghost hunters, or people that investigate haunted locations, use full spectrum cameras to see "ghosts".
Macro photography involves a macro lens or, preferably, a bellows.
When a subject or scene has detail in very bright parts and detail in very dark parts, a camera can not capture both with great success. Instead, the camera will try to find a middle ground losing detail in the bright areas and losing detail in the dark areas making them blotchy and void of the detail that the eye sees. An example is photographing a object with the sun behind it. The sky is very bright and the object much less so. The camera must choose a middle value losing detail in the object and losing detail in the sky so the object will be underexposed and the the sky, overexposed. This can be overcome by having the camera overexpose by one or more stops which will wash out the sky but make the subject properly exposed or by using a flash to fill in the subject's lighting. The alternative is high dynamic range (HDR) photography.
RAW images have no post-processing done to them. They can only vary by exposure and noise. If they vary by color/white balance then they are probably being adjusted by the post-processing software as they are opened because the white balance settings are saved in the RAW image.
An alternative to HDR photography is to shoot RAW and at a low ISO and then to increase the shadows in post processing. Expose for the highlights when doing this by using a highlight-weighted meter. If the camera does not have one of these meters then bracket the exposure. Post processing requires software such as Adobe Lightroom, Capture One Pro, AlienSkin Exposure X4, Skylum Luminar, or ON1 Photo RAW, just to name a few.
RAW images were used in the histogram article.
Diffraction and Sensor Size
Smaller sensors are more sensitive to diffraction than larger ones. This is why Ansel Adams shot his large format (8x10") camera at f/64. Diffraction begins to cause problems around f/5.6 for smaller sensors and f/8 for larger sensors, like full frame/35mm.
The formula to find the diagonal measure = √width² + height²
The aperture stop f/4 means the focal length (f) divided by four so a
f=50mm f/4 lens would have a maximum aperture diameter of 50/4 = 12.5mm, and a
f=50mm f/2 lens would have a maximum aperture diameter of 50/2 = 25mm. This can
be written several different ways and still have the same meaning; 50mm/2, f=50mm
1:2, f=50mm/2, 50mm f/2, etc. all have the same meaning. One aperture stop
is from f/2 to f/1.4 or f/2.8 to f/4, multiplied by
each stop decrease and divided by
each stop increase. Common aperture stops, or f-stops, are f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 and f/22 which are
√2 risen to the 1st, 2nd,
3rd, 4th, 5th, 6th, 7th, 8th and 9th powers, respectively. To find out half
or third stops, increase the power by 0.5 for a half-stop and 0.3333 for a third-stop. For example,
raised to the 1st power is f/1.4 and a half stop above that would be
raised to the power of 1.5 which is f/1.6817928305074292, or simply f/1.7. To be clear,
here are the exact f-stops:
√20 = f/1.0
√21 = f/1.4142135623730951
√22 = f/2.0
√23 = f/2.8284271247461907
√24 = f/4.0
√25 = f/5.656854249492382
√26 = f/8.0
√27 = f/11.313708498984766
√28 = f/16.0
√29 = f/22.627416997969533
√210 = f/32.0.
Now, use algebra to find out the difference between two apertures. For example, f/3.5 is 0.6147 stops slower than f/2.8:
Each greater aperture stop lets twice as much light into the camera so the shutter speed has to be halved for each increased aperture stop. Likewise, it has to be doubled for each decreased aperture stop. The technical reason for this is that the area of the aperture is doubled with each aperture stop increase and halved for each aperture stop decrease . The math for the area of the aperture -- a circle -- given the diameter is AREA = (π/4) × DIAMETER². So, (π/4) × (50mm/1.4)² = 1000mm², (π/4) × (50mm/2)² = 500mm², (π/4) × (50mm/2.8)² = 250mm², 125mm², 62mm², etc. But these numbers are rounded. To be precise and to demonstrate the accuracy in the math:
- f/1.4: (π/4) × (50mm/1.4142135623730951)²=
- f/2: (π/4) × (50mm/2)²=
- f/2.8: (π/4) × (50mm/2.8284271247461907)²=
- f/4: (π/4) × (50mm/4)²=
- f/5.6: (π/4) × (50mm/5.656854249492382)²=
- f/8: (π/4) × (50mm/8)²=
- f/11: (π/4) × (50mm/11.313708498984766)²=
- f/16: (π/4) × (50mm/16)²=
- And so on…
The math to figure out the number of stops to change shutter speed is: SHUTTERSPEED × 2stops.
1/1000 × 22 = 1/250
1/1000 × 20 = 1/1000
1/1000 × 2-2 = 1/4000
Now at +10 stops (such as when using a 10 stop ND filter):
1/125 × 210 = 8 sec.
Or, simply use the shutter speed calculator.