In photography, think of the light as rain (a shower of photons). We are interested in the amount of rain we collect (the exposure). This water volume (exposure) depends on the rate or intensity at which the rain falls (the light level or "scene luminance"), the size of our collection opening (aperture) & the length of time we collect it (the shutter speed). Also, a digital camera can electronically amplify the amount, after collection and conversion to an electronic signal, by increasing the ISO sensitivity (turning up the gain).
For a certain light level, exposure (EV or Exposure Value at ISO100) is composed of the combination of 2 parameters:
f-number: a relative measure of how big the aperture opening is inside a lens
Shutter Speed: how long the shutter is open
At a set light level, only these two affect the amount of light captured by the camera's sensor. However it is useful to consider ISO sensitivity too since we often operate away from ISO100:
ISO Sensitivity : how much you turn up the gain - like turning up the volume control on an amp: the music gets louder, but so does the noise/hum.
Note: the sensitivity of the sensor is fixed, so it doesn't change as you change the "ISO Sensitivity". It's the gain/amplification of the signal from the sensor that changes.
Manufacturers calibrate the metering in DSLRs to produce a certain rendered image brightness in response to a certain exposure level and a certain amount of signal gain. So, the main role of ISO Sensitivity is to adjust the brightness of the image. Obviously, if the exposure is low, (less photons collected during the exposure period), more gain will be required to render the outputted image with acceptable brightness.
EV is specified at ISO100, whereas the LV (Light Value) incorporates changes in ISO sensitivity. LV at ISO100 has the same value as EV. LV is the average scene luminance (the amount of light reflected off a subject). This is the "brightness" of the scene. Another term for it is Bv (Brightness Value).
It is easiest to think of changes to all 3 Light Value parameters in terms of Photographic Stops. Stops are a power-of-two (doubling/halving) relationship.
Sensitivity: ISO100 -> ISO200 (+1 stop increase in gain) -> ISO400 (+2 stops from ISO100) -> ISO800 (+3 stops) -> ISO1600 (+4 stops)
Shutter Speed: 1/125s -> 1/250s (-1 stop decrease in shutter duration) -> 1/500s (-2 stops) -> 1/1000s (-3 stops) -> 1/2000s (-4 stops)
F-number to double the area of the aperture, you increase the diameter by root-2 (approx. 1.4).
Consider an increasing root-2 sequence:
1 (root-2^0) -> 1.4 (root-2^1) -> 2 (root-2^2) -> 2.8 (root-2^3) -> 4 (root-2^4)
Each step represents the relative increase in diameter of 1.4 i.e. the doubling of the area of a circle. Now with f-numbers it's the ratio of focal length/entrance pupil diameter, not entrance pupil diameter/FL, so as the f-number increases, the aperture area decreases. For example, 1/2 (one-half) is bigger than 1/4 (one-quarter), just as ƒ/2 (aperture diameter is one-half FL) is a bigger iris area than ƒ/4 (aperture diameter is one-quarter FL).
It is easy to remember the full-stops numbering sequence by remembering just two values: 1 & 1.4. Consider the doubling of each:
1 2 4 8 16 32
1.4 2.8 5.6 11(rounded down) 22
Now combined: 1 1.4 2 2.8 4 5.6 8 11 16 22 32
With this understanding of f-numbers:
Aperture: ƒ/1 -> ƒ/1.4 (-1 stop decrease in aperture area) ->ƒ/2 (-2 stops) -> ƒ/2.8 (-3 stops) -> ƒ/4 (-4 stops)
Here is how varying the exposure parameters affects the output image brightness:
f-number:
Smaller aperture (larger f-number) means less light hitting the sensor, so you either have to increase shutter duration or increase sensitivity (or both) to compensate.
Bigger aperture (smaller f-number) means more light hitting the sensor, so you either have to decrease shutter duration or decrease sensitivity (or both) to compensate.
Shutter Speed:
Faster shutter speed means less light hitting the sensor, so you either have to open up the aperture (smaller f-number) or increase sensitivity (or both) to compensate.
Slower shutter speed means more light hitting the sensor, so you either have to close down the aperture (bigger f-number) or decrease sensitivity (or both) to compensate.
Sensitivity:
Bigger ISO = more gain, so the image brightness increases, as does the noise.
Lower ISO = less gain, so the image brightness decreases, as does the noise.
Now consider the following exposure settings:
1/500s, ƒ/5.6, ISO400.
Assuming the same LV, and keeping ISO constant for the moment, if we increase one parameter by 2 stops, we must decrease the other parameter by 2 stops to maintain the same exposure. The following settings produce the same exposure:
1/125s (+2 stops), ƒ/11 (-2 stops), ISO400
1/500s, ƒ/5.6, ISO400
1/2000s (-2 stops), ƒ/2.8 (+2 stops), ISO400
Now including a change in ISO as well, the following parameters produce the same output image brightness:
1/500s, ƒ/5.6, ISO400
1/1000s (-1 stop), ƒ/8 (-1 stop), ISO1600 (+2 stops)
I've written an LV calculator so you can use the exposure settings recorded in a image's Exif to roughly determine the light level at a scene. Knowing this will help you to learn which combinations of aperture, shutter speed & sensitivity will work in a particular situation e.g. at the beach or in a circus, so you can plan ahead for the next time.
Excel 2003 LV Calculator
Once you understand stops/LV/EV you can easily determine the amount an exposure changes as you make adjustments. For example, say you want to take a early-morning picture of a water fall or cascade and you want to get a "silky water" effect. This typically requires a shutter speed of 1s or longer. Say your current exposure settings are:
1/8s, ƒ/11, ISO100.
According to the LV calculator, the scene light level that matches these settings is approx. LV 10.
The required change from 1/8s -> 1s for "silky water" = +3 stops because
1/8s -> 1/4s (+1 stop) -> 1/2s (+2 stops) -> 1s (+3 stops)
To get a longer shutter speed there are 4 options:
1. Shoot earlier i.e. pre-dawn, when the LV is lower.
2. Close the aperture further (increase the f-number). A -3 stop reduction in light passing through the aperture is ƒ/11 -> ƒ/16 (-1 stop) -> ƒ/22 (-2 stops) -> ƒ/32 (-3 stops). Now there are two problems here: a) some lenses don't stop down this far
b) diffraction softening, the reduction in sharpness caused by light passing though a small hole, gradually rises on lenses as the aperture gets smaller and becomes more significant with openings smaller than about f/9.5 on APS-C cameras. By the time we stop down to f/32, we're dealing with a pinhole and diffraction softening is pronounced. On the other hand, with this type of scene, that softening may be acceptable.
3. Decrease the ISO. However many cameras have ISO100 as their lowest ISO so we usually can't go any lower here.
4. Use a Neutral Density Filter on the front of the lens. An ND8 filter = 3 stops light reduction because 2^3 = 8. The exposure setting with the ND8 filter mounted is then:
1/8s, ƒ/11, ISO100 (LV 10 approx.) - 3 stops (ND8 filter)
= 1s, ƒ/11, ISO100 (LV 7 approx.)
Please note that only the scene luminance, the f-number & the shutter speed determine the actual exposure i.e. the total number of photons falling on the sensor. Increasing ISO sensitivity is different. It does not increase the number of photons hitting the sensor. Instead, it amplifies the voltage after the photons have been converted to electrons by the sensor. Hence, boosting the ISO sensitivity increases the voltage fed into the Analogue-to-Digital Converter (ADC). This boosting also means that any noise that occurs before the ISO sensitivity programmable gain amplifier (PGA) stage, e.g. sensor read noise & photon noise, is also boosted.
Light itself is noisy, as individual photons arrive irregularly. (An irregularity in a supposedly steady signal can be expressed as the presence of a noise component.) At low light levels, photon noise (aka shot noise) is larger compared to the wanted signal, because photon noise rises at the square root of the number of photons captured by the sensor. The reason why a high exposure (more photons captured) is less noisy is that individual photon irregularity tends to be smoothed out as more photons arrive in the collection period.
For example, say only 100 photons (a low exposure level) are captured during the collection period by a sensor's photo-electric element ("sensel"). The square root of that is 10, so the photonic signal-to-noise ratio is 100:10 = 10:1.
Now say 40,000 photons are captured. (A high exposure level - a K-5 sensel can hold a max. of approx. 47,000 electrons.) The photonic SNR in this case is 40,000:200 = 200:1.
Amplifying the sensor output voltage in an attempt to make the digital image look brighter, brings up this photon "shot noise" too. So, in the example above, the low sensel exposure of 100 photons, amplified 400x (+8.6 stops) would still have a photonic SNR of 40,000:4,000 = 10:1.
See the examples of different numbers of photons per sensel here: File:Photon-noise.jpg - Wikipedia, the free encyclopedia
Think about this further. Consider 2 situations:
An average of 32,000 photo-electrons output per sensel, followed by a gain setting of ISO100
An average of 2,000 photon-electrons output per sensel, followed by a gain of ISO1600 (16x relative to ISO100)
Both should result in a similar output brightness level from the ADC. Say the camera has been calibrated so that this is the "Full Scale" (FS) level, i.e. we can't go any higher without clipping the ADC output. As ISO is increased, the number of captured photo-electrons is decreasing (i.e. a lower exposure). Hence the reason why we are increasing the ISO is to increase the brightness of the stored/rendered value in the raw or JPEG image file.
Now compare the shot noise in the two captures:
32000 : root(32000) = 179:1
2000 : root(20000) = 45:1
The reason that the high-ISO (ISO1600) example is noisier than the low-ISO (ISO100) example is not usually because of the use of 16x gain (particularly if the PGA has low noise and minimally degrades the amplified sensor output signal). Rather it is due to the smaller number of photo-electrons capable of being captured before the ADC output clips. As we've seen, a smaller number of photo-electrons means a worse SNR.
This irregularity in the nature of light means that even if a sensor, PGA and ADC all had no other sources of noise, a low exposure will always have more noise, after amplification, than a proper exposure. So the exposure rule is: get the best exposure possible for the required/acceptable depth-of-field & sharpness (the aperture setting) and the required/acceptable level of motion blur (shutter speed), and only increase ISO sensitivity when absolutely necessary.
Finally, the term "exposure". This term is used loosely and incorrectly by many people, probably due to reading books written in the age of film. In the digital age, exposure refers to the amount of photons captured by the sensor, not how brightly the image is rendered after capture. Users who fail to make this distinction consider ISO Sensitivity to be part of the "Exposure Triangle", a term popular in film-era books. But it's not.
The Exposure Triangle is: Scene Luminance; Shutter Speed; f-number.
Dan.
|
