A. Aperture

Aperture
The opening of the lens which controls the amount of light entering the camera



F-Number, F-stop, Focal Ratio, Relative Aperture
Quantitative unit used to measure the opening of aperture. Changing the f-number means  changing the aperture. F-number is commonly notated using a hooked f i.e. f/N where N is the f-Number. As denoted in the formula for f-number, it has a inverse relation with aperture opening. A low f-number means larger opening and more light.

N=f/D
f= focal length
D= diameter of diaphragm

Stop
Stops are units used to quantify exposure. Changing one stop means doubling or halving the exposure. It is also known as exposure value (EV)

F-stop
F-stops are exposure values of aperture. Changing aperture by one f-stop means doubling or halving the light by doubling or halving the aperture opening.



Depth of Field
The distance between the nearest and the farthest point in a scene that is recorded completely in focus.



B. Shutter Speed
When you press camera shutter button, what happens is camera shutter opens up to allow light to hit the camera sensor. Shutter speed denotes the amount of time the camera shutter remains open when a photograph is clicked. Although it is named as speed, it is basically a measurement of time. To avoid the confusion of how aperture is measured in f-numbers and shutter speed in time, all the increment changes in aperture and shutter speed are denoted as stops.

Reciprocal Relationship
Shutter speed and aperture shares inverse relation when it comes to exposure. If we increase aperture by one stop, we have to decrease shutter speed by one stop to maintain same exposure.

Increase by one stop of aperture = Double the exposure = Reducing f-number by one stop

Increase by one stop of shutter speed= Double the exposure = Increasing the exposure time by one stop

C. ISO
A measure of the sensitivity of the sensor/film. High ISO means more sensitive sensor/film and less amount of light is needed to make the exposure.

Noise
Presence of grains creating a speckled effect, most of the times due to high ISO
 

D.  Metering
A photograph is made of light exposing a imagery on a surface. One of the basic factor in obtaining a good image is deciding the right amount of light required to make the image. Over the time various methods and instruments have been invented to determine the correct exposure.
 

Metering
In photography metering refers to the mechanism a camera adopts to determine what would be the right amount of light for the particular scene.

Values/ Tones/ Dynamic Range
Values are basically various levels of greys. If we follow Ansal Adams model, there are 11 different groups of greys, ranging from 100 % black to 100% white.

18% Grey
18% grey is the grey tone which reflect 18% of the light. It is considered as the average of the entire tonal range. All the modern meters are based on assuming 18% grey as the average grey.

Incident Light Meter
It meters the light falling on the subject to determine the correct exposure. It is set according to the 18% grey reflectance irrespective of the subject.

Reflected Light Meter
It meters the light reflected from the subject to determine the right exposure for a image. Most of the DSLRs use this to determine the exposure. It works good in diversity of tones. It doesn’t work that good when there are extreme tones or tones which belongs majorly to one extreme.

TTL Reflected Light Metering
It meters all the tones in the image and adjusts the exposure to bring the average to 18% grey. There are various ways for the camera to evaluate the scene and calculate the average.

Evaluative Metering
It takes into account all the pixels of the image and average them out to determine the exposure.

Centre-Weighed Average
It gives more weightage to the parts near the centre of the image in calculating the average.

Partial Metering
It takes into account a circle with centre at the mid of frame.

Spot-Metering
It only analyses the pixels around the centre to determine the exposure.
 

E. Histogram
It is the graph of various values of grey versus number of pixels. It shows the distribution of various grey values with respect to the number of pixels with that grey value.



Highlights/ Shadows/ Mid-tones
The dynamic range of the camera is commonly divided in three segments. Highlights are the white areas, shadows are the dark areas and mid-tones are the grays in between.



F.  Exposure compensation
Technique for adjusting exposure indicated by exposure meter. It is used in conditions when meters are unable to give right reading or we consciously want a underexposed or over exposed image.