Glossary of terms – James Allan


What do all these numbers mean?  Why are the scales non-linear?


Glossary of Terms

I acknowledge Mark Pedlar who wrote an article, “Demystifying the numb3rs in photography”.  These notes are based on his definitions and are intended as a reference for beginner photographers..

Exposure –

Exposure relates to the amount of light falling on the film or sensor in the camera.  It is determined by the size of the aperture (f-stop) and the duration that the shutter is left open (Shutter speed).   It is like baking a cake.  The cake needs to be in the oven at the right temperature for the right duration.  The correct exposure will result in a satisfactory image.  Over exposed has had too much light and under exposed too little.

Focus and Focal length –

Focus occurs when the subject is sharply defined on the sensor.  The lens will focus a subject on the film or sensor when it is  placed at the correct distance away (i).  This distance is determined by the focal length of the lens (f) and the distance to the object (0).  This can be calculated using the formula 1/o + 1/i  = 1/f.  The focal length is a fairly constant characteristic of a lens that relates to how much it is curved and consequently how much it will bend rays of light.


ISO and ASA –

These numbers relate to the sensitivity of the film (ASA) or sensor (ISO) to light.  The higher the number the greater the sensitivity and less light is required to form an image.  The sequence of numbers is exponential.  50-100 – 200 – 400 – 800 – 1600 – 3200 – 6400 –  and so on.  Each step up the scale is 1 stop less light required and corresponds to a similar 1 stop jump in the f-stop or shutter speed.

NB There are often 3 sub divisions between the stops.  This creates an expanded scale as follows 50-64-80-100-125-160-200-250-320-400-500-640-800-1000-1250-1600-2000-2500-3200.  In this scale 1 stop is 3 steps up or down the scale.

F-stop –

The term ‘f stop’ refers to the size of the hole. It is the diameter of the hole as a fraction of the lens focal length. So, at F8 the diameter of the hole on your 200mm lens is 25mm (1/8 focal length).  However at the same f-stop (F8) the diameter of the hole on your 50mm lens is only 6.25mm (1/8 focal length).  The sequence of numbers is non linear.  2.8 -4 – 5.6 – 8 – 11 – 16 – 22 and so on.  Like with knitting needles the smaller sizes have bigger numbers.  Each step up the scale allows 1 stop less light into the camera and corresponds to a similar 1 stop jump in the ISO or shutter speed.


NB There are often 3 sub divisions between the stops.  This creates an expanded scale as follows 2.8 – 3.2 – 3.5 – 4 – 4.5 – 5 – 5.6 – 6.3 – 7.1 – 8 – 9 – 10 – 11 – 13 – 14 – 16 – 18 – 20 – 22 – 25 – 28.  In this scale 1 stop is 3 steps up or down the scale.

Shutter speed –

Shutter speed denotes how long the shutter is open; or how long the light is allowed to fall on the film / sensor. Again the numbers are fractions. So 125 is 1/125th of a second.   The sequence of numbers is exponential.  8sec – 4 sec – 2sec – 1 sec – 1/2 – 1/4 – 1/8 – 1/16 – 1/32 – 1/64 – 1/128 – 1/256 – 1/512 – 1/1024  and so on.  However there is a bit of short hand  so that 1/512 bcomes 1/500 and 1/1024 becomes 1/1000.  Each step up the scale is 1 stop less light being allowed to fall on the sensor and corresponds to a similar 1 stop jump in the f-stop or ISO.


The amount of light increases as you go to the left (open up) and decreases when you go to the right (stop down).  The change in aperture is equal and opposite to the change in shutter speed.   The final exposure (EV) has not changed.

Depth of field (DOF) –

Depth of field is the distance between the nearest and farthest objects in an image that appear acceptably sharp. In reality the lens can precisely focus only one distance at a time. The decrease in sharpness is gradual on each side of the focused point.  The DOF, refers to the area of un-sharpness on either side of the focal point that is imperceptible to the human eye.  In the picture below the DOF is just less than the width of one line of text.


Some lenses have markings that will estimate the DOF for you.  Look at the picture below.  There are 3 settings that will alter the lenses DOF.  Firstly the distance of the object from the camera .  As you focus closer you get less depth of field.  Secondly the aperture and thirdly the focal length (if using a zoom lens).  In the lens below the distance from the camera is determined as you turn the barrel to focus the lens.  You can see that the distance markings are not a linear scale.  The focal length is set by moving the barrel in and out to the desired focal length.  As the lens zooms out the lines (and DOF)  becomes less.  Finaly the aperture is determined by the colour of the line.  For an object 60 meters away at this focal length  and aperture (f3.5 = green line), the lens would have a DOF from 40 to 75 meters.


Exposure value (EV) and Stop –

EV (Exposure value) represents a combination of a camera’s shutter speed and f-number.  These steps are commonly referred to as stops.   An interval of 1 stop on the EV scale corresponds to a doubling of the amount of light entering the camera.  Two stops is 2 doublings (4x as much light) and 3 stops is 3 doublings (8x as much light).  All combinations that yield the same exposure have the same EV.  If you move up one stop in shutter speed and down one stop in aperture you have not changed the overall EV.  You can see from the following graph the combinations of f stop and shutter speed that give the same EV value. Just trace up or down the diagonal red line.


The area bound by the two green lines represents the favorable operating range for most dSLR cameras.