Photography & Imaging

Light and lenses: exposure, f-stops, depth of field, hyperfocal distance and image resolution

Photography & Imaging formulas connect the physics of light and optics with the practical choices photographers, students, and imaging professionals make every day. From f-stops, exposure value, and flash guide numbers to depth of field, angle of view, and image resolution, these equations help you predict how a lens will behave, set up shots accurately, and solve real-world exam or fieldwork problems with confidence.

F-Number

N = \frac{f}{D}

Aperture f-number is focal length divided by aperture diameter.

Exposure Value (EV)

NEWIntermediate

EV = \log_2\!\left(\frac{N^2}{t}\right)

Combines aperture and shutter speed into one exposure figure.

Depth of Field (approx.)

DoF \approx \frac{2 N c\, s^2}{f^2}

Range of distances that appear acceptably sharp.

Hyperfocal Distance

H = \frac{f^2}{N c} + f

Focus distance that keeps everything from H/2 to infinity sharp.

Lens Magnification

NEWIntermediate

M = \frac{f}{s - f}

Image magnification from focal length and subject distance.

Angle of View

NEWIntermediate

\alpha = 2\arctan\!\left(\frac{d}{2f}\right)

Field of view from sensor size and focal length.

Equivalent Focal Length

f_{eq} = f \times C

Focal length on a crop sensor expressed in full-frame terms.

Ground Sample Distance

GSD = \frac{H \cdot p}{f}

Real-world size of one pixel in aerial/drone imagery.

Megapixel Count

MP = \frac{W \times H}{10^6}

Total pixels of an image in millions.

Diffraction-Limited Spot

d = 2.44 \lambda N

Smallest resolvable spot size set by diffraction at aperture N.

Flash Guide Number

NEWIntermediate

GN = N \times d

Relates flash power, aperture and subject distance.

Exposure Reciprocity

NEWIntermediate

N^2 \propto \frac{ISO}{t}

Aperture, shutter and ISO trade off to keep exposure constant.

Thin Lens Equation

NEWIntermediate

\frac{1}{f} = \frac{1}{u} + \frac{1}{v}

Relates focal length to object and image distances.

Motion Blur Length

b = v \times t

Blur streak length from subject speed and shutter time.

Print Resolution (PPI)

PPI = \frac{W_{px}}{W_{in}}

Pixels per inch when a photo is printed at a given width.