# An approximate lens formula: i/f = o/d

Use this formula to get an idea of what lenses will work in what space, what area of subject a lens will cover at closest focusing distance.

## The Lens Formula

Approximate lens field of view calculation:

`i / o = f / d`

where:

i = image size (mm)
o = object size (mm)
f = focal length (mm)
d = distance (mm)

## Reference Values

• Full Frame: 36mm x 24mm
• APS-C: 24mm x 16mm
• Micro Four Thirds: 17.3mm x 13mm
• Medium Format (e.g. Fujifilm GFX 50): 44mm x 33mm

## Example Calculations

• Nikon 135mm f/2.0 DC – size of subject at closest focal distance (1.1m): O[h] = 36 x 1100 / 135 = ~295mm
o[v] = 24 x 1100 / 135  = ~195mm i.e. at closest focal distance, the lens can achieve a tight head shot 30cm x 20cm.
• Nikon 300mm f/2.8 – what distance is needed for a full length fashion shot (~2m tall model): d = 300 x 2000 / 36 = ~16700 i.e. you need almost 17m.

## One thought on “An approximate lens formula: i/f = o/d”

1. […] in the meantime, I can use my approx. lens formula and some simple ratios to work out an approximate difference in distance from camera to subject […]