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.

[…] 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 […]