Photography glossary

Definitions

Depth of field

Depth of field (DoF) is the slab of space that still looks sharp on a given print or screen. It is not a hard wall in the scene — it is defined by a circle of confusion. Longer glass, wider apertures and closer focus all shrink DoF. Hyperfocal focus pushes the far limit to infinity so the near limit is as close as that lens and f-stop allow. Confirm on a test shot at the intended viewing size; pixel-peeping a 45 MP file is harsher than an 8×10 print.

Hyperfocal distance

Hyperfocal distance H is the closest you can focus and still hold infinity inside the circle of confusion: H ≈ f² / (N c) + f. Focus there and everything from roughly H/2 to infinity is “sharp enough” for that CoC. Landscape shooters use it to keep a foreground rock and a distant ridge both usable. It is a print-size decision, not a property of the lens alone. Recalculate if you crop hard or hang a huge print.

Circle of confusion

The circle of confusion (CoC) is how large a point source may smear on the sensor before you call it unsharp. A working stills default is diagonal / 1500 (about 0.030 mm on full frame, 0.019 mm on APS-C). Tighter divisors (1730, 2000) match pixel-peeping or big prints; looser ones match small web JPEGs. Every DoF and hyperfocal number is only as honest as this CoC. Pick the viewing, then pick the circle.

Crop factor

Crop factor compares your sensor diagonal to a 36×24 mm frame (diagonal ≈ 43.3 mm). APS-C is typically 1.5× or 1.6×, Micro Four Thirds 2×, many 1-inch compacts ~2.7×. A 50 mm on APS-C frames like a 75 mm on full frame. Depth of field and equivalent aperture also shift with crop if you match field of view and f-number. Crop factor does not magically add reach — it only describes the smaller window on the same optic.

Exposure value

Exposure value compresses aperture and shutter into one stop-scaled number. At ISO 100, EV = log₂(N² / t): f/16 and 1/125 s is about EV 15, classic Sunny 16. EV at working ISO subtracts log₂(ISO/100), so the same light at ISO 400 reads two stops lower. Use EV to compare scenes, ND stacks and flash fill — not as a substitute for a meter reading on a job that cannot miss.

ND filter

A neutral-density (ND) filter cuts light without intending to shift colour. Optical density 0.3 is one stop; ND8 / 0.9 is three stops; ND1000 / 3.0 is ten. New time = base time × 2^stops. Long-exposure water and bright-sun video at 180° shutter both live here. Cheap filters leak colour and sharpness; stack polarisers only if you accept the extra stops and the risk of cross-polarisation. Confirm on a test frame — meters behind heavy ND lie.

NPF rule

The NPF rule (Nénette / Pixel / Focal) estimates max untracked seconds as accuracy × (35N + 30p) / f, with N the f-number, p pixel pitch in µm and f the focal length in mm. Accuracy 1 is pixel-sharp; 2 allows a little trailing. It beats the 500 rule on dense modern sensors. Declination, tracking error and aggressive sharpening still move the goalposts. Use it as a starting shutter, then inspect a 100 % crop of a test star.

Shutter angle

Shutter angle is the cinema way to talk about how long each frame is open. Time in seconds = angle / (360 × frames per second). A 180° shutter at 24 fps is 1/48 s; at 60 fps it is 1/120 s. Smaller angles look staccato; larger angles smear. It is a pacing and blur choice, not a stills EV substitute. On a rolling-shutter sensor, angle does not fix jello — it only sets exposure time.

Inverse-square law

For a compact source in free space, illuminance scales as 1/d². Move a bare speedlight from 1 m to 2 m and you lose two stops; 1 m to 3 m is a bit over three. Guide numbers assume this falloff on-axis with no bounce. Softboxes, walls and the sun (effectively at infinity) break the simple square. Use it to pre-visualize flash distance, then meter or chimping a test shot before the client walks in.

Guide number

A guide number is the manufacturer’s shortcut for flash reach: GN (metres, ISO 100) ≈ f-number × subject distance. GN 32 at f/8 is 4 m; at ISO 400 the same head behaves like GN 64. Bounce, zoom head, modifier and TTL ceiling all change the real number. It is a bare-bulb, on-axis estimate, not a studio plot. Never point a laser meter or high-power strobe at eyes to “check” GN.

Diffraction

Airy-disk diameter on the sensor is about 2.44 λ N (λ in the same units as the diameter, N the f-number). At 550 nm, f/16 already throws a ~21 µm disk — larger than many pixel pitches and often larger than a 0.030 mm CoC. Stopping down for DoF is real; so is the mush. The “diffraction limit” is a comparison, not a brick wall. Check a test chart at the print size you actually deliver.

Magnification

Reproduction ratio (magnification) m = image height on the sensor / subject height. True 1:1 macro is m = 1: a 20 mm insect fills 20 mm on the chip. Effective f-number grows with m, and DoF shrinks roughly with 1/m², which is why f/11 at 1:1 is still a sliver. Pupil factor (exit/entrance pupil) tilts front vs rear DoF on retrofocus glass. Focus stacking exists because the formula is cruel, not because you metered wrong.