Crop factor calculator

A 35 mm lens on a Super 35 cinema camera does not frame like the same lens on a full-frame stills camera. The lens has not changed. The sensor behind it is smaller, so it records a smaller rectangle from the middle of the image, and the view looks tighter, as if you had reached for a longer lens.

Crop factor puts a number on that. It compares a sensor with the 36 x 24 mm full frame of 35 mm stills photography, still the reference most lens talk uses.

How the crop factor calculator works

Four formulas, all measured against full frame:

• Crop factor = full-frame diagonal ÷ sensor diagonal. The full-frame diagonal is 43.27 mm.
• Equivalent focal length = focal length × crop factor: the full-frame lens that would give the same view.
• Equivalent aperture for depth of field = f-number × crop factor. Exposure does not change.
• Horizontal angle of view = 2 × arctan(sensor width ÷ (2 × focal length)), for an ordinary rectilinear lens focused at infinity.

The sensor menu uses common reference sizes, and real sensors differ from them by fractions of a millimetre: full frame 36 x 24 mm, Super 35 24.89 x 18.66 mm, APS-C 23.5 x 15.6 mm for most makers and 22.3 x 14.9 mm for Canon, Micro Four Thirds 17.3 x 13 mm, Super 16 12.52 x 7.41 mm and the 1-inch type at 13.2 x 8.8 mm. The preview draws the chosen sensor inside a full-frame outline.

Worked example: a 35 mm lens at f/2.8 on Super 35

The tool opens with Super 35 (24.89 x 18.66 mm), a 35 mm lens and an aperture of f/2.8. It reads:

• Crop factor: 1.39x
• Full-frame equivalent focal length: 48.7 mm
• Depth of field matches full frame at: f/3.9 (exposure stays f/2.8)
• Horizontal angle of view: 39.1°

The Super 35 diagonal is 31.11 mm, and 43.27 ÷ 31.11 = 1.39. Multiply the lens by that and a 35 mm frames like 48.7 mm on full frame, close to a 50. Multiply the aperture and f/2.8 gives the depth of field of f/3.9 on full frame. Exposure is still f/2.8, so the ISO and shutter speed you metered stay as they are. Across its width the sensor sees 39.1 degrees.

A 35 mm lens on each format

The chart under the tool repeats the sum for every sensor in the menu. Here it is for a 35 mm lens, with the f/2.8 depth-of-field and angle-of-view figures from the same formulas added:

Read across any row and the trade is plain. The smaller the sensor, the longer the lens looks and the deeper the focus at the same f-number.

Depth of field equivalence

Keep the framing and the f-number the same, and a smaller sensor gives deeper focus. To match a full-frame look, multiply the f-number by the crop factor. It works in both directions: f/2.8 on Super 35 already looks like f/3.9 on full frame, and getting the full-frame f/2.8 look on Micro Four Thirds takes f/1.4.

For documentary that is often a gift rather than a loss. An operator pulling their own focus on someone who moves, in a room nobody has lit, is helped by a little more depth of field. Shallow focus is a choice, not a mark of quality, and the interview guide shows how much of a good frame comes from the chair, the light and the background instead.

What the calculator cannot tell you

Crop factor uses the diagonal, so it is exact only between sensors of the same shape. The Super 35 reference size is close to 4:3. Measured across its width it crops 1.45x, and a 35 mm lens matches the width of about a 51 mm on full frame, a touch tighter than the 48.7 mm the diagonal suggests. For width, trust the angle-of-view line. If you will crop the frame for delivery, the aspect ratio calculator handles that part.

Many cameras use less than the whole sensor in some resolutions or at high frame rates, and the tool only knows the seven sizes in its menu; check your manual for the active area of the mode you shoot. Focal reducers and anamorphic lenses change the maths and are not modelled. The angle of view assumes focus at infinity. At close focus the view shifts, and some lenses breathe visibly as focus is pulled.

Questions

What is crop factor?

How many times smaller a sensor's diagonal is than the 36 x 24 mm full frame. Multiply a lens's focal length by it to find the full-frame lens that gives the same view.

Does a crop sensor change the focal length of a lens?

No. A 35 mm lens is 35 mm on any camera. The smaller sensor records less of the image, so the view matches a longer lens on full frame: about 48.7 mm on Super 35.

What is the crop factor of Super 35?

1.39x, using the 24.89 x 18.66 mm reference size. Super 35 sensors vary by maker and by recording mode, so check your camera's figures for precise work.

Does crop factor change exposure?

No. f/2.8 lets in the same light per square millimetre on any sensor, so it meters as f/2.8. Only the field of view and the depth of field change.

Are APS-C and Super 35 the same?

Close, but not identical. In the tool, APS-C from most makers is 23.5 x 15.6 mm with a 1.53x crop, and Super 35 is 24.89 x 18.66 mm with a 1.39x crop. Many video cameras sold as Super 35 use an APS-C sized sensor, so the maker's own dimensions are the ones to trust.