Crop Factor and Focal Length Equivalence

Updated October 2026

How to find the lens that gives the same shot on a different sensor, why the width, height and diagonal crop factors disagree, and what equivalence does and does not carry over: perspective, depth of field and exposure.

12 min readIntermediate

Crop Factor
Focal Length Match
Multi-camera
Depth of Field

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What crop factor means

A crop factor is the ratio between the sizes of two recorded areas. Divide a focal length by it and you get the lens that frames the same way on the smaller format; multiply and you go the other way. It is usually quoted against full frame (36 × 24mm), but it works between any two cameras, which is how it is used on set: what lens on the B camera matches the 40mm on the A camera?

It works because the angle of view depends only on the ratio of sensor size to focal length (see the field of view guide). Scale both by the same factor and the angle does not change.

Equivalent Focal Length

f₂ = f₁ × d₂ ÷ d₁ | Crop factor = d₁ ÷ d₂

d is the dimension you match on, normally the recorded width. The result is exact for that dimension: the two frames have the same horizontal angle of view.

Where

f₁: Focal length on the original camera(mm)
d₁: Recorded width on the original camera(mm)
d₂: Recorded width on the other camera(mm)

Example: 85mm on full frame, matched on Micro Four Thirds (17.3mm wide)

Calculation:: 85 × 17.3 ÷ 36 = 40.8mm; crop factor 36 ÷ 17.3 = 2.08
Result:: A 40mm or 42mm on Micro Four Thirds gives the same horizontal view

The free focal length match calculator does this in your browser, with width presets for common formats.

Width, height or diagonal: three crop factors

When two formats have the same shape, their width, height and diagonal ratios are identical and there is only one crop factor. When the shapes differ, as with a 3:2 full-frame sensor against a 4:3 Micro Four Thirds sensor, each ratio gives a different answer.

Format	Size	Diagonal	Width crop	Height crop	Diagonal crop
Full frame	36 × 24mm	43.27mm	1.00	1.00	1.00
Super 35 (35mm full aperture)	24.89 × 18.66mm	31.11mm	1.45	1.29	1.39
Micro Four Thirds	17.3 × 13mm	21.64mm	2.08	1.85	2.00

Crop factors against full frame. Micro Four Thirds is the familiar 2× only on the diagonal.

Width is the convention for motion pictures. Shots are framed and cut on the horizontal: two cameras on the same scene must show the same amount of set from side to side. CineMath's Focal Length Match calculator matches on width.
Height is the one to use when the delivered frame is limited by height on one camera, for example when one camera crops a 16:9 frame from a taller sensor and the other records a wider aspect.
Diagonal is the stills convention, and the one behind marketing figures like "2× crop" and "1.5× crop". It splits the difference between width and height, so it is exact for neither.

Equivalent focal lengths across formats

Matched on width, these focal lengths give the same horizontal angle of view as the full-frame lens in the first column. Real lens sets come in fixed steps, so round to the nearest focal length you carry and accept a small difference, or adjust the camera distance to make it up.

Full frame (36mm wide)	Horizontal angle	Super 35 (24.89mm)	Micro Four Thirds (17.3mm)
18mm	90.0°	12.4mm	8.7mm
24mm	73.7°	16.6mm	11.5mm
35mm	54.4°	24.2mm	16.8mm
50mm	39.6°	34.6mm	24.0mm
85mm	23.9°	58.8mm	40.8mm
135mm	15.2°	93.3mm	64.9mm

Rounding costs less than it looks. Swap the exact 34.6mm for a 35mm on Super 35 and the frame is about 1% tighter; moving the camera back by the same 1% (about 4cm at 3m) restores it exactly.

Match the frame you deliver, not the sensor

What has to match is the picture that reaches the edit, so compare the areas each camera actually records and the extraction you will frame for. Three common cases:

Both cameras record the delivery aspect across their full width. Match on width and the frames match exactly, top to bottom too.
A 2.39:1 extraction from both. If both extractions use the full recorded width, match on width. If one camera is height-limited (a 4:3 or 6:5 mode for anamorphic, say), match the widths of the two extraction areas, not of the sensors.
One camera records open gate, the other 16:9. Width usually still matches, but the open gate camera has extra height above and below. Frame both for the delivery frame lines, then match on width.

How each recording mode changes the frame, including anamorphic squeeze, is set out in open gate vs 16:9 vs anamorphic.

What equivalence doesn't carry over

Perspective comes from distance

Matching the focal length matches the angle of view. Perspective, the relative size of near and far things, depends only on where the camera stands. Put both cameras at the same distance with equivalent lenses and the shots match in perspective as well as framing. A "longer" lens on a bigger sensor does not compress the background if it frames the same from the same spot.

Depth of field does change

At the same framing, distance and stop, a smaller format gives more depth of field, because its shorter focal length matters more than its smaller circle of confusion. To match depth of field too, divide the f-number by the same crop factor.

Equivalent Aperture for the Same Depth of Field

N₂ = N₁ × (d₂ ÷ d₁)

Holds when the circle of confusion scales with the format, as it does when both images are viewed at the same size. It matches depth of field and background blur, not exposure.

Where

N₁: F-number on the original camera(f-number)
d₂ ÷ d₁: Target width over original width: one over the crop factor

Example: 50mm at f/2.8 on full frame, focused at 3m, matched on Super 35

Calculation:: 2.8 × 24.89 ÷ 36 = f/1.94 on the 34.6mm
Result:: Both sharp from 2.73 to 3.33m. At f/2.8 the Super 35 camera holds 2.62 to 3.51m instead

In stops, the gap is 2 × log₂(crop factor), and how big it is depends on which Super 35 you mean. For the 24.89mm-wide film gate used in this guide it is about one stop (1.06 by width, 0.95 by diagonal); for a 27.99mm-wide digital Super 35 sensor such as the ALEXA 35 in open gate it is about two-thirds of a stop (0.73 by width, 0.70 by diagonal). So full frame to Super 35 costs between about ⅔ and 1 stop, and Micro Four Thirds is about two stops from full frame. The depth of field guide works through the near and far limits, and CineMath's Depth of Field calculator runs them. Its circle-of-confusion presets follow common published values (0.030mm for full frame, 0.025mm for Super 35), which are not in exact proportion to sensor size: with them, matching a full-frame shot on a 27.99mm Super 35 sensor takes about nine-tenths of a stop rather than two-thirds. The free depth of field calculator also takes a custom circle of confusion, so you can check each format at the stop you plan.

Exposure doesn't change

A T-stop sets image brightness on any format: T2.8 on Super 35 exposes exactly like T2.8 on full frame at the same ISO and shutter. The equivalent aperture above is only about depth of field. If you open up a stop on the smaller camera to match depth of field, take a stop out with ND or the shutter to hold exposure; the Exposure calculator handles that trade.

Worked example: matching a B camera

Case study

Full-frame A camera, Super 35 B camera

Two-camera dialogue
Intermediate
Worked example

Scenario

A dialogue scene is covered by two cameras side by side, both delivering 16:9 across their full recorded width. The A camera records the full 36mm width of a full-frame sensor; the B camera records 24.89mm across.

Challenge

The A camera holds a two-shot on a 40mm at T2.8, 3m from the actors. The B camera needs a matching two-shot from the same distance, with similar background softness, and the B camera kit is a set of Super 35 primes in 18, 25, 32, 40, 50 and 75mm.

Solution

Work out the exact matching focal length on width
Round to the nearest prime and check the difference in framing
Work out the stop that matches depth of field
Hold exposure with ND

The numbers

Exact match: 40 × 24.89 ÷ 36 = 27.7mm (crop factor 1.45)
A camera frame at 3m: 3 × 36 ÷ 40 = 2.70m wide. B on a 25mm: 2.99m; on a 32mm: 2.33m
On the 25mm, move the B camera in to 3 × 25 ÷ 27.7 = 2.71m for a 2.70m-wide frame
Matching depth of field: T2.8 × 24.89 ÷ 36 = T1.9, about one stop more open, so add ND 0.3 to hold exposure

Result

A 25mm at T1.9 with ND 0.3, about 30cm closer than the A camera, gives the B camera the same frame width, near enough the same depth of field, and the same exposure.

Key takeaways

Match on the recorded width of each camera, not the format name
Round to the lens you have and correct with distance
Moving 30cm closer changes perspective very slightly; at 3m it rarely shows in the cut
Equivalent aperture matches depth of field; ND keeps the exposure

Questions

What is the crop factor of Super 35?

It depends on which Super 35. The 35mm film gate, 24.89 × 18.66mm, is 1.45 against full frame by width and 1.39 by diagonal. A 27.99 × 19.22mm digital Super 35 sensor, such as the ALEXA 35 in open gate, is 1.29 by width and 1.27 by diagonal. Use the recorded width of the mode you shoot.

What lens on Super 35 matches a 50mm on full frame?

Multiply 50 by the Super 35 width and divide by 36. On a 24.89mm-wide Super 35 frame that is 34.6mm, so a 35mm; on a 27.99mm-wide ALEXA 35 open gate it is 38.9mm, so a 40mm is the closest common prime.

Does crop factor change the f-stop?

Not for exposure: T2.8 exposes the same on any format. It does change depth of field. To match depth of field at the same framing, divide the f-number by the crop factor: f/2.8 on full frame matches about f/1.9 on a 24.89mm-wide Super 35 frame.

Why is Micro Four Thirds called a 2× crop when 36 ÷ 17.3 is 2.08?

Because 2× is the diagonal crop factor: 43.27 ÷ 21.64 = 2.00. By width it is 2.08 and by height 1.85, since a 4:3 sensor has a different shape from a 3:2 full-frame one. For matching shots, use the width.

Does crop factor change perspective?

No. Perspective depends only on where the camera stands. Two cameras at the same distance with equivalent lenses give the same framing and the same perspective; a longer lens only compresses the background when the camera also moves back to keep the framing.

Focal Length Match in CineMath

CineMath's Focal Length Match calculator takes the original focal length, the original sensor width and the target sensor width, and returns the equivalent focal length and the crop factor between the two, with the formula and your numbers shown. Width presets cover 36, 24.89, 23.6, 22.3, 17.3 and 13.2mm, and you can type any recorded width from the sensor size chart.

It matches on width, as camera departments do. For a height or diagonal match, enter the heights or diagonals in the two width fields: the ratio works the same way. Then check the new frame in Field of View, which gives both angles and the coverage at your distance.

The free web version works the same way in your browser.