Crop Factor Explained

Why does a 50 mm lens look like a 75 mm lens on one camera but not on another? The lens is the same, but the sensor behind it is smaller. A typical APS-C sensor covers only about 43 percent of the area of a full frame sensor. That smaller window crops the picture, and the crop factor tells you by how much.

What Is Crop Factor?

Crop factor is a number that compares your camera’s sensor to a full frame sensor. It equals the full frame diagonal divided by your sensor’s diagonal. A bigger number means a smaller sensor and a tighter view from any lens.

Full frame is the reference because it matches the classic 35 mm film frame of 36 by 24 mm. Its diagonal is 43.3 mm, so full frame has a crop factor of exactly 1. Every other format is measured against it.

Think of the lens as a projector that throws a round image onto the back of the camera. A full frame sensor catches a large rectangle of that image. A smaller sensor catches only the middle part, just like cropping a print with scissors.

The Crop Factor Formula

The core formula is short: crop factor equals 43.3 mm divided by the sensor diagonal. You find the diagonal with the Pythagorean theorem, using the sensor’s width and height. For an APS-C sensor of 23.6 by 15.6 mm, the diagonal is 28.3 mm.

Divide 43.3 by 28.3 and you get 1.53, which everyone rounds to 1.5. Two more formulas follow from that one number:

• Equivalent focal length = real focal length x crop factor.
• Equivalent aperture = real f-number x crop factor (for depth of field and total light only).

Notice that the table uses the diagonal, not the width. Sensors with different shapes, such as the squarer 4:3 Four Thirds format, still compare fairly this way.

A Worked Example on an APS-C Body

Say you mount a 50 mm f/1.8 lens on a 1.5x APS-C camera for a portrait. Check framing, background blur, and exposure.

Framing

Multiply 50 by 1.5 to get 75 mm. The shot frames like a 75 mm lens on full frame, a classic short portrait length. The horizontal angle of view shrinks from 39.6 degrees to 26.6 degrees.

Picture a wall 5 m away. Full frame with this lens covers a strip 3.6 m wide. The APS-C sensor covers only 2.36 m of that same wall. For angle math on any lens, the field of view calculator gives exact degrees.

Background Blur and Exposure

Multiply f/1.8 by 1.5 to get about f/2.7. At 2 m, this setup keeps about 11 cm in focus, the same as a 75 mm f/2.7 lens on full frame. Exposure does not change, so you still meter and shoot at f/1.8.

Does a Smaller Sensor Change Aperture and Depth of Field?

The f-number stays the same for exposure. For the same framing, though, a smaller sensor gives more depth of field and less total light. The equivalent aperture captures both effects.

Exposure Stays the Same

An f-number describes light per unit of sensor area. So f/2.8 at 1/125 s and ISO 100 gives the same brightness on every sensor size. Your meter and shutter speeds work exactly as before.

Depth of Field Grows

Compare two cameras framing the same subject at 3 m, both at f/2.8. A 50 mm lens on APS-C keeps about 39 cm sharp. A 75 mm lens on full frame keeps only about 25 cm sharp.

Stop the full frame lens down to f/4.2, which is 2.8 times 1.5. Its zone grows to about 38 cm, a near match. Our guide to how depth of field works covers the near and far limits behind these numbers.

Total Light Drops

An APS-C sensor has about 43 percent of the area of full frame, so it collects about 1.2 stops less total light. Micro Four Thirds collects about 2 stops less. That is why small sensors show more noise at the same ISO setting.

Where Crop Factor Math Goes Wrong

Most errors come from applying the crop factor to the wrong thing. These five show up again and again in forums and camera reviews.

• Believing the lens changes. A 50 mm lens is 50 mm on every body. Its focal length comes from its glass, not the sensor, so only the framing changes.
• Changing the exposure setting. Do not set f/2.7 because the math says so. Keep f/1.8 for exposure and use f/2.7 only to compare blur.
• Mixing up 1.5 and 1.6. A 200 mm lens frames like 300 mm at 1.5x but 320 mm at 1.6x. Check your camera’s exact sensor size.
• Treating reach as free detail. A crop sensor does not magnify anything. Cropping a 24 MP full frame image to the APS-C area leaves about 10.2 MP. Read about what image resolution really measures to see why pixel count matters here.
• Applying it twice. Phone specs often list focal lengths as full frame equivalents already. Multiplying those again gives nonsense numbers.

A good habit is to write both numbers down: the real value for settings, and the equivalent value for comparing systems.

Which Sensor Size Fits the Way You Shoot?

Choose a larger sensor for low light and shallow focus, and a smaller one for reach and compact size. No format wins at everything, so match it to the subjects you shoot most.

Wildlife and sports shooters often like crop sensors. A 300 mm lens on Micro Four Thirds frames like 600 mm, in a far smaller and lighter package. Landscape shooters enjoy the extra depth of field that small sensors give at wide apertures.

Portrait and night shooters lean toward full frame. It gives thinner focus and about 1.2 stops more total light than APS-C. Wide-angle fans also benefit, since an 18 mm lens on APS-C frames only like 27 mm.

To convert any lens and f-number in seconds, run it through our crop factor and equivalent focal length tool before you buy.

Common Questions About Crop Factor

References

This article is general photography education. Depth of field figures use a 0.029 mm circle of confusion for full frame, scaled by the crop factor for smaller sensors. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated September 27, 2026.