You may have heard that a 50mm lens "becomes" a 75mm lens on an APS-C camera. That is a useful shortcut, but it causes a lot of confusion. The lens does not change at all. What changes is how much of its image the sensor records. This idea is called crop factor. This lesson explains what crop factor really means, how to calculate equivalent focal lengths, how it affects depth of field, and what it does not change.
What is crop factor?
Crop factor is the ratio between the size of a sensor and the size of a full-frame sensor, measured across the diagonal. Full frame has a crop factor of 1. A smaller sensor has a larger crop factor: about 1.5× for most APS-C sensors, about 1.6× for Canon APS-C and 2× for Micro Four Thirds, as listed in sensor formats.
A smaller sensor records only the central part of the lens's image circle, which you met in how camera lenses work. The result looks like a cropped version of what a full-frame camera would record with the same lens, hence the name.
Equivalent focal length
To describe what field of view a lens gives on a smaller sensor in familiar full-frame terms, multiply the focal length by the crop factor:
| Lens | On APS-C (1.5×) | On Canon APS-C (1.6×) | On Micro Four Thirds (2×) |
|---|---|---|---|
| 16mm | 24mm | About 26mm | 32mm |
| 35mm | About 52mm | 56mm | 70mm |
| 50mm | 75mm | 80mm | 100mm |
| 200mm | 300mm | 320mm | 400mm |
This is what people mean by "a 50mm becomes 75mm". The lens is still 50mm; it simply gives the same field of view a 75mm lens would give on full frame. Phones describe their lenses this way, such as "24mm equivalent". Focal length and angle of view were covered in focal length and angle of view.
What crop factor does not change
| Changes with crop factor | Does not change |
|---|---|
| Field of view (how much you see) | The lens's actual focal length |
| Depth of field at the same framing | Exposure: the same f-number, shutter speed and ISO give the same brightness |
| Total light collected for the whole image | Perspective from the same position, as explained in perspective |
Warning:An f/2.8 lens is always f/2.8 for exposure. You do not need to change your settings because of the sensor size.
Crop factor and depth of field
To get the same framing on a smaller sensor, you use a shorter focal length (or stand further back). That gives deeper depth of field at the same f-number. To compare background blur across formats, photographers use an equivalent aperture: multiply the f-number by the crop factor.
| Setting on smaller sensor | Similar depth of field on full frame |
|---|---|
| 25mm f/1.4 on Micro Four Thirds (2×) | 50mm f/2.8 |
| 35mm f/1.8 on APS-C (1.5×) | About 52mm f/2.7 |
| 17mm f/1.2 on Micro Four Thirds | 34mm f/2.4 |
Equivalent aperture describes depth of field and total light, not exposure brightness. This explains why smaller sensors find very shallow depth of field harder, and why they are useful when you want more of the scene sharp. Depth of field was covered in shallow and deep depth of field.
Advantages of crop factor
- Telephoto reach: a 300mm lens on Micro Four Thirds frames like 600mm on full frame, in a smaller, lighter, cheaper lens.
- Deeper depth of field: useful for landscapes, macro and video.
- Smaller lenses: lenses designed for small sensors can be smaller.
Wide angles are harder: to get a very wide view on a small sensor, you need very short focal lengths, such as 7mm on Micro Four Thirds for a 14mm-equivalent view.
Common misunderstandings
- "Crop factor changes the focal length": it changes the field of view, not the lens.
- "An f/2.8 lens becomes f/4.2 on APS-C for exposure": no; equivalent aperture applies to depth of field and total light only.
- "Crop sensors magnify": they crop; the result looks magnified only when shown at the same size.
- Mixing up formats when comparing reviews: always check whether focal lengths are real or equivalent.
How professionals use crop factor
Professionals think in equivalent focal lengths when they switch between systems, so they know instantly what view a lens will give. Wildlife and sports photographers sometimes choose crop-sensor bodies for reach. Portrait photographers using smaller sensors choose faster lenses to keep background blur. They always remember that exposure settings stay the same, whatever the format.
Practical examples
Choosing a portrait lens for APS-C
- Situation
- You want the classic 85mm portrait view on your 1.5× APS-C camera.
- What to do
- You choose a 56mm lens.
- Why it works
- 56mm × 1.5 is about 84mm equivalent.
- Result
- A natural portrait framing like an 85mm on full frame.
A bird with a Micro Four Thirds camera
- Situation
- You need about 600mm of reach on full frame terms for small birds.
- What to do
- You use a 100–300mm lens at 300mm on Micro Four Thirds.
- Why it works
- 300mm × 2 gives a 600mm-equivalent field of view.
- Result
- Bird photos with long reach from a lens much smaller than a full-frame 600mm.
Key points
- Crop factor compares a sensor's size with full frame: about 1.5× APS-C, 1.6× Canon APS-C, 2× Micro Four Thirds.
- Equivalent focal length = focal length × crop factor; it describes field of view.
- The lens and exposure do not change: f/2.8 is always f/2.8 for brightness.
- Equivalent aperture (f-number × crop factor) compares depth of field at the same framing.
- Crop factor helps telephoto reach and depth of field but makes ultra-wide views harder.
Frequently asked questions
What is crop factor?
How do I calculate equivalent focal length?
Does crop factor change aperture?
Is crop factor an advantage?
Conclusion
Crop factor describes how a smaller sensor records a narrower part of a lens's image. Use it to calculate equivalent focal length and to compare depth of field, but remember that the lens and exposure never change. Next, you will learn what resolution really means for detail in your photos.
Finished reading? Track your progress through Sensors and RAW.