Focal Length
Written by Abin Alex and Anu Joseph.
Abstract
Focal length, measured in millimetres, is the distance from the lens to the sensor when the lens is focused at infinity. Together with sensor size it sets the angle of view: a short focal length shows more of the scene and a long one shows less. This guide covers angle of view, crop factor and equivalent focal length, the relation of focal length to perspective, the standard ranges, and the choice between prime and zoom lenses. It closes with the focal lengths suited to each kind of photography. It is written for photographers and cinematographers.
Keywords: focal length; angle of view; crop factor; perspective; prime lens
What is focal length?
Short answer. Focal length is the distance, in millimetres, from the lens’s optical reference point to the sensor when the lens is focused at infinity (Canon Europe, n.d.-a). It is not the length of the barrel. A longer focal length magnifies the subject more and shows less of the scene.
- Canon describes it as the distance between the focal plane and the lens’s nodal point when the lens is focused at infinity (Canon Europe, n.d.-a). A simple thin lens model uses the optical centre instead.
- The number printed on the lens, such as 50 mm or 24-70 mm, is the focal length. One number means a prime lens. A range means a zoom.
- Focal length is an optical distance, not a physical one. A telephoto design can be shorter than its focal length because its glass is arranged to bend the light in a compact way.
- The thin lens equation 1/f = 1/u + 1/v links the focal length f, the object distance u and the image distance v (Ray, 2002). For a distant subject u is very large, so v is almost equal to f.
- A 50 mm lens focused on a subject 1 metre away needs an image distance of 52.6 mm. The lens moves about 2.6 mm away from the sensor to focus.
Cham tipRead the focal length on the lens before you shoot. If you do not know your lens, you cannot predict your frame.
Cham taskLook at your lens and write down its focal length or range. Take one photo from the same spot at the shortest and the longest setting and compare them.
Check yourself: A 50 mm lens is focused on a subject 1 metre away. How far is the lens from the sensor?
About 52.6 mm. From 1/f = 1/u + 1/v, 1/v = 1/50 minus 1/1000, so v = 52.6 mm.
What is angle of view?
Short answer. Angle of view is how much of the scene the camera records, measured in degrees. A shorter focal length gives a wider angle and a longer one gives a narrower angle. The sensor size matters too, because a smaller sensor records a narrower angle from the same lens (Ray, 2002).
- The formula is α = 2 × arctan(d ÷ 2f). Here d is the sensor dimension and f is the focal length (Ray, 2002).
- Use the matching dimension. The width gives the horizontal angle, the height gives the vertical angle and the diagonal gives the diagonal angle. Makers usually quote the diagonal (Canon Europe, n.d.-a).
- A full frame sensor is 36 × 24 mm, so its diagonal is 43.3 mm. A 50 mm lens gives 2 × arctan(43.3 ÷ 100) = 46.8 degrees diagonally.
- Longer lenses see a narrower slice. At 200 mm the diagonal angle is only 12.3 degrees, and at 400 mm it is 6.2 degrees.
- Table 1 gives the angles for common focal lengths on full frame.
| Focal length | Horizontal | Vertical | Diagonal |
|---|---|---|---|
| 14 mm | 104.3° | 81.2° | 114.2° |
| 24 mm | 73.7° | 53.1° | 84.1° |
| 35 mm | 54.4° | 37.8° | 63.4° |
| 50 mm | 39.6° | 27.0° | 46.8° |
| 85 mm | 23.9° | 16.1° | 28.6° |
| 135 mm | 15.2° | 10.2° | 18.2° |
| 200 mm | 10.3° | 6.9° | 12.3° |
| 400 mm | 5.2° | 3.4° | 6.2° |
Table 1. Angle of view on a full frame sensor (36 × 24 mm), worked from the formula for a lens focused at infinity (Ray, 2002). Real lenses differ a little because of their design.
Cham tipShoot the same scene at the widest and the longest end of your zoom without moving. Notice what leaves the frame.
Cham taskUse your zoom lens from one spot. Take photos at the widest setting, the middle and the longest. Use Table 1 to write the angle of view you expect for each.
Check yourself: What is the diagonal angle of view of a 200 mm lens on full frame?
About 12.3 degrees. 2 × arctan(43.3 ÷ 400) = 12.3 degrees.
What is crop factor and how do I find the equivalent focal length?
Short answer. Crop factor compares a sensor with a 36 × 24 mm full frame sensor. A smaller sensor records only the centre of the lens’s image, so the view is narrower, as if the focal length were multiplied by the crop factor. The lens itself does not change. Canon’s APS-C crop factor is 1.6 (Canon Europe, n.d.-b) and Nikon’s DX crop factor is 1.5 (Nikon, 2024).
- A lens projects a round image circle. The sensor records a rectangle inside it. A smaller sensor records less of the circle (Canon Europe, n.d.-b).
- The focal length stays the same. A 50 mm lens is still a 50 mm lens on every camera.
- Crop factor is the diagonal of a full frame sensor divided by the diagonal of the camera’s sensor (Nikon, 2024). For Canon’s 22.2 × 14.8 mm sensor that is 43.3 ÷ 26.7 = 1.6.
- On Canon APS-C a 50 mm lens gives the view of an 80 mm lens on full frame (Canon Europe, n.d.-a). On Nikon DX it gives the view of a 75 mm lens (Nikon, 2024).
- Micro Four Thirds sensors are smaller again, with a crop factor of about 2, so a 25 mm lens gives the view of a 50 mm lens on full frame.
- The crop factor changes only the angle of view. It does not change the lens’s real focal length or its aperture. For how sensor size affects blur, see the Depth of Field guide.
| Sensor | Size | Crop factor | A 50 mm lens gives the view of |
|---|---|---|---|
| Full frame | 36 × 24 mm | 1.0 | 50 mm |
| Canon APS-C | 22.2 × 14.8 mm | 1.6 | 80 mm |
| Nikon DX | about 24 × 16 mm | 1.5 | 75 mm |
| Micro Four Thirds | about 17.3 × 13 mm | about 2 | about 100 mm |
Table 2. Common sensor sizes and crop factors (Canon Europe, n.d.-b) (Nikon, 2024).
Cham tipWhen you compare lenses across cameras, compare the angle of view, not only the millimetres.
Cham taskFind your camera’s sensor size. Divide 43.3 mm by its diagonal to get the crop factor, then work out the full frame equivalent of your lens.
Check yourself: What view does a 35 mm lens give on a Canon APS-C camera?
The view of a 56 mm lens on full frame, because 35 × 1.6 = 56 mm.
Does focal length change perspective?
Short answer. No. Perspective, the size relationship between near and far objects, depends on where the camera stands. Focal length only changes the crop. If you zoom from the same spot, the relationships stay the same. If you step back and zoom in, the background changes size relative to the subject.
- Perspective comes from the distance between the camera and the subject.
- Near the camera, the foreground looks much larger. Far from the camera, the difference in size between near and far shrinks.
- A longer lens from the same spot gives the same picture as cropping the wide shot.
- To change perspective, move. To change framing, zoom.
- A wide lens used very close can make a face look stretched. The cause is the closeness, not the lens.
- A long lens seems to compress depth because you stand far back to fit the subject in.
- So choose the viewpoint first and the lens second.
| What you do | Framing | Perspective |
|---|---|---|
| Zoom in from the same spot | Tighter | Unchanged |
| Step back and zoom in to keep the subject the same size | Subject same size | Background looks larger and closer |
| Step closer and use a wider lens to keep the subject the same size | Subject same size | Background looks smaller and farther |
Table 3. Zooming changes the crop. Moving changes the perspective.
Cham tipWalk around your subject before you choose a lens. Your feet decide perspective.
Cham taskPhotograph a friend with a 24 mm lens from close. Then step back and zoom to 85 mm until the face is the same size. Compare the faces and the backgrounds.
Check yourself: You zoom from 35 mm to 70 mm without moving. Does the perspective change?
No. Only the crop changes. Perspective changes only when the camera moves.
What are the standard focal length ranges?
Short answer. Lenses are grouped by how much they show. On full frame, fisheye is about 8 to 16 mm, ultra-wide is below 24 mm, wide is 24 to 35 mm, normal is 40 to 60 mm, telephoto is 70 to 300 mm and super-telephoto is above 300 mm. These are conventions, not hard limits.
- Makers draw the lines a little differently. Canon calls lenses up to about 35 mm wide, about 50 mm standard and about 85 mm and longer telephoto (Canon Europe, n.d.-a).
- A normal lens gives a view close to natural. On full frame it is about the sensor diagonal, 43 mm, so 40 to 60 mm looks normal. On APS-C it is about 35 mm and on Micro Four Thirds about 25 mm.
- A fisheye lens uses a different projection, and straight lines bow away from the centre. This is a design choice, not a fault.
- Macro lenses are described by magnification. 1:1 records the subject life-size on the sensor. Read the maximum reproduction ratio, not only the minimum focus distance.
- Tilt-shift lenses tilt the plane of focus and shift the lens to control framing and converging verticals. They need a larger image circle.
| Name | Focal length on full frame | Usual use |
|---|---|---|
| Fisheye | 8 to 16 mm | Special projection, creative effect |
| Ultra-wide | Below 24 mm | Interiors, architecture, big landscapes |
| Wide | 24 to 35 mm | Landscape, street, environment |
| Normal | 40 to 60 mm | Natural view, general work |
| Telephoto | 70 to 300 mm | Portraits, events, sport, wildlife |
| Super-telephoto | Above 300 mm | Distant wildlife, sport |
Table 4. Practical full frame ranges. Treat the edges as soft.
Cham tipLearn the names, but think in angle of view. A lens is wide or long only relative to the sensor behind it.
Cham taskList every lens you can use. Next to each, write its range name and the kind of job it suits.
Check yourself: Is an 85 mm lens on full frame wide, normal or telephoto?
Telephoto, a short one. It is in the 70 to 300 mm range and is a popular portrait length.
Should I use a prime lens or a zoom lens?
Short answer. A prime lens has one fixed focal length and a zoom lens covers a range. Primes are often smaller and have wider maximum apertures. Zooms let you change the frame without moving. Neither is better. Choose by the job, the light and whether you can move.
- A prime has one focal length. It is often compact, and it is often fast, with a maximum aperture such as f/1.4.
- A zoom changes focal length by moving groups of glass. Compensating groups keep the picture in focus as you zoom.
- The maximum aperture shows lens speed. A smaller f-number means a larger opening and more light. The Aperture guide explains the f-number.
- A marking such as 1:3.5-5.6 means a zoom with a variable maximum aperture: f/3.5 at the wide end and f/5.6 at the long end.
- Cinema lenses are marked in T-stops, which count the light actually transmitted. F-stops come from geometry.
- A teleconverter multiplies the focal length. A 1.4× converter loses one stop and a 2× converter loses two. It can also reduce sharpness and slow autofocus.
Cham tipIf you are new, learn on one prime such as 35 or 50 mm. It teaches you to move your feet.
Cham taskShoot for a day with one focal length only, either a prime or your zoom locked at one setting. Notice how you move.
Check yourself: A lens is marked 1:3.5-5.6. What does that tell you?
It is a zoom with a variable maximum aperture, f/3.5 at the wide end and f/5.6 at the long end.
Which focal length suits each kind of photography?
Short answer. Choose the viewpoint first, then the lens. As starting points on full frame: landscape 16 to 35 mm, architecture 16 to 24 mm or a shift lens, portraits 50 to 135 mm, events 24 to 70 mm plus 70 to 200 mm, wildlife 100 to 400 mm and longer, and macro with a dedicated macro lens.
- Landscape: 16 to 35 mm to show the whole scene.
- Architecture and interiors: 16 to 24 mm, or a shift lens to keep vertical lines straight.
- Portraits: 50, 85 or 135 mm, with some distance so faces look natural. For a group in a small room use 24 to 35 mm from as far back as you can.
- Events and weddings: 24 to 70 mm and 70 to 200 mm zooms, with 35 and 85 mm primes in low light.
- Sport: 70 to 200 mm for close action and 400 mm or more for field sports.
- Wildlife, birds and reptiles: 100 to 400 mm and longer, so you keep a safe distance from the animal.
- Insects and macro: a dedicated macro lens. A longer macro lens gives more working distance.
- Product: a normal to short telephoto lens avoids exaggerated shapes. Use a macro or tilt-shift lens for small products.
- Fashion: 70 to 135 mm for flattering compression, and 35 to 50 mm for wider environmental frames.
- Automotive: 24 to 70 mm for details and surroundings, and 70 to 200 mm for compressed and rolling shots.
- Photojournalism: 24 to 70 mm or a 35 mm prime, working close to the action.
- Cinematography: 24 to 35 mm for establishing the place, 40 to 60 mm for natural dialogue and longer lenses for close-ups.
- A longer lens also magnifies camera shake. A common rule of thumb is a shutter speed of at least 1 divided by the focal length for handheld shots, but stabilisation changes that.
Cham tipStart with distance, access and the picture you want. Then pick the millimetres.
Cham taskPick two kinds of photography from the list. Shoot each with the suggested lens, then with a very different one, and note which one gave the better picture.
Check yourself: You need to photograph a group in a small room. Which focal length and why?
A wide lens of 24 to 35 mm, used from as far back as the room allows. It fits everyone in and avoids stretching faces.
Questions beginners ask
What does 50 mm mean on a lens?
It is the focal length. On full frame a 50 mm lens shows about 47 degrees diagonally, which looks close to natural.
Does a longer focal length make the background blurrier?
It magnifies the background, so the blur looks stronger. How shallow the depth of field is also depends on the aperture and the distances. The Depth of Field guide explains this.
What focal length is best for portraits?
Many photographers use 85 mm on full frame. The 50 to 135 mm range works well when you have some distance from the subject.
Is crop factor the same as zoom?
No. The lens does not change. A smaller sensor records only the centre of the image, so the view is narrower.
Does a wide-angle lens distort faces?
Not by itself. Shooting very close creates the stretched look. From a normal distance a wide lens shows the scene wider without distorting faces.
References
- Canon Europe. (n.d.-a) Understanding focal length in photography. www.canon-europe.com/pro/infobank/understanding-focal-length/
- Canon Europe. (n.d.-b) APS-C vs full-frame. www.canon-europe.com/get-inspired/tips-and-techniques/aps-c-vs-full-frame/
- Nikon. (2024) What is crop factor and how do I calculate it? DX and FX explained. nikon.pt/pt_PT/learn-and-explore/magazine/tips-and-tricks/what-is-crop-factor-and-how-do-i-calculate-it-dx-and-fx-explained
- Ray, S. F. (2002) Applied Photographic Optics, 3rd edn. Oxford: Focal Press.
How to cite this guide. Alex, A., & Joseph, A. (2026). Focal length: A guide for photographers and cinematographers. Creative Hut Academy. https://creativehut.org/library/focal-length/
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