The lens focal length of an optical system is the inverse of the system’s optical power; it measures how strongly the system converges or diverges light. A system with a positive focus length converges light, whereas one with a negative lens focal length diverges light.
Many people ask me, “Hey Ravi, what lens do you use?” And I respond by adding, “I use such and such a lens” (Example: 50mm f1.4 FX lens, 14-24mm FX lens, or a 70-300mm lens). It’s crucial to understand not just what lens I use, but also what camera body I use. It is the sensor size, not the type, that I use. The reason for this is that the output/image of a 50mm on a full-frame camera and a 50mm on a cropped sensor camera is not the same, despite the fact that the lens focal length is the same.
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What is focal length in photography?
When a lens is focussed at infinity, its lens focal length is calculated. The angle of view, how much of the scene will be captured, and the magnification, how large individual elements will be, are determined by the lens focal length. The smaller the angle of vision and the greater the magnification, the longer the lens focal length.
The lens focal length of your lens is the distance (in millimeters) between the point of convergence and the sensor or film that records the picture. Your film or digital camera lens’ focal length determines how much of the scene your camera can capture.
Lens Focal length size guide
There are a variety of lenses available, including Fisheye, Wide-angle, Telephoto, Prime, and others. The lens focal length indicates the user’s field of view and how wide the lens/camera may be extended to let light in. Let’s take a look at a 70-300mm f4.5-5.6 lens as an example. This lens has a zoom range that starts at 70mm and extends to 300mm.
At a lens focal length of 70mm, the maximum opening of the lens is f4.5, while at a lens focal length of 300mm, the maximum opening is f5.6. Let me offer you another example of a high-end lens with a 70-200mm f2.8 focal length. Regardless of the lens focal length, the maximum opening of the aperture stays at f2.8.
|Lens Focal Length||Type of Lens||What is it used for?|
|4mm – 14mm||Fisheye||Abstract, creative|
|14mm – 35mm||Wide-angle||Landscape, architecture|
|35mm – 85mm||Standard||Street, travel, portrait, wedding|
|85mm – 135mm||Short telephoto||Street, portraits, candid wedding|
|135mm+||Medium telephoto||Sports, wildlife, action|
|300mm+||Super telephoto||Sports, wildlife, astronomy|
What are mm and F in the lens?
When you see “mm” after a number or a pair of numbers on a lens, you’ll know that the number refers to the lens focal length (single number) or focal range (multiple numbers) (pair of numbers). When you look through the viewfinder of your camera, the focal length is what you see.
What does focal length affect?
The angle of vision is affected by the focal length. And your camera lens’s angle of view determines how much of the world it can see. A lens with a lower focal length, such as 24mm, can capture more of the scene than one with a longer focal length, such as 300mm. Lenses with shorter focal lengths capture a wider field of view.
Impact of sensor size
It’s very important to consider the sensor size when we speak about focal length. Check out my previous article, where I have written about sensor size – Which one to buy, a camera with a Full frame or an APSC sensor? If I consider my 50mm f1.4D on a full-frame FX body, the image output that I get is equivalent to the image output that I get using a lens with a 35mm focal length on a DX camera.
I captured some random landscape photographs with a full-frame camera and a 14-24mm lens in the images above. The image has been saved in raw format. Both were taken at the same moment with the same focal length of 24mm, the same aperture of f/9, the same shutter speed of 1/320 sec, and the same ISO 100. The sole difference is that the first image was taken in full-frame FX mode, while the second image was taken in DX mode with the identical camera. Even though there is no change in focal length while shooting in DX mode, the mountain and clouds in the backdrop are now zoomed in.
This is why some wildlife photographers use a DX body or DX mode on a full-frame camera to get that extra zoom in on the subject, even if they’re using a super zoom lens like 600mm with a teleconverter to obtain a very close-up view of the subject. Keep in mind that a telephoto lens may compress the backdrop as you move away from the subject.
Hope you find this article on lens focal length useful. Also, check out more on the gadgets that I use on my YouTube channel and support my work by subscribing. You may signup for my fortnightly newsletter by joining my Friend Zone.