Understanding Lens Focal Length

Ravindra Joisa Photography adjusting focal length

Many ask me this question – Hey Ravi, what lens do you use? And I reply to them saying – I use so and so lens (Example: 50mm f1.4 FX lens, 14-24mm FX lens or a 70-300mm lens). In addition to knowing what lens I use it’s important to know the camera body that I use as well. Not which model I use, but the sensor size. The reason is a 50mm on a full frame camera and a 50mm on a cropped sensor camera output/image is not the same even though the focal length remains the same.

There are different kinds of lenses available like the Fish eye, Wide angle, telephoto, prime etc. The focal length describes its user with the field of view and how wide it can be opened to let the light enter lens/camera. As an example let me consider a lens 70-300 f4.5-5.6. This lens has a zoom range starting at 70mm and can go all the way to 300mm. The aperture reading of f4.5 is the max opening of the lens at focal length of 70mm and f5.6 the maximum opening at focal length of 300mm. Let me give another example of an expensive lens which has a focal length of 70-200mm f2.8. Here the max opening of aperture remains the same at f2.8 irrespective of the focal length.focal length fx vs dx - ravindra joisa photography

Subscribe to Newsletter

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 Full frame or an APSC sensor? Cameras like the Nikon D610, D810, D4S, Canon 6D, 5D and 1D series etc, these are the full frame cameras with 35mm sensor size. Cameras such as Nikon D3300, D5500, D7200, Canon 70D, 7D etc have a cropped sensor with a crop factor of 1.5x/1.6x. So, if I consider my Nikon 50mm f1.4D on a full frame FX body – Nikon D600, the image output that I get is equivalent to the image output that I get using a lens with 35mm focal length on a DX camera.

In the above images, I have used Nikon D600 and a Nikon 14-24mm lens to capture some random landscape shots. The image is in raw format. Both were captured with the same focal length of 24mm, same aperture f/9, same shutter of 1/320 sec, same ISO 100 and at the same time. The only difference is, in the first image I have captured using Nikon D600’s full frame FX mode and the second image is using the same camera, but in DX mode. As you can see, the mountain and clouds in the background are now zoomed in even though there is no change in focal length when shot in DX mode.

This the main reason why some wildlife use a DX body or use DX mode in their full frame body to get that extra zoom in to the subject even though they use a super zoom lens like 600mm and a teleconverter, so that they can get a very close-up shot of the subject. Also, note that a telephoto lens can compress the background from the subject as you move away from the subject. If you are still not very clear about what I had explained then checkout the video below to understand it even better. Don’t forget to follow me on social media to get more updates of this kind.