Hi all,
I’m trying to better understand relative focal length. I shoot with a Nikon D500 (APS-C) and up until now have done mostly sports photography. I’m trying to get more into street photography but trying to understand what I’ll be shooting with based on the lenses that I currently own and I’m a little confused. Can you please confirm if my thinking here is correct.
Here are the lenses I currently own and (my assumption about the relative focal length with this crop sensor camera):
- Nikon AF-S 70-200mm f/2.8E FL ED VR (105-300mm)
- Nikon AF-S 50mm f/1.8G (75mm)
- Nikon AF-S 35mm f/1.8G DX (35mm since it’s a “DX” lens)
Are my assumptions correct? If so, do I not have a true 50mm focal length lens for this camera body? I was looking to shoot with a 50mm to start out my street photography journey.
Thank you for the help.
Focal length is a property of the lens, not the sensor. The 35mm on your camera will appear as a 52.5mm lens (35 x 1.5) would on a full frame camera.
The thing that makes it a crop lens is the smaller image circle projected out the back to cover a smaller crop sensor. It allows the whole design to be smaller in size and weight than a lens made for full frame. But it’s the same 35mm’s as any other 35mm lens whether designed for full frame, crop, 4/3rds, or medium format.
The 35mm lens will have a FFE of 52.5mm. Focal length doesn’t change.
What’s really happening is “focal length” is being used as a proxy for “angle of view”. So the 35mm lens on crop sensor has the same angle of view as a 50mm lens on full frame, which is about 40˚. People relate to 50mm more easily than 40˚.
It actually goes further: a 50mm lens on full frame is about the same as a 150mm lens on 4x5, a 210mm on 5x7 and a 300mm lens on 8x10. On each of those formats the angle of view is about the same so the picture will turn out with about the same framing.
There is no such thing as a crop sensor. Well, unless it is literally a sensor placed in a cornfield or something. Maybe you took a large sensor and cut it down. There you cropped it. If you bought a camera that is equipped with an apsc format sensor, it is just a sensor. The 35 millimeter is no more a crop of a medium format sensor than a phone sensor is a crop of a micro four thirds sensor.
For understanding crop sensors I like this diagram. Seeing it this simplified makes it very clear what the difference is. The amount of light coming in is always the same. The only difference is the size of the sensor that sits at the focal plane is smaller
Here is the basic equation explaining the relationship:
Focal length / Sensor width = Distance to subject / Width of field at subject
A normal lens for a camera is defined as one whose focal length equals (more or less) the width of the sensor. Often, the width of a sensor is measured from opposite corners. A focal length equal to the sensor width will give you a width of field—at the subject—equal to the distance to the subject. Doubling the focal length halves the width of view, doubling the sensor width doubles the width of view, etc.
