So a few weeks ago we were gathered at Rusty's place for a photo tech day and there were a few conversations about focal length and what relation it plays with the size of sensor that you have in your camera. Also I have seen these questions pop up from time to time on here as well as other places so I figured that we could take a moment to discuss.
I guess we could start this off by actually define focal length in a very easy to understand way. Focal length is the distance between the point of convergence inside the lens, and the sensor of the camera.
So what does that mean in the real world? First it controls how wide or narrow of a field of view that your camera sees. A longer focal length means that you will have a narrower field of view. A shorter focal length will give you a wider field of view.
The second biggest thing that focal length gives you is a different perspective. Along with perspective and certainly not to be forgotten is lens compression. At 24mm you can obviously see that the cans are separated by a certain distance but at 300mm that separation is virtually gone.
This is a very handy thing to know when shooting portraits, landscapes, or even motorbike travel shots. When shooting a portrait of someone with a large nose, do you want to make it appear smaller? When taking a photo of your bike near the sign of some National Park, to you want it to appear closer to the sign instead of off in the distance? If so, lens compression is your friend.
Finally, since focal length is a measurement between the convergence point on a lens and it's sensor, it also means that no matter what camera body you put a particular lens on, the focal length does not, and will not change. A 50mm lens is a 50mm lens no matter what you size sensor it is sitting in front of.
I know, there are lots of folks that will chime in to say that when they pull a 50mm lens off of their full frame camera and put in onto one with a smaller sensor the effect is that the lens is more zoomed in. While it may appear this way, it is just because the crop body has just that, a smaller sensor. It is not using the entire image that the lens is giving it, but it is most certainly not changing the "zoom" of the lens. It is more accurately changing the field of view, which is an all-together different thing.
So putting a 50mm lens on a smaller sensor does not in fact give you a 70mm lens. It merely gives you a 50mm lens with a decreased field of view. The very same thing can be accomplished by taking a full frame camera and simply cropping the image after the fact using any photo editing software that you choose. Remember, there is a reason they call them "crop bodies" and not "zoom bodies".
For more info on any of this stuff, here are some handy links:
https://en.wikipedia.org/wiki/Perspective_distortion_(photography)
http://www.photocrati.com/the-curious-case-of-lens-compression/
http://imaging.nikon.com/lineup/dslr/basics/19/01.htm
I guess we could start this off by actually define focal length in a very easy to understand way. Focal length is the distance between the point of convergence inside the lens, and the sensor of the camera.
So what does that mean in the real world? First it controls how wide or narrow of a field of view that your camera sees. A longer focal length means that you will have a narrower field of view. A shorter focal length will give you a wider field of view.
The second biggest thing that focal length gives you is a different perspective. Along with perspective and certainly not to be forgotten is lens compression. At 24mm you can obviously see that the cans are separated by a certain distance but at 300mm that separation is virtually gone.
This is a very handy thing to know when shooting portraits, landscapes, or even motorbike travel shots. When shooting a portrait of someone with a large nose, do you want to make it appear smaller? When taking a photo of your bike near the sign of some National Park, to you want it to appear closer to the sign instead of off in the distance? If so, lens compression is your friend.
Finally, since focal length is a measurement between the convergence point on a lens and it's sensor, it also means that no matter what camera body you put a particular lens on, the focal length does not, and will not change. A 50mm lens is a 50mm lens no matter what you size sensor it is sitting in front of.
I know, there are lots of folks that will chime in to say that when they pull a 50mm lens off of their full frame camera and put in onto one with a smaller sensor the effect is that the lens is more zoomed in. While it may appear this way, it is just because the crop body has just that, a smaller sensor. It is not using the entire image that the lens is giving it, but it is most certainly not changing the "zoom" of the lens. It is more accurately changing the field of view, which is an all-together different thing.
So putting a 50mm lens on a smaller sensor does not in fact give you a 70mm lens. It merely gives you a 50mm lens with a decreased field of view. The very same thing can be accomplished by taking a full frame camera and simply cropping the image after the fact using any photo editing software that you choose. Remember, there is a reason they call them "crop bodies" and not "zoom bodies".
For more info on any of this stuff, here are some handy links:
https://en.wikipedia.org/wiki/Perspective_distortion_(photography)
http://www.photocrati.com/the-curious-case-of-lens-compression/
http://imaging.nikon.com/lineup/dslr/basics/19/01.htm