Focal length confusion

[davidjpope1]

I think I am getting a little confused about focal length of lenses on a crop sensor body.

I understand that a 50mm EF lens will have an effective focal length of 80mm on a crop sensor body like my 650d but what is the focal length of an EFS lens at 50mm on a crop sensor body? Is it still 80mm or is it 50mm because the lens is designed for a crop sensor?

Can somebody shed some light on this please!?

 

[PMN]

A 50mm EF-S lens is just the same as a 50mm EF lens in terms of what you see, the only difference is the size of the image the EF-S lens gives is smaller to suit the APS-C sensor, and in turn the lens itself can generally be more compact.

Both EF-S and EF lenses give the same field of view if they're the same focal length.

 

[PaulButler]

The focal length of a lens is always the same irrespective of what the sensor size may be. So a 50mm lens on a FF camera has the same focal length as a 50mm lens on a crop body. What changes is the effective field of view, on crop bodies the effective field of view is longer than on FF bodies.

So to answer your question

...
I understand that a 50mm EF lens will have an effective focal length of 80mm on a crop sensor body like my 650d but what is the focal length of an EFS lens at 50mm on a crop sensor body? Is it still 80mm or is it 50mm because the lens is designed for a crop sensor?
...

They would both have the same focal length of 50mm - the image produced on the crop sensor would have an effective field of view as an image taken with an 80mm lens on FF.

I'd upload some images to demonstrate but I am unable to do so just yet (perhaps 'cos I'm new here).

 

[maninsuitcase]

Yes a 50mm is still 50mm on a crop camera, but it's field of view will be similar to an 80mm on a FF camera, assuming a Canon, Nikon is more like 75mm. The image properties will still be that of the 50mm though.

The focal length of a lens is a physical property, this doesn't change depending on the sensor size. What does change with sensor size is the field of view. The image projected to the sensor will be the same (as the focal length is the same), difference being the smaller sensor only samples a smaller area of that image changing the apparent field of view for the same lens.

The difference between a crop (EF-S) and a full frame lens (EF) is the image circle. This is basically the size of sensor that lens can provide light to before vignetting happens. An EF lens will have a large image circle, but to save money for an EF-S lens they use less glass etc and only provide a smaller image circle.

But ultimately it's not really worth worrying too much about it.

 

[Krisstiffer]

Unless you have a full frame camera (or plan on getting one in the near future) then I'd suggest you forget what happens on one of those because it doesn't matter squat to you what happens on a type of camera you don't own.....

Just remember that on YOUR camera a 50mm EF lens gives exactly the same field of view as a 50mm EF-S lens. In the same way a 28mm EF lens will give you exactly the same FOV as a 28mm EF-S lens. And guess what - a 200mm EF lens will give you the same FOV as a 200mm EF-S lens. Familiarise yourself with the results different focal lengths give you on your camera and don't worry about all this equivalence stuff.

 

[davidjpope1]

YOUR camera a 50mm EF lens gives exactly the same field of view as a 50mm EF-S lens. In the same way a 28mm EF lens will give you exactly the same FOV as a 28mm EF-S lens.

Thank you, that answers my question.