Focal Lengths

Davidaghaian

Hey everyone,

I'm continuing in my efforts to learn as much as possible about my Nikon D5100 and I had a really quick question. For portrait purposes, regardless of focal length, if both are set to the same aperture then you will be getting the exact same amount of light. What I was curious about is why would you use a 300mm f/2.8 versus say a 50mm f/2.8? Does one allow for more blurring of the background? I mean I could understand if flash was being used then being further away from your subject might help reduce the harshness of light, but if no flash is used then what is the advantage of one focal length versus the other? Thanks everyone!

Dr_M

Given the choice of 300mm versus 50mm, everything else being the same, the 50mm will give you the option of more blurring of the background. For macro pictures where you want maximum blurring of the background you would not use a 300mm versus a 50mm with everything else beeing the same. I am a novice too, but that is what I think from my readings in books.

GerardH

Actually, that's not correct. Without getting into the physics of it, a longer lens, in this case the 300mm, has a more limited depth-of-field than the 50mm lens. In other words, more of the foreground and background would be blurred with the longer lens. For example, if you were to open the 300mm's aperture all the way and focus on your subject's nose, the ears would start losing detail. Conversely, a shorter lens has a greater depth of field and will tend to keep more of everything in the frame in focus, with everything else being equal.

Davidaghaian

I see, so whereas my 50mm still does a superb job of blowing out the background, the 300mm would be even better at blowing out the background/foreground.