depth of Field & the media?

Chile Chef

Now that I finally know what depth of field is, I can start using the correct lens,

However since I've started going to college I'm finally learning why you need a big L lens and a small lens such as a 28mm, but I don't get some of the media people and their big fancy l lens, when the depth of field can be lost.

I can understand using a L lens if your subject is farther away then you anticipated but your not going to get the characteristic's of that subject in your shot and it would be more of a snap shot.

So after that being said, I'll be carrying around 3 lens only a white 70-200 4.0 USM lens, a kit 35-80mm lens, And a tameron 17-55mm Lens.


The rest of my lens can stay home and sit in their bag.




Anyways can you get a shallow depth of field with a 70-200 4.0 lens?


And why do they call the lens 4.0 for the aparture? I know thats the distence that light needs to travel threw the lens to the prisum in the camera.

Candid Arts

Chile Chef wrote:

And why do they call the lens 4.0 for the aparture? I know thats the distence that light needs to travel threw the lens to the prisum in the camera.

They give the lens a 4.0 rating for it's aperture because of the max size of the opening in relation to the focal length. Even with a fixed aperture lens, as the focal length changes the physical size of the aperture will change, but at the same ratio to give it a f/4.

EX: 70mm lens with a f/4 has an opening size of 17.5mm where as a 200mm lens with a f/4 has an opening size of 50mm. So as you can see, even though the aperture stays constant at f/4, while the focal length changes, so does the size of the opening. f/4 is not a set size, more of a ratio of focal length to size of opening. In case you're wondering what the formula is, it's focal length(m) / aperture(f) = size of opening(o), so m/f=o.

Chile Chef

Candid Arts wrote:

They give the lens a 4.0 rating for it's aperture because of the max size of the opening in relation to the focal length. Even with a fixed aperture lens, as the focal length changes the physical size of the aperture will change, but at the same ratio to give it a f/4.

EX: 70mm lens with a f/4 has an opening size of 17.5mm where as a 200mm lens with a f/4 has an opening size of 50mm. So as you can see, even though the aperture stays constant at f/4, while the focal length changes, so does the size of the opening. f/4 is not a set size, more of a ratio of focal length to size of opening. In case you're wondering what the formula is, it's focal length(m) / aperture(f) = size of opening(o), so m/f=o.

Thank you very much Candid, That explains a lot. I've asked my teacher and she gave me a short answer, She said shes not a gear freak but more of what she uses when its handy.