General Lens Question

P

payal_choksi

Well-known member
hi all,

i have a very simple question to ask, so let me just jump straight to the question,

i want to know whether the focal plane changes when the lens is zoomed in or when it is wide? e.g 24-70 lens with f2.8 aperture when zoomed to 70mm @ f2.8 the focal plane is very narrow but when it is at wide 24mm f2.8 will the focal plane remain the same or will it increase keeping more area in focus?

in short will the focal plane remain constant when set at f2.8 irrespective of tele or wide?

kindly help,
 
I think you're saying focal plane but you mean depth of field. Taking that on board...

The simple answer is "no": depth of field will be much shallower on long focal lengths than on wide focal lengths. If you've got a FF camera with a 24-70 f/2.8 lens, and you're focusing at something that is 2m from you, at 24mm DoF will be 103cm, at 70mm it will be 11cm. Huge difference!

Now there are two elements in this answer:

* With the longer focal length, you're taking a much narrower shot of your subject. At 2m, a 24mm shot is a medium-to-wide shot (height of image at focal distance: 1.69m), whereas a 70mm shot is a head-and-shoulders shot (height of image at focal distance: 58cm). If you know what kind of shot you want (e.g. a head-and-shoulders shot) and will adjust distance to subject when you change focal length (so you'll be at 2m when using 70mm, but at only 68cm if using 24mm; both options give you a height of image at focal distance of 58cm) then depth of field is the same with the 24mm or the 70mm (it is 11cm in both cases).

* Perspective: even if the depth of field is the same 11cm when shooting with a 70mm at 2m or with a 24mm at 68cm, the background will look a lot more blurred in the 70mm shot, because a smaller piece of said background will be filling the image. The fact that it is stretched in this way makes it more difficult to recognize any features.

More here:
http://www.similaar.com/foto/dof/dof_notes.html
http://www.similaar.com/foto/tuten/320.html

And some DoF calculations here:
http://www.similaar.com/foto/dof/dof.php?f_x=1&f_r=1.77&f_mm=24&f_f=2.8&f_d=2
http://www.similaar.com/foto/dof/dof.php?f_x=1&f_r=1.77&f_mm=70&f_f=2.8&f_d=2
http://www.similaar.com/foto/dof/dof.php?f_x=1&f_r=1.77&f_mm=24&f_f=2.8&f_d=0.68
 
No it will not... The Canon zooms are not par focal and you will have to reset the focus after adjusting the focal length.
 
well what i actually meant was the area that remains in focus. when aperture is at 2.8 very thin area remains in focus, but when the aperture is at f8 more area is in focus.

so what i wanted to ask was does this area change when the aperture is at f2.8 at 24mm and 70mm? will the area that remains in focus increase keeping the aperture constant and changing the zoom from 70mm to 24mm?
 
You must log in or register to reply here.
Back
Top