Back to DOF Basics ( arrgh)

windoze

I just cant grasp this concept and it makes me wanna throw the camera out the window.......

using a Canon 30D and 17-55 MM

envision a young girl or boy 5 feet away and trees back in the distance.

you might have a better way to do this but..this is what i want to know:

i want wide DOF from my subject 5 feet away to "infinity" or whatever....
if i preset my distance scale to 5 feet, turn off AF, and i use f/22, manually set exposure....when i compose my scene, do i still have to focus on the "young person" or can i recompose knowing that im preset anyway at 5 feet? Because i dont want the youn person in the middle but rather off to the side.



troy

pyrtek

You didn't mention what focal length you're shooting at. This is extremely
important, since the difference between 17mm and 55mm is huge when it
comes to depth of field. At 17mm if you set your distance to 5 feet and your
aperture to 22, you'll get everything from about 2 feet to infinity perfectly
sharp. On the other hand, at 55mm you're not ever going to get everything
from 5 feet away to infinity perfectly sharp. Check out the depth
of field calculator for more information.

windoze

pyrtek wrote:

You didn't mention what focal length you're shooting at. This is extremely
important, since the difference between 17mm and 55mm is huge when it
comes to depth of field. At 17mm if you set your distance to 5 feet and your
aperture to 22, you'll get everything from about 2 feet to infinity perfectly
sharp. On the other hand, at 55mm you're not ever going to get everything
from 5 feet away to infinity perfectly sharp. Check out the depth
of field calculator for more information.

using 17 mm,
ok, i understand what you said but does it matter where in my scene i "focus"?


troy

pyrtek

windoze wrote:

using 17 mm,
ok, i understand what you said but does it matter where in my scene i "focus"?

Yes, but not much if you're using 17mm at 5 feet and f22. Notice that at this
focal length with the distance to your subject being roughly 5 feet you're
guaranteed a sharp image from about 2 feet to infinity. That means the slight
difference in distance that you'll get by shifting your position is not going to be
relevant.

Antonio Correia

Troy.
Please follow my modest tutorial here.
Try. You will do what you want.

Don't use f/22 ! Stop at f/11:D
See the table !

OK. I used f/22 in that photo but don't. I was wronge because it causes diffraction.

Stu Engelman

Hi Windoze,

What you are trying to do really isn't that difficult, and hopefully these points will help. All points below assume use of full manual exposure, and your choice of manual or automatic focusing.

Getting good DOF from 5 feet to a far distance of infinity becomes more likely as you narrow your aperture and shorten your focal length. So, to begin with, try to use the widest zoom you can that is consistent with your desired composition. Then, set your aperture as narrow as possible, maybe at F18 or F20. Avoid the very smallest aperture (usually F22), as it can cause light that bends around the shutter window to fall on the camera sensor and cause noise (this is called diffraction). After doing the above, if you have bright sun, set ISO to 100 and meter for the coorect shutter speed. In darker scenes you might need to use an ISO of 200 or 400 (the idea is that you want the shutter to be fast enough to prevent momentary sensor overheating that causes chroma blotches to appear on your image; try to get your shutter speed at 1/60 or faster).

Getting things sharp from 5 feet to inifinity also requires proper focusing. Whether using manual or automatic focus, an ideal focus point is going to be about one-third between the near and far focal distances within the composition (closer to the near end). You could pre-focus there first, recompose, and then shoot (don't hold the shutter half way down first if using auto-focus, for obvious reasons). If your aperture is sufficiently small, and your zoom length sufficiently short, you may not have to pre-focus in this manner (assuming the central point of your composition provides a logical "middle of the range" focusing distance using the actual composition), since the range of clear focus may more than cover your composition.

Stu Engelman