question of DOF

[kitkatdubs]

so i took this photo of my daughter today- just quickly snapped it as practice b/c I'm trying to learn about DOF. my question is, how come the background (the trees) aren't more out of focus? She's standing really far from them and i shot this photo at a f/3.5 . what should i be shooting at to have the trees be more out of focus?

 

[snowbear]

Depth of field is dependent on not only the aperture, but also the magnification, the distance to the subject (from the camera, the focal length and the size of the film or sensor. Wide angle lenses, having less magnification, will have a deeper DOF than a longer focal length all other things being equal.

Have a look at this DOF calculator to get an idea of what happens (without getting into the math).

 

[kitkatdubs]

ok so i was using my 24mm lens. would it be better to use my 50mm lens?

 

[snowbear]

ok so i was using my 24mm lens. would it be better to use my 50mm lens?

Maybe. On a cropped sensor (I think the 50D is one), I understand you probably want something between 50mm and 80mm for a portrait (I don't shoot portraits). I would shoot both and compare them.

Something else, though it's not really DOF - get closer to your daughter(?), and try portrait mode (turn the camera on it's side). It will also make a better photo if you kneel down so you are at eye level with her, and try to leave a little more space between her feet and the bottom of the frame.

 

[FITBMX]

I have a crop sensor, and shot portrait type photos with a old 135mm, and it does a great job on DOF.
Anything less than 80mm and you're not going to get the background out of focus much.

P.S.
You have a very cute little girl.

 

[jake337]

I have a crop sensor, and shot portrait type photos with a old 135mm, and it does a great job on DOF.
Anything less than 80mm and you're not going to get the background out of focus much.

P.S.
You have a very cute little girl.

You can throw the background out of focus with much shorter focal lengths than 80mm

 

[FITBMX]

You can throw the background out of focus with much shorter focal lengths than 80mm

I met that's what I like, but I did type that wrong! LOL
Thanks for correcting me, I don't want to be confusing.

 

[Derrel]

how come the background (the trees) aren't more out of focus? She's standing really far from them and i shot this photo at a f/3.5 . what should i be shooting at to have the trees be more out of focus?

The reason the background is not more out of focus is that the focal length of the lens is relatively short, and the camera that made the photo is a relatively small format camera, and the image is what one would call a low-magnification image. So, there are three factors at work. The shorter the lens focal length, and the smaller the camera's film or sensor size, and the lower the magnification, the more depth of field there is.

Assuming an APS-C sensor size d-slr, that size of sensor has pretty deep depth of field compared against say, a 4x5 sheet film camera. A smart phone sensor is so small that it offers tremendously deep depth of field, even when its tiny little 3 to 4mm length lens is wide-open at f/2.8! With your camera, and a lens foal length that is as short as the one you used, when the camera is set up so that it frames a small girl full-length in the foreground and the horizontal orientation shows a 65-foot-wide background area, the "magnification" is pretty low...because 1) the lens is short, and 2)the subject is not really close, but is at a moderate distance. The shot of your daughter is a low-magnifcation, wide-area, short focal length, semi wide-angle type photo. It is very difficult to get shallow depth of field looks with your camera and an aperture of f/3.5 on a semi wide-angle type shot.

To get a family group with an out of focus background, you'd want to do the following in about this numerical order of descending importance 1)place the camera as close as you can, within reason 2)use a wide lens aperture like f/3.5 3) use the longest focal length that you can, within reason 4)have the background as far behind the group as is practical and 5) pose the group fairly tightly and 6)frame the portrait as a vertical, not a horizontal, to force the area covered to be smaller, rather than larger.