Pizza and focus

[adam613]

So I picked up the infamous Canon 50mm f/1.8 lens this morning; I figured it would be a fun toy, since it's cheap, and I've never used a prime lens, and I've never used a fast lens.

The following shot made me curious:

http://farm3.static.flickr.com/2021/2196910746_e075ba3bca_o.jpg

ISO 400, f/7.1, 1/50 sec, on a monopod, shooting through a window.

The focus point was the big chunk of pineapple in the middle of the frame. So why is the focus so spotty away from the center of the picture? Specifically, the near crust of the chicken/broccoli pizza and the far crust of the background pizza are very blurry. I've never had a picture come out this way by accident before.

I figured I'd get plenty of depth of field at f/7.1. So what happened here? Is this a near/far focus issue due to the fact that I was kinda shooting across the pizza?

(This is fun.)

[moose135]

Depth of field is relative...while f/7.1 will give you more DOF then f/1.8, at a close focus distance to your subject, you will still get a short distance that is in focus. Check out this on-line DOF calculator:

http://www.dofmaster.com/dofjs.html

I plugged in Canon 10-40D line, 50mm focal length, f/7.1, and a distance of 5 feet, and got these results:
Depth of field
Near limit: 4.63 ft
Far limit: 5.43 ft
Total: 0.8 ft

On the other hand, using the same settings, with a subject distance of 50ft, you get:
Depth of field
Near limit: 27.4 ft
Far limit: 282.2 ft
Total: 254.7 ft

Given your distance to the subject, even at f/7.1, you will only have a relatively narrow band that is in focus.

[adam613]

Oh interesting. I knew DoF was relative, but not THAT relative. Thanks for the link! I gotta learn more about this stuff.

[FlyingColors]

Depth of field is relative...while f/7.1 will give you more DOF then f/1.8, at a close focus distance to your subject, you will still get a short distance that is in focus. Check out this on-line DOF calculator:

http://www.dofmaster.com/dofjs.html

I plugged in Canon 10-40D line, 50mm focal length, f/7.1, and a distance of 5 feet, and got these results:
Depth of field
Near limit: 4.63 ft
Far limit: 5.43 ft
Total: 0.8 ft

On the other hand, using the same settings, with a subject distance of 50ft, you get:
Depth of field
Near limit: 27.4 ft
Far limit: 282.2 ft
Total: 254.7 ft

Given your distance to the subject, even at f/7.1, you will only have a relatively narrow band that is in focus.

Great info and link Moose!

[jran225]

Mmm, I spy two slices of Hawaiian. They look too tasty! :D

Greets,
-Omar S.

[lijk604]

Now I'm craving pizza. Thanks!

[adam613]

Guys, that pizza is more than two months old now...I'd stay away... :P