Depth Of Field Refresher

[Winterpaws]

I've got myself a little confused about my depth of field in my photos.

I took this photo and it hasn't really got the depth of field I was looking for. I really only wanted Charm to be on focus

Contrast that to the photo of Kallie which is getting much better and has what I was looking for.

I used the same lens for both photos....

Charm was 55 focal length and Kallie was 214. Kallie's aperture 5.6 and Charm 5.0

Does that mean to have gotten a bit more DOF with Charm I should have moved back a bit more???

[Luke W]

Couple of things to clarify..

1. Who is Charm and who is Kallie ?

I assume Charm is the first one...

2. More DOF means that the background is sharper - not more blurrier.

In your examples, the first shot has more DOF (depth of field than the second).

OK..

Depth of field is tricky...

The short answer

With wider focal lengths and more subject in the frame, it's much harder to get shorter (narrower, more blurred background) depth of field. You need to get closer to the subject and use a wider aperture.

The long answer:

Depth of field varies with focal length, aperture and distance to subject. Each one of these factors are intertwined.

Focal length and distance to subject can be combined to give 'subject magnification'

The greater the subject magnification the smaller the DOF (blurrier background).

The greater the aperture the smaller the DOF (blurrier background).

So...let's say the horse is 2m long (ok, it's a shetland) and it's represented on the sensor as 20mm in length. That's a mag factor of 1/100.

Now lets say you zoom in on the horses head (either by changing the focal length, or the distance to subject by walking toward the horse), in each case, you'll change the magnifcation factor. The horse head (20cm...ok it's a shetland with a tiny head) is represented on the sensor as 20mm in length. That's a mag factor or 1/10. The DOF in this short will be much smaller (blurrier).

There are online DOF calculators that can perform the sums given the variables.

Here's one. http://www.dofmaster.com/dofjs.html

So....

Bottom line...

If you want a blurry background (small DOF), zoom in tight and shoot with an aperture as wide as you can and use a lens with a long focal length.

[rubiton]

If you want the horse in focus and the background blurry you have to move the horse away from the background HOWEVER the closest you can stand to the horse is so you can fit the whole horse in at 50mm on a lens (no less or you get big head short legs syndrome).

SO you should have had Charm closer to you and further away from the background and you will have the effect you want. However being a horse on a slight angle try not to go below 5.6 aperture or not all the horse will be in focus as the DOF will be too short. Alternatively standing closer to the horse would also help if the ground isnt suitable over clser to you.

Nowadays at the yearling sales I try to take the pics between the barns as you have a long way before a solid background.

Edited March 10, 2009 by rubiton

[Joan of Arc]

I'm dong a digital photography course at TAFE and we went to the local cemetary last Wednesday night to work on DOF shots it was great. I will try and post some simple shots tomorrow.