Why the DoF difference?

[jukas]

Ok, I'm sure this is probably a dumb question, but I noticed when doing some quick photographing of my dad's harley for a classified ad, that there was a drastic difference in DoF between two different lenses.

I started out using my 70 - 200 f/2.8 L at 2.8 and noticed I wasn't getting the blurring I wanted in the background, so I slapped on my 50 f/1.8 at 2.8 and got much more acceptable background blur at the same f-stop. To be sure I was doing the test right I setup the camera on a tripod and made sure to take the images from the exact same distance, in the same light, with the same focal point.

Now I understand there's a difference in focal length, but can that alone really cause the difference I was seeing? As much as I'd like to show you examples, those images are no long gone thanks to a faulty HDD.

Is this a basic photo 101 thing I missed here?

[CyberDyneSystems]

Another major factor in dof... is distance of camera to subject.

Where "background blur" is concerned,. the distance of camera to subject relative to distance of subject to background plays are larger ingredient in the total effect than aperture does in many cases.

The rules are..

1) the closer to the camera the subject is the more blur you will have,
2) The more distance between the subject and the background, the more blur you will have.

This is because;
1) The closer the camra is to the focal point, the narrower your DOF will be.
2) The farther the background is from the focal point, the more out of focus/out of your DOF the background will be.

The third factor of course is Aperture.. where the wider the aperture the shorter your DOF will be.


What haopend is that when you swapped to the 50mm.. you got physically closer to the subject/focal point... thus reducing your DOF.. and throwing the background further out of focus.

[CyberDyneSystems]

Oops... I didn't read the "the distance remains the same" part..

My bad...

Well er.. I have two thoughts.. but they are only theories.. (number of iris blades, focal length, and telephoto compression... )

....here I admit defeat

[jukas]

What haopend is that when you swapped to the 50mm.. you got physically closer to the subject/focal point... thus reducing your DOF.. and throwing the background further out of focus.

Actually CDS it seemed to be the opposite.

I took two pictures from the same spot, so theoretically, with the 70 - 200 I was about 6 inchs closer due to the length in the lens. The distance between the camera and the subject never changed.

[CyberDyneSystems]

Yep.. I'm stumped ? :oops:

[jukas]

Doh!

I'll have to try and re-create it this weekend. Oh well gives me something to do! :shock:

[Jesper]

Here is an online DOF calculator (http://www.dofmaster.com/dofjs.html).

[Mark Kemp]

I have to admit that I sometimes find Dof a bit strange too.

CDS is perfectly correct about the theory and most of the time it seems to work, but sometimes I wonder.

I seem to get odd results that don't exactly fit the theory every so often.

I have always just accepted them and assumed that I made a mistake or something wasn't set up as I thought it was.

Maybe I will be more careful and investigate a bit more in the future.

[jyrgen]

Background blur is not DOF. It's, in a way, everything else but DOF (save for foreground blur). There are formulas to describe blur, too. Here (http://hobbymaker.narod.ru/English/Articles/blur_eng.htm) are few, but you can google for some more.

Of course I don't want to say that DOF and background blur are not interrelated, they very much are different parts of the same fuzzy formula. But it's easy to see that using different focal lenghts, one can get equal DOF on the same subject (say a Harley) at the same magnification (say Harley filling a frame), but totally different backgrounds and hence background blurs.

[Tom W]

Somebody here ran across this depth-of-field simulator a couple of months ago. Here's the URL:

http://www.liquidsculpture.com/dof.htm

[drisley]

Tom, that simulator is awesome! :shock: