Depth of Feild-please explain

[super duper]

Hi, I've read the depth of feild is supposed to be twice as large behind the subject as it is infront of the subject. However, I recently snapped a pic of this bird and the handrails suggest the depth of feild is larger in the front, not the back........why???



Thanks for your help!

[ricktas]

firstly it is spelt : field!

DOF is the area of your photo that is in-focus, front to back. There is information here: http://www.ausphotography.net.au/for...-_why_is_it_so

Basically for any given F-Stop, there is a depth, that the photo will be in focus from and to. That depth changes with either, a change of f-stop and/or change in distance to the subject. BUT, as lenses are not perfect, there will always be discrepancies in that for each individual lens. I would suggest in your photo, that the actual point of focus is slightly in front of the bird, or your lens 'front focuses' a bit, resulting in the appearance of mroe in focus in front of the bird. As the bird is within the DOF, it to has gained focus.

[ElectricImages]

What Rick said. Also: remember to take perspective into account. The depth behind your point of focus may *appear* smaller because the space in front of it is closer to you and appears larger. In this regard, if your focus was on the bird's eye, in this photo the railing in front of the bird is in focus, as is the railing behind the bird. The depth of field *behind* your focus point is [width of bird's head/space between railings]+[railing], which is actually greater than the depth of [railing] in front of it...

[super duper]

firstly it is spelt : field!

Sorry! Speeling has never been my strong suit

The depth of field *behind* your focus point is [width of bird's head/space between railings]+[railing], which is actually greater than the depth of [railing] in front of it

Nicely explained

Thanks for the help guys

[super duper]

While I think of it, another question has been swirling around the back of my brain: I read the DoF is proportional to the square of distance to the subject and inversly proportional to the square of the focal length.......so, does this mean the DoF varies linearly with magnification? ie-if I take a picture then alter both the distance and the zoom to acheive the same magnification and take another picture, will the DoF be the same for both photos? (hope I'm making sense!)

[fillum]

Perhaps I'm missing something, but I don't get the "1/3 in front, 2/3 behind" DOF thing. As the subject distance approaches the hyperfocal* distance, the in-focus area behind the subject approaches "infinity" and the in-focus area in front of the subject approaches half the hyperfocal distance. So if the hyperfocal distance is 20m, focusing at 20m gives 10m in front of the subject and "infinity" behind the subject, hardly a 1:2 ratio. Perhaps it works as a general rule of thumb when using narrow apertures (eg landscapes)?

I take a picture then alter both the distance and the zoom to acheive the same magnification and take another picture, will the DoF be the same for both photos? (hope I'm making sense!)

It won't be linear because the relative size of the distance to subject (squared) will be much larger than the change in focal length (squared). In practice what this means is that if you shoot a full body portrait at f/4 with a 400mm lens you will have a much smaller DOF than shooting a full body portrait at f/4 with a 24mm lens.

*[The hyperfocal distance is the subject/focus distance that gives greatest DOF and depends on focal length, aperture and (basically) sensor/film size].


Now my head hurts...



Cheers.

[ameerat42]

While I think of it, another question has been swirling around the back of my brain: I read the DoF is proportional to the square of distance to the subject and inversly proportional to the square of the focal length.......so, does this mean the DoF varies linearly with magnification? ie-if I take a picture then alter both the distance and the zoom to acheive the same magnification and take another picture, will the DoF be the same for both photos? (hope I'm making sense!)

Short answer: No!

Long answer: No! But you knew because you tried it, right?

[abitfishy]

Also keep in mind your focusing. Are you using the selectable focus points? I know with larger apertures when I was focusing on a point (ie eyes) and re-composing the image, the slight movement in camera, me, and subject prior to then getting the photo often meant cr*p focus. I have now found I'm not as scared to stop down a tad and by habit got for the focus point button each time I point - its made a hell of a difference and I can actually get a bunch of photos of a model all with her eyes and face in focus!! Its great.

[super duper]

it is spelt ** more **

lol

Are you using the selectable focus points?

Yep

But you knew because you tried it, right?

I really should have. I just assumed that the DoF would be the same, and wanted some quick confirmation in here. But I will be playing the concept more tomorrow (just for fun now I have all the answers).

Now my head hurts...

Mine too! Very interesting, there is so much more maths to it then I ever expected.

It won't be linear because the relative size of the distance to subject (squared) will be much larger than the change in focal length (squared).

That makes sense. I just assumed there was another constant in there to make the DoF's equal.


Thanks Guys-very helpful

[Roosta]

Hay Super Dude, Try this site, it make it as interesting and as layman as needed.

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

[Kym]

I zoomed up and you are definitely focused in front of the bird (closer).

Just to clarify, there is only one focal point, i.e. where things are sharpest.
The DoF is the range of distance where things have acceptable focus, i.e. appear in focus.

The definition of acceptable is based on a concept called the circle of confusion, which is a highly technical concept, used a lot in lens design.
Gory detail here: http://www.rags-int-inc.com/PhotoTechStuff/DoF/

[super duper]

Thanks blokes-youse are a wealth of information