Focal Length - Depth of Field?

[EchoingWhisper]

I thought: Aperture controls depth of field, then why does focal length control depth of field too?

I used to think that the longer the focal length, the lesser the depth of field, but not anymore.
Since longer focal length compresses the distance between objects, it increases the size of everything further away. Thus, the circle of confusion (bokeh/blur) gets larger too, which makes the background LOOKS more blurry. In reality, the depth of field is just the same, only that the defocused area has larger circle of confusion. The defocused area in shorter focal lengths is not able to be seen because of diffraction/anti aliasing etc.

I might be wrong so, point out any mistakes of my point of view. I don't really understand what I'm talking... lol

 

[EchoingWhisper]

Wait, I seem wrong.

 

[480sparky]

Focal length DOES affect DOF. A 20mm lens on an APS camera, focused at 10 feet, at f/4 will have a DOF from 5'-8" to 41'. A 200mm lens with the same settings will have a DOF from 9'-11" to 10'-1"

 

[EchoingWhisper]

Focal length DOES affect DOF. A 20mm lens on an APS camera, focused at 10 feet, at f/4 will have a DOF from 5'-8" to 41'. A 200mm lens with the same settings will have a DOF from 9'-11" to 10'-1"

I think I understand what I am talking about now, if your subject (point of focus) is the same size at all focal length, the depth of field should be the same. Circle of confusion in defocused area of longer focal lengths are larger, but depth of field should be the same.

 

[480sparky]

Focal length DOES affect DOF. A 20mm lens on an APS camera, focused at 10 feet, at f/4 will have a DOF from 5'-8" to 41'. A 200mm lens with the same settings will have a DOF from 9'-11" to 10'-1"

I think I understand what I am talking about now, if your subject (point of focus) is the same size at all focal length, the depth of field should be the same. Circle of confusion in defocused area of longer focal lengths are larger, but depth of field should be the same.

How does 5'-8" = 9'-11" and 41' = 10'-1"?

If you have the 20mm focused at 10', an object at 20' will be in focus. Switch to a 200mm, and it won't be.

 

[Rephargotohp]

Why, focal length "doesn't matter" is because YOU MOVE. To get the same framing on your subject as you change focal lengths, you move the reciprocal distance.

So the three things that affect DOF, Aperture, Focal Length and Distance to subject, when you frame the subject the same, Focal Length and Distance to subject cancel each other out. ( there actually can be a slight change in DOF but not one that is noticeable.\

The reason a Long lens seems to have a shallower dof is perspective compression

 

[Big Mike]

 

[Rephargotohp]

QUOTE=480sparky;2434341]Focal length DOES affect DOF. A 20mm lens on an APS camera, focused at 10 feet, at f/4 will have a DOF from 5'-8" to 41'. A 200mm lens with the same settings will have a DOF from 9'-11" to 10'-1"[/QUOTE]
ONLY if your distance to subject stays constant

Put in your DOF calculator f/4 50mm 10 feet to subject, then put in 100mm 20 feet to subject. DOF will remain the same

Focal length Does affect DOF, But in practice, It doesn't because we move

 

[EchoingWhisper]

Why, focal length "doesn't matter" is because YOU MOVE. To get the same framing on your subject as you change focal lengths, you move the reciprocal distance.

So the three things that affect DOF, Aperture, Focal Length and Distance to subject, when you frame the subject the same, Focal Length and Distance to subject cancel each other out. ( there actually can be a slight change in DOF but not one that is noticeable.\

The reason a Long lens seems to have a shallower dof is perspective compression

Yep, that is what I am trying to say.

 

[480sparky]

Since when did we start moving?

 

[Rephargotohp]

Since when did we start moving?

When mom said we couldn't stay home anymore, I drank all her beer

I have a cardboard box outside Starbucks now, It's nice

 

[Crollo]

The reason a Long lens seems to have a shallower dof is perspective compression

I believe he already stated that about 3 times now.

 

[Rephargotohp]

The reason a Long lens seems to have a shallower dof is perspective compression

I believe he already stated that about 3 times now.

Actually, no
He said the CoC gets larger, It doesn't

I'm sorry I tried to clairify his statement..that you needed to take offense to it

 

[Iron Flatline]

He said the CoC gets larger, It doesn't

Hehehe... Beavis, he said CoC.

 

[table1349]

Understanding Depth of Field in Photography