Understanding DOF

[monkeyboyadams]

Hi all,

I have hit a bit of a stumbling block, on what appears to be a fairly simple subject. I'm hoping someone can clear this up for me as it is hurting my head a little.

Basically, does depth of field work in layers of distance? For example, if I have my settings set up to generate a shallow depth of field, will everything that is a certain distance from the camera be in focus? Or just the subject that I have focused on? To be even clearer, if I have 2 subjects that are say both exactly 3 metres from the camera, but are 2 metres apart (side by side), will they both be in focus? Even if the foreground and background are out of focus, or will it just be the one subject that I specifically focus on?

I hope that makes sense and I hope someone can clear this up for me?

Thanks in advance

 

[BrentC]

As long as they are at the same distance from the camera then yes.

 

[SCraig]

Depth of Field is, as the name implies, a DEPTH and not a well-defined line. It is assumed to be the depth of "Acceptable" focus, and is the distance between "xx" feet and "yy" feet measured from the camera. For example if the DOF is 70 feet minimum distance to 80 feet maximum distance everything between 70 and 80 feet from the camera will be in "Acceptable" focus.

 

[TCampbell]

You will occasionally hear (read) someone refer to "focus & recompose" techniques. The idea is that perhaps you want your main subject off to one side of the frame (e.g. "rule of thirds" composition techniques) but you've selected the center focus point instead of a focus point off to the side. So you point the the camera straight at the subject, "focus" the lens, then "recompose" with the subject off to one side. They're in focus and you get the composition you wanted.

Sounds great, right?

But you'll also read some advice that if you have a very shallow depth of field, that technique might not result in a tack-sharp subject.

This is because the focused "field" usually has a tiny bit of curvature. So the focused distance at the center of the frame is not quite the same as the focused distance at the sides or corners of the frame. Lens manufacturers typically do try to create a relative "flat" field, but that's really just a "flatter" field... not a perfectly flat field.

So yes... "everything" at roughly that same distance from the lens will be reasonably well focused. But if you are working with an extremely shallow depth of field and need critical focus... just remember that the field isn't perfectly flat (compose first, then focus... rather than focus and then recompose.)

 

[BrentC]

There are plenty of phone apps that will calculate DOF. I have Android and use 'Photographers Tool's'.

 

[monkeyboyadams]

You will occasionally hear (read) someone refer to "focus & recompose" techniques. The idea is that perhaps you want your main subject off to one side of the frame (e.g. "rule of thirds" composition techniques) but you've selected the center focus point instead of a focus point off to the side. So you point the the camera straight at the subject, "focus" the lens, then "recompose" with the subject off to one side. They're in focus and you get the composition you wanted.

Sounds great, right?

But you'll also read some advice that if you have a very shallow depth of field, that technique might not result in a tack-sharp subject.

This is because the focused "field" usually has a tiny bit of curvature. So the focused distance at the center of the frame is not quite the same as the focused distance at the sides or corners of the frame. Lens manufacturers typically do try to create a relative "flat" field, but that's really just a "flatter" field... not a perfectly flat field.

So yes... "everything" at roughly that same distance from the lens will be reasonably well focused. But if you are working with an extremely shallow depth of field and need critical focus... just remember that the field isn't perfectly flat (compose first, then focus... rather than focus and then recompose.)

Thanks Tim, that's a super helpful and equally insightful answer to my question.
I have always opted for focus first and then recompose, so this new bit of info will definitely make me rethink this in future.
Thanks again for your in depth reply.

 

[fmw]

OP, only one plane at one distance from the camera is in true focus. Everything else is in relative focus ranging from acceptably sharp to out of focus. The field from near to far becomes relatively more focused depending on camera to subject distance, aperture and lens focal length. About 2/3 of the field extends beyond the plane of focus and 1/3 in front of it.

longer subject to camera distance, smaller apertures and shorter lenses put more of the field into acceptable focus and the opposite parameters have the opposite effect. That was a quick description of how it works but it is fairly complete.

I suggest buying a monitor magnifier. There is no better way to determine the depth of field than actually looking at the image.

 

[480sparky]

As long as they are at the same distance from the camera then yes.

This may not be true, at least technically. In an ideal world, the focus distance would be a flat plane parallel to the image sensor / film plane. If one subject were in the exact middle of the image, another subject the same distance from the camera would be out-of-focus.

This, of course, assumes the ideal flat-place focus. But not all lenses achieve this and have a 'spherical dome' focus plane.

 

[BrentC]

As long as they are at the same distance from the camera then yes.

This may not be true, at least technically. In an ideal world, the focus distance would be a flat plane parallel to the image sensor / film plane. If one subject were in the exact middle of the image, another subject the same distance from the camera would be out-of-focus.

This, of course, assumes the ideal flat-place focus. But not all lenses achieve this and have a 'spherical dome' focus plane.

Agree. And like tcampbell mentioned, how shallow the DOF would come into play as well.

 

[benhasajeep]

Distance plays a role as well. The closer the subject is the narrower the depth of field is for a given setting. The farther the subject is the wider dof using the same setting.
For the most part. Any point that is in sharp focus will be in focus parallel in all directions to your sensor or film plain. Then as you move further from this point towards and away from your camera. The objects get less sharp in focus.

In the good ole days all the lenses had DOF scales on them. Today your lucky if the expensive pro glass has one.

 

[KmH]

Understanding Depth of Field in Photography
Depth of filed is distributed both in front of and behind the point of focus.
The distribution changes relative to the focal length of the lens used and for all but short focal length lenses is close to 50%/50%. The often stated 1/3 in front, 2/3 behind distribution only applies to short focal length lenses.

 

[480sparky]

..........In the good ole days all the lenses had DOF scales on them. Today your lucky if the expensive pro glass has one.

What gets me is why DOF can be displayed on the monitor of even entry-level DSLRs, given all the gee-whiz technology we have these days. Even if it's an option to display it and can be toggled on/off through menus, there's no reason it cannot be calculated by the camera the same way 450 trillion smart phones can. After all, the camera can do it much much much much faster as it already has focal length, focus distance and aperture fed into it automatically. And circle of confusion would be a fixed number based on the camera body. To me, this would be a no-brainer.

But alas. Canon, Nikon, Pentax etc don't listen to me.

 

[benhasajeep]

..........In the good ole days all the lenses had DOF scales on them. Today your lucky if the expensive pro glass has one.

What gets me is why DOF can be displayed on the monitor of even entry-level DSLRs, given all the gee-whiz technology we have these days. Even if it's an option to display it and can be toggled on/off through menus, there's no reason it cannot be calculated by the camera the same way 450 trillion smart phones can. After all, the camera can do it much much much much faster as it already has focal length, focus distance and aperture fed into it automatically. And circle of confusion would be a fixed number based on the camera body. To me, this would be a no-brainer.

But alas. Canon, Nikon, Pentax etc don't listen to me.

You better go to the patent office right now! Put my name down as contributor!

 

[480sparky]

You better go to the patent office right now! ..........

Kinda hard patenting something that's already on 450 trillion phones.

 

[benhasajeep]

You better go to the patent office right now! ..........

Kinda hard patenting something that's already on 450 trillion phones.

That's it though. This is for DSLR, Bridge, Mirrorless, and Compact cameras! Not phones or laptops, or Pads.