Depth of field

Goldcoin79

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I have been playing around with the depth of field calculated in dofmaster.com and after reading the explanation of the calculated dof it shows that the dof is the distance behind and infront if the subject.

I have only been getting in to phototography in the last few months so I am still on a learning curve but thought the dof was from the centre of the frame going outwards (left and right as you look at the shot)

My question to someone who knows better than me is, does dof also work outwards as I thought above?

If dof is only distance behind and infront of the subject does that mean that if you have a shallow dof and you had a number of people standing to the left and right of each other (exactly inline with each other) and looking at the camera they would all be in focus but the background would be out of focus?

An clear explanation of what's correct and not will be appreciated.

James
 

SCraig

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Depth of field is from close to distant. Field of view is left and right. They are not the same at all.

If you have a thin depth of field a line of people going left to right would all be in focus however a line of people going toward and away from the camera would not.
 

amolitor

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It does not go side-to-side in any appreciable way, no. Depth of Field (or Depth of Focus) is strictly front-to-back.

There IS some softening of the image as you move laterally away from the center of the frame, for most lenses, but this is a different phenomenon, it's usually quite minor, and lens makers try hard to minimize or eliminate it. You should not notice this effect in normal usage of a reasonably modern reasonably good lens. It is possible this is what you are remembering, though.
 

TheKenTurner

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DOF usually involves something blurry in Roth the foreground or background with a sharp object. This is controlled by aperture. Put your camera on a tripod, put in it manual focus to make sure it's always focused on the same spot, then test out f/1.8 f/2.8 f/4 f/5.6 etc up until probably f/20 or however high your lens goes.

-Ken Turner
 
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Goldcoin79

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Thank you all for your replies, I now understand. This is what is so great about this forum, you gan get an answer to a question so quick with the information you want.

Top resource.

One more question if i may, how does dof work regarding long distance eg mountains in the distance. Obviously you would use a small aperture to get a large dof but if the mountains a very far away why does it not blur, is this to do with a term I have read called hyperfocal distance. If it is what exactly is hyperfocal distance?
 
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Derrel

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It does not go side-to-side in any appreciable way, no. Depth of Field (or Depth of Focus) is strictly front-to-back.

There IS some softening of the image as you move laterally away from the center of the frame, for most lenses, but this is a different phenomenon, it's usually quite minor, and lens makers try hard to minimize or eliminate it. You should not notice this effect in normal usage of a reasonably modern reasonably good lens. It is possible this is what you are remembering, though.

I wonder if any technically-minded TPF'er will step in here and administer 30 lashes with a wet noodle for the inaccurate use of the term "Depth of Focus" as being synonymous with depth of field. lol. Anyway, some good replies already. Just wanted to encourage those who have not seen it to check out amolitor's photography blog...it's worth stopping by his site. "This isn't your fathers's Oldsmobile ,errr, I mean this is not your father's blog!"

Where are those vintage Hasselblad brochures, with the leaf-shutter lenses with their wonderful distance scales, and lens-mounted depth of field scales and aperture rings, with the nifty "red lines" in front of, and behind, the point of focus, and the awesome PICTURE THAT SHOWED how f/4 had narrow depth of field, and how f/16 had deep depth of field?
 

Helen B

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I wonder if any technically-minded TPF'er will step in here and administer 30 lashes with a wet noodle for the inaccurate use of the term "Depth of Focus" as being synonymous with depth of field.

Who would do such a thing?
 

KmH

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DOF This is controlled by aperture.
and distance to the point of focus, and the lens focal length. http://nicolesyblog.com/2010/01/29/lens-compression/

In fact, focus point distance affects DoF more than lens aperture does.

Yep, close to far, which is why it's called depth-of-focus.
http://en.wikipedia.org/wiki/File:Depth_of_field_diagram.png
http://en.wikipedia.org/wiki/Depth_of_field

Replace the word 'subject' with 'point-of-focus' in the statement "the dof is the distance behind and infront if the subject."

It is helpful to be aware that most lenses have some amount of field curvature, so the DoF distances tend to be slightly different as the sides of the image frame are approached.
 
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TCampbell

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I have only been getting in to phototography in the last few months so I am still on a learning curve but thought the dof was from the centre of the frame going outwards (left and right as you look at the shot)

That's called the "angle of view". It's based on the lens and also the crop-factor of the camera.

My question to someone who knows better than me is, does dof also work outwards as I thought above?

If dof is only distance behind and infront of the subject does that mean that if you have a shallow dof and you had a number of people standing to the left and right of each other (exactly inline with each other) and looking at the camera they would all be in focus but the background would be out of focus?

An clear explanation of what's correct and not will be appreciated.

James

There are three issues here:

1) The field of view you can see in the frame is called the "angle of view". A lens has it's own angle of view, but if you have a crop-frame camera then you don't see everything the lens sees... because your camera is only imaging a smaller area in the center of what the lens sees. Sensor size will impact the true angle of view.

2) The depth of field is just that... a DEPTH. It's the range of distances at which subjects that fall within that range will appear to be acceptably focused (they technically are not "perfectly" focused... but the error is small enough that it shouldn't be apparent.)

3) And then you brought up the topic of people left and right of center being out of focus. This gets a little more complicated. I'll describe below.

Suppose you focus your camera to a distance of 20'. NOW suppose you have a 20' long piece of string. If you attach one end of the string to the end of your lens, and hold the other end of the string in front of the camera so that it's tight, you are now standing at the "focused distance". If you walk left and right (keeping the string tight) you will be walking in an ARC -- not a straight line. The focused distance is technically a curve... not a flat plane. If you were to try to photograph art (suppose the art is painted on the side of a building), you might notice that the corners of the image are going a bit soft when the center is sharply focused.

Lens makers, it turns out, actually do endeavor to compensate and create a somewhat "flat" field, but it's never PERFECTLY flat. Lenses go soft at the edges and corners of the frame. ALL lenses do this -- though some are clearly better than others.

This left-right focus is plotted on a graph that they call the MTF curve (MTF = Modulation Transfer Function). Understanding what they mean and how to read them is a whole topic unto itself. It's an objective measurement of a lens' optical quality (one of many ways to grade a lens.)
 
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Goldcoin79

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Thanks for that T Campbell, that was a clear explanation which has helped me understand more.
 

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