Depth of Field

[David Mullen ASC]

Everyone should note that Depth of Field describes an acceptable range where an object will look in focus -- it does not necessarily describe how out-of-focus a background will look, which is another visual issue to deal with, but one that is more about the perception of shallow focus rather than the level of Depth of Field.

A wide-angle lens makes background smaller in detail and size, making the degree of focus back there harder to judge compared to a telephoto image where the background is magnified and the perspective is compressed, making it easy to see just how soft it is. But these are somewhat separate issues compared to the range in which the subject can move slightly forward and back and still look in focus.

 

[Eki Halkka]

So 3 elements which affect depth of field all come back to one thing; aperture.

Of course - that makes perfect sense. Should have thought of it right off the bat. To keep any given f-stop, the physical size of the aperture grows along with focal length. This will enlarge the CoC. Thanks!!

@ David:

Changing focal length doesn't really compress space, or change perspective, moving the camera to match framing does. If this sounds silly, see my post on the first page of this thread for an actual example case...

 

[Alan Skinner]

Great Article!

Thanks!

A very good tutorial about the topic of this tread:

Understanding depth of field by Cambridgeincolour.com>>>

 

[David Mullen ASC]

Changing focal length doesn't really compress space, or change perspective, moving the camera to match framing does. If this sounds silly, see my post on the first page of this thread for an actual example case...

You're being too pedantic -- of course changing your focal length during a scene affects the perspective, or why else, other than for FOV reasons, does a director and a DP discuss whether the room should be shot on a wider-angle lens or a more telephoto lens, assuming they have room to back up the camera? Putting a wide-angle lens on the camera and moving it closer or farther back does not make it look like a telephoto shot. Everyone here knows the classic look of a telephoto shot with its compressed space versus a wide-angle shot with its elongated space.

You're talking about the affect on depth of field, I'm talking about the look of wide-angle versus telephoto lenses on a space.

But in regards to depth of field, one of the reasons why people think that shooting the scene with wide-angle lenses will give them a deep focus look (or conversely, shooting the scene on telephoto lenses will give them a shallow focus look) is the way that these lenses make the background look relative to the subject.

 

[Dan Hudgins]

focal length to subject distance ratio

focal length to subject distance ratio

One thing that is not looked at enough for DOF is the focal length to subject distance ratio.

If you have an 8mm camera with a 12mm lens the focal length is small in ratio if the subject is 3 feet away, but if you have an 8x10 camera with a 250mm lens the bellows extension required to focus on a subject 3 feet away will cause the focal length to extend.

When you figure the DOF you should take the "bellows extension" into account as it changes not only the DOF but the angle of view and perspective. As the subject get closer the focal length increases, but not in the same ratio for all formats, larger formats have more increase for the same subject distance.

==

DOF is not a property of a good lens, a good lens will focus much smaller spot size than you can see at normal viewing distance. It is the limit of the resolution of the film, projector being out of focus, and what your eye can see that makes the resolution have a plateau.

People in the last row "see" more depth of field than people in the front row, since people in the front row see less depth of field if the projector is in focus.

Devices that act like a low pass filter, such as TVs, make a plateau in the focus of the lens, you can see the image go softer as you focus out and then there is an area where you cannot SEE a change then you see the focus go out focusing inward. If you had a good lens and were very close to the image you could not see a plateau there would be no DOF.

The lens itself does have limits to the contrast or MTF it can produce at various f/ stops and circle of confusion limits. Some lenses can produce a minimum spot size that is smaller at a given contrast level than others.

You need to take into account the actual spot size when you figure the DOF tables, since a softer lens will excede the circle of confusion limits sooner as it is focused off the "center" distance.

 

[DS Williams]

But how exactly do longer focal lengths inherently have "thinner" depth of field? Every professor I ask says, "Well they compress foreground and background, and magnify a smaller portion of background, emphasizing the out of focus region."

That's all fine and great, but those are just characteristics of perspective, and optical compression, ect. I want real answers.



I've always wondered, that if you take a 1/3" inch sensor image, and a 35mm still frame, with the EXACT same field of view and 'focal length equivalent', will the background in the 35mm image look closer and this more out of focus, even though the foreground subject remains the same size????

 

[Daniel Browning]

But how exactly do longer focal lengths inherently have "thinner" depth of field?

If you really want to understand "how exactly" (and "when exactly"), then it would be best for you to read one the references already mentioned, such as "The INs and OUTs of FOCUS."

Every professor I ask says, "Well they compress foreground and background, and magnify a smaller portion of background, emphasizing the out of focus region."

That does not answer "how" longer focal lengths have thinner DOF, but they did point out correctly that it's possible to increase the magnification of background blur by increasing focal length.

I've always wondered, that if you take a 1/3" inch sensor image, and a 35mm still frame, with the EXACT same field of view and 'focal length equivalent', will the background in the 35mm image look closer and this more out of focus, even though the foreground subject remains the same size????

The background will not look closer or further away, but the same in both (same perspective and angle of view).
DOF will be thinner if the f-number is the same.
DOF will be the same if iris diameter and bellows factor are the same.

 

[DS Williams]

Thank you Daniel

There is obviously something I'm missing. I thought the only reason longer focal lengths had shallower DOF is because of background magnification. And wide angle lenses appear to have greater depth of field because of the size and detail of the background compared to foreground (not magnified). At least that's what I gathered from David Mullen's post.

But obviously there are other factors at play.

 

[Tim Griffith]

I used these articles to get my head around depth of field when in my large format days. it is based on large format photography but really gets into the nuts and bolts of DoP. I must say that the second article is quite heavy and involves alot of mathematics but you can skim over it if you wish.

An introduction to depth of field: http://www.largeformatphotography.info/articles/IntroToDoF.pdf

Depth of field in depth: http://www.largeformatphotography.info/articles/DoFinDepth.pdf

Hope this helps and good luck with your thesis.

Tim.