Out of focus drop off

-markie-

-markie-

Suspended / Banned
Messages
653
Name
MG
Edit My Images
No
What determines the fall off of the out of focus area in a picture

For example (to keep it simple we will use the same focal length) if you compared the two:

A 85mm fix focal length lens shot at 2.8 and had say 60cm of focus

Then shot the same shot (exactly) at f4 say you had 120cm of focus (so say you brought the person forward 60cm so the fall off area starts at the same point).

Would the fall off be the same?
 
•

Disambiguation


Generally, fall off is related to light of whatever nature.
It is not used for focus consideration but drop off is.

Different languages, cultures, social circles and working
environments twisted the meaning of it quite a bit!
 
Last edited:
Kodiak Qc said:
•

Disambiguation


Generally, fall off is related to light of whatever nature.
It is not used for focus consideration but drop off is.

Different languages, cultures, social circles and working
environments twisted the meaning of it quite a bit!
Click to expand...
Thanks, edited the title to reflect.

I am referring to drop off
 
markgodley said:
What determines the fall off of the out of focus area in a picture

For example (to keep it simple we will use the same focal length) if you compared the two:

A 85mm fix focal length lens shot at 2.8 and had say 60cm of focus

Then shot the same shot (exactly) at f4 say you had 120cm of focus (so say you brought the person forward 60cm so the fall off area starts at the same point).

Would the fall off be the same?
Click to expand...
The person would be oof
People coming from landscapes often think in terms of a zone of acceptable focus, but for shooting people (or most subjects), the subject should be on the plane of focus, otherwise it looks odd.

Sorry, it doesn't answer your question.
 
•

Let's see…

A few things will have a direct influence on DoF.

  1. the aperture, of course. ƒ 1 will render shallower DoF than ƒ 22
  2. the focal length too. A shorter focal length (say 14mm) will have
    a much greater DoF than a longer one (say 600 mm)
  3. the distance to the subject. Any / all lens DoF will be shallowed
    the closer the subject is to the lens.
To remember…
Both DoF and sharpness are features of a lens. However, they don't work
together. Greater DoF will not equate to greater sharpness.
 
markgodley said:
Would the fall off be the same?
Click to expand...

No it won't. You will find that if you bring your subject closer and shoot at F4, your DoF will actually reduce.

Using DoF calculator http://www.cambridgeincolour.com/tutorials/dof-calculator.htm

On FF (35mm):

85mm@2.8 focused at 2m: DoF = 0.1m
85mm@4 focused at 1.4m: DoF = 0.07m

Also bear in mind that DoF and degree of background blur are not the same thing
More background blur doesn't necessarily mean narrower depth of field.

Many photographers confuse the two, hence the misconception that longer focal length reduces DoF.

The fact is:
If you take exact same shot of the subject (e.g. head and shoulders) with say 50 and 135 mm both at say 2.8
You will get the same depth of field, but 135mm will give you more background blur.
 
Last edited:
I think you are talking about blur radius which is essentially a factor of DOF.
Increasing the DOF does not change the point at which falloff occurs, it always extends forward/backward from the point of focus. Ii.e. you wouldn't move the subject closer, that would make it a different image w/ potentially less DOF. And having more DOF dictates that the falloff (blur radius increase) is more gradual.

The factors controlling DOF are aperture, FL, and subject distance; the *approximate* effects are:
2x aperture has a 2x effect on DOF
2x distance has a 4x effect on DOF
2x FL has a 4x effect on DOF, in the opposite direction (i.e. 1/4).

Because FL and distance have *approximately* the same magnitude of effect in opposite directions, for any composition/framing the DOF tends to remain effectively constant (when beyond HFD). Which leaves aperture as the primary control for the actual DOF.
However, the actual/technical DOF is not the only factor affecting how the OOF portions convey... i.e. how the falloff is perceived. Size is another significant factor that is typically overlooked.
 
Last edited:
gunnar said:
Larger blur radius from longer lenses creates a narrower perceived depth of field (e.g. all the wildlife shots with 'paper' background).
However if measured, depth of field on the same shots taken with different FL will be virtually the same
Click to expand...
I was actually responding to the OP... :)
The link I provided allows you to put in multiple variables to see how it affects the blur radius, the primary factor being BG separation/distance.

But this has always perplexed me to some extent... DOF is defined as the "perception of sharpness." So if when using a longer lens the BG elements are reproduced larger (with a greater blur radius), and are perceived to be less sharp, why isn't the DOF technically shallower? :confused:

Edit: I *believe* the answer is because the DOF defines what is "acceptably sharp," and the BG elements in question are not/never were within that tolerance.
 
Last edited:
gunnar said:
No it won't. You will find that if you bring your subject closer and shoot at F4, your DoF will actually reduce.

...
Click to expand...
Not if the OP doesn't refocus - and keeps the same focus distance, which was his question as I understood it.
 
You must log in or register to reply here.
Back
Top