Focus and DOF differences.

[reanimated838uk]

I understand DOF varies with different f values (small values means large aperture, meaning less DOF, and larger values means smaller aperture and more DOF) but i just cant distinguish the difference between that (DOF) and focus.

For me, if a picture is blurred or not as sharp it could be a smaller DOF but it can also mean focus wasnt locked properly. Hope this makes sense.

[PhotosGuy]

Hope this makes sense.

Not sure. DOF extends 1/3 toward you & 2/3 away from the point of focus.
Except in close-ups, where it's 1/2 & 1/2. Does that help?

[reanimated838uk]

heh, i'll try and explain what i mean again.

http://www.digitalcamera.com.au/tutorials/sharpness/sharp7.shtml

at the first image, how would i know if the other matchsticks are blurred because of lack of focus or incorrect DOF.

(blah i cant seem to explain what i mean again)

[Scottes]

The image will be at it's sharpest for anything that is the same distance as it's focus point. So if you take a picture of a perfectly straight picket fence and the camera is exactly 90-degrees to the fence and the focus is perfectly on the fence then every picket will be sharp - exactly as sharp as any other.

Now all that exactness is pretty difficult, so let's say that the fence is perfectly straight but the camera isn't. In fact, let's say that you are at a 45-degree angle to the fence, and you pick one picket for the focus point and the camera does in fact focus perfectly on that one picket. That picket will be perfectly sharp, but the other pickets will be out of focus depending on the distance from that picket. This is where Depth Of Field comes in.

As Photosguy said, about 1/3 of you DoF will extend towards you, and some pickets that are closer than the focused one will appear in focus. More pickets that further from that one will appear in focus, approximately twice as many.

A key point of DoF is that it is the area that is acceptably sharp. Only the one picket is truly in focus, but the others are sharp enough to appear to be in focus. If you zoom in - or hold a print closer - you will see that the closer and further pickets aren't really as sharp as that one picket. But as you zoom out, or hold the print further away, you'll find that more and more pickets look sharp, and appear to be in focus.

This distance thing shows another very important part of Depth Of Field - the Circle of Confusion.


Michael Reichmann - from this article (http://www.luminous-landscape.com/tutorials/understanding-series/dof.shtml) on Depth Of Field.

You can't understand Depth of Field until you understand COF (Circle of Confusion). The human eye has a finite ability to see fine detail. This is generally accepted as being 1' (minute) of arc. Translating this to the practical world, this means that at a normal reading distance the smallest object that a person with perfect eyesight, under ideal conditions can see is 1/16mm in size. If you place two dots smaller than this next to each other they will appear to be just one dot.

I'm definitely too tired to explain CoF. The above article will help, but this article (http://members.aol.com/Photoinfo/dof.html) is really a good primer on DoF for real world situations and without getting more technical than is really necessary.

[Scottes]

But to answer your question now that I think I understand it... Something will appear sharp when it is perfectly in focus. But let's say that the camera, for some reason, focused on a point slightly closer than what you thought. Then your subject will appear slightly out of focus - slightly because it's not perfectly in focus but still remains within the Depth of Field.

Let's say you shoot something round, like a tree. You aim at the front of the tree and choose that as your focus point. However, the camera requires contrast to shoose a focus point, AND the focus point is usually larger than you think it is. So the camera chooses the edge of the tree - nice contrast between the dark tree and the bright sky. If your aperture is too large you'll have a small DoF, and the front of the tree will appear out of focus.

If you had used a small aperture then the entire tree would be acceptably sharp, and you'd think that it was in focus just fine.


So how to tell in your link? If more matches are in focus then a small aperture was used to get a large Depth of Field. If less matches are in focus then a large aperture was used, which results in a small Depth of Field.

[robertwgross]

As Photosguy said, about 1/3 of you DoF will extend towards you, ...


Be really careful with terminology here.

Some folks will read that and think that you are saying that the DoF extends 1/3 of the distance towards you. Of course, that is not the case.

Instead, 1/3 of the DoF is in front of the point of focus, and 2/3 is in back of it. Some of us know that, but there may be beginners reading.

---Bob Gross---

[PhotosGuy]

Actually, I said, "DOF extends 1/3 toward you & 2/3 away from the point of focus. Which, to me, was perfikally klear!

[reanimated838uk]

But to answer your question now that I think I understand it... Something will appear sharp when it is perfectly in focus. But let's say that the camera, for some reason, focused on a point slightly closer than what you thought. Then your subject will appear slightly out of focus - slightly because it's not perfectly in focus but still remains within the Depth of Field.

Let's say you shoot something round, like a tree. You aim at the front of the tree and choose that as your focus point. However, the camera requires contrast to shoose a focus point, AND the focus point is usually larger than you think it is. So the camera chooses the edge of the tree - nice contrast between the dark tree and the bright sky. If your aperture is too large you'll have a small DoF, and the front of the tree will appear out of focus.

If you had used a small aperture then the entire tree would be acceptably sharp, and you'd think that it was in focus just fine.


So how to tell in your link? If more matches are in focus then a small aperture was used to get a large Depth of Field. If less matches are in focus then a large aperture was used, which results in a small Depth of Field.

ive just woke up so please bare with me :)

so whatever part i focus on anything within the same distance within a perpendicular plane of the focus will be sharp?.

[Scottes]

As Photosguy said, about 1/3 of you DoF will extend towards you, ...


Be really careful with terminology here.

Some folks will read that and think that you are saying that the DoF extends 1/3 of the distance towards you. Of course, that is not the case.

Instead, 1/3 of the DoF is in front of the point of focus, and 2/3 is in back of it. Some of us know that, but there may be beginners reading.

---Bob Gross---

"1/3 of your DoF" is nothing like "1/3 of the distance to the subject". Stop looking for things to pick on. It was clear.

Luckily we have few newbies who are persnickety engineers bent on dictating that the English language shall be used in a way as to be perfectly exact to other persnickety engineers.

[reanimated838uk]

http://homepage.ntlworld.com/silentgriffinwingz/pic.gif

how what im trying to say in previous post.
Note the three objects (if u can call them that) are lined up straight, the 2nd one is just having the camera at an angle.

[Scottes]

In figure 1 all three are lined up and approximately the same distance from the camera, so they're in focus. In the second, the front and back ones may be out of focus because they're different distances from the camera.


DoF is measured as distances from the camera, not distances around an object:

http://www.itsanadventure.com/postimages/pic2.gif

[PhotosGuy]

so whatever part i focus on anything within the same distance within a perpendicular plane of the focus will be sharp?

Yes, IF the plane of the back of the camera is what you're using for reference.

[robertwgross]

"1/3 of your DoF" is nothing like "1/3 of the distance to the subject". Stop looking for things to pick on. It was clear.


Actually, that was exactly the terminology problem that came up in a photo seminar last year.

---Bob Gross---

[Tom W]

In figure 1 all three are lined up and approximately the same distance from the camera, so they're in focus. In the second, the front and back ones may be out of focus because they're different distances from the camera.


DoF is measured as distances from the camera, not distances around an object:

http://www.itsanadventure.com/postimages/pic2.gif

Hopefully, its safe to add that the red lines that Scottes drew would be closer together when using a larger aperture (smaller numerically) such as f/2.0 or f/2.8, and they would spread farther apart as you stopped down to smaller diameter aperture settings such as f/8 or f/11.

Learning to use the aperture for this function is a great tool as it can be used to give enormous in-focus depth (like when your subjects are at varying distances from the camera), or to give soft blur to the background while isolating the primary subject (as one might do with a portriat).

[PhotosGuy]

DoF is measured as distances from the camera, not distances around an object:

True, but who carries a DOF table & tape measure when they shoot? (Don't tell me his/her name - I never want to meet them!)

The 1/3-2/3 factor is an approximation for those of us who live in the real world. Personally, I'd use the DOF preview & bracket the F-stops +1 & -1 around the F-stop I picked & then pick the best shot on the computer. Much quicker, & I'll be taking pics while someone else is still measurbating the DOF at 4' 6" for F-6.3 @ 52mm! :wink:

[Scottes]

DoF is measured as distances from the camera, not distances around an object:

True, but who carries a DOF table & tape measure when they shoot?

From the original drawings it seems to me that the poster thought that DoF was measured around the subject in all directions. I was just trying to make it clearer that it's distance from the camera, not the subject.

So if you're 10 feet from a subject and have a DoF of 3 feet, then everything from a distance of 9 feet to 12 feet will be inside the DoF, regardless of how far away something is from the subject. Even if something is 5 feet to one side, if it's 10 feet from the camera it will be in focus.


One thing I'd love on a camera is DoF/Shutter Speed Bracketing. Take three pictures all at the same equivalent exposure, but vary the aperture and shutter speed so that DoF is different in each shot. I think that would be useful as hell. Not only for DoF, but for those questions about "how fast should the shutter speed be to slightly blur propellers" and things like that.

[reanimated838uk]

i wanted to post this a couple hrs ago but it timed out: so here it is.

DoF is measured as distances from the camera, not distances around an object

not entirely sure what you mean. By the circle i mean the object which the camera has focus on. The DOF line in my pic represents the distances away from the camera where everything will be sharpish. So in the 2nd pic if another object was alongside the middle one along the line area it be in the DOF.

Another question:
if i focuses on a object far away and had a short DOF will it have the object sharp but quickly blur over the distances infront and behind DOF? So a large DOF will bring objects in front and behind DOF more sharp?[/quote]

[Tom W]

Another question:
if i focuses on a object far away and had a short DOF will it have the object sharp but quickly blur over the distances infront and behind DOF? So a large DOF will bring objects in front and behind DOF more sharp?

Generally, yes.

[Scottes]

i wanted to post this a couple hrs ago but it timed out: so here it is.

DoF is measured as distances from the camera, not distances around an object

not entirely sure what you mean.

"So if you're 10 feet from a subject and have a DoF of 3 feet, then everything from a distance of 9 feet to 12 feet will be inside the DoF, regardless of how far away something is from the subject. Even if something is 5 feet to one side, if it's 10 feet from the camera it will be in focus."

[reanimated838uk]

i wanted to post this a couple hrs ago but it timed out: so here it is.

DoF is measured as distances from the camera, not distances around an object

not entirely sure what you mean.

"So if you're 10 feet from a subject and have a DoF of 3 feet, then everything from a distance of 9 feet to 12 feet will be inside the DoF, regardless of how far away something is from the subject. Even if something is 5 feet to one side, if it's 10 feet from the camera it will be in focus."

i assume the 3 feet DOF means 1/3 of it is in front and 2/3s behind the point of focus.


From the original drawings it seems to me that the poster thought that DoF was measured around the subject in all directions. I was just trying to make it clearer that it's distance from the camera, not the subject.

i thought DOF was the area in front of the focus point to the camera.

So would i be right in assuming to take a picture of say a hill, and to have both foregrond and background in sharp view, i should have or use the infinite DOF or would that be the infinite focus.

[Belmondo]

[
ive just woke up so please bare with me :)

I will be happy to bear with you, but I don't think I want to bare with you. (I'm such a prude.) :oops:

[Scottes]

"So if you're 10 feet from a subject and have a DoF of 3 feet, then everything from a distance of 9 feet to 12 feet will be inside the DoF, regardless of how far away something is from the subject."

i assume the 3 feet DOF means 1/3 of it is in front and 2/3s behind the point of focus.

i thought DOF was the area in front of the focus point to the camera.[/quote]

No. 1/3 of the DoF will be in front of the focus point, that is, between the camera and the focus point. 2/3 of the DoF will be behind the focus point, that is, from the focus point and an area further away from the camera.

Take a look at your link to the matchstick picture. The matchstick with the mark is the focus point. Some of the closer matchsticks are in focus, but even more behind it are in focus.


So would i be right in assuming to take a picture of say a hill, and to have both foregrond and background in sharp view, i should have or use the infinite DOF or would that be the infinite focus.

That would be closer to Infinite DoF, which isn't possible. But to get the most possible area to be in focus you can set the focus point to the Hyperfocal Distance.

http://www.dofmaster.com/hyperfocal.html

[reanimated838uk]

[
ive just woke up so please bare with me :)

I will be happy to bear with you, but I don't think I want to bare with you. (I'm such a prude.) :oops:

eh! :shock:

[reanimated838uk]

"So if you're 10 feet from a subject and have a DoF of 3 feet, then everything from a distance of 9 feet to 12 feet will be inside the DoF, regardless of how far away something is from the subject."

i assume the 3 feet DOF means 1/3 of it is in front and 2/3s behind the point of focus.

i thought DOF was the area in front of the focus point to the camera.

No. 1/3 of the DoF will be in front of the focus point, that is, between the camera and the focus point. 2/3 of the DoF will be behind the focus point, that is, from the focus point and an area further away from the camera.

Take a look at your link to the matchstick picture. The matchstick with the mark is the focus point. Some of the closer matchsticks are in focus, but even more behind it are in focus.

its just ur reply thats confusing me now:
thought that DoF was measured around the subject in all directions. I was just trying to make it clearer that it's distance from the camera, not the subject...
howd ya mean by around the subject in all directions?

[Scottes]

I give up.

[reanimated838uk]

:roll: well i appreciate the answers so thanks for them :) im a bit wiser now thanks to you lot. Should i take over the world, u know who to blame. :wink: 8)

[PhotosGuy]

The DOF line in my pic represents the distances away from the camera where everything will be sharpish
I would be more like a radius drawn from the cam instead of a line (straight).

so this is the best explaination so far:
"So if you're 10 feet from a subject and have a DoF of 3 feet, then everything from a distance of 9 feet to 12 feet will be inside the DoF, regardless of how far away something is from the subject. Even if something is 5 feet to one side, if it's 10 feet from the camera it will be in focus."

ive just woke up so please bare with me Smile
I will be happy to bear with you, but I don't think I want to bare with you. (I'm such a prude.)
What? I wasn't supposed to do that, belmondo?

Scottes: "I give up."
Me, too!
:lol:

[reanimated838uk]

http://209.196.177.41/04/04-04.htm < more info for others