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  • Hey all, just changed over the backend after 15 years I figured time to give it a bit of an update, its probably gonna be a bit weird for most of you and i am sure there is a few bugs to work out but it should kinda work the same as before... hopefully :)

Why does more Light increase DOF?

Depth of Field is affected by three factors - aperture, focal length, and distance to the subject.

Increased light would force you to shoot at a higher aperture, hence increasing your depth of field.
 
Ah - well if you're talking about different formats then it's a different story.
To get the same shot with different formats you're either going to have to change the focal length of the lens or the distance to the subject (in order to get the same field of view).

Doing either of those is going to change the depth of field (you will wind up with a narrower depth of field in 65mm than 35mm).

By increasing the amount of light (and thus forcing yourself to shoot at a higher aperture) you will negate the difference and wind up wiht the same equivalent DOF.

Make sense?
 
Luis Caffesse said:
Depth of Field is affected by three factors - aperture, focal length, and distance to the subject.

Increased light would force you to shoot at a higher aperture, hence increasing your depth of field.
Click to expand...

Newb question: Does focal length affect the DOF, technically speaking? I know the out of focus-areas becomes more blurry the longer the lens is, but what's in focus (DoF) is the same on a 50 mm and a 100 mm with the same aperture. Or am I wrong?
 
Four things WILL change your depth of field.

1. Distance to Subject
2. Aperture
3. Focal Length
4. Image plane size (corresponding with the lens design)

oh and the fifth

5. Jim Jannard with his mind.

Optics is geometry. DOF is about tolerances.
 
Tabula Rasa said:
Newb question: Does focal length affect the DOF, technically speaking? I know the out of focus-areas becomes more blurry the longer the lens is, but what's in focus (DoF) is the same on a 50 mm and a 100 mm with the same aperture. Or am I wrong?
Click to expand...

Nope- The DoF at the same distance is different when using either lens. For example, shooting 35mm film, 2.40:1 with a 50mm lens at f/4 focused at 10' yields a DoF of 2'6". A 100mm lens with the same specs yields a DoF of 7".

Exactly why a good focus puller is essential.

By the way, the resolving power of the imaging medium - i.e. circle of confusion - also figures into the depth of field.
 
3. Focal Length
Click to expand...

No it doesn't. This has been covered to death, and yet for some reason it's the biggest misconception out there.

Look at it this way, if you did a wide shot on an 18mm at an f 5.6, almost everything would be in focus. Now if you took a 300mm at the same stop, and backed way the hell up to get the same frame, surprise surprise, same depth of field.

Nick
 
J. Eric Camp said:
Four things WILL change your depth of field.

1. Distance to Subject
2. Aperture
3. Focal Length
4. Image plane size (corresponding with the lens design)
Click to expand...

If you ignore the bellows factor and post-processing, there are really only two factors:

1. Focus distance
2. Aperture

Note that aperture does not mean "focal ratio". Aperture is the focal length divided by focal ratio.

The image plane and focal length only affect the field of view, not the depth of field. So if you change the image plane and focal length by the same amount, the field of view stays the same, and if the aperture stays the same, then DOF will also be the same.

All have the following have the same 16.4-foot focus distance, 40 degree AOV, and 6.7 feet DOF:

Scarlet 2/3": 14mm f/0.9
4/3" DSLR: * *24mm f/1.6
Super35: * * *34mm f/2.2
Still FF35: * 49mm f/3.2
645: * * * * *76mm f/5.0
617: * * * * 220mm f/14

EDIT: Notice how they also have the same aperture: 15mm.
 
arrinick said:
for some reason it's the biggest misconception out there.
Click to expand...


The reason is that most people confuse "background blur" with "thin DOF". A lot of them don't even like thin DOF ("yuck, only one eye is in focus"). Longer lenses let them have the deep DOF they really want along with background blur.
 
arrinick said:
No it doesn't. This has been covered to death, and yet for some reason it's the biggest misconception out there.

Look at it this way, if you did a wide shot on an 18mm at an f 5.6, almost everything would be in focus. Now if you took a 300mm at the same stop, and backed way the hell up to get the same frame, surprise surprise, same depth of field.

Nick
Click to expand...

Focal length is absolutely a factor that determines depth of field. What you are talking about is the specific instance of maintaining the same framing with 2 different focal lengths. When you move the camera way back to get the same framing with that 300mm that you had with the 18mm, what changes is one of the other factors that determines DoF, which is focus distance. The two kind of cancel each other out in that case. The longer lens has less DoF at the same focus distance, the same DoF at the same framing. Perspective also changes in that situation, though. So the longer lens will still appear to have less DoF because the out of focus background objects will be magnified. The DoF is technically the same, but the image will not look the same.

Longer focal lengths having less depth of field is the reason that larger formats have inherently less DoF for a given f-stop. To get the same field of view, at the same focus distance, on a larger format, you have to use a longer lens.
 
Paris Remillard said:
Focal length is absolutely a factor that determines depth of field.
Click to expand...

You are both saying the same thing with different words.

1. "For a given perspective and focus distance, you can change the angle of view..."

2. "For a given angle of view, you can change the perspective and focus distance...".

That is why I prefer to use a much more simple definition: aperture and focus distance. That's it. Both of your scenarios are covered that way.

If you change focal length and keep the focal ratio, your aperture just got wider, so that's covered.

If you change focal length and back up to keep the same field of view, the aperture increases, but the focus distance compensates, so they are the same, that is covered.

If you change just the focal ratio, aperture increases, so that is covered.

The only thing that isn't covered is the bellows factor and other CoC effects (e.g. sensor resolution, AA filter, post-processing).
 
Daniel Browning said:
If you ignore the bellows factor and post-processing, there are really only two factors:

1. Focus distance
2. Aperture

Note that aperture does not mean "focal ratio". Aperture is the focal length divided by focal ratio.

The image plane and focal length only affect the field of view, not the depth of field. So if you change the image plane and focal length by the same amount, the field of view stays the same, and if the aperture stays the same, then DOF will also be the same.

All have the following have the same 16.4-foot focus distance, 40 degree AOV, and 6.7 feet DOF:

Scarlet 2/3": 14mm f/0.9
4/3" DSLR: * *24mm f/1.6
Super35: * * *34mm f/2.2
Still FF35: * 49mm f/3.2
645: * * * * *76mm f/5.0
617: * * * * 220mm f/14

EDIT: Notice how they also have the same aperture: 15mm.
Click to expand...

Then I suppose that one should say f-stop, or relative aperture, instead of aperture. In this example the size of the aperture remains constant, but the f-stop changes as it is a ratio of aperture to focal length. At the same f-stop, and the same focus distance, the longer lens will have less depth of field than the shorter one.

EDIT: I just read your response above. Yes, what you are saying is true and it covers all of the bases, but I'd guess that most people mean f-stop( focal ratio, relative aperture) when they say aperture. So while your description is correct, it is probably a bit more confusing to most people in most situations. But yeah, if you are just going by the effective aperture, as in the actual diameter of the aperture regardless of focal length, then focus distance and aperture would be the only determining factors, since the same f-stop on a longer lens is actually a larger effective aperture divided into a longer focal length to come to the same f-number. I'd never thought about it like that. Thanks Daniel.
 
arrinick said:
No it doesn't. This has been covered to death, and yet for some reason it's the biggest misconception out there.

Look at it this way, if you did a wide shot on an 18mm at an f 5.6, almost everything would be in focus. Now if you took a 300mm at the same stop, and backed way the hell up to get the same frame, surprise surprise, same depth of field.

Nick
Click to expand...


Nick,

In your example you changed one factor to compensate for the other.

If I change to the 300mm (focal length for the kids at home) and do not change my distance to the subject then my DOF has changed. Albeit along with my field of view.

Also the DOF is not exactly the same when we match field of view.
Experiment:

Constants:
35mm
F-stop: 5.6

Variables:
Focal Length: 18mm

Distance: 5 ft
Near: 3.02 ft
Far: 14.6 ft
Total : 11.6

2 ft in front of subject
9.6 in the rear.


Focal Length: 300mm

Distance: 110.5 ft
Near: 105 ft
Far: 116.6 ft
Total : 11.6

5.5 ft in front of subject
6.1 in the rear.

So in my humble opinion, it does have an affect.

I also mentioned image plane size to illustrate to the kids at home that 15mm on a 16 camera is not the same as 15mm on a 35 camera.

I hope this is taken in the form of healthy discussion. If I am wrong, then I am interested in how.
 
Paris Remillard said:
I'd guess that most people mean f-stop( focal ratio, relative aperture) when they say aperture. So while your description is correct, it is probably a bit more confusing to most people in most situations.
Click to expand...

Yes, you're right. Unfortunately, there is no longer any word to describe "aperture". Well, aperture would be fine, but as you said, the meaning has been co-opted to mean something else. There is no longer any word to convey the meaning.

Maybe we need to think of a new word?

  • Aperture II
  • Aperture With a Vengance
  • Live Free or Aperture
  • Aperture Reloaded
  • The Aperture Revolutions
  • Aperture: The Next Generation
  • Aperture Redux
  • Return of the Aperture
  • Aperture: we really mean it this time
  • Aperture: not focal ratio

:)
 
Aperture 2: Electric Boogaloo

And yes, we are using the same term to cover two concepts.
 
I think that Apple has Aperture II, so maybe Aperture: we really mean it this time. Electric Boogaloo is hard to pass up, though.
 
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