Confused with DOF and f stop relationship

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w i l l

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I'm not a photographer, but a 3d visualiser (http://www.willusher.com) trying to get my head
around 'f stop' and 'depth of field' - something is confusing me and
i'm wondering if someone could help me out.

THIS IS HOW I UNDERSTAND IT:

Increasing the f stop increases the DOF.

If f stop is the focal length divided by the diameter of the lens then
surely increasing the size of the focal length will increase the f
stop (increase DOF) and increasing the diameter of the lens will
decrease the f stop (decrease DOF).

BUT:

I read that both decreasing the focal length and decreasing the lens
size will increase the DOF.

How does this work? - I dont understand the relationship.

Any help appreciated!

Cheers

Will
 
First we have to all be on the same page as to what each part of your question means or we go around in circles.. For example what do you mean by increasing the dof? I knwo what i mean. But people have different ideas.

Increasing the f stop from f2.8 to f8 for example will give you a bigger DOF ..ie more in front/behind the subject will be in focus

Give us an actual example?
 
f numbers are a measurement of apature and range from f/1.0 upwards. Depth of field is not increaced or decreased, it's shallower or wider.

The lower the f numer, the shallower the depth of field. So if you had a lens that you set at f/2.8, then it would have a small focus point meaning that everything else would be blurred or out of focus

Example - say you had a pencil sitting on your desk and you set your lens to f/2.8 and focused on the pencil, everything around it would be blurred. Similarly, if you set the lens to f/22, pretty much everything in the image would be sharp / in focus.

Every lens has a maximum apature (wide open, the widest the apature on said lens will open to). This means more light is let into the lens and the depth of field is shallower.
 
I Edited my post exact time you posted 8utters... oops :)
 
w i l l said:
If f stop is the focal length divided by the diameter of the lens
Click to expand...
The f-stop is the FL divided by the diameter of the aperture, i.e. the iris in the lens, not the diameter of the lens itself. This varies, but the f-stop quoted in the description of a lens is the maximum size of the aperture, not the only size (except for catdioptric (mirror) lenses).

So, when talk about the DoF varying with the f-stop you should add "for the same focal length". So, for a prime (fixed focal length) lens, or a zoom at any given focal length, reducing the aperture (larger f-stop) will increase the DoF - because the circles of confusion get smaller the small the aperture is.
 
Try this, it's an interactive camera demo, that lets you adjust the aperture/shutter/speed and iso to see what result it has on a picture
 
makes u wonder why they made the F number and the aperture size inversely proportional, as in theory, the largest aperture u could have is infinite, whereas the minimum aperture is definite, and could be defined as 1 or 0 !!!!!!

anyone wanna sign a petition
 
hey what software do you use, i am a visualiser too, i work for

www.biglolly.com

if you use 3d studio max with vray i may be able to help with settings
 
Ok but if f stop is the focal length divided by the diameter of the aperture then
in relation to this, increasing the size of the focal length will increase the f
stop (increase DOF).... but this is wrong because increasing the focal length decreases the DOF (so i've read).

I understand what should happen and some of the posts above state what should happen but what i dont understand is the relationship to this original... f stop is the focal length divided by the diameter of the lens formula.
 
The way i try to remember it is this.. I hope I get it correct..

Consider the f number as the number you divide the aperture

High Number = Smaller Aperture = Less Light = Longer Shutter = More Data
Low Number = Larger Aperture = More Light = Faster Shutter = Less Data

Now this is the silly bit.. for longer shutter I imagine having my eyes open for a while and I could see lots and take more in.. For short shutter I imagine a fast open and close of the eyes and i might see one or two things but the rest is a blur..

Am I crazy :help:
 
w i l l said:
Ok but if f stop is the focal length divided by the diameter of the aperture then
in relation to this, increasing the size of the focal length will increase the f
stop (increase DOF).... but this is wrong because increasing the focal length decreases the DOF (so i've read).
Click to expand...

When you say "increasing the size of the focal length" do you mean in a zoom lens, or two different lenses with different focal lengths?

In the latter, to get the same maximum f-stop for a longer focal length the diameter of the lens has to be greater - look at some of the monster lenses pro photographers use, some must be 8" diameter, and that is so you can have a larger maximum aperture, e.g. f/2.8, with a long, e.g. 500mm, lens.
 
leon1p said:
The way i try to remember it is this.. I hope I get it correct..

Consider the f number as the number you divide the aperture

High Number = Smaller Aperture = Less Light = Longer Shutter = More Data
Low Number = Larger Aperture = More Light = Faster Shutter = Less Data

Now this is the silly bit.. for longer shutter I imagine having my eyes open for a while and I could see lots and take more in.. For short shutter I imagine a fast open and close of the eyes and i might see one or two things but the rest is a blur..

Am I crazy :help:
Click to expand...



and by the time you hit the shutter button the cars will have done another lap :lol: :lol:
 
parish said:
When you say "increasing the size of the focal length" do you mean in a zoom lens, or two different lenses with different focal lengths?

In the latter, to get the same maximum f-stop for a longer focal length the diameter of the lens has to be greater - look at some of the monster lenses pro photographers use, some must be 8" diameter, and that is so you can have a larger maximum aperture, e.g. f/2.8, with a long, e.g. 500mm, lens.
Click to expand...

I dont know i'm referring to what i read online i.e.

I. Focal length of the lens

The depth of field is inversely proportional to the focal length of the lens; that is, the smaller the focal length number of the lens, the greater the depth of field. For example, a 28mm lens has the ability to capture more of the picture in sharp focus than a 100mm lens.
 
w i l l said:
The depth of field is inversely proportional to the focal length of the lens; that is, the smaller the focal length number of the lens, the greater the depth of field. For example, a 28mm lens has the ability to capture more of the picture in sharp focus than a 100mm lens.
Click to expand...

Correct, but with that same lens the DoF will vary depending on what you set the aperture to on that lens.

So, using the 28mm lens, you'll have a greater DoF at f/22 than you will at f/4.

If you now switch to a 200mm lens, the DoF will be less than with the 28mm one but, again, the DoF of that lens will vary with the f-stop used.

So yes, both f-stop and focal length affect the DoF.

The is some complex maths that will allow you to calculate the DoF for a given distance (i.e. the point you are focused on) at a given focal length, and a given f-stop.

Edit: It's all to do with circles of confusion. The light rays from only one distance away are focused to a point on the film/sensor (the focal plane), light from objects closer and further away forms small circles rather than points at the focal plane and the DoF is the minimum and maximum distances where those circles are small enough that they are perceived as points by the human eye. i.e. they appear to be in focus, but in reality they aren't.
 
I'm going to pop up here and add some confusion.

DOF is not related to focal length in the strict sense of the term.

DOF is a function of aperture and magnification. A 60mm lens and a 180mm lens giving 1:1 reproduction at the same aperture will have the same DOF...despite the focal length of one lens being 3x that of the other.

DOF is calculated based on the reproduction of the image on a nominal 10"x 8" plate and viewed from a certain distance. The human eye can perceive whether a portion of the image is OOF at the approximate threshhold of 0.03mm for 35mm (0.018 for APS-C sensors)....the so called cirlce of confusion.

I would encourage folk to read this excellent description rather than trying to decipher my insane rambling. http://www.luminous-landscape.com/tutorials/understanding-series/dof.shtml

Bob
 
parish said:
Correct, but with that same lens the DoF will vary depending on what you set the aperture to on that lens.

So, using the 28mm lens, you'll have a greater DoF at f/22 than you will at f/4.

If you now switch to a 200mm lens, the DoF will be less than with the 28mm one but, again, the DoF of that lens will vary with the f-stop used.

So yes, both f-stop and focal length affect the DoF.

The is some complex maths that will allow you to calculate the DoF for a given distance (i.e. the point you are focused on) at a given focal length, and a given f-stop.

Edit: It's all to do with circles of confusion. The light rays from only one distance away are focused to a point on the film/sensor (the focal plane), light from objects closer and further away forms small circles rather than points at the focal plane and the DoF is the minimum and maximum distances where those circles are small enough that they are perceived as points by the human eye. i.e. they appear to be in focus, but in reality they aren't.
Click to expand...

Ah ok i think I understand (my brain is a bit slow from my days work) - this is the problem with trying to understand something from a maths/text on google point of view before trying it in real life.

Thanks for your help fellow Wiltshireton!
 
Canon Bob said:
I'm going to pop up here and add some confusion.

DOF is not related to focal length in the strict sense of the term.

DOF is a function of aperture and magnification. A 60mm lens and a 180mm lens giving 1:1 reproduction at the same aperture will have the same DOF...despite the focal length of one lens being 3x that of the other.

DOF is calculated based on the reproduction of the image on a nominal 10"x 8" plate and viewed from a certain distance. The human eye can perceive whether a portion of the image is OOF at the approximate threshhold of 0.03mm for 35mm (0.018 for APS-C sensors)....the so called cirlce of confusion.

I would encourage folk to read this excellent description rather than trying to decipher my insane rambling. http://www.luminous-landscape.com/tutorials/understanding-series/dof.shtml

Bob
Click to expand...

I previously read these.

http://www.azuswebworks.com/photography/dof.html

http://www.mir.SPAM/rb/photography/fototech/htmls/depth.html
 
Canon Bob said:
DOF is not related to focal length in the strict sense of the term.

DOF is a function of aperture and magnification. A 60mm lens and a 180mm lens giving 1:1 reproduction at the same aperture will have the same DOF...despite the focal length of one lens being 3x that of the other.
Click to expand...

In order to achieve that am I correct in thinking that you would have to be 3 times further away from the subject with the 180mm lens?
 
8utters said:
f numbers are a measurement of apature and range from f/1.0 upwards. Depth of field is not increaced or decreased, it's shallower or wider.
Click to expand...

I have to disagree. If for example at f/2.8 the DoF is 0.5 metres in front of the subject and 0.75 metres behind the subject and then changing my aperture to say f/8 and my DoF changes to +/- 2 metres from the subject then the DoF has increased, moving from 0.5 to 2 is an increase of 1.5 metres. So I say DoF can increase and decrease.
 
parish said:
In order to achieve that am I correct in thinking that you would have to be 3 times further away from the subject with the 180mm lens?
Click to expand...

That's correct :thumbs:..in theory at least. In practice it isn't actually the case. The focal length of a lens is always stated with the lens focussed at infinity. As focus gets closer then the focal length will decrease.

Bob
 
ThisPhotoGuy said:
I have to disagree. If for example at f/2.8 the DoF is 0.5 metres in front of the subject and 0.75 metres behind the subject and then changing my aperture to say f/8 and my DoF changes to +/- 2 metres from the subject then the DoF has increased, moving from 0.5 to 2 is an increase of 1.5 metres. So I say DoF can increase and decrease.
Click to expand...

So 8utters is saying that Depth of field doesn't get longer but it gets wider..
Your saying that it can get longer
I thought it got deeper :thinking:

Are we not just all correct :D

Head hurts
 
I never said longer.

Now if the DoF gets larger by going to a smaller aperture the only thing that happens is that the area of acceptable focus becomes greater (The DoF increases in size).

If I am looking through my lens, the subject is in front of me. If the DoF gets wider that implies that the DoF is increasing left to right and not front to back of the subject. So I say the depth of field increases and decreases by changing the aperture appropriately. Deeper and shallower makes sense. Wider just doesn't make sense to me. :thinking:
 
Sorry I retract that statement :)

I suppose most diagrams show a side on view so maybe that's one reason people think of it as a width.
 
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