CIRCLE OF CONFUSION

[J Costantini]

Hi.

I still have some doubts regarding the relation between circle of confusion, depth-of-field, formats and magnification on screen.

 

What exactly dictates wider or shallower depth-of-field when we compare formats using same lens and aperture?

If I use a 25mm lens with a 2.0 opening shooting 35 and then 16mm, why do I get shallower depth of field on 35?

 

Any help is appreciated.

 

Thanks

Edited July 23, 2005 by nillo

[David Mullen ASC]

Depth of field is determined by the circles of confusion, the f-stop, the focal length, and the distance focused at.

 

Trouble is that the variable is the circles of confusion, which most people arbitrarily set because there are too many factors that can modify it, so some sort of gross simplication of thinking has to happen. I mean, you can't honestly know how close someone is going to sit next to the screen, or how large every screen is going to be, etc.

 

The film format does not directly determine depth of field, but the size of the target area affects the field of view of a given focal length, and the focal length affects the depth of field. This is why smaller formats have a greater depth of field generally, because they use shorter focal lengths on average to achieve the same field of view as in 35mm.

 

You DON'T get a shallower depth of field when you use a 25mm lens on a 35mm camera instead of a 16mm camera. A 25mm lens is a 25mm lens. You get a shallower depth of field because you have to use a 50mm lens on a 35mm camera to get the same field of view as a 25mm lens on a 16mm camera, and a 50mm lens has less depth of field than a 25mm lens at the same f-stop focused at the same distance.

[J Costantini]

But if I use the same lens shooting 16 or 35mm won´t the different magnifications on screen give the feeling of different depth-of-fields??

[David Mullen ASC]

But if I use the same lens shooting 16 or 35mm won´t the different magnifications on screen give the feeling of different depth-of-fields??

<{POST_SNAPBACK}>

 

But the same lens doesn't give you the same image, so how can you compare them? With a 25mm lens on both cameras, you'd be comparing a medium focal-length effect in 16mm (with the subject looking bigger) to a wide-angle image in 35mm (with the subject looking smaller in frame). How are you really going to compare how they look in terms of depth of field? They aren't the same shot!

[drew_town]

David,

 

You should really write a book. I know that you have admitted the market is saturated with books about the technical aspects and the creative and practical aspects of filmmaking. BUT I've read a good number of books and they fall into one of two categories: they are either too vague and generalize things (which normally leads to a book filled with the author's own objectionable biases) or they are too technical and are difficult to digest. I think you've got an ability to be able to explain things (such as the DoF example above) in a complete and concise manner that most anyone will be able to comprehend. I think in another post you explained that you weren't ready to write a book yet, but I would be well interested to read one.

[Kyle Geerkens]

this may soujd dumb,

 

but I've never heard of the circle of confusion before.

 

could someone please briefly explain? Have i missed a huge idea all my life?

[David Mullen ASC]

http://en.wikipedia.org/wiki/Circle_of_confusion

http://tangentsoft.net/fcalc/help/CoC.htm

http://www.northnet.org/jimbullard/CoC.htm

 

Yes, some people will traditionally pick a different CoC for 16mm (assuming a greater degree of enlargement). The charts in the ASC manual just pick .001" (1/1000") but say that for sharper lenses of higher contrast, or for 16mm, one could use .0005" (5/10,000"). To do that on the chart using a CoC of .001", just read the DOF figures as if you had opened-up by two stops from what you were really shooting at.

 

Still, you should be comparing different focal lengths giving you the same image on 16mm versus 35mm to get a better idea of the difference in depth of field, since the same lens on both formats creates such different pictures.

[J Costantini]

Right, thanks for the links.

[Guy Meachin]

Wow I think I just grasped CoC for the first time!

 

But I'm still not sure why it is important to know when it comes to cinematography. Is it just a concept that you can understand if you like or is it something you really need to know which will influence you craft?

 

I'm guessing that it's for understanding why your aperture influences depth of field. Am I right?

 

When you're driving down the road on a wet night and you look up and see the street lights through the car window - sometimes the lights seem like fuzzy circles - I believe this is the CoC. So is the reason you see these because the water and the glass have effectively created a lens which your eye sees through? Why can't we focus on the street lights, is it because our eyes cannot focus through a long enough range of focus?

 

Am I just confusing myself and talking rubbish? More than likely!

 

Guy

[Jason Debus]

When you're driving down the road on a wet night and you look up and see the street lights through the car window - sometimes the lights seem like fuzzy circles - I believe this is the CoC. So is the reason you see these because the water and the glass have effectively created a lens which your eye sees through? Why can't we focus on the street lights, is it because our eyes cannot focus through a long enough range of focus?

<{POST_SNAPBACK}>

 

You are referring to 'bokeh' I believe, which is different than circle of confusion.

 

http://en.wikipedia.org/wiki/Bokeh

[Manu Anand]

As David has already stated . The Dof is dependant on

 

1 Focal length

2 Aperture

3 Subject Distance

4 Circle of confusion

 

It also depends on viewing distance of the image but lets assume that the viewing distance is a constant.

 

In a particular format the circle of confusion is the same so for a particual format DOF is only dependant on the other three and the viewing distance

 

http://en.wikipedia.org/wiki/Depth_of_field

 

Here you will find the relevant formulas as im unable to type them clearly not having an equation editor software. With the formulas you will clearly see what depth of field is a function of; and the variables involved; and how changing the variables changes depth of field.

 

One common myth is that depth of field changes with focal length. Yes it does but not if the image size is kept the same.

 

Dof field doesnt change if the Image size is kept the same in the frame.

That means that if I change my lens from a wide angle to a telephoto but i keep my object size the same my DOF doesnt change because the subject distance also changes which is another variable in the Dof equation.

There are exceptions to the this when the subject distance tends to equal the Hyperfocal distance as well as the Focal length of the lens ( macro basically)

 

As David has pointed out that Dof has to many parameters that have to be kept constant while changing formats and comparing but a general rule of thumb is

 

"All things being equal depth of field is inversely proportional to film format size"

 

This because the circle of confusion is a variable in the Dof equation and as other other variables are kept constant the Dof changes in relation to CoC

 

The key phrase is "All things being equal."

 

So if you compared equal sizes of projected 16 mm and 35 mm images same subject to camera distance same field of view(not same focal length)..viewed from the same distance etc then 16 would have more Depth of field as compared to 35mm (when the subject distance is less than the hyperfocal distance.)

 

Here are some sites explaing the above but with comparisons from still photography formats

 

http://www.photo.net/learn/optics/dofdigital/

http://en.wikipedia.org/wiki/Depth_of_field

 

Manu Anand

Bombay

Edited July 29, 2005 by Manu Anand

[Stephen Williams]

There are exceptions to the this when the subject distance tends to equal the Hyperfocal distance as well as the Focal length of the lens ( macro basically)

 

<{POST_SNAPBACK}>

 

 

Hi Manu,

 

When the Subject distance equals the hyperfocal length you are not talking about MACRO. In other respects you are correct!

 

Stephen Williams DP

 

www.stephenw.com

[Algis Kemezys]

Example of 28mm on 35mm camera. Neo Crete Sculpture.

Edited July 29, 2005 by Algis Kemezys

[Algis Kemezys]

I agree with Drew, David Mullen seems too selfless and knowledgable,How lucky we are to have such a valuable associate, a true teacher too.... So I am passed impressed with him and more like in awe.

 

The term Coc has eluded me as well with 35 years of still photography under my belt. Those above links et el are wonderfull. One can really sink ones teeth into this digital medium and find plenty of room to explore. I just want to say that almost in all cases using 35mm film results in some area being slightly to rightly out of focus. The wider the lens , the more depth of field. The older 35mm cameras used to have a scale on the barrel showing what you got in focus and what was not. You could then adjust your focus anywhere within that range. You could start focus at the plane of important subject matter and let it go as far behing that point as possible, or place the focus ending at that determined subject matter so everything in the forground has the chance of being in focus.

 

I hope and could only wish to shoot a feature in 35mm.

[Manu Anand]

Hi Manu,

 

When the Subject distance equals the hyperfocal length you are not talking about MACRO. In other respects you are correct!

 

Stephen Williams DP

 

www.stephenw.com

<{POST_SNAPBACK}>

 

Hi Stephen

Those are two instances im talking about

1) is when the subject distance tends to equal the hyperfocal distance

 

and i said "as well as"

 

which is 2

 

2) when the subject distance tends to come close to the the focal length.

which is what i referred to as macro.

 

Sorry for being grammatically a bit ambiguous there

 

Manu Anand

Bombay

[Stephen Williams]

Hi Stephen

Those are two instances im talking about

1) is when the subject distance tends to equal the hyperfocal distance

 

and i said "as well as"

 

which is 2

 

2) when the subject distance tends to come close to the the focal length.

which is what i referred to as macro.

 

Sorry for being grammatically a bit ambiguous there

 

Manu Anand

Bombay

<{POST_SNAPBACK}>

 

 

Hi,

 

In Macro changing a lens has no effect on DOF if the image size is and F stop are constant.

 

Stephen