Re: DOF and angle of view or focal length (long)

[Peter J. Alling]

The Circle of confusion required for an acceptable depth of field 
changes depending on the format.
The smaller the sensor size, the smaller the cof must be.  Also the 
desired enlargement factor
should also be taken into account, (although that last is difficult).  
This unfortunately isn't hard science
it's more like cooking. 

William Robb wrote:

----- Original Message ----- 
From: "Jens Bladt"
Subject: DOF and angle of view or focal length (long)

 

Some of you people are very knowledgeable when it comes to optical
   

science.
 

So, I would like to ask you this:

On the internet there is an ongoing discussion about this subeject.
Some say, that smaller formats have greater DOF (Photonet). They
   

say that in
 

order to get comparable images, I must use shorter focal length to
   

go with
 

the smaller format, thus achieving greater DOF. They are using
   

circle of
 

confusion (COF) theories to support their point of view.

I (and Photozone) say, that smaller formats only show a part of the
   

image,
 

captured by a specific focal lenght. If I shoot the same scene
   

twice with
 

the same camera, same lens (focal length( and same aperture and
   

focus point,
 

you will get identical images on let's say APS and 35mm film - that
   

is for
 

the part, that is covered by the smaller format (e.i. APS). I say
   

that the
 

DOF of these two identical images - is exactly the same. I say that
   

focal
 

length, aperure and focal distance determins the DOF.
   

This part is true, to an extent.
However, if you are going to use the same focal length, but change
formats, then really, camera to subject distance pretty much has to
change to be photographically useful.
Or, if you are going to change formats, but not camera to subject
distance, then you need to change lenses.
In theory, I think you are correct.

In practice, I find that I have to stop down a bit more on 6x7 to get
similar DOF as I get with 35mm, presuming a similar angle of view is
being done on both cameras.
 

IMO COF theories are somewhat subjective, because the point to
   

where a point
 

looks like a disc, depends on the degree of enlargement. I think
   

that the
 

smaller image, captured by a shorter focal length needs more
   

enlargement,
 

thus less appearing less sharp.
   

Well, yes. Of course.

William Robb