Depth of Field

[zigipha]

I have read alot about DoF.

In a coulple of CiC articles, it states that magnification has an effect on DoF (which is why zooming in seems to change DoF - not because of change in focal length but due to change in angle of view).

i have not nees a write up as to why zooming in changes DoF..any pointers?

Thanks!

zigi

[mikesan]

The definition of DoF states that it is the range over which the focus is acceptably sharp. "Acceptably" being the operative word. The various equations by which DoF are calculated all assume a certain magnification (or print size) of the original image. That is why zooming in can result in a decrease in DoF. The more you enlarge the image the less acceptable the blur.

[IzzieK]

Welcome to CiC...If you will take the time to go to the top of this message, you will find a My Profile option. If you click on that button, you can fill up your details as to where you are from and what you want us to refer to you as just like most of us do. It will help you a lot when you need response to your questions too.

[zigipha]

Mike,
Thanks for the reply, but.....

I have seen comparisons of FF cameras taking a pic with 100mm lens and a mft taking same pic with 200mm lens, same F stop and the mft is definitely sharper (here for example), and in the CiC article here, focal length changes dof due to "magnification". So the magnification referred to is in the optical enlargement before it hits the sensor, not due to printing. I have read quite a bit on this, and this point is eluding me.

[dje]

Hi Zigi

You may need to delve into the mathematics of it if you are so inclined to get a proper idea of what is happening. This article here might help, it's not overly mathematical.

Dave

[jcuknz]

i have not nees [seen ? ] a write up as to why zooming in changes DoF..any pointers?
zigi

If we ignore the mis-use of terms on this thread

To zoom-in one is changing the focal length of the lens in use ... the angle of view is decreasing resulting in a smaller circle of confusion hence a greater DoF.
When enlarging one is increasing ones view of the size of the CoC which results in less apparent DoF.

But we come back to the well established fact than given a certain object size photographed to produce a given image size there is little or no difference in DoF whatever focal length is used.

[xpatUSA]

Mike,
Thanks for the reply, but.....

I have seen comparisons of FF cameras taking a pic with 100mm lens and a mft taking same pic with 200mm lens, same F stop and the mft is definitely sharper (here for example), and in the CiC article here, focal length changes dof due to "magnification". So the magnification referred to is in the optical enlargement before it hits the sensor, not due to printing. I have read quite a bit on this, and this point is eluding me.

DOF is wonderful subject, often implying that there is some exact magic number (CoC) outside of which everything is blurred and inside of which everything is perfectly focused and super-sharp. Often, the literature quotes said "CoC" as it were an absolute fixed number for a particular camera.

My own research indicates that the said CoC, the basis of all DOF calcs, is pretty much anything you want it to be (admittedly, a slight exaggeration).

You might this reference of interest:

http://kronometric.org/phot/iq/DepthOfField-Lyon.pdf

Rather than skimming over the surface, as on-line calculators tend to do and rather than simply quoting some fabled Zeiss formula, it actually goes into the subject with some depth and is recommended for reading by those whose eyes are glazed over by all the standard cant in the literature.

[zigipha]

If we ignore the mis-use of terms on this thread

To zoom-in one is changing the focal length of the lens in use ... the angle of view is decreasing resulting in a smaller circle of confusion hence a greater DoF.
When enlarging one is increasing ones view of the size of the CoC which results in less apparent DoF.

But we come back to the well established fact than given a certain object size photographed to produce a given image size there is little or no difference in DoF whatever focal length is used.

But that is exactly what is NOT shown in the first link i provided. It shows the same image taking up the same angle of view, one with ff/200mm and one with mft/100mm f2.8, and the mft is sharper.

Thanks to others for replying; i will look into these links

[Manfred M]

But that is exactly what is NOT shown in the first link i provided. It shows the same image taking up the same angle of view, one with ff/200mm and one with mft/100mm f2.8, and the mft is sharper.

Thanks to others for replying; i will look into these links

Don't forget that you are enlarging the micro four-thirds (mft) (crop factor 2x) twice as much as the full-frame (FF) image to show the same image. Ted's explanation about the multiple variables that are play is correct. A simple (but not completely accurate) view that lets you figure this out is to apply the crop factor to the DoF as well as the effective focal length.

Just as a 100mm lens on a mft sensor has the same magnification of a 200mm lens of a FF factor, you should also do that to estimate the change of DoF. So a f/2.8 lens on a mft camera would have an "equivalent" DoF of 2.8 x 2 = f/5.6; i.e. more or less the same DoF of shooting the FF lens at /f5.6 or 2-stops more DoF. Doing the inverse; to get the same DoF as f/2.8 on the FF camera, you would need to shoot 2.8/2 = f/1.4 on the mft; i.e a mft lens that is 2 stops faster than the FF lens.

This is making Cosina some money with their fast, manual lenses aimed at the video market with their f/0.95 Voigtländer prime lenses. These are primarily aimed at the low light and shallow DoF crowd.

http://voigtlaender.com/mft.html

[William W]

[1.] I have seen comparisons of FF cameras taking a pic with 100mm lens and a mft taking same pic with 200mm lens, same F stop and the mft is definitely sharper (here for example),

[2.] and in the CiC article here, focal length changes dof due to "magnification". So the magnification referred to is in the optical enlargement before it hits the sensor, not due to printing. I have read quite a bit on this, and this point is eluding me.

These are two facts extracted from two DIFFERENT scenarios and that is probably why you are confused.

In the first link the camera FORMATS are NOT the same: hence the CoC (Circle of Confusion) will be different.

In the CiC tutorial it is assumed that the Camera Formats will be the same when discussing the 'magnification' apropos Depth of Field.

WW

[JHzlwd]

DOF is almost entirely a function of magnification (no matter how obtained) and F-Stop. Keeping that in mind helps clarify the concept.

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

There is a common misconception that DOF has something to do with lens design. It does not. Similarly, it has nothing to do with sensor size. Some commentators factor sensor size into the concept but that's only because they change focal length (hence magnification) to get the same field of view amongst sensors of different sizes. Doing this is misleading.

[William W]

DOF is almost entirely a function of magnification (no matter how obtained) and F-Stop. Keeping that in mind helps clarify the concept.

I agree that's a good general position to articulate the concept.

But a couple of small (and not general) points here:

. . . There is a common misconception that DOF has something to do with lens design. It does not.

It can: indirectly by way of camera design, for example a Camera with movements.

Directly (probably more pertinent to more general and modern photography forum conversations) with Tilt-Shift Lenses designed for modern Cameras

*

Similarly, it has nothing to do with sensor size. Some commentators factor sensor size into the concept but that's only because they change focal length (hence magnification) to get the same field of view amongst sensors of different sizes. Doing this is misleading.

My view is that there is on one hand the concept of DoF - and on the other hand the computation of DoF.

The latter can serve a practical purpose in day to day photography.

Some folk might not ever want to understand the concept: but they might want to know how to use the application to use as a useful guide to their planning of a shot.

It is certainly not misleading to instruct those folk how to achieve that outcome - and that requires them pumping into a DoF Calculator (or doing it long hand):
The Focal Length;
The Subject Distance;
The Aperture;
and an acceptable Circle of Confusion (based on Camera Format).

WW