Help with Depth of Field

You're defining magnification as an abstract size in space, which doesn't tell you much about the final picture until you know crop size.

I'm defining magnification as the size of the projected image size relative to the sensor. That tells you right away how the final image will look.

1.) The projected image size is 15 MM. What does that tell you? Not much from the photographer's viewpoint, only interesting to mathematicians.

2.) I want the object to occupy 50% of the final print size. Much more useful, someothing anyone can relate to.

So, definition (2) is more useful and immediate. To repeat: if you want your image to occupy a certain percentage of the final print, the only factors that affect DOF are 1.) the object size 2.) aperture 3.) sensor size. I think most photographer's can relate to this rule very easily.

tko said:

You're defining magnification as an abstract size in space, which
doesn't tell you much about the final picture until you know crop
size.

Click to expand...

No, actually I'm defining magnification in the strict sense. If an ant is 3 mm long, the image of it on the sensor will be 3 mm at 1:1, and 1.5 mm at 1:2. By your definition it would be impossible to talk about magnification, because it all depends on sensor size.

tko said:

I'm defining magnification as the size of the projected image size
relative to the sensor. That tells you right away how the final
image will look.

Click to expand...

Unless you crop it afterwards, right? You confuse enlargement with magnification.

tko said:

1.) The projected image size is 15 MM. What does that tell you? Not
much from the photographer's viewpoint, only interesting to
mathematicians.

Click to expand...

If the car is 15 mm high on the sensor, then no matter the focal length, DOF will be the same, and that's what I'm saying. The photographer may know nothing about the size of his sensor, let alone relative to the car, but if he tries to obtain more shallow DOF by using a tele, it won't help if the car occupies the same amount of space on the sensor.

tko said:

2.) I want the object to occupy 50% of the final print size. Much
more useful, someothing anyone can relate to.

Click to expand...

Read above.

tko said:

So, definition (2) is more useful and immediate. To repeat: if you
want your image to occupy a certain percentage of the final print,
the only factors that affect DOF are 1.) the object size 2.)
aperture 3.) sensor size. I think most photographer's can relate to
this rule very easily.

Click to expand...

So subject distance and focal length don't matter? I could have sworn they did!

... actually defends my point of view. There are more than one set of A and B photos; look again.

The question what F-number we need to be able to decipher the license plate of the car would yield the same answer for both lenses. Indeed, this aspect of blur, the question to what degree the various elements in the background are individually recognizable, does not depend on the focal length.

"Depth of Field" means the range of distances at which objects are acceptably sharp in a print.

"Background Blur" refers to parts of the image that are far from being sharp. Although the underlying phenomenon (defocus blur) is the same, there are some differences in practice.

Most notably, whereas DOF (when shallow) is a function of aperture, sensor size, and framing, background blur is strongly affected by focal length as well. Paul van Walree has a nice example, 100mm f/4 vs. 28mm f/4 (on 35mm format):
http://www.vanwalree.com/optics/dof.html#backgroundblur

Note that while the size of the blur circles relative to the background objects is unchanged, they are larger relative to the subject, or in absolute size on a print, at the longer focal length.

Although the OP used the term DOF, he was clearly referring to background blur. Many of the responses, having ignored this distinction, are misleading.

An interesting alternative to the usual DOF calculations:
http://www.trenholm.org/hmmerk/DOFR.html

Depth of field and sensor size:
http://www.photo.net/learn/optics/dofdigital/

--
Alan Martin

Klipsen said:

... actually defends my point of view.

Click to expand...

Actually, it supports both points of view. Your point of view describes "relative blur", while mine describes "absolute blur". I think the OP wants to maximize absolute blur: it doesn't matter if the license plate can be read, only that it looks blurry!

Humbre: if you've followed the discussion this far, please take a look at the first set of A/B photos in the "Background Blur" section here:
http://www.vanwalree.com/optics/dof.html#backgroundblur
and tell us if you consider A to be preferable to B for your purposes.

There's a similar example with less blur a little further down:
http://www.vanwalree.com/optics/dof.html#magnification2

--
Alan Martin