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# Thread: Depth of field question

1. The article I referred to is by Ctein, not Mike Johnston. An opinion that Johnston's blog is "pretty" doesn't make Ctein wrong.

I posted this: http://www.apug.org/forums/forum206/...tml#post824775 to a similar thread earlier, which was a way of checking the stated 'rule' against the depth of field spreadsheet provided by Schneider Optics. It supports Ctein's observations.

There are apparently a couple of folks posting to this thread who are on my ignore list, so I should bow out and let you guys carry on. No need for me to create a lot of confusion and cross-talk. People who are concerned can research things for themselves.

Lee

2. OK, here's one more explanation, cited in the wikipedia article sirius glass cites:

A rule of thumb for depth of field is:

Depth of field is the same for all lenses when the image size is constant and the same f-stop is used.

This rule of thumb is approximately true when the focus distance for the shortest lens is less than about 1/4 of the hyperfocal distance for that lens.

... It can be shown mathematically that the rule is not exactly correct for any situation.
http://www.dofmaster.com/dof_imagesize.html

Lee

3. Once again, DoF calculations are derived from considerations concerning hyperfocal distance.
Then yes, you get the impression that there is a double dependency on focal length.
But it's plain wrong to confuse hyperfocal distance with DoF.

What depends on focal length (besides image scale) is the degree of increase of blur. The DoF range however is the same at same image scale, no matter what lens.

4. "It is not exactly true ..." because they used the wrong basis for their approximation. When magnification [same size image] is used then it is true, because that is the algebraic substitution that was made. By changing it to one quarter of the focal length the derivation does not apply. That would be like saying that your car got X mpg [or Y km/l], and then using that information to determine whether or not one should have orange juice for breakfast. The efficiency of the vehicle will have no bearing of the choice of beverage - of course it would not be exactly true.

Steve

5. Originally Posted by Sirius Glass
When magnification [same size image] is used then it is true, because that is the algebraic substitution that was made. By changing it to one quarter of the focal length the derivation does not apply.
The cited web page does use the same image size [magnification of an object on the film] for both the short and long focal length lenses. The change is not to make a given object in the image one quarter of each focal length, the statement is that the 'rule' you've stated only holds approximately when the shorter lens is focused closer that 1/4 of the hyperfocal distance for that lens at a given aperture. At longer subject distances the 'rule' you've stated doesn't even hold as an approximation. Both long and short focal lengths are used at a distance from the plane of best focus in the image that sets that plane at the same magnification.

Lee

6. Originally Posted by Lee L
[...] At longer subject distances the 'rule' you've stated doesn't even hold as an approximation. [...]
Only when you begin to mix in that pesky thing called infinity, i.e. start thinking in terms of hyperfocal again.
What's the scale of an image of something 'at infinity'? Yet that image has a size???

Etcetera.

I think it's time again to remind ourselves that DoF is in most part a fictional entity, and that how we perceive it depends on way too many variables. No mathematical description will be true much anyway.

7. Originally Posted by Q.G.
Only when you begin to mix in that pesky thing called infinity, i.e. start thinking in terms of hyperfocal again.
What's the scale of an image of something 'at infinity'? Yet that image has a size???
Infinitesimal?

8. Originally Posted by Lee L
The cited web page does use the same image size [magnification of an object on the film] for both the short and long focal length lenses. The change is not to make a given object in the image one quarter of each focal length, the statement is that the 'rule' you've stated only holds approximately when the shorter lens is focused closer that 1/4 of the hyperfocal distance for that lens at a given aperture. At longer subject distances the 'rule' you've stated doesn't even hold as an approximation. Both long and short focal lengths are used at a distance from the plane of best focus in the image that sets that plane at the same magnification.

Lee
It is still a useful rule of thumb to have. If you cannot get the depth of field you want for photographing a relatively near object, do not bother changing lenses => either stop down more and lengthen the exposure or stop down more, lengthen the exposure, and use a tripod.

Warren J. Smith thought enough of this to put it into his college level optics text books. Do you know more about optics than he did??

Steve

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