L-Lens DoF question

[felix_ing]

Hi,

 

Recalling years ago when I began photography, I remember reading something

about the f-number is the ration between lenght of the lens and the size of

aperture size (correct me if I'm wrong).

 

This got me thinking: how does the depth of field compare between f/2.8L and

f/4L lenses (i.e. 24-70mm f/2.8L USM v.s. 28-105mm f/4L IS USM)? (clearly

f/2.8 will still be a faster and more expensive lens)

 

What do owners of such lens(es) think?

Thanks.

[saskphotog]

Depth of field (I hope you are using that term as I understand it) depends upon the f-stop and the point of focus for any given lens. If two images are shot with the same lens, at a specific focus point, and at f2.8 and f4, the f2.8 image will have a shallower depth of field.

 

Whether the lens is expensive or not, the physics of light are the same.

 

If you change the focal lengths and do the same comparison, the variables become much more complicated. More complicated than I can be sure to get right.

[bobatkins]

Simple answers

 

(1) - You get less DOF shooting at f2.8 than shooting at at f4

 

(2) - All lenses of the same focal length have the same DOF at the same aperture. Doesn't matter if it's a prime or a zoom, an "L" lens or not.

 

Complicated answer - read the DOF articles and use the DOF calculators at http://www.bobatkins.com/photography/technical/

[christopher hartt dallas]

I have both lenses and to add even a bit more to the equation, even at f4, the bokeh (background blur) is different for these two lenses because Canon uses different refusing molds for the f4 than the f2.8. A matter of preference, I suppose, but I strongly favor the f2.8 bokeh over the f4, at identical apertures.

[felix_ing]

What I mean is this (which hugely depends if my recollection is correct - being that f-stops is calculated based on length of lens / opening of aperture) : say if both lens having the same aperture opening, say a 50mm diameter, but because one lens has a longer lenth, say 200mm vs a 100mm (hence f/4 vs f/2), then will we get the same depth of field?

 

Or as I'm gathering from the above answers, f-stops are absolute numbers, being that an f/2.8 will have the same depth of field with all lenses?

[mark u]

If you set two zoom lenses to the same focal length (as evidenced by the actual field of view) and use them at the same aperture focussed at the same distance using the same camera the depth of field will be essentially identical (you could argue that the lower quality one delivers slightly less depth of field due to extra aberrations). Depth of field depends on a number of variables, not only on aperture. Read Bob's links or explore here:

 

http://www.vanwalree.com/optics/dof.html

[mike_earussi1]

For the same magnification, say 1/10 lifesize, all f stops will have aprox. the same DOF regardless of focal length. The apparent differences in comparing different formats (35mm vs 645 vs 67, etc.), i.e. the larger the format the less DOF, come from the different magnifications on the film. The apparent differences between different focal lengths in the same format (28mm vs. 50mm vs. 100mm, etc.), which give the illusion of less DOF of telephoto lenses--hence their preference for portrait work, come from the way that the background is expanded in telephotos to cover the same area of film, which creates a smoother background. And yes, you are correct. The f stop is a ratio of the lens opening to the focal length--a 50mm opening in a 100mm lens will yield an f stop of 2 (100/50 = 2, 100/25 = 4, etc.).

[bobatkins]

"... say if both lens having the same aperture opening, say a 50mm diameter, but because one lens has a longer lenth, say 200mm vs a 100mm (hence f/4 vs f/2), then will we get the same depth of field?.."

 

Simple answer - No.

 

DOF depends on F-stop, not physical aperture size.

 

Infinity background blur when focused well inside the hyperfocal distance depends on physical aperture size (see http://www.bobatkins.com/photography/technical/bokeh_background_blur.html)

 

Background blur and Depth of Field are entirely different animals, as that article explains.

[William Michael]

> What I mean is this (which hugely depends if my recollection is correct - being that f-stops is calculated based on length of lens / opening of aperture) : say if both lens having the same aperture opening, say a 50mm diameter, but because one lens has a longer lenth, say 200mm vs a 100mm (hence f/4 vs f/2), then will we get the same depth of field?

Or as I'm gathering from the above answers, f-stops are absolute numbers, being that an f/2.8 will have the same depth of field with all lenses?<

 

 

I believe you are confusing two mathematical formulae:

 

The relative aperture of the diaphragm of any lens is expressed: N = F/D

 

Where:

 

N = the relative aperture (prefixed by`f`)

 

F = Focal length of the lens

 

D = Diameter of the effective aperture of the diaphragm

 

Thus, yes, there will be a 50mm effective aperture at f2 on a 100mm lens and also at f4 on a 200mm lens and at f8 on a 400mm lens.

 

But the formula for calculating depth of field is NOT a SIMPLE divisible ratio, but rather a formula which relies upon, in part, the focal length being raised to the power of 2

 

The Depth of Field formulae I cannot adequately (easily) print here, you can find it in any standard text.

 

Suffice to say, taking the specs in your question the following is reasonably accurate:

 

For a 35mm camera, a uniform circle of confusion and with the subject at 50 feet and lens focussed on the subject at 50 feet:

 

ndf = near distance focus

fdf = far distance focus

DoF = Depth of Field

 

100mm lens at f2 ndf: 45.8 fdf: 55.0 DoF: 9.2

 

200mm lens at f4 ndf: 47.8 fdf: 52.4 DoF: 4.6

 

400mm lens at f8 ndf: 48.9 fdf: 51.1 DoF: 2.2

 

I think this answers what you were asking.

 

WW

[bueh]

<b><a href="http://www.tangentsoft.net/fcalc/" target="_NEW">f/Calc</a></b> is your friend for all questions regarding photographic formulae.

[dogbert]

Depth of field varies with aperture being used, which is not always the same thing as the aperture of the lens (faster f-stop - smaller number - equals less dof)

 

Depth of field also varies with the focal length being used (shorter focal length = more dof).

 

And the distance to the subject (closer = less dof)

[scotty m]

The only thing I want to add to the answers above is to keep in mind that some lenses have a

bigger difference between the t stop and the f stop and that the DOF is based on the f stop

while exposure is based on the t stop. So a lens that loses more light, having a greater

difference between the physical f stop and the amount of light transmitted would require you

to shoot at a wider f stop to get the same exposure (same t stop), thereby creating less depth

of field at the same exposure.