Lens depth of field calcs

[swmcl]

Hi,

I'm wondering if I could ask one of the fine LF members to compare two lens DOFs for me. I have no calculator now coz I bought a new computer with Win7. I used to use the Bob Atkins calculator on Win XP.

One would be a Xenotar 150mm @ f2.8 and the other would be a Xenar 178mm f3.5. Assuming the same head-and-shoulders shot on 4x5 film.

Otherwise, I wonder if there is another calculator for Win7 of the same capability as the older Bob Atkins one ?

Cheers,

Steve

[Lachlan 717]

Hi, Steve.

Have a captain here:

http://www.csgnetwork.com/depthoffieldcalc.html

Should work.

[swmcl]

Thanks for the link but how do I know the magnification figure? I'm presuming I put the figures in for say the first lens... say 2 metres to subject, 150mm focal length, f2.8, 4x5 film ... this will have a 'magnification'. I then fiddle with the subject distance in the second calc to arrive at the same magnification yes ? So it might be something like 178mm focal length, f3.5, 4x5 film and say 2.3m to subject...

Then I compare the two DOFs.

The other thing that used to be in the Bob Atkins calculator was a Circle of Confusion figure which I have forgotten.

I've got a feeling nothing beats the Xenotar for lack of DOF that's why they cost so much!!

Cheers,

[Emmanuel BIGLER]

Hello to all people who hate the "group f/64" approach where everything is (supposed to be) equally sharp from one foot to infinity

The computations are simple ; hence I hardly ever use any DOF calculator, I always prefer to re-start from the usual formulae and cook my own soup according to what I need.
For example you'll have hard times finding a DOF calculator where your input is a magnification ratio and not a distance ;-)

Dof chart for f=150mm and f=178mm at a given distance between 1 metre and 3 metres.
DOF-chart-f150-f178-distance-2metres-CoC100microns.pdf

Dof chart for f=150mm and f=178mm at a given magnification ratio (image size)/(object size) between 0.05 (=1/20) and 0.33 (1/3)
DOF-chart-f150-f178-CoC100microns-MAG-0.05-0.3.pdf

same in the range of magnifications between 0.1 (1/10) and 0.33 (1/3)
DOF-chart-f150-f178-CoC100microns-MAG-0.1-0.3.pdf

For all calculations I have taken 100 microns for the diameter "c" of the circle of confusion.

As it can be seen, in this portrait setup we are already close to the macro regime where total DOF is independent from the focal length at a given magnification ratio and depends only on the f-stop and the magnification ratio. Hence, if you keep the same magnification ratio for both lenses, a same f-number will yield exactly the same total DOF. Eventually the 2.8 Xenotar will win, but by half a f-stop only

My computations are made at zero tilt / no Scheimpflug, objvet and film are ideally plane and parallel and perpendicular to the optical axis. The usual formulae are valid only for quasi-symmetrical lenses (formally, those formulae are not valid for telephotos, but xenotar and xenar lenses are very close to a symmetrical design, at least as far as DOF computations are concerned; pedantly, we'll no be afraid on this forum to mention that their pupillar magnification ratio is close to unity )

Those formulae read as follows:

(1/p_{1,2}) = (1/p) {+ or -} (1/H).(1-f/p)
p_1 and p_2 are the near and far limits of acceptable sharpness around the subject located at a distance "p" for a given value of "f","N","c" and distance "p".
total DOF = p_2 - p_1

f is the focal length
H is the hyperfocal distance computed as usual H=f^2/(N*c)
N is the f-number
c is the diameter chosen for the CoC
A starting value for "c" is equal to the diameter of the format divided by 1720, this yields a limit of resolution of about 2 minutes of arc for a print examined at a distance equal to its diagonal.
For 4x5", the diagonal of the format is bout 150 mm hence a starting value for c = 150/1720 = 87 microns.
100 microns is slightly less stringent but here we only need a rough value for the DOF.

It should be noticed that when the magnification ratio M = (image size)/(object size) is greater than about 0.1, we gradually enter into the macro regime where the focal length has no longer any direct effect on DOF.
In this regime, the approximate form for the previous formulae yields

total DOF for M > 0.1 = (p_2 - p_1) ~= 2 N c . (M+1) / (M^2) = 2 N_{eff} c . 1/(M^2)

where N_{eff} is the effective f-number = N(1+M) taking into account in the macro regime that the distance between the lens and the image becomes greater than the focal length.
In the macro regime, DOF extends approximately symmetrically around the object; which is not true for far-distant objects.

In this macro regime DOF becomes exceedingly easy to compute, no need for any software, no need even for a computer

Ex. tight head-and-shoulder portrait, assume that we fill a 20x24" subject frame on a 4x5" film format ; M=1/5=0.2 > 0.1
1/M = 5

Hence the approx formulae for macro are valid, and total DOF @ f/2.8 whichever focal length, for M=0.2, 1/M = 5, and c=0.1 mm is equal to
2 ... N ... c .. (M+1)(1/M)(1/M)
2 x 2.8 x 0.1 x (1.2) x 5 x 5 = 17 mm

Outch! Can't get the eyes and the ears sharp at the same time!!

[Ed Richards]

If you have a smart phone, there are cheap/free apps that do the calculation. I use them in the field when I am not using a Sinar. (Sinar's have a built in DOF calculator.)

[Jim Jones]

The difference in focal lengths is not that great in the two lenses cited above. Nor should the difference in those lens's formulae make much difference. The DOF should be similar. Rather than use a calculator, consider downloading and printing a DOF chart. It's handier while studying and doing photography.

[swmcl]

Hello again,

Once again the community provides in spades ! Thank you Emmanuel. I thought I couldn't quite get the Xenotar results. Hi Ed, I have only a very basic phone. Jim I haven't heard of these charts, I shall indeed have a look for some.

Cheers guys.

[Leigh]

DoF Master http://www.dofmaster.com/dofjs.html

- Leigh

[Emmanuel BIGLER]

Rather than use a calculator, consider downloading and printing a DOF chart

I totally agree with Jim. Print charts, use slide rules like the Rodenstock DOF calculator (a kind of of a circular slide rule) or whichever simple thing with 0% electricity that suits you.
My arguments would be: if I insist on using a large format film camera fitted with a mechanical leaf shutter and 0% electricity on board, I do not want to be tethered to any kind of computer
Yes, I certainly need a good exposure meter, and modern ones look very similar to smart-phone, but I could always use the sunny-16 rule in case the batteries are dead

I thought I couldn't quite get the Xenotar results
Well, except for half a f-stop, 3.5 instead of 2.8, you get extremely close to it with your xenar lens for many good reasons explained by various forum friends above.

Large format photography likes to challenge us in what we had been used for decades with small format cameras. At least for those of us who have started with a small format camera.
The idea that as soon as you get close to your subject, closer than about ten times your focal length (hence with magnification ratios greater than 1/10), DOF no longer depends on the focal length at a given magnification ratio & f-number, is something totally counter-intuitive.
And this can make a relatively large distance: for example if you work in the 8x10" format with a standard 300 mm lens : 10x300mm makes 3 metres, about 10 feet. This distance does not look like a "macro" range at all !!
And do not forget that for any kind of picture, perspective rendition depends only on the absolute distance to the subject and not at all on the focal length nor magnification.

[Leigh]

DoF charts and calculators are sort of meaningless for LF since there is no distance scale on the lens/camera.

Sinar probably has the best "calculator" built into its focusing system. The GG is the only reliable measure.

- Leigh