Depth of field for Medium formats, 645 and 67

[benjamin_kim1]

<p>I know the DOF for 645 that any F2.8 lens is similar to 1.4 in 35mm. But What about 67 with F2.8 lens? I have no idea what DOF of 67 have. Since I don't know about the DOF for 67, it's hard to control the DOF for portrait. Can anyone tell me the DOF of 67 for f 2.8, 4, 5.6, 8, and etc?</p>

[SCL]

<p>Hese you go: http://bobatkins.com/photography/technical/dofcalc.html</p>

[mukul_dube]

<p>I'll be glad to send a couple of DOF calculators if you get in touch by e-mail. Offhand, I'd say that for portraits you'll be better served by tests at f/2.8, f/4 and f/5.6, with all the focal lengths you use. Dry tables are not so useful in judging the real effect.</p>

[ellis_vener_photography]

Every lens for every 6x7 SLR that I amaware of has a DoF scale,except for the Mamiya RB-67 and RZ-67 and those

cameras have DoF scales on the bodies.

[chris_waller]

<p>DOF = 2u*2 NC/f*2 where u = the subject-camera distance, N = the relative aperture, c = the diameter of the circle of confusion for that format, and f = the focal length of the lens. All dimensions in millimetres. For medium format the circle of confusion is 0.06 mm.</p>

[ondebanks]

<blockquote>

<p>I know the DOF for 645 that any F2.8 lens is similar to 1.4 in 35mm.</p>

</blockquote>

<p>The f-stop difference between 35mm and 645 is not as large as that.</p>

<p>The crop factor/focal length multiplier from 35mm format to 645 format is 1.6. So a 50mm lens on 35mm is equivalent to an 80mm lens on 645 (this is convenient, as those two lenses actually exist, as the "standards" for their format).</p>

<p>The aperture for equivalent DOF must also change by 1.6x. Since 1.4x is a difference of 1 stop and 2x is a difference of 2 stops, this change falls between 1 and 2 stops, and closer to the 1 stop end. It amounts to this: an f1.4 lens on 35mm format has a 645 equivalent of f2.2. </p>

<p>For 6x7, the equivalent lens would be f2.9...so f2.8 is fine to use in this case. </p>

[rodeo_joe1]

<p>The DOF formula given above is very simplified and will go badly wrong at close distances. It also takes no account of the asymmetrical nature of DOF, which increases behind the subject at a greater rate than in front as the subject distance increases, or rather as the magnification decreases.</p>

<p>The simplest option is to use an online calculator that uses a more complex and accurate formula, such as this one: http://www.dofmaster.com/dofjs.html<br>

which also gives separate figures for the near and far limits of good focus.<br>

The online calculator above allows you to choose the CoC from a drop down menu. For 6x7 it should be around 0.06mm, and for 645 around 0.047mm.</p>

<p>The CoC is usually taken to be proportional to the diagonal of the format used. So the CoC used for 6x7 should be 1.3x greater than that used for 645. But then CoC figures are based on a very loose "standard" (as is any DoF calculation), and should really be geared to the final viewing size of the image.</p>

[john_stockdale2]

<p>"But then CoC figures are based on a very loose "standard" (as is any DoF calculation), and should really be geared to the final viewing size of the image."</p>

<p>Absolutely correct. DoF calculations generally are based on a postcard sized print. Why use medium format if you want to print postcards? Generally we're looking for something better (bigger).</p>

[craig_shearman1]

<p>"Since I don't know about the DOF for 67, it's hard to control the DOF for portrait."<br /><br />Are you shooting with an SLR? If you are, why do you need any numbers or tables or calculations at all? You look through the lens and you instantly see how much DOF you have. If you're shooting with a rangefinder, that's a different story, but a test roll or two would give you the answer without having to do any math.</p>

[MTC Photography]

Majority DOF calculation used simplification, hence cannot be used at close range.

 

Without any simplification, a precision depth of field formula can be found easily with Maple

 

The equations are:

 

> h := H = F^2/(N*coc)+F;

 

E0 := 1/d+1/D0 = 1/F;

 

E1 := 1/(d+e)+1/D2 = 1/F;

 

E2 := a/(d+e) = coc/delta;

 

E3 := a = F/N;

 

Let e=delta, delta being change in lens extension, get the depth of field near limit Dn

 

Dn= -D0*(-H+F)/(D0+H-2*F)

 

Let d=-delta, obtain depth of field far limit Df

 

Df = D0*(-H+F)/(D0-H)

 

The solution is the exact solution obtain simply by using Maple, which solves complex algebraic equations

with only a few lines of code

 

Note the focusing distance D0 cannot > the hyperfocal distance H, if D0>H, you will get minus nummber

which means the far limit is beyong infinity,(ie, the DOF far end bar | is beyong infinity treat it as

infinity

[MTC Photography]

For any one interested in detail attach the Maple code

 

eq := {E0, E1, E2, E3, h};

 

var := {D2, N, coc, d, delta};

 

e := -delta; solve(eq, var);

 

e := delta; solve(eq, var)

[MTC Photography]

For any one interested in details, attach the Maple code

 

eq := {E0, E1, E2, E3, h}; var := {D2, N, coc, d, delta}; e := -delta; solve(eq, var); e := delta; solve(eq,

var);

[MTC Photography]

The usual coc =1/30 was suggested by Leica for 35mm negative enlarged 8x at a viewing distance of 25cm

 

For Tessina subminature camera, which has a small format negative, 8x enlargement is 4.4x6.6 inch

postcard size, that is too too small, hence you need double it to 16x, thus the coc must be halved to coc

=1/60;

 

For medium format if you still use 8x magnification aka 35mm format, you get a size of 18.8 x 18.8 inch

enlargement to view at 25mm, which is apparently too big for photo magazine or books, hence the

enlargement

factor need to scale down to 8 x 8 " size, that is you need only 3.3x enlargement, hence

coc for Rolleiflex TLR is 3.3 x 1/30 = 0.11