Depth of Field ain’t so Tuff

Depth of Field ain’t so tuff.

Recently I needed to take some photographs of the antique glassware that my spouse collects.  This is easy with a point n’ shoot, since its depth of field is pretty good thanks to the short focal length that most point ‘n shoots have in their built-in lenses.  The point ‘n shoots usually have a wide angle (anywhere from 6 to 15 mm – but sometimes variable up to higher focal lenghs with fixed zoom offsets).

The manufacturers figure that the point ‘n shoot is for Benny’s Birthday, and wide angle is good for kids gathered around the BDay cake.  The typical DSLR comes with a kit lens of 50 mm – right away not wide angle enough to be optimal for Benny’s party.  The DLSR is much more likely to have actual glass in the lens however, and a more pristine image possibility.

So, the manufacturers figure that instead of Benny’s BDay party, you’ll be taking your DLSR or Mirrorless out to the great outdoors to photograph a mountain, or sheep on the hillside.

I needed to take the glass pictures indoors, with an antique lens.  I figured I should take the pics of the antique glass with an antique lens – it seemed appropriate to do that.  So – a DoF figure doesn’t appear magically in the viewfinder because the antique lens provides no feedback to the camera.  I’m in fairly low light, which means opening up the aperture.  And – the lens barrel has DoF markings but they’re way too coarse for the small numbers I’ll be seeing under the described conditions.

The cavalry could come in the form of a table, or an equation.  The equation is more fun to talk about – and really pretty easy to use.  Just tape the equation to the bottom of your camera, carry a small four function calculator, and you’re in business.

So all the values in the equation must be in meters for it to work (can’t be using mm mixed up with meters or will get a wrong result).  One can see that the distance to the object plays a big part (d, or distance, is squared in the equation).  Focal length plays a big part, cuz it’s squared in the devisor.  The F-number aperture could help by being bigger, but my low light situation means it’ll be small (like 2.0).  And CoC (Circle of confusion) – will be set primarily by my sensor, which I cannot do anything about (other than use another camera).  For the typical 35 mm DSLR, the CoC is about .028 – .030 mm.

I have a crop sensor camera.  So – the focal length of the lens (which was cut for full frame) – must be multiplied by the crop factor to get the effective focal length for the shoot.  So, 50 mm * 1.5 = 75 mm.

So, my 50mm lens and distance of 1.5 meters gives me:

  • DoF = (2 * 1.5m * 1.5m * 2 * .000028m) / (.075m * .075m)
  •  .000252 / .00564 = .045m = 45 mm = 4.5 centimeters.

Note that .028mm CoC is .000028 when converted to meters!  My figures are based on a distance of 1.5m, aperture of 2.0,  CoC of .000028m,  and an effective focal length of 75.

The first couple times running this equation takes a little bit of time, but after a dozen cycles it’s pretty easy and automatic.  OK – so 4 centimeters is a poor depth of focus/field for glassware shots.  Some of the antique glass pieces will be 12 centimeters (or more) in diameter.  So what should I do?

I could use a wider angle lens of course, and I happen to have an antique 35 mm lens.  With the crop factor, that is 52 mm effectively.  So, let’s see what that does for our equation:

  • DoF = (2 * 1.5m * 1.5m * 2 * .000028m) / (.052m * .052m)
  • .000252 / .00271 = .093m = 93 mm = 9.3 centimeters. 

Dang! We’re still not there. OK, so what about my 28mm lens?  Yup – got one of those that’s 35 years (or so) old – not quite antique, but hey – it’ll work in my plan.  WIth the crop factor it is 42 mm effectively.

  • DoF = (2 * 1.5m * 1.5m * 2 * .000028m) / (.042 * .042)
  • .000252 / .00176 = .143 m = 143 mm = 14.3 centimeters.

Yes!!  I thought of one thing while I was writing this.  The aperture should also be multiplied by the crop factor, making my 2.0 f-stop aperture a 3.0 f-stop aperture effectively.  So, I’ll rework the equations shown above when I’ve got time.  It’ll make a little difference, but the numbers shown are still pretty ballparkish.

So, what happens when we play with distance?  We’ll leave that for our next installment …

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