Comments on: A better depth of field table

By: Nate Davis

Nate Davis — Mon, 02 Dec 2013 22:39:23 +0000

Great. I plan to rebuild the chart in ft. and inches, since that is what I am used to. I also will just do the middle of the chart, because I won’t use the short end and, as you have mentioned, the long end is usually all in focus anyway. Thanks for answering my question and for posting the article.

By: bernie

bernie — Sun, 01 Dec 2013 15:04:10 +0000

> Would it be as simple as using the 1.5 height to width ratio that my canon crop sensor uses?

It would be exactly that simple! The chart requires the length of the long edge of your frame, so width in landscape or height in portrait. If you know that the short edge of the frame (height in landscape or width in portrait) is say 2 meters, then multiply it by 1.5 and read along the 3 meter row on the chart.

By: Nate Davis

Nate Davis — Fri, 29 Nov 2013 04:56:22 +0000

This is brilliant! Thank you. If I shoot landscape, then I would measure the length of the bottom of the frame at the plane of the subject, correct? Portrait seems easier, because I can use the height of the subject to calculate the height of the frame. Any pointers for how to make the calculation for landscape? Also, could another chart be built using the height of the frame while shooting landscape? Would it be as simple as using the 1.5 height to width ratio that my canon crop sensor uses? Thanks again.

By: bernie

bernie — Tue, 14 May 2013 15:43:11 +0000

You make a good point. Basically, this guide is most useful for macro photography, still life, close-up portraits and such like. With large objects a long way away, the whole scene tends to be in focus so this chart isn’t really needed anyway.

By: Hankk

Hankk — Tue, 14 May 2013 13:56:23 +0000

Nice! I like the approach here.

You say not to trust the values in the grey. But for things > 2 m and apertures at f/2.8 or slower, in the region of the table bounded by these values, the table is over 90% grey! What approximations are involved in the DOF equation, and can they be done to higher precision, so as to make the table more useful?

Also, whether intentional or not, the greys don’t seem to appear on the PDF versions of the table.

By: bernie

bernie — Sun, 10 Jul 2011 03:20:50 +0000

OK, I think I see the issue. The difference between Wikipedia’s equations and mine is some spurious additions and/or subtractions of focal length.

The reason for this is that Wikipedia measures subject distance from the sensor, and I measure it from the Nodal point of the lens. In a simple lens design, the nodal point will be the same distance from the sensor as the focal length.

If the distance to the subject is large compared to the focal length of the lens, .e.g when you’re taking a picture of a person, the difference doesn’t matter. For closer subjects it starts to be important where you measure the subject distance from.

Bernie :o)

By: Sam

Sam — Fri, 08 Jul 2011 04:02:14 +0000

Thank you so much for the reply. Sorry for didn’t make it clear. I get the equation from this page’s inline script.

function getDistanceForSubjectSize(size, sensorSize, focalLength) {
return size * (focalLength / sensorSize);
}

I was studying your script and tried to rewrite it in python, the only language I know (with options for coc, sensor, subject sizes, unit conversion, colors and etc) to help myself understand DOF.

At the same time I was reading the DOF page on wikipedia, gathering equations, and attempted to generate a table to compare magnification dependent dofs and focalLength dependent dofs of a given subjectSize. That’s how it all started.

Sam

By: bernie

bernie — Fri, 08 Jul 2011 01:58:02 +0000

Sorry you’ve lost me. Where are you getting these equations from?

If subjectDistance = size * (focalLength / sensorSize) then operating a zoom lens (changing focal length) would either alter the distance to your subject or the size of the subject. This would be quite a remarkable effect, and I have not observed it in the field ;o)

By: sam

sam — Thu, 07 Jul 2011 19:02:31 +0000

Thank you so much for putting some practical sense to the use of DOF. One thing I do not understand is the the formula that you use for subject distance is:

size * (focalLength / sensorSize)

However, given
m = f / (s – f) # from wikipedia
m = sensor size /subject size # from bernie
then
sensorSize / subjectSize = f / (s – f)
s – f = f * (subjectSize / sensorSize )
then s (subjectDistance)
s = (f * (subjectSize / sensorSize )) + f # is it right?

I am just gettting into the subject of DOF, so please correct me if I am wrong. Thanks again.

Sam

By: holroy

holroy — Thu, 21 Apr 2011 09:53:21 +0000

I like this table as it gives me the depth of field directly even when I’m varying my focal length, which is a constant variable in most of the other DoF tables. Here you can simply focus on how large your subject is and find the approximate depth of field. More than close enough for my use!