Calculating depth of focus


eoinandpaula

Link Posted 22/07/2013 - 13:10
Does anyone know of a simple(ish) formula which would allow me to calculate the probable depth of focus from the distance to the focus point, the aperture setting and any other item that would need to be factored in? Or is the calculation either too difficult to be done with mental arithmetic or does the depth vary from lens to lens and therefore not easily quantifiable?

Regards to all

Eoin Hanley

johnriley

Link Posted 22/07/2013 - 13:19
What you are referring to is Depth of Field. Depth of Focus is something else entirely and refers to the amount of leeway available in the actual location of the sensor plane.

There are depth of field tables in books and no doubt on the web, but if you want a simple "rule of thumb" for landscapes then just use a small aperture, such as f/16, and focus on a distance about a third of the way in to a scene.

A thorough understanding of depth of field is useful, so a suitable book on lenses would be the way to go IMHO.
Best regards, John

gwing

Link Posted 22/07/2013 - 13:23
I'll leave it your judgment as to whether you consider the formula's simple but have a look at http://en.wikipedia.org/wiki/Depth_of_field

Depth of field does indeed vary according to the lens, and the aperture, and how far away you have focused so working this out isn't very practical. Some folks have carried around pre-printed depth of field tables that save some maths or, I guess, the modern equivalent of a calculator or phone with the formula plugged in.

I find that in practice the best way of handling aperture and depth of field reasonably accurately in the field is to use one of the older prime lenses that shows depth of field on the lens thus making it much, much easier. It is however all an approximation so, once you have done that enough, you can probably make a decent stab at guessing what depth of field is likely to be even with a modern lens omitting those DOF scales. Alternatively you can in theory stop down the lens to view at taking aperture and actually see in the viewfinder what will be in focus - that hasn't ever worked well for me though as I find it too dark and too difficult to judge by eye.

McGregNi

Link Posted 22/07/2013 - 14:18
A good read up is definitely good advice, if just for general knowledge. For critical work you might need to do some maths or use lens scales (if included on yours). But for general photography when you don't want to add too much time to your shooting technique, then the 'hyperfocal' distance approach is perhaps most practical.

Again, this distance would need calculating for exactness, but as John said above, in most situations getting something sharp at a third of the way into the scene would work in many situations.

f16 would certainly pull most of the foreground into focus at many focal lengths, but I find that there aren't many times I could really use f16 - it generally would cause too slow a shutter speed, unless you're in very bright light, or using a tripod then that won't matter. Also 'diffraction' has to be a concern at this aperture surely.

If foreground is important (eg for wideangle shots) then you could get away with a wider aperture, f11 or f8 even - you may have to pull your focus point in a bit (in front of 1/3rd) to get the critical elements of the shot all within the DOF.

We've got DOF preview on the DSLRs of course (maybe even on the other Pentax types ?). This has to be selected in the menus first (as an option instead of digital preview), but in reasonable light it provides a great way to check the exact effect you're going to get. Digital preview is more versatile for checking a lot of things, but is perhaps not so precise for checking DOF on the LCD. But, you can't have both at once!
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Defragged

Link Posted 22/07/2013 - 14:43
AP ran an article I believe in a recent issue (week b4 last I think) on maximising depth of field. It included depth of field tables for different apertures as well as instructions for calculating DoF. Might be worth locating a copy and having a read. Other than that, check out the 'DOFMaster Depth Of Field Calculator' here:- link
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MrB

Link Posted 22/07/2013 - 15:04
The rules and calculations are not simple but there are several free apps for smartphones to work out depth of field for you instantly after entering the camera, focal length, aperture and subject distance. E.g. For Android phones open Google Play and search for "depth of field calculator"; I expect there will be similar for iPhones.

Philip

johnriley

Link Posted 22/07/2013 - 15:04
Quote:
Also 'diffraction' has to be a concern at this aperture surely.

Fortunately, in practice it's not a problem and you'll get crisp shots at f/16. They won't be as crisp as the optimum aperture maybe, but still absolutely good enough. f/22 and beyond is another story, but if you need the DOF then, as always, there's a trade off.
Best regards, John

eoinandpaula

Link Posted 22/07/2013 - 15:17
Hi All and thanks

John - I always understood that the proper term was, as you pointed out, depth of field but I remember a posting from a few months ago which stated depth of focus - but I may well have remembered incorrectly.

Thanks for the 2 links - not quite what I had hoped but lenses are complex so I was expecting too much.

I recently got the Sigma 150-500 and was surprised by the shallow depth of field at f/6.3 when photographing birds at relatively close quarters (10 metres). Obviously I can easily go to f/8 or beyond but the speed suffers.

I believe that I will download Defragged's calculator and see if I can derive some simple tables for the lenses that I am likely to use on safari in South Africa in 3 weeks time.

Now all I need is an answer to my other post about the continuous autofocussing on the K5II

Regards

Eoin

JAK

Link Posted 22/07/2013 - 17:19
You're probably are already of of these two points, but in case it helps I'll mention them...

1. Many of the older prime lenses actually show the depth of field available at each aperture such as this M50mm f2.



Obviously this is harder to implement in a zoom lens.

2. Don't forget most DSLRs have an optical preview button to save fathoming out the maths!

John K
John K

gwing

Link Posted 22/07/2013 - 17:36
If you are trying to get reasonable DOF with a long lens, particularly at close distances ...

It is going to be impossible, DOF will always be shallow.

To improve things as much as you can:
a) Increase camera ISO to as high as you can tolerate noise-wise.
b) Stop down the lens a bit. Changing down one stop is generally always going to be a good thing as it'll give you a little more DOF and probably improve the lens's IQ as well.
c) Check the resulting shutter speed. If its faster than you need than hey presto you have some leeway to play with. Either stop down more, or increase the shutter speed or decrease the ISO - whichever is most important for the shot you are taking.
d) If it's too slow bad luck. Something (ISO,shutter or aperture) is going to have to be sacrificed. Or use a flash ...

SteveEveritt

Link Posted 22/07/2013 - 18:26
If you have an iPhone I recommend TrueDoF it you input your focal lens' length then you use the slider for aperture and it will display the approximate DoF.
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MrB

Link Posted 22/07/2013 - 20:26
JAK wrote:

Many of the older prime lenses actually show the depth of field available at each aperture such as this M50mm f2.

The depth of field scales on these older lenses assume a full frame image (35x24mm on film). However, with a Pentax K-series crop frame sensor (24x16mm), the actual depth of field is only similar to that on the scale for wide apertures (e.g. f/2.8 ) and short subject distances, at which the D.o.F. is unreadable anyway, as it is almost nil.

Philip

simonkit

Link Posted 23/07/2013 - 09:30
If you have an android mobile have a look at the DOF CALC app, I use it all the time

Simon
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JAK

Link Posted 23/07/2013 - 11:53
MrB wrote:
JAK wrote:

Many of the older prime lenses actually show the depth of field available at each aperture such as this M50mm f2.

The depth of field scales on these older lenses assume a full frame image (35x24mm on film). However, with a Pentax K-series crop frame sensor (24x16mm), the actual depth of field is only similar to that on the scale for wide apertures (e.g. f/2.8 ) and short subject distances, at which the D.o.F. is unreadable anyway, as it is almost nil.

Philip

Not quite sure what you are saying here. If I pick up one of my prints taken with a 35mm film camera plus 50mm lens, take a pair of scissors to it (the print!) and crop the sides off so it is now half its original size, has its depth of field suddenly changed somehow?

John K
John K

johnriley

Link Posted 23/07/2013 - 12:00
JAK has made a good point. The DOF at a given aperture will always be the same, regardless of the format behind it.

What changes is the the field of view has changed, so to get the original image back needs a wider lens. Conversly, the cut down picture is now a mild telephoto shot with the same DOF as a standard lens on the bigger format.
Best regards, John
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