Depth of field

When you focus on a particular point you have an area of acceptable focus in front of where you are focusing and an area behind where you are focusing.  Most photographer know this already – it is your depth of field.

What you may not know is that the distance behind your focus point that is in acceptable focus approximately twice the size of that the distance in front.   In other words approximately two thirds of the depth of field lies behind the focus point and only one third in front of it.

I say “approximately” because the rule breaks down as we approach infinity as our focus point.  And of course, if you focus on “infinity” you are effectively throwing away two thirds of your depth of field – because there is nothing beyond infinity.  (Despite what Buzz Lightyear might try to tell you!).

Hyperfocal Distance

Landscape photographers use the knowledge that there is a distance behind the focus point that is in focus to get the maximum depth of field for their images.

If focusing at infinity ‘wastes’ the depth of field that is behind your focus point, logically there must be a distance short of infinity that you can focus on where objects behind the focus point all the way to infinity will be in acceptable focus.  That point is known as the hyperfocal distance, and it gives the maximum depth of field for the aperture you are shooting

One definition of hyperfocal distance is:

“The hyperfocal distance is the closest distance at which a lens can be focused while keeping objects at infinity acceptably sharp; that is, the focus distance with the maximum depth of field. When the lens is focused at this distance, all objects at distances from half of the hyperfocal distance out to infinity will be acceptably sharp.”

Lenses with Hyperfocal Distance Scale

Some lenses have a hyperfocal distance scale on them (less common these days). To use the scale you set the “infinity” distance on the focusing ring to the marker indicating the current f-stop.  When set to this we get the maximum depth of field, and all objects at infinity will be acceptably in focus.

Above image is public domain via Wikipedia image created by “Mark Sweep”)

 

The above image shows a lens with a hyperfocal distance scale on it.  The markings are for f/22, f/16 and f/11.  Assuming the lens was set to an aperture of f/22, we would set the infinity marker to the “22” mark on the left of the focus indicator.  The nearest point in focus could then be read by looking at the “22” marking to the right of the focus point indicator, ie 0.7m

The same scale can also be used to calculate depth of field for any setting.  As shown this lens is set to f/11.  We can then see from the scale that everything between 1m and 2m will be in acceptable focus.

The Photographer’s Friend

Modern lenses no longer include a DOF scale, or have hyperfocal distance markings inscribed on them.  Even those that do are not that precise and don’t cover every f-stop you might want to work with.  However iPhone, and Android apps are available to accurately calculate the hyperfocal distance.

The app I recommend is called “The Photographer’s Friend” – it’s an iPhone/iPad app created by photographer Martin Bailey.

The app is available in the Apple App Store and costs $3.99

Using the app is pretty simple, firstly select “DoF Cac” from the menu at the bottom of the screen.

there are four ‘dials’ in the lower third of the display.  The first dial, “format” is used to select the sensor size you are working with.  I use a full frame camera so I set that to “35mm” if you have a crop sensor you would select CF1.5 (for Nikon) or CF1.6 (for most Canon) or CF1.3 (for some early Canon DSLRs).  The app can also handle medium and large format film sizes as well as 4/3 sensors.

The second dial is for your aperture.  Available settings go from f/0.7 through to f/256, with both 1/3 and 1/2 stop intervals available.

Next, we have the focal length dial.  If you are using a prime lens simply set the focal length, if it is a zoom lens and you are not at one of the extremes of the zoom, you will need to estimate the focal length you are working at.  Again the possible range of settings are wider than any photographer is ever going to need going from 5mm through 1200mm.  If you have a crop factor camera, use the actual focal length as given on the lens – don’t convert it to the 35mm equivalent value.

The final dial allows you to input the distance at which you are focusing.  If you are using the app to calculate hyperfocal distance you do not need to put anything here.  However, if you want to calculate the depth of field when focusing at a particular distance this where you would enter the distance you are focusing at.

With all the values entered the app then calculates the depth of field based on the focus distance you entered.  The top section of the display shows the extremes of the depth of field and with the centre box mirroring the focus distance that you entered.

Below the graphic we have, the depth of field in the middle and either side of it the hyperfocal distance and the hyperlocal depth of field.

To use the hyperlocal information, simply set your lens to manual focus and set the focusing distance to the value shown in the hyperfocal distance box.

If your lens doesn’t have a focusing distance on it, you will need to estimate something at approximately that distance away from you and focus on that.  And that is exactly what I did to create the image at the top of this blog post.   I wanted to make sure that the hills in the distance and the jetty were both in focus.  The screen shot of “The Photographer’s Friend” was made as was creating the image.  So the settings it shows are the settings I had on my camera.  I estimated the end of the jetty was about 12-15m away from camera position so I used that as my focus point, safe in the knowledge that the hills would still be in acceptable focus.

The Photographer’s Friend can also be used to calculate exposure times when working with and stacking neutral density filters and has a building countdown timer if you are doing long exposures in ‘bulb’ mode.

You can find the full details of the app over on Martin’s website here:  https://www.martinbaileyphotography.com/photographers-friend-app-for-ios/

App Giveaway

Martin has very kindly given me codes to giveaway three copies of “The Photographer’s Friend”.  I am making the codes available to three members of my mailing list.  If you would like a free copy simply email me with the subject “Photo Friend” – if you are not already on my mailing list I will add you.  I will make the draw on Thursday 14th December 2017.

Non-iPhone Options

For those who don’t have an iPhone there are apps available for Android which can be used to calculate hyperfocal distance.  There are also print-out-and-keep tables for those who prefer a more low-tech solution.  See: http://www.dofmaster.com/ for details of all of these.

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