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Depth of Field explained

February 5th, 2011

DOF

Depth of Field (DOF) confuses many people, especially in relation to medium format cameras Vs 35mm format cameras.  Why do lenses that give essentially the same view on their respective format, say and 80mm on medium format and a 50mm on 35mm format,  exhibit dramatically different DOF, even at the same f-stop?

Firstly, what is Depth of Field?

The focal point of a lens is in fact a focal plane, a flat 2 dimensional field out in front of the lens where everything is actually and truly sharp (assuming a good quality lens), that is, a point in our subject focuses to a point on our film or sensor.  This plane shifts forwards and backwards as we move the focusing ring of the lens and is usually parallel with the film or sensor plane of the camera.  The DOF is the range fore and aft of this focal plane that appears to be sharp in our final image.  It is not actually sharp, in the strict definition of the word, but each point (from our subject) forms a circle of confusion on our sensor that is sufficiently small as to appear as a point in the final image, thus it looks sharp. This Depth of Field, also known as the range of acceptable sharpness,  is effected by 5 primary factors…

  1. The focal length of the lens:  The shorter the  focal length, (wide angle lenses), the greater the inherent DOF at all relative apertures.
  2. The aperture used:  The smaller the aperture, the smaller the Circles of Confusion will be.  Rendering more of the image with the appearance of being sharp… up to a point.  Beyond that point (this will differ for each lens), diffraction through the lens will start to have a significant effect, this will soften the overall focus regardless of the DOF increase.  Once the aperture declines below a point where the DOF blur is smaller than the diffraction blur, the image will begin to increase in sharpness again.  So some lenses with a big aperture range  can actually become sharper  at the extreme end of their  scale (such as f45 or f64).  Many large format lenses, particularly those made for landscape photography, are actually optimised for maximum f-stop use, and can be rather soft when used with less than this extreme aperture.
  3. The subject distance:  The closer the subject is to the lens the less is the apparent DOF, especially when taken to extremes in macro photography, when DOF can be measured in fractions of a millimetre.
  4. The format of the camera used:  The larger the format of sensor or film used the higher its resolving power, and the greater the level of detail it can record.  To create a print of the same size the medium format image requires far less enlargement, thus the larger the Circle of Confusion size can be and still appear sharp (in the same size final print).  Thus, for the same focal length, at the same subject distance, the same f stop, the greater the apparent DOF in the final print for the medium format sensor over the smaller format for the same size print.
  5. The enlargement of the image:  The more an image is enlarged the less DOF it will exhibit.  The circles of confusion that appear sharp in smaller prints (or screen enlargements) will become more obviously soft, both forward and behind the actual focal plane, effectively narrowing the DOF.

Depth of Field and Medium format cameras

If the same image is taken with two different format cameras, say a 35mm camera and a medium format camera, at the same subject distance, the same f-stop, with the same focal length lens  and the resulting images are printed  to the same size, the medium format image will have greater apparent DOF.

An 80mm lens is an 80mm lens  whichever camera its on, the focal length at infinity is 80mm and its DOF is a constant factor of the f-stop used.  What changes is how much of the image projection is included in the frame,  the angle of view.  A 35mm camera has a film area (or sensor) of 36mm x 24mm, whereas a medium format camera has a much larger area of film or sensor, usually between 60mm x 45mm to 60mm x 70mm. This is a considerably bigger area so the sensor “sees” a much wider view of  the image the lens is projecting, which changes the perspective to a wider field of view.

So effectively, the same focal length lens can be a telephoto with one format, but a wide angle in another, as illustrated in the example below.  This is an image shot on a 6×17 format camera with a 105mm lens.  It gives a very wide field of view, and a 105mm is considered a wide angle lens on this format camera.

The cut out outlined in black is a 35mm film format size.  What you would get if you shot the same scene, from exactly the same place with a 105mm lens (with the same exposure settings) on a 35mm camera, and you can clearly see it is a medium telephoto field of view.  Obviously both will have the same DOF.  This is a real example, its actually what you would get if you stood in the same place with the two cameras and took the same shot on both with the 105mm lens… Or, you could simply take it on the 6×17 camera, develop the film, and take a pair of scissors and cut out a 24x36mm rectangle, and get exactly the same result.

Fortescue fallsin Karijini NP used to show the differences between large and small format cameras

A 35mm perspective size cutout, in a shot taken with a 105mm lens on a 6x17cm large format camera

 Large Depth of Field

What seems to be the difficult part for people to understand here is the field of view, and how the DOF relates to it… in short, it doesn’t relate at all.  Although a 50mm lens on a 35mm format camera and an 80mm lens on a Medium format camera give you a similar field of view, they will not give you the same DOF for a given aperture.   Its an easy mistake to make, to assume because your angle of view looks the same, your DOF will be the same too… But DOF is dependent on Focal length, not field of view, so the 80 mm lens on the medium format in fact has the same DOF as an 80mm lens on 35mm format (for the same aperture).

The Way it was

Back in the good old days (I’m showing my age here) when photographers shot on a variety of formats it was no problem.  We all understood that how wide a lens’s field of view is, is determined by the format of the camera its used on.  We also understood that DOF was not determined by this field of view, but by the actual focal length of the lens (and the f stop used).  So you would just shift gears with each change in camera format, and understand that, for example, a 90mm lens on a 5″x4″ (12 x 10cm) camera was a really wide lens, whereas on a 6x6cm medium format it was a slight telephoto, but the DOF at the same aperture would be the same (excluding the enlargement factor).

The new Issue

Since the ascendence of 35mm cameras they have become the benchmark and since most people never use other formats they never see the possible variance.  Thus a normal lens is 50mm, a 100mm is a telephoto and a 35mm is a wide angle… all the time.  While everyone is shooting the same 35mm film format that is fine, but then the camera companies started coming up with APS c sensors, micro 4/3 sensors and all the other tiny sensors in smaller point and shoot cameras and phones.

My point and shoot camera has a zoom lens that they have very kindly converted to 35mm speak as 28-112mm, which is about what it looks like on the screen at the back.  The 28mm end of the scale looks nice and wide and pretty much how you would expect a 28mm lens field of view to look, and the 112mm end of the zoom gets up nice and close like a small zoom should…

But then I read the fine print in the manual and the lens is in fact a 7.1-28.4mm zoom… so what?  Well, what it means is that at full zoom, at what looks like 112mm (in 35mm speak), I get the DOF of a 28mm lens, which has just about everything in focus, even with the aperture wide open, which is almost never what you want with a telephoto lens.  So you see now we have the opposite problem to previously, now people are getting far more DOF than they expect and our images are looking flat…

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