Beginner’s Guide to Depth of Field

This article is written mainly for those who want to know the very simple concepts which allow camera operators to manipulate the Depth of Field in order to make them look more cinematic.

Achieving the correct choice of lenses is paramount – there are complete ranges of film & DSLR lenses with such strange concepts as sensor coverage & T stops or F stops and these can make an enormous difference to how the images look and choice can be overwhelming and a little confusing!

This article will look at the issues below and give a very simplified view of what the basic concepts are in order to help the aspiring cinematographer make good decisions in their choice of equipment.

You have several factors to consider and these issues will be introduced and then discussed briefly, after a short précis of the article.

  • Lenses make a massive impact to the production values of a given production.
  • Film lenses are capable of greater production values than DSLR lenses as a result of superior optics and minimised focus breathing and better focus control for pulling focus.
  • Generally, prime lenses are generally capable of a more shallow depth of field than zoom lenses (only if used at the wider apertures that they are capable of, compared with zooms) but primes will need to be frequently changed instead of simply resizing the image with zoom lenses.
  • The size of the sensor affects both the image field of view and also the equivalent depth of field significantly.
  • 35mm adapters come in many different shapes and sizes and these can work very well with a multitude of cameras with a interchangeable mounts or by a suitable adapter to allow lenses with alternative mounts to be fitted in order to widen the choice available.
  • There are several ranges of 35mm lenses available and sometimes, the choice of lens groupset is simply one of aesthetic look, rather than capability of working with a shallow depth of field.

WHAT AFFECTS DEPTH OF FIELD

There are 3 factors to affect the depth of field.

  1. Changing Sensor Size
  2. Changing Iris
  3. Changing Focal Length

1. SENSOR SIZE

The Size of Sensor and effect on image size

The size of the sensor will affect the image angle of view, so that a 25mm Prime lens will look different if viewed on a variety of image formats.

Thus a 25mm lens will have a mid shot when used on 35mm film (HDTV 1.78:1 16:9 aspect ratio), close up on HD Camcorders and big close up on smaller semi-professional, ‘prosumer’ camcorders such as PMW-200 or a Go Pro etc which have ½″ sensors and ¼″ sensors respectively.

The same focal length will have different field of view when using different sensor sizes.

The popularity of acquiring in Full Frame makes this even more dramatic, since the large sensor coverage will cause lenses to seem even wider than you would expect in S-35 format.

Full Frame vs S-35 coverage is very dramatic for lens coverage.

This explains therefore why a given focal length prime lens will have quite a marked difference in viewing coverage, when mounted on cameras with differently sized sensors.

Here is a very useful Field of View App provided by Abelcine.

Here is a useful sensor coverage compatibility web app provided by CVP.

Conversion of focal lengths between camera formats

Even though lenses are designed to work with different negative and sensor sizes, the convention is that lenses are still referred to by their focal lengths and/or also their zoom ratios in the case of zoom lenses.

A very useful trick to know is that to convert 35mm focal lengths to 2/3″, just divide by 2.5. and to convert Super16 to 2/3″, divide by 1.6.

Thus a 25mm PL mount film lens has the same field of view as a 10mm 2/3″ lens.

The same lens in Super16 has an equivalent focal length of 16mm and has the same field of view as a 10mm 2/3″ lens.

Size of Sensor and the effect of Depth of Field

The size of the sensor also greatly influences the Depth of Field.

Simply put, the larger the sensor or negative, the narrower is the depth of field.

The ubiquitous ‘film-look’ has a shallow depth of field and is called this because this is easy to perform when shooting on a 35mm negative. As the size of the negative or digital sensor is reduced, so the depth of field commensurately increases for the equivalent shot and the same iris settings.

Thus, a traditional ENG Camcorder with a 2/3″ CCD sensor under the same conditions has a larger depth of field than a 35mm camera and an GoPro camera with a 1/2″ CCD has a larger depth of field still.

Of course it also means that under the same conditions, a camera with Full Frame sensor has a more shallow DoF than one with a S-35 sensor size.

This also explains why Go Pros always shoot beautiful images with loads of depth of field and also why a shallow depth of field is almost impossible to achieve using them.

The larger the sensor (or negative), the shallower is the depth of field for given conditions.

In order from largest sensor size to smallest are the following basic groups.

2. IRIS

The Effect of Iris on the Depth of Field

Many years ago at School, we made a pinhole camera. This was a magnificent experiment because it illustrated the relationship between depth of field and the size of iris beautifully.

You can make a pinhole camera very easily like this:

  1. Take a cardboard box and cut one side of the box off.
  2. Stick a sheet of tracing paper over the open side with tape.
  3. Make a small hole in the side opposite the tracing paper.
  4. Point the hole towards a window on a bright day

In doing this, you will see a perfectly focussed image (upside down) projected on the tracing paper.

What this illustrates is:

  • An infinitely small iris (hole) has an infinite depth of field, since the entire image will be in focus.
  • It also illustrates why the quality of lens of a very small iris is not at all important, since you have no more basic lens than just a hole and this can deliver perfect pictures!

Taking this argument a stage further, explains how smart phones equipped with most basiclenses, can produce excellent images on sunny days – since in a well-lit environment, you will shoot with a very small iris.

Contrary to this example is what happens when you shoot in low light. In low light, you must open the iris considerably and in doing so, the depth of field becomes much shallower, so poor quality lenses struggle when used in these testing conditions.

A practical example of this is a glasses-wearer’s difficulty whilst driving at night. Driving at night in low light conditions causes their iris to be wide-open to let in maximum light and the wide iris causes the depth of field to be very shallow. With the impaired sight of a glasses wearer, these challenging conditions cause the viewers sight be blurred when their irises are wide-open, yet during the day their same same eyes give perfectly focussed images.

This also explains why mini smartphones equipped with cheap lenses produce lovely images during the day, yet give soft images when shooting in drama-type situations in low light with a wide-open iris.

It also helps to explain why DPs are so passionate about lenses because they perform well in all situations and why really great lenses are so reassuringly expensive!

3. FOCAL LENGTH

A shallow Depth of Field is always seen in a close-up shot. You never see a shallow Depth of field with a house in focus and a mountain out of focus but instead a subject in a close up or medium close up shot, as it is easier to create the conditions of a shallow depth of field with a shot of this nature, as the Depth of field is affected by the focal length of the lens.

Put simply, zooming-in will narrow the Depth of field at the same iris setting but you have other techniques available to you which can help to achieve this same result.

By using the longer end of a zoom lens i.e. zooming in, or using a longer focal length prime (say an 85mm instead of a 50mm), you will increase the focal length of the lens that you are using and the Depth of Field will reduce. Of course that this also assumes that to create the same shot you will also have to move further away from the subject.

This image has been reproduced courtesy of IMAGO to show how at the same iris, the depth of field reduces with longer focal lengths.

These examples used ARRI Signature Primes and ARRI Alexa LF cameras shooting in ARRI Large Format, which is a bit larger than Full Frame.

Another technique is to change the point of focus to reduce the perceived depth of field.

This principal requires a bit of explanation:

The longer the magnification, the flatter the image appears and so paradoxically, shallow DoF may be more apparent on wider lenses focused close on their subjects because of the amount of background ambient information available to the spectator. This principle can be seen in the many variations of the ‘contra-zoom’ principle, using a zoom lens to smoothly ramp the magnification whilst the physical adjustment of the camera on the dolly maintains the relative subject size.

This example from DP Mark Moreve demonstrates this extremely effectively.

This is a great example shot by DP Mark Moreve to show that just by changing the point of focus of the lens, the perceived depth of field can be manipulated very effectively.

CONCLUSION

Manipulating Focal Length and Iris to adjust Depth of Field

Cinematographers have long been using the Kelly Calculator for calculating depth of field charts for various lenses but a recent App called Artemis Pro does even more than this, by also making your smartphone into a Director’s Viewfinder. ‎Artemis Pro on the App Store

1. Decide on the Sensor Size

Clearly, the first question is to decide on the camera sensor size and to ensure that the lenses chosen are compatible with the image sensor size of the camera. Each camera on the VMI website clearly gives details of both the lens mount and sensor size and similarly with the lenses too.

2. Choose lenses with an appropriate maximum Aperture size

Lenses are classified by their maximum aperture capability and this is sometimes referred to as ‘speed’. It is given as a ‘F’ or ‘T’ rating, to gives an indication of how large the maximum aperture of the lens is. The smaller the number, the faster is the lens, so the more light the lens can let through to the lens.

The reason why this is important is that the larger the maximum aperture of the lens, the greater is the capability of producting a very shallow depth of field, so a Sigma High Speed Prime lens with a rating of T1.5 is 1.5 stops faster than Sigma Classic Prime Prime lens speed of T2.5. This means that with its iris opened at the widest aperture, the Sigma High Speed prime is capable of a shallower depth of field than the Sigma Classic prime for the same focal length of lens.

However, the Sigma Classic primes will have more ‘character’ – more on this later.

Thus ‘faster’ lenses (those with a lower T rating) are more useful for creating narrow depth of fields than ‘slower’ lenses with a higher T rating.

There is a technical difference between the meaning of T or F for maximum iris, in that T ratings are for Transmission, rather than Iris size. A useful explanation of the differences between T ratings and F ratings are on this site.

3. Adjust the Iris

By cutting down on the amount of light entering the lens, you can open the iris and this will necessarily reduce your depth of field. By inserting Neutral Density (ND) filters in front of the lens will necessarily reduce the amount of light entering the lens, so to perfectly expose the image would cause you to open the iris and in doing so the depth of field will narrow somewhat.

The amount that you can do this will be limited by the speed of the lens that you are using. ‘Faster lenses’ (lenses with a large maximum iris capability) mean that you have more scope to create shallow DoF images.

4. Zoom-in, or use a longer focal length

By using the longer end of a zoom lens i.e. zooming in, or using a longer focal length prime (say an 85mm instead of a 50mm), you will increase the focal length of the lens that you are using and the Depth of Field will reduce. Of course that this also assumes that to create the same shot you will also have to move further away from the subject.

Different types of lens

There are an enormous range of lenses available but it may be helpful for the reader to consider the differences between film lenses and DSLR lenses, though both of these types of lenses are available as zooms or primes.

Motion Picture Film Lenses

Film lenses are built with a far greater build quality to DSLR lenses and to have extremely clear focus and iris markings. They are built with precision optics and sturdy housings with standard lens gears, to make them compatible with follow focus devices.

Shooting with stills lenses

Stills lenses are a much lower cost option of lenses than motion picture lenses.

If you buy a quality stills prime set, then you can expect great image quality at a stills price but the operator needs to be aware of certain compromises.

  1. They aren’t built to withstand the rigours of motion picture film production, which means that they are built in much more lightweight lens housings.
  2. As a result of this, the screws holding a lens together are not designed to withstand the forces applied when using follow focus units etc. If you use a lens control system to control the focus, then the power of the servo units is sufficient to rip the screws of a stills lens apart!
  3. The focus gear pitch is different to PL mount lenses – you can substitute an appropriate gear to make it compatible but even then, the fine thread makes it susceptible to slippage.
  4. The focus thread runs in the opposite direction to motion picture film lenses, so your focus puller either needs to learn to pull focus in the opposite direction, or alternatively use a follow focus unit with an idler gear. Which reverses the motion of the follow focus correct for stills lenses.
  5. Differing lens diameters meaning standard matte-box donuts or clip-on matte boxes won’t fit – besides which the lens will not be strong enough to hold a clip-on matte box without putting undue pressure on the lens mount and housing.
  6. Stills lenses are most commonly available in Canon EF, Nikon FZ or Sony E mounts (also known as Alpha, Micro 4/3 or MFT) mounts. Many cameras are available with these native mounts or have lens adapters to allow them to be fitted, which widens the choice of lenses available to the cinematographer enormously.

Even considering all of the above, if you have a low-budget short film and want to narrow the depth of field of your images and are using a low-cost camcorder, then it is still worth considering a set of stills primes to work with your system.

DSLR Lenses allow Auto Focus Capability (AF or EOS)

The other benefit is that modern AF lenses offer superb autofocus capability which might be very important to your production and which is currently not possible with film lenses.

What do the numbers on the lenses mean?

Traditionally, film lenses are known by their focal lengths only 14mm T1.3, 16mm T1.3 etc and the film zooms also by their focal ranges – e.g. 15-40mm T2. The sensitivity of the lens as already explained is critical in understanding how much light the lens will transmit to the negative and is an essential aid for the DoP in be able to manipulate Depth of field.

The character of the lenses is a very important consideration

The enormous variation of lenses, lens design, coatings, spherical vs anamorphic, vintage vs modern, coated vs uncoated and other factors, will vary the ‘look’ or ‘character’ that a given lens groupset will produce.

Thus the cinematographer must consider the character of a given set of lenses in addition to its maximum aperture capability.

This is a very long subject and many other VMI articles concentrate on this specific question.

CONCLUSION

  • Choice of lenses make a massive impact to the production values and ‘look’ of a given production.
  • DSLR or Film lenses are suited to different production methods.
  • Only suitable EF lenses with compatible cameras are presently capable of autofocus capture.
  • The size of the sensor/negative affects both the field of view of the image and influences the equivalent depth of field enormously.
  • Increasing Iris by introducing ND filters in front of the lens will reduce depth of field.
  • Increasing focal length, either by zooming-in or using longer focal length lenses, will also reduce depth of field.
  • There are several ranges of 35mm lenses available and all have their own benefits and drawbacks when being considered for an electronic film production.

Barry Bassett,
Managing Director, VMI November 2020

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