An outline guide to HD and Film lenses

This essay is written mainly for Producers and cameramen who want to improve the production values of their programmes to create the ubiquitous 'film look' and want to have more options than just to use 35mm lenses and a Pro35 lens adapter!

Achieving the correct choice of lenses is paramount - there are complete ranges of HD video-style zoom lenses, HD Primes lenses and cine-style zoom lenses designed specifically for HD, as well as many different 35mm lens adapters including optional mounts to be able to fit motion picture film and also stills lenses...

All of these promise to achieve quality production values, so the choice can be overwhelming and a little confusing!

This article will look at the issues below and give a summary of current lenses commonly available and their compatibility with current HD and digital film equipment.

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

WHAT AFFECTS THE DEPTH OF FIELD

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:

Taking this argument a stage further, explains how mini DV cameras equipped with very basic, very cheap lenses, 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. Poor quality lenses struggle when used in these testing conditions.

A practical example of this is a glasses-wearers difficulty whilst driving at night. The short-sighted nature of the person means that his/her eye lenses are slightly imperfect, or rather his/her lenses are slightly imperfect. Driving at night in low light conditions causes the irises to be wide-open to let in maximum light and the wide iris causes the depth of field to be very shallow. With the imperfect nature of the lenses, these challenging conditions cause the viewers sight be blurred when the irises are wide-open, yet during the day these same eyes give perfectly focussed images.

This also explains why mini DV camcorders equipped with cheap lenses shoot 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 DoPs are so passionate about lenses because they perform well in all situations and why really great lenses are so reassuringly expensive!

THE EFFECT OF 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 EX-3 and Z1 etc which have ½″ sensors and ¼″ sensors respectively.

Images created from scans of Pcam software running on a Palm Tungsten. www.davideubank.com

This explains therefore why if you mount an HD lens onto a ½″ camcorder (such as Sony PDW-F355) without an optically corrected mount, that a wide angle acts like a telephoto and why the focal lengths of wide angle lenses of ½″ lenses are always smaller than those used for 2/3″ lenses.

Here is a useful sensor size chart:

For 1/3-inch CCDs: H = 4.8 mm, V = 3.6 mm
For 1/2-inch CCDs: H = 6.4 mm, V = 4.8 mm
For 2/3-inch CCDs: H = 8.8 mm, V = 6.6 mm
For 1-inch CCDs: H = 12.7 mm, V = 9.5 mm

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, 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 Broadcast HD Camcorder with a 2/3″ CCD sensor under the same conditions has a larger depth of field than a 35mm camera and an HDV prosumer camera with a 1/3″ CCD has a larger depth of field still.

This also explains why miniDV Camcorders always shoot beautiful images with loads of depth of field and why a shallow depth of field is almost impossible to achieve without using a 35mm adapter – more on this later.

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

ACHIEVING A SHALLOW DEPTH OF FIELD

Manipulating Focal Length and Iris to adjust Depth of Field

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 our 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.

If you are not shooting on a 35mm format size (say 2/3″ HD cameras with 2/3″ lenses), we have already established that your depth of field will be greater than the equivalent 35mm format but don't despair – you have 2 options in your armoury to be able to achieve a shallower depth of field.

Cinematographers have long been using the Kelly Calculator for calculating depth of field charts for various lenses but Douglas software have produced a very useful electronic application written for the Palm and iphone is available for shareware download at: www.davideubank.com. I used this application to create the images above and work out the depth of field calculations as used.

Another application written for the pocket pc, palm, windows or mac and is downloadable on http://www.photo-software.com/cameramodule.htm?gclid=CJaX1MitnJcCFYoH3godRlKmDA

1. 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 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 mean that you have more scope to create shallow DoF images – see prime section next.

'Speed' of the lenses and their affect on Depth of Field

Lenses are classified by their 'speed' rating and this 'F' or 'T' rating gives an indication of how large the maximum iris 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 iris of the lens, the shallower is the depth of field, so a Canon 9mm HD Prime lens with a speed of T1.5 is a stop faster than a Canon HJ21x7.5KLL cine-style lens with a speed of T2.1. This means that with a wide-open iris, the Prime lens is capable of a shallower depth of field than the zoom lens at the same focal length.

Thus faster lenses are more useful for creating narrow depth of fields than slower lenses.

2. Zooming-in

By using the longer end of the lens i.e. zooming in, 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.

3. Use 35mm lenses with a suitable adapter

By using 35mm lenses with a suitable adapter will permit you to fit 35mm stills and motion picture lenses to your digital camera and create the same depth of field as 35mm film. This is discussed later.

SHOOTING FOR PORTABILITY OR PRODUCTION VALUES

The difference between cine-style and video-style lenses

Canon HJ22ex7.6 BIRS T2.1 Video-style lensConventional video lenses are called 'ENG' (Electronic News Gathering) lenses and are designed to be used shoulder-mount or tripod-mounted. As a result, they permit the camera operator to operate or adjust focus whilst shooting ('pull' focus) him/herself. In order to distinguish them from the cine-style zoom lenses, they are also called video-style lenses

The physical limits of movement of a person's arm limits the focus adjustment through the entire focussing range of a video-style lens in one movement, to around 1/3 of a turn. Therefore for practical reasons, the focus adjustment on a 'video-style' lens is limited to around 1/3 of a turn – very good for shoulder mount operation but not so good for drama where the need for fine focus adjustment is greater.

Drama needs are quite different from ENG documentary shoots.

Firstly, drama lighting tends to be low and moody requiring the camera to be frequently used a wide-open iris in order to minimise depth of field.

Secondly, at this exposure, focus adjustment is critical, so the focussing is frequently carried out by a separate person – enter the focus puller.

In order to hit accurate focus marks when operating with a shallow depth of field means that they need to use lenses with a large focussing latitude, as the adjustment is critical. This is much harder to achieve using video-style lenses, which only have around 1/3 of a turn of total focus adjustment.

As a result, a range of lenses exists called 'cine-style' zoom lenses, which are designed to have a much larger focus latitude than video-style lenses (around 300 degrees) to enable critical focus control with a wide-open iris.

Canon HJ21x7.5KLL T2.1 Cine-style lensThey are also designed to minimise focus 'breathing' (zoom effect whilst focussing) and maximise performance at each fixed focal length, as opposed to compact zoom lenses that are designed for portability and performance.

Also, there focus markings on the lens are very much more accurate than video-style lenses which reflects the need of the focus puller in high production-values environments.

However, there exists a video-style cinematographic zoom lens exists (Canon HJ21x7.5 BIRS) that offer very similar optical performance to cine-style lenses but with the format of an ENG lens – pulling focus is still as critical as with other ENG style lenses (same 1/3 of a turn focus range) but image quality and production values is similar to the cine-style equivalent.

X2 Range Extender

HJ21x7.5BIRS video-style cinematography lensOne last difference exists between video and cine-style lenses – the x2 range extender. Video-style lenses have a x2 range extender built into the lens as standard, allowing the operator to double the zoom capability by pressing one knob. This cuts down the light by around a stop and a half but is a useful function nonetheless which does not exist on cine-style lenses, as these functions are not commonly used and additional unused elements compromise optical performance of the lenses.

External x2 and x1.4 range extenders exist which can be fitted to both cine-style zooms and HD Prime lenses to extend their range when needed without having unnecessary lens elements being used during regular operation.

HD Prime lenses

Zeiss DigiPrime T1.5 HD Prime lensesWhilst HD zoom lenses offer very good performance over a range of focal lengths, this versatility comes at a cost. They are never as fast as their equivalent prime lenses (as the lenses contain more glass and the coatings are purportedly inferior), and their optical performance is never as good at either the widest or the longest ends of the focal range, nor at the extremities of the frame.

HD Prime lenses usually come in sets of 5 or 6 lenses of discrete focal lengths. Each lens is designed to be optically maximised at each particular focal length and is usually measurably superior compared to the equivalent zoom in terms of barrel distortion, lens ramping, chromatic aberration and lens breathing.

Most importantly, the lack of optical elements compared to a zoom makes primes usually significantly faster as well than the equivalent zoom lens. For example, the Zeiss DigiPrime HD Prime lenses and Canon FJ HD Primes have a speed of T1.5, which is around a stop faster than the equivalent Canon cine-style HD zoom lenses which are T2.1.

Each stop requires a doubling of the quantity of light transmitted through the lens, so it can be appreciated how much faster these lenses are and how much better in low light they perform.

The colour balance of the Canon and Zeiss zooms match closely the Canon cine-style zoom lenses well (within 150K), so are often used together. Zeiss also now make high quality HD zoom lenses called DigiZooms to complement the DigiPrime range of lenses.

Whilst the prime lens is always going to be better than the zoom, it is important to emphasise that the zoom lens performance of the Canon HD cine-style lenses are still very good and are frequently used for dramas and features as a result of their high quality and convenience value compared to the HD Primes which come at a significant cost premium to the zoom lenses and the DoP must carefully balance the improvement in performance against the additional cost.

Ultimately, what lenses to use are very much a question of personal choice and budgetary choice.

Checking Back Focus

Century Optics HD ColimatorVideo lenses work by splitting coloured light through a prism and coloured filters to make 3 discrete beams of light in the Red, Green and Blue primary colours.

As a result, any tiny imperceptible variations in expansion of the lens and/or camera housing caused by small temperature variances or movements in the lens mount can cause focussing problems.

There is a very subtle adjustment which takes into account these variations called the back focus adjustment and although the DoP can perform this check by eye or using a back focus 'flange' chart, it is much easier and quicker to perform this check using a lens collimator.

You do not need to collimate lenses on the RED, they work just like a film lens on a film camera - the back focus or flange depth as it in known in the film world should be set by a camera engineer and not touched by the film crew.

Issues when using heavy lenses - Back focus drift

Larger HD lenses, such as Canon HJ21x7.5KLL must be used with lens support brackets secured against the 15mm or 19mm bars to ensure that back-focus drift is minimised during normal operation. Correctly fitted, the camera operator should not suffer any back-focus drift caused by any lens movement.

However, it is still worth checking the back focus adjustment every morning and also if the temperature or humidity changes significantly and frequent checks – especially when changing locations or moving from indoors to outdoors or when the temperature changes significantly are always advised.

35mm Lens Adapters

P+S Technic Pro35 adapterShooting with an image size larger than 2/3″ does make it easier to create a shallow depth of field. P+S Technic make an industry-standard adapter called a Pro35 which allows motion picture and stills lenses to be mounted onto 2/3″ digital video cameras, such as the Sony HDW-F900R etc.

They work by focussing the image of the lens onto a ground glass that is the same size as a 35mm negative. The image is projected onto the ground glass and this glass spins in an elliptical manner at preset speeds to avoid imperfections in the ground glass being visible. Then the light from the ground glass image is reflected through a prism to turn it the correct way around and then through some more lenses in order to focus this image onto the digital image block of the digital camera.

The result of using this adapter is that conventional 35mm lenses can be mounted onto Broadcast 2/3″ Camcorders and images recorded with the same field of view and depth of field as when mounted onto 35mm format film cameras, solving the problem of shooting with a shallow depth of field.

It actually works very well, creating images on the screen looking incredibly filmic just as if they were shot on film! However, as is often the case, you never get something for nothing and there is a price to be paid for this:

P+S Mini 35 adapter3 x variants of the P+S Technic adapter exist at present:

Low Cost Film lens adapters

Letus Extreme 35mm adapter on Canon XL1HLow-cost 35mm adapters are available for semi-professional/prosumer camcorders such as the Sony Z1/Z5/Z7, EX1/3 or Panasonic HVX 200/201 etc such as the Letus, Red-Rock and others.

These are very much cheaper than the equivalent P+S Technic Mini-35 adapter however, they are less sturdily built than the P+S unit and are not designed to withstand the punishment of surviving a rental environment, which explains why they are not commonly available to hire from rental companies.

If however, you choose to purchase one of these units exclusively for private use, they deliver a shallow depth of field in common with the P&S Technic Mini35 unit and even have less stop-loss than the Mini-35. Horror stories are sometimes heard of lightweight low-cost units simply falling apart on location, so extreme tender care is required when using these products!

Motion Picture Film Lenses

Zeiss T1.3 SuperSpeed lensesZeiss SuperSpeed T1.3 series lenses are a very fast set of PL mount lenses which have been superseded by the Zeiss UltraPrime and MasterPrime series.

These are an older generation speed of which the MkIII are the best set to use. In spite of this, there are very fast lenses (T1.3), compact size and light weight makes these still very popular as a combination with the P+S Technic Pro35 lens adapter.

The standard set includes a 18mm, 25mm, 32mm, 50mm and 85mm; all are T1.3.

A drawback of these lenses is that their relatively large minimum focussing distance to the lens means that for commercials and pack shots, the UltraPrimes and MasterPrimes are often preferred.

Zeiss T1.9 UltraPrime lensZeiss UltraPrime T1.9 series is the later version of the Zeiss SuperSpeed lenses. The same Zeiss precision optics and closer focussing distance than the older SuperSpeeds make this a very popular lens.

The conventional set includes 18, 25, 35, 50 & 75 all T1.9.

Zeiss MasterPrime T1.4 series are faster than the Zeiss UltraPrime series and this makes them more expensive to buy and hire than the UltraPrimes. The same Zeiss attention to detail and quality precision optics ensures that these are both very popular lens choices for DoPs.

The conventional set includes 18, 25, 35, 50 & 75 all T1.4.

Cooke S4 T2 series lenses are a common favourite with DoPs precisely because they are not as clinically precise as the Zeiss optics. Cooke lenses are frequently described to have a 'beautiful softness that is missing in Zeiss lenses'. Whatever your preference, they look beautiful and have a speed rating of T2.

RED recently produced a set of Cooke RED lenses which have some RED markings on them. Cooke advise that these have identical housings and markings, the cost of purchase is the same and the decision upon ordering them is whether you want the RED or Black markings!

The conventional Cooke S4 set includes 18, 25, 35, 50 & 75 all T2

The Cooke 'RED' set is frequently supplied with a short zoom (15-40 T2) plus a 50mm, 75mm & 100mm T2.

Cooke T2 S4 Prime lenses

Other lens manufacturers exist such as Panavision who make the world-class Primo range and also Angenieux who make the world-class Optimo zoom lenses and others. I have only concentrated on the mainstream film lens models commonly available.

Shooting with stills lenses

Zeiss 35mm stills lensStills lenses are a much lower cost option of lenses than motion picture lenses.

If you buy a quality stills prime set such as quality Nikon or Canon set, then you can expect great image quality at a stills price. In contrast, a set of Cookes or UltraPrimes will cost circa £100,000 compared with sub £5,000 for a set of stills prime lenses.

"Why doesn't everybody use them?" I hear you ask! Well the reasons are similar to why more people don't use the low-cost cameras or low-cost 35mm adapters:

  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 have non-standard lens mounts, so you will need to fit suitable adapters to work with existing cameras and 35mm lens mounts.

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. Be prepared for your crew to complain audibly about having to work with non-standard lenses or adapter and above all, be gentle with the equipment! You don't want them falling apart on you whilst on production (or indeed at any other time either!).

Achieving a 'film-look' Depth of field using 2/3″ lenses – a practical example

Although it is widely considered that a narrow Depth of Field is only possible using 35mm lenses. Here is a practical example to demonstrate that this is not the case.

Shooting on a 35mm camera (or 2/3″ Camera with Pro35 adapter) with a 25mm Prime lens will give you a viewing angle of 50 degrees. Exposing a subject 3ft away at F5.6 will give a depth of field of approx. 38cm. (Settings are based on 20 Micron circle of confusion and 1.85:1 HDTV safe)

Shooting now on a 2/3″ HD Camcorder using HD Zoom or Prime lenses, in order to shoot the same viewing angle, you must substitute the lens for a 10mm HD Prime or Zoom lens (B4 mount 2/3″ lens). With this lens fitted, the viewing angle is a very comparable 47 degrees. Exposing at the same F5.6 will give a larger depth of field as expected but opening the iris to F2, will give an equivalent narrow depth of field of just 34cm. (Settings are based on 10 Micron circle of confusion and 16:9 HD aspect ratio)

(Calculations used using the excellent Pcam software working on a Palm Tunsgsten T3 www.davideubank.com )

This means that if you are able to shoot with the iris just 2 stops more open than on 35mm film and using an alternative HD lens to give an equivalent viewing angle, then shooting on HD with HD lenses will generate a depth of field comparable with 35mm film and 35mm lenses.

There are issues which some DoPs may have with shooting wide open which are beyond the scope of this essay but specific books such as Paul Wheelers excellent text, 24p and HD Cinematography, Focal Press, covers this in detail. http://www.amazon.co.uk/High-Definition-Cinematography-Paul-Wheeler/dp/024052036X/ref=sr_1_1?ie=UTF8&s=books&qid=1228398224&sr=8-1

This examples assumes of course that you are working on F5.6 on 35mm and not more wide open and thus that you have 2 x stops of iris to play with on your HD lens.

HD cine and video-style zoom lenses are relatively fast with a sensitivity usually of T2.1 but the dedicated HD Prime lenses are faster still with a speed of T1.5, which gives an additional stop of sensitivity.

This means that in practise, that you will be capable of producing a much shallower depth of field using an HD Prime lens compared with an HD zoom lens, as well as the images being of higher quality.

What do the numbers on the lenses mean

Cooke 15-40mm T2 zoomTraditionally, 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.

Video lenses have a different terminology and have a format whereby the first number refers to the zoom multiple of the lens and the second number is the minimum focal length of the lens.

For example an HD Wide angle lens has the label: HJ11x4.7BIRS

This means that the lens works over the range 4.7mm – 51mm (4.7*11=51.7mm)

The BIRS or BIASD at the end of the lens says that it is a video-style lens and cine-style lenses have the suffix KLL.

Thus a long HD Canon cine-style lens has the label: HJ21x7.5KLL

This means that the lens works over the range 7.5mm – 157mm (7.5*21=157mm)

The KLL suffix means that this is a cine-style lens.

THE TRADE-OFF BETWEEN QUALITY AND COST

How are HD lenses better than SD lenses?

They are better than SD lenses in every way – improved barrel distortion, reduced chromatic aberration, lens ramping, focus breathing, speed and coatings and the difference if shooting HD, is both noticeable and justifies the cost differential compared with SD lenses.

In answer to the common question of whether you will see the difference in image quality between a SD lens and an HD lens, the answer is categorically YES!

SD lenses can fit HD camcorders, as they share the same B4 2/3″ CCD lens mount, so there is nothing to stop you from using these lenses with HD Camcorders but your image quality will be compromised. You will also be breaking the terms of any HD commissioning contract and VMI do not recommend this.

There is a new fashion amongst DoPs who have appreciated the additional quality of HD lenses compared with the older SD lenses, to fit these to SD cameras for the additional production values that they afford. Many dramas today still shot on SD are shot using HD lenses.

CONCLUSION

Barry Bassett,
Managing Director, VMI December 2008

 

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dr asok 8 months ago

good information in asmall package

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