The Video Enabled Digital SLR

Until very recently if you needed to shoot video you purchased a video camera and if you needed to shoot stills you bought a stills camera. However, over the last couple of years manufacturers have started to release stills SLR cameras that can also capture video. With many of the middle and higher range camera models now offering video capture we should look at this new feature and try to identify its strengths and weaknesses. This document is aimed at anyone who has or is about to purchase an SLR camera and needs to know if the video option is of use to them.

Introduction

More and more digital stills cameras are offering video capture. In many respects these cameras are limited when compared to the video features offered by even the most basic dedicated video camera - however, there is still considerable interest in the technology from some user groups.

The camcorder has evolved to meet the specific needs of the filmmaker, while the SLR has a very different heritage. By bringing these two media types together there were always going to be compromises. This document is mainly concerned with the video features offered (or not) by these new SLRs - it is therefore worth having a look at the main features found in the typical camcorder and comparing them to those in the video enabled SLR.

The standard video camera

Camcorders come in a wide range of shapes and sizes from small pocket video cameras through to fully featured professional models. While the range of features may vary widely between camcorders, there are some elements that are common to all.

• The lens is normally incorporated into the body of the camera, though a few high end models offer detachable optics
• Zoom lenses are controlled by a camera mounted rocker switch which give video clips a smooth transition as the lens zooms between focal lengths
• The autofocus system is fast and noise free
• the camera may also offer vibration reduction to reduce any movement introduced by the operator
• The camcorder may have built in neutral density (ND) filters to reduce overexposure when filming under bright lighting

The lens projects the image onto the camera's light receiver. This is normally a single light sensitive chip though higher end camcorders often employ 3 sensor chips (red, green and blue) to improve image quality and resolution. Most camcorders use sensors significantly smaller than the sensors used in stills photography.

The captured image can be viewed on a small externally mounted LCD screen or via an electronic viewfinder. The LCD screen is useful for viewing recorded footage or for navigating through menus while the eyepiece covering the electronic viewfinder allows the operator to concentrate on the scene, it also eliminates reflections when used under bright conditions.

'Prosumer' camcorders usually offer features such as:

• Zebra striping to warn of exposure issues
• Gain to amplify the image signal in low light conditions
• Controls over sound recording with the option of using an external microphone. The microphone is normally found in a prominent forward facing position to reduce the risks of recording the sound of hand movements, etc. 

Built in microphone on video camera

These features are housed in an ergonomically designed camera body which lends itself to the process of recording moving images and sound.

Typical prosumer camcorder

The video enabled digital SLR

The SLR is primarily designed as a still image capture device, and its video capabilities are a by-product of the Live-View feature. At the time of writing the video feature is in its infancy - however, it is being widely adopted by filmmakers and it is inevitable that manufacturers will improve the video capabilities in future cameras and extend the feature to their entry level models.

In contrast with the typical video camera, the interchangeable lens is a key feature of the SLR camera. The main camera and lens manufacturers produce a huge range of lenses, some of which are highly specialist such as tilt-shift lenses with no real equivalents in the video world.

The focal length of zoom lenses is controlled directly on the lens - in contrast with the power zoom switch on camcorders, a smooth zoom is not a key requirement in stills photography and so a zoomed video clip shot on an SLR may therefore appear jerky.

The autofocus system in SLR cameras is designed to work in the standard mirror down camera state and is highly responsive and very fast, with the mirror up to record video the autofocus system is slow and inaccurate and at the time of writing most serious filmmakers using digital SLRs switch to manual focus. The reliance on manual focus means at present it is impractical to film rapidly moving objects with a video enabled digital SLR.

The sensors used in digital SLRs are considerably larger than those used in prosumer camcorders. When wide aperture settings are used the result is a cinematic 'shallow depth of field' look associated with professional film or video cameras costing tens of thousands of pounds. It is mainly the shallow depth of field that can be achieved with these cameras which is generating so much interest from filmmakers.

The images below illustrate how a subject can be isolated from the background with a shallow depth of field.

Still taken from camcorder showing the minimum depth of field (widest available aperture)

Still taken from video SLR showing minimum depth of field (widest available aperture)

The image taken with the standard camcorder (top image) shows some isolation though it is still possible to identify background detail. The image taken with the video SLR (bottom image) shows a much shallower depth of field - the background is almost unrecognisable and the subject is clearly isolated from it.

To capture video, the SLR's mirror has to be held in its raised position to allow the sensor to record a continual image stream. This blocks light from passing through to the eyepiece and renders the viewfinder redundant. Like the video camera the viewfinder provides an accurate and direct impression of the scene, without it the user must rely on the LCD display. This however is less accurate and prone to reflections, particularly in bright conditions.

 
Left, reflections on LCD screen. Right, shaded eyepiece used to eliminate reflections

In terms of specialist video controls at present these are very limited, though it may be possible to use existing stills features such as histogram warnings rather than the video equivalent of zebra stripes to warn of exposure problems.

The camera's built-in microphone should be used with care, its position next to the lens picks up the sound of fingers operating the camera and lens as well as the internal motors driving the autofocus or image stabilisation systems.

Built in microphone on video enabled digital SLR (3 small holes on centre of frame)

Ideally an external microphone should be connected to the camera, if this isn't possible a common solution is to record sound on a separate device and synchronise the video and sound at the editing stage with a visual and audio cue such as a hand clap at the start of a clip.

Video enabled digital SLR with connection for external microphone

At present it is not possible to monitor the sound as it is being recorded using a pair of headphones, and the built-in speaker - like the microphone - is basic.

Like the camcorder the ergonomics of the digital SLR are designed around its primary function as a still imaging device. For serious filmmakers additional accessories can be attached to the camera to allow the camera to be used in a more film like way.

Follow focus systems can be added which offer smoother and more control over manual focus - the follow focus is often controlled by the focus puller. Matt boxes can be added to shade the lens and rubber eyepieces or separate screens can be used to view the shot.

While the additional cost of these accessories can easily equal the price of the SLR, the total cost is still significantly less than the price of a professional movie camera.

Some of the sound and video capture controls on a prosumer video camera

The movie capture settings offered by a video enabled digital SLR (Nikon D300s)

Using a video enabled digital SLR

Previous experience with a video camcorder does not really prepare the user for a video enabled SLR. The few capture controls available are buried in menus and the focus and zoom control is basic to say the least.

Camera operation will vary between model. Below is basic workflow for the Nikon D300s, some or all of this may be appropriate to other camera models.

• To reduce focus issues attach the camera to a steady support, ideally a tripod with a video head
• Switch focus to manual and switch image stabilisation off to reduce camera generated noise
• If possible connect an external microphone and switch it on
• Choose the video resolution and video storage location
• Select the white balance setting
• Choose an aperture setting
• Click on the Live View button
• Compose the image on the rear LCD screen
• Click on the centre button to start recording, click again to stop

If the focussing or focal length of the lens must be altered during the shot care should be taken to do this as smoothly as possible.

What are the advantages and disadvantages?

If you were to compare the video specifications of the typical camcorder to a video enabled digital SLR, then the camcorder would at first appear to be the clear winner. However for some users the decision isn't that straightforward. The lists below highlight a few of the advantages and disadvantages of the video SLR.

Advantages

• Compact stills and motion capture system
• Larger sensor when combined with large apertures produces 'cinematic' shallow depth of field
• Access to huge range of additional and sometimes highly specialist photographic lenses
• High quality video capture
• Value when compared to equivalent film/video camera
• May have greater dynamic range and low light performance than dedicated video camcorder

Disadvantages

• Rear LCD screen is prone to reflections and hard to use for focussing
• At present auto focus is very slow and focus hunting may be distracting; manual focus is normally more pleasing but requires practice
• Limited audio control
• Limited capture controls
• No power zoom
• No zebra striping
• >Camera form factor designed around stills capture
• Flickering image under some artificial light sources
• The built-in audio does not match the quality of the video, some cameras offer an external mic input however this is 3.5mm jack rather than the professional standard XLR connection.
• No audio monitoring
• Some video SLRs have a limited maximum clip length of 5 minutes per shot

Conclusion

Video enabled digital SLRs are opening up new opportunities to both stills photographers and filmmakers, however they lack the familiar ergonomics and user friendliness of the typical camcorder.

The novice filmmaker only requires a minimum of training to produce quite usable results from a typical consumer camcorder but may struggle to produce good quality footage with a video SLR.

Therefore, if the primary requirement is for a quick and easy to use video camera then you should probably stick with a dedicated camcorder. If you are planning to purchase a camera primarily for stills photography with the potential to shoot video in the future you should carefully consider the issues discussed in this document before making your decision.

If you hope to purchase a camera that is capable of capturing stills, video and audio media with aplomb you may have to wait some time.

This technology is in its infancy and there is little doubt that manufacturers will be competing to resolve many of the issues mentioned in this document and improve on the qualities that are already in evidence.

The short film below demonstrates some of the issues covered in this document.

The video enabled digital SLR from JISC Digital Media on Vimeo.

Published in: Creating

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