| Video microscopy trials with
the USB Live View output of a DLSR camera.
by David Walker
'Live View' from the main sensor is an increasingly popular feature on the digital SLRs offered by most makers. The live image can be viewed in a number of ways: on the camera's LCD, on a TV via the video output(s) and also on a computer screen via a USB cable with the maker's remote control software (if available).
The author has been using a Nikon D300 with its live view feature for critical focussing in photomicroscopy (see January 208 Micscape article) and wondered if the PC screen video stream could be captured, so that the DSLR effectively became both a still image camera and a video camera.
I was unfamiliar with what software could capture the video stream on screen but an online search showed that live view video capture from DSLRs was already being explored e.g. by photographers interested in video macroscopy. One useful forum discussion 'How to capture video from 40D?' in a DPreview forum discussed software that users had been trying. The software that most appealed to me, both in effectiveness and price, was ZDSoft's 'Video Recorder'. A 30 day free trial is available and good value to buy at $29.
The videos below were all captured using this software and the Nikon D300, but any DSLR that has control software that presents live video on the PC screen should be fine. The software was very robust and gave no problems at all with Nikon's software.
Video recording alternative routes: The video output from a DSLR with live view can of course also be captured via the composite video output (and/or high definition if offered) to a suitable video recorder. This should give the best quality but the potential benefit of capturing the USB video stream is for computer and web page use without the need to play back the video recorder and use a video capture board.
ZDSoft's Video Recorder screen shown right is very intuitive. The software detects the video codecs on the PC and offers them in a drop down box. The parameters available for each codec can be altered via the 'Config' button, e.g. compression rates etc.
When the Start button is pressed the arrowed film reel icon in the status bar rotates to indicate recording is in progress. The button to stop and start video can be configured to suit.
By default video files are saved with sequential file names so no danger of overwriting them.
If the PC has a built in microphone or one is connected to the PC's audio card, an audio commentary could be added.
The 'Live View' box of Nikon Capture Control Pro 2 using a Nikon
D300 on a microscope. Before starting the video recording software, the
camera's red focus box 'A' can be put in a corner so as not to be in
video field. When the video recorder Start button is pressed, a
floating crop area is enabled 'B' that allows the user to select the
screen area to be captured. Once selected the software starts recording.
Notice that the 'Magnification Ratio' 'C' is set to 'Full Screen' to capture the camera's full sensor area.
One of the benefits of the user selectable video capture area is that non-standard aspect ratios can be selected to suit the subject. The maximum capture area is defined by the camera's control software, in this case 640x480, but that is more than sufficient for most computer use.
A potentially useful feature is to use the 'Magnification Ratio' box to
fill the screen without changing mag. This admittedly is a type of
digital zoom but as the sensor has a huge pixel count if used with
caution, image quality didn't seem to be affected. I didn't find much
above 25% worth doing, an increase in microscope objective mag would be
Videos - to play a video of interest below, click on an image and it will play in an external window using the default avi player on the local PC. Note that the Xvid compression codec is used. Readers may need to download the codecs from www.xvid.org if not on local PC.
Clips edited with free Virtual Dub. Apart from cutting out frames and compression, the videos are as seen on the PC screen from the Nikon D300's Live View window. The video quality on Live View screen was higher than shown here because of compression and frame rate reduction to save file size. (fps in captions = frames per second).
Pond life - Vorticella
Zeiss 16x objective, brightfield, contrast enhanced lighting. Not a
lot of movement in this video but shows fine tone retention and liked
the arrangement of the individual specimens.
Clip 10 secs, 564 kbytes, 10 fps from original 20 fps. One of the benefits of the ZDSoft video capture is that the capture box can be fitted to suit the subject being viewed, rather than having to adopt a fixed format.
Vorticella - close-ups
of above colony with the same Zeiss 16x objective but with 25% Live View zoom-in
on screen of same field as above . Brightfield, contrast enhanced lighting.
The three specimens obligingly remained extended and in same plane of
focus to show both cilia crown and side view.
As remarked in the introduction, the zoom-in function of the camera's Live View can be used with care to magnify the subject without changing objective, thus retaining the depth of field of the lower power.
Clip 10 secs, 1.7 Mbytes, 10 fps from 20 fps master. Monochrome set in camera. The compression parameters need to be chosen with care. Those for Xvid are dauntingly extensive and have yet to gets to grips with what they all do to optimise video clips for web use. Some 'stepping' of fine gradations in backgrounds can occur during compression during master recording to hard drive then compressing again after frame editing.
detail, phase - clip 10 secs at 10fps from original 20fps, 1.0 Mbyte. Zeiss
16x phase objective, 25% zoom-in. When I use my Moticam 1000 (1995 model) for phase video clips
I find its limited dynamic range bleaches halo edges. The DSLR's
superior dynamic range copies the tonality more faithfully. Recorded
a little on dark side here for contrasty internal detail, but
increasing illumination is tolerated by the DSLR sensor.
Another benefit of the DSLR cf some video cameras is its superior noise performance, especially with a low key video like the above.
This was an early trial where rather fierce compression was used so there's noticeable 'stepping' in the even tone backgrounds of this video and to some extent in the others with default compression settings.
Pond life - Collotheca rotifer
Clip - 10 secs, 10 fps from original 20 fps, 397 kbytes. Zeiss 16x objective. While scanning plant stems for the above Vorticella I was delighted to find the rotifer Collotheca, and which made an obliging film star.
Clip - 40 secs at 10 fps from original 20 fps, 4.0 Mbytes. Brightfeld, contrast enhanced lighting. Zeiss 16x objective, 25% zoom-in of same field as above.
Clip - resized and
reduced to 5 fps from 20 fps master, 1.6 Mbytes. Zeiss 16x phase objective,
A longer sequence for readers interested in Collotheca's
typical antics. Depth of field is small so there is focus through
and a sequence of it contracting and expanding both the 'crown'
Ciliate - Zeiss 16x objective, brightfield, contrast enhanced lighting, 25% zoom in. 10 secs, 10 fps from 20 fps master, 2.9 Mbytes. Lighting level lowered to maintain colour depth. The sensor of the D300 does start to show noise on screen at low light levels so also captured on video but not too objectionable.
acetate (Biosil slide), Zeiss 4x objective. Rotation of the analyser
with a lambda plate and polariser. Just a bit of fun making a static
Clip, 20 sec, 30 fps, 6.7 Mbytes. The hue and saturation was increased in the camera before video start to intensify the colours.
Stacking video frames for increased depth of field
If a still image was desired with increased depth of field, stacking the video frames as focus through the subject could be tried. The image quality compared with a still image from the DSLR would be compromised of course and size limited to the Live View screen size, typically 640x480, but a quick and easy way of creating such an image e.g. for a web page.
Various types of camera can capture both stills and video e.g. webcams and dedicated microscope cameras; the quality being dependent on cost and specs. From the tests above with a Nikon D300, a DSLR with Live View of main sensor also seems to offer reasonable quality video typically up to VGA size in addition to hi-res stills. A dedicated high performance microscope camera can probably exceed the DSLR video quality captured via the screen but the author does not own one for comparison as can be very expensive.
The results above are from the D300 where the Live View is taken from the main sensor. Some maker's offer DSLRs where the Live View is taken from a separate sensor. If the computer control software presents this video on screen, the same video capture method should be possible. I'd be interested to hear from users of other DSLRs with Live View either from the main or subsidiary sensor as to the quality of such video clips.
DSLR prices are steadily dropping and Live View is becoming a standard feature on the 'budget' models in a maker's range, e.g. the Canon 1000D (Rebel XS) just announced and current Canon 450D. So the ability to additionally capture video off screen and/or save to a dedicated video recorder may be a useful option. Some consumer digicams with remote control software and Live View should also be able to do this.
Ideally saving the master video to the hard drive uncompressed would minimise compression artifacts if the clip is to be edited and further compressed, but no compression can generate huge files. The author used a modest compression on the master videos and heavier compression for the edited clips for a reasonable web file size, but careful choice of video codec / compression needs to be made to keep artifacts to a minimum.
Comments to the author David Walker are welcomed.
Published in the June 2008 edition of Micscape.
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