Using your DSLR / ILC as a "real" 1/4 inch sensor webcam.


(last updated: 2014-03-08)

In particular for planetary and to a lesser degree lunar photography, stacking of hundreds, if not thousand, of images are required to produce detailed images of the highest quality. Hence the desire to use video with its many frames rather than single pictures for such work.

DSLR / ILC cameras and most webcams used for lunar and planetary astrophotography have one thing in common, namely the size of the sensor's pixels. As at this time of writing, webcam pixel pitches typically lie in the range 0.003 mm (HD video) to 0.005 mm (VGA video), while APS-C sensors used in DSLRs and ILCs typically have pixel sizes close to 0.005 mm.

So why is it that webcams with their tiny 1/4" sensors are so superior to DSLR / ILC video with their APS-C (or full frame) sensors when it comes to video for planetary photography?

The answer lies in the compression required for capturing DSLR / ILC frames for video. It is completely impractical to make a video from some 5000 x 3300 pixels on the APS-C sensor without a drastic compression of the image data. In fact, the pixel data from each block of about 8 x 6 pixels on the sensor will have to be represented by one, single pixel in an VGA video file.

Webcams, on the other hand are "born" with 640 x 480 up to some 1080 x 800 pixel sensors and no compression is needed as illustrated in the following diagram:

Figure 1: Schematic camera vs. webcam video capture process

Even with Full HD video (1920 x 1080 pixels) videos from APS-C sensors are so compressed that they cannot compete with the uncompressed video from a "simple" 640 x 480 pixel webcam.

If only we could crop the image on the sensor before capturing the images for video frames, all would be well. It could be done in firmware, but few DSLRs have had that feature, and Pentax isn't one of them.

However, there is a way around, in principle at least. With a videograbber, one can capture the signals to the live view LCD screen and record that output as an AVI or MPEG video.

How this is done can be seen by clicking on the next image.

Figure 2: Live view on a PC using a videograbber


So, here we have my artificial test-planet, a lamp on a building some hundreds of yards away from my balcony:

Figure 3: "Artificial Planetary Test-target" at 700mm f/11

This image was taken with my K-5, Tamron SP 350mm f/5.6 mirror lens and a Tamron SP F-series 2X teleconverter.

A 100% non-resized 640 x 480 px crop of the image looks like this:

Figure 4: Non-resized 640 x 480 pixel crop of test-target shown in Figure 3

The lamp is a 135 pixels in diameter equal to a physical image diameter of 0.625mm on my sensor. I note in passing that this image size would correspond to photographing Jupiter when it is closest to Earth (48 arc seconds in angular diameter) with a 2,700mm telescope.

Next a frame grab from an VGA video (also 640 x 480 pixels) taken with the same set-up:

Figure 5: Single frame grabbed from VGA video recorded in-camera.

Here, the lamp is meagre 20 pixels in diameter. We see the effect of the strong compression as previously discussed.

Now then, I cannot crop my sensor output in-camera, but I can crop the part of my sensor image to be displayed on the LCD screen. More so, I can choose what portion of the sensor image I want to crop to in steps of 1X, 2X, 4X, 6X, 8X and 10X (8X and 10X only available with manual lenses - which includes telescopes):

Figure 6: LCD signal recorded with videograbber. Full sensor image displayed on the LCD

Figure 7: Same as above but with 2X cropping

Figure 8: 4X cropping

Figure 9: 6X cropping

Figure 10: 8X cropping

Figure 11: And finally 10X (maximum) cropping.

So, for each of those "magnifications" - which are actually croppings - I can record my videos just as any webcam planetary astrophotographer would do. The down-side is that I cannot record the cropped sensor output directly; I have to go via the signals to the LCD display (1 million dots/pixels by the way) and live view "assumes" that I want these signals to be more and more - and automatically - amplified the darker the overall scene is.

A small Jupiter in a sea of dark sky would be hopelessly overexposed that way. Fortunately I have found a way to control the LCD signal amplification: I simply illuminate my telescope / lens with a diode lamp at such an angle that the brightness of the LCD is adequately dampened and then lock that amplification level by pressing the AE-L button on my K-5. I am not sure that this will work on any Pentax digital camera but as said, it works on my K-5!

And now I can do something that the webcam photographers cannot do:
1) I can start out with no cropping, find my target in relative comfort and ease and the centre as I go up in "magnification"/ degree of cropping
2) I can always use the four way controller to position my target at any "magnification" - thus I can pane over the Moon for example.
3) As already said, I can do video at different crop settings, depending on whether I am shooting the Moon or Jupiter and whether I shoot at prime focus of my telescope or with a Barlow or with eyepiece projection.

Now, can you beat that?

Well....I still have to prove that it works in practice. Jupiter is at its closest these nights. It is there, straight over my head - and so are the clouds that have covered my skies almost constantly since November...........

Addendum 2014-03-08 - First Real-life Test

Well, well,well, finally an evening with several of hours of (reasonably) clear skies. I had no chance of setting up my equatorial mount and telescope for Jupiter, but nearby Moon provided an ample target for some tests with my camera and Tamron 700 mm f/11 system on a tripod. So, here's some, if not "proof" then "evidence":

Figure 12: First find your target

Figure 13: Then fool the sensor with a dose of LED light

Figure 14: That's better!

Figure 15: Now set the crop-factor o an appropriate value.

As it turned out, on my Pentax K-5 8X cropping provide close to a 1:1 representation of pixels on my sensor to pixels in my video frames. I only to a handful of still images and seeing was still not too good, but here is a representative crop to the same part of the Moon as shown in Figure 15 above.

Figure 16: 100% non-resized crop of single exposure taken with Pentax K-5, Tamron SP 350mm f/5.6 Model 06B lens and Tamron SP F-series 2X teleconverter. 700 mm focal length; f/11; 1/30 s; ISO 200

Figure 17: Downsized version of the full image used for Figure 16 above. Click on image to see larger version.

Of course, with the video I have the luxury to have several frames to choose from, but the fact that I could indeed find frames as good as the one in Figure 15 demonstrates, what this was all about: Videos obtained this way should indeed be suitable for planetary photography and stacking.

Jupiter is still waiting.......






Copyright 2014 - Steen G. Bruun