Bit Twiddler's Rant

“Any sufficiently advanced technology is indistinguishable from magic.” – Arthur C. Clarke

Gently, you squeeze the shutter release.  You hear a satisfying “clack” and your DSLR captures the perfect slice of time.  As if by magic, an image appears on the camera’s LCD and gets written to a flash card.  We don’t know how all the parts work together, but we’ve come to accept that they do – and we expect they’ll keep doing it, over and over, for hundreds of thousands of clicks.

But beyond the electronic gadgetry that we see, DSLRs are mechanical marvels crammed with complex movements.  It’s impressive how they can mass-produce a device with so many moving parts, at a relatively “cheap” price, and still deliver the long-term reliability and accuracy we’ve come to expect.

I have an appreciation for the engineering involved (and more than a little curiosity), so this is the first in a series of articles to explore the inner-workings of a DSLR – specifically, my Nikon D2H guinea pig.  Ideas for future articles include the mirror box mechanicals under-the-hood, a detailed analysis of Nikon’s shutter assembly, and a look at shutter lag along with a hack for ultra-low shutter lag.  And I’m open to suggestions… got something you want to see?  Leave me a comment below!

Freeze!

Like so many projects, this one involves a newly acquired gadget – in this case, a Camera Axe triggering timer.  This slick little tool allows for very precise timing of photos (adjustable to 1/10,000 sec), making it possible to capture very specific points in time.  Combined with some speedlights to freeze the action, it’s possible to systematically capture every stage of a DSLR’s mechanical process.

Lights, Camera, Action!

To show how the different mechanical parts work together as a system, 118 photos were taken – each one 1/1000 sec later.  The total time elapsed is 105 milliseconds, or 0.105 seconds.  The shutter speed on the D2H is set to 1/200 sec – slightly slower than its maximum flash sync speed.

This slideshow requires JavaScript.

The subject camera is a Nikon D2H, which is renowned for its low shutter lag and short viewfinder blanking time – even years after its release, at 41 milliseconds the D2H is still much faster than most modern DSLRs, which range from 50ms to 150ms shutter lag.

The first step you’ll see is the aperture stop-down arm being lowered on the left side of the photos.  This arm links up with a lever on the back of the lens that controls the aperture diameter.  When this arm moves downward, the aperture is no longer being forced wide-open, and a spring inside the lens pulls the aperture closed.

Since we don’t have a lens attached, the camera assumes an older manual-focus lens is attached.  For these lenses, the aperture value is controlled by an aperture ring on the lens, so the camera’s aperture stop-down arm simply moves out of the way and leaves control up to the in-lens mechanism.

Next, the mirror stars lifting.  This step takes slightly less time than the aperture arm, then the shutter fires.  The shutter is open for 0.050 seconds, which lines up to the 1/200 shutter speed – notice that the exposure time is from start of open to start of close, and the curtains sweep in the same direction to ensure the entire frame gets equal exposure.

As soon as the shutter closes, the reset sequence begins for the aperture arm, mirror, and resetting the shutter for the next shot.  Total elapsed time: 0.102 seconds.  Add a little breathing room and the camera is ready to fire again at 0.125 seconds, yielding 8 frames per second in high-speed mode.

Doin’ the Jitterbug

What was that?!  Does the shutter really bounce back closed after it opened?

It looks that way, but no.  That’s due to an effect called “jitter” – these photos were taken from 118 separate D2H shutter clicks; the image was timed using an external triggering system that first fires the D2H, pauses, then fires the flash to freeze the shot.  Although the triggering system is precise, the shutter release on the D2H apparently isn’t – sometimes the D2H actually fires about 1/2000 sec later.  This inconsistency in the timing is called jitter, and when it happens it causes the shutter cycle to be running late when the image is captured; in a series of photos like this, jitter makes the shutter movement appear to freeze or step backward briefly, then jump forward.  You’ll see it happen a few times here, prior to the shutter opening fully.

To overcome the jitter, I used a different triggering technique for the second half of the cycle – once the shutter was open.  Instead of starting the external timer when the D2H’s shutter release was pressed, I wired the timer to start when the D2H triggered its external flash.  This technique only works for capturing the second half of the shutter cycle, but it got rid of the jitter in the timing and delivered consistent results for images of the shutter closing and the mirror lowering.

This entry was posted on 18 May 2011 at 22:24 and is filed under Photography with tags Camera Axe, Camera mods, D2H, D2X, DIY, Hacking, Nikon, Shutter, Teardown. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.