Learnings (Part 3): Using Predictive Autofocus

Any modern DSLR camera supports (or should support) predictive autofocus tracking. While they might have this capability, there is an art to using it effectively, and therefore getting in-focus shots of moving subjects.

This is actually something I only learnt on the 19th of August, 2009.  Yes, I remember the date, it's an important date.  This is after having a DSLR many years, and never being able to effectively shoot moving subjects.  I would resort to doing things like:

• Stopping down, therefore increasing the depth of field, such that the moving subject might still be in focus.
• Increasing the shutter speed ('cause clearly I didn't understand the difference between motion blur and just plain out-of-focus).
• Pre-focusing in front of a moving subject.

These can give acceptable results, but why not get the camera do what it's supposed to do.

Music of Manolo Camp

I've been looking at Creative Commons licensed music for use with my videos.  There are a number of licenses, but typically they allow sharing and use in non-commercial products.

There is a lot of web sites dedicated to free music, and I've found some music by Manolo Camp that I really like.  Here's two web sites with his music for download:

• Opsound.org
• ccMixter.org

For example try Morning Coffee or Ships in the fog.  I'm already picturing some video that I can shoot for that second track.  Anyway, hope you like it.

Update 11/12/10: I've published my first video using Manolo Camp's music "Medieval". I think the music really adds polish to the video.

Learnings (Part 2): Time-lapse battery, shutter and aperture life

Time-lapse movies are cool.  Especially when they are really professional looking.  Some of my favourite time-lapse movies are made by these people:

• Tom @ TimeScapes
• Philip Bloom
• Ross Ching

I've made a couple of time-lapse movies which you can find in high-definition on YouTube.  Perhaps not in the same league as those guys, but it's still fun and rewarding anyway.

I hope to do a lot more, and have recently learnt some new techniques to maximise battery life and minimise wear on the camera.

On a SLR camera, every photo causes the mirror to flip up, the shutter opens and closes, then the mirror flips back down.  Being moving components, the mirror and shutter have a finite lifetime.  During a time-lapse you may be taking thousands of photos, therefore reducing the life expectancy of your camera.

These techniques can be used with the Canon 7D and 5D mk II cameras (and possibly others) to reduce mirror and shutter activations.

Live View mode eliminates the mirror flapping since the viewfinder is disabled.  Unfortunately as I found out here, under normal Live View operation (Silent shooting Mode 1) the shutter is activated twice for every photo.  If you select Silent shooting Mode 2, then a sensor reset is used instead of a first-curtain shutter meaning that only a single shutter activation occurs.  Eliminating mirror flapping not only extends the life of the mirror mechanism, but also saves on battery power consumption.

Using Live View in itself does use more power because the LCD is lit between each photo.  Here is a technique to turn off the LCD, simply connect the supplied A/V cable to the AV Out/Digital socket (the weird USB socket) on the side of the camera.  This will trick the camera into thinking you're using an external monitor instead of the LCD monitor and switch it (permanently) off.  Note using a HDMI cable doesn't work.

The battery life is (apparently) very long when the camera is used in this manner.  But, I'm yet to try it.

For every photo, the camera will also switch the aperture between two positions (except if you are using your lens wide open).  Surprisingly, people report success by half unscrewing their lens which results in the aperture staying in the desired position.  Instead, a better solution is to operate in Movie mode instead of traditional Live View.  This might result in a longer aperture life and/or some power saving.  The real gain from this technique though is to reduce flicker in the time-lapse image sequence.  Flicker can occur where the aperture does not open to the exact same position every time.

I hope these techniques work for you.  Please post links to your time-lapse movies.

Read about other things I've learnt here.

Learnings Index

Remember when you were young and you used to know everything.  Yeah, me too!

Now, even when I'm an expert on a subject, I'm constantly learning new things.  I guess it seems more profound or gratifying when you do have an in-depth subject knowledge, and then discover a new dimension.

This is a collection of things that I've learnt, and hopefully they're things you'll find interesting too.

I'll add to this post when/if I learn anything else.

The index:

• Part 1: Fractional ISO 
• Part 2: Time-lapse battery, shutter and aperture life
• Part 3: Using Predictive Autofocus

That's it for now.  Check back later.

Learnings (Part 1): Fractional ISO

The ISO setting on a digital camera controls the gain applied to a digital sensor (making it appear more sensitive to light) - in the same way that ASA film speed specified the film sensitivity.

Having previously owned a Canon 20D, I was familiar with traditional ISO values (associated with powers of two), for example: ISO 100, 200, 400, 800, 1600.

Due to my computer background I had assumed that boosting the ISO was achieved through bit-shifting - or digital amplification/gain.  It turns out that this is wrong.  ISO gain is achieved by analog means prior to analog/digital conversion.  That said, ISO expansion modes such as ISO 12800 on the Canon 7D and ISO 3200 on the Canon 20D are achieved by digital gain (bit shifting).

My Canon 7D supports fractional ISO values such as 125, 160, 250, 320 etc.  There has been discussions circulating on the Internet saying that the 2/3 of a stop values result in less noise.  While there is an apparent loss of noise, this actually a result of over exposing the photo.  Specifically:

• ISO 160 is actually photographed as ISO 200 (therefore with analog overexposing) before digitally underexposing by 1/3 of a stop.  In this case you are more likely to have hard clipping of highlights.
• ISO 125 is actually photographed as ISO 100 (therefore with analog underexposing) before digitally overexposing by 1/3 of a stop.  In this case you are likely to lose some shadow detail and dynamic range.

Highlight Tone Priority (or HTP) is a another camera feature which alters ISO behavior.  This is enabled through Custom Function II-3 (on the 7D) and is often recommended for video work.  In this mode, camera will intentionally underexpose (analog) by one stop.  For example if you choose ISO 200 (the minimum), then the camera actually shoots at ISO 100 (therefore halving the exposure).  In a (digital) post processing step, the image has digital gain applied (normalising the histogram), but this time with smarts to better handle highlight clipping.  Or as Canon says "The graduation between greys and highlights becomes smoother".

Both Fractional ISO and Highlight Tone Priority are particularly useful for shooting movies, and JPGs where the image bit-depth is being reduced (in camera) from 14 bits to 8 bits.  On the other hand when shooting RAW you want the extra latitude (i.e. bits) to do manual adjustments.

By combining Fractional ISO and Highlight Tone Priority you can get the benefits of extra highlight headroom, and also some reduction of noise.  For example, consider shooting at ISO 320 with Highlight Tone Priority enabled.  In this scenario the camera will effectively take the shot with ISO set at 200, so it is (analog) underexposing by 2/3 of a stop, then apply the digital gain increase (and avoid highlight clipping).

I believe the Canon 5D mk II behaves the same, but other Canon models or other brands may not.  You need to do your own research.

If your camera does behave this way, then by understanding how fractional ISO works, it can help you use it effectively or when appropriate not use it.

Additional References:

• Unfortunately this link is dead: http://www.cryptobola.com/PhotoBola/Canon7D_ISO.htm
• However a Google Search will find many informative pages linking to that site

Read about other things I've learnt here.