Why don't DSLRs use laser rangefinders for autofocus?

Question

I own a simple laser rangefinder for home repairs purposes which is pretty great - it can measure the distance to any object with an astounding accuracy (down to 1cm for distances up to 100m). It's fast, works in any light conditions and doesn't require the object measured to have any contrast.

This brings me to my question — why not include a laser rangefinder in DSLRs? This would let the camera focus in the worst possible conditions when the standard methods fail. It also shouldn't be too expensive as cheap laser meters cost as little as $10. Or perhaps I'm missing something and such systems do exist already?

Nikon does produce a series of portable laser rangefinders, but nothing similar for DSLRs. And in case you're wondering - laser rangefinders can use wavelengths invisible to the human eye, so you won't disturb your subject by measuring the distance to them. And you won't be hurting anyone's eyes as most laser rangefinders use Safety Class 1 lasers.

Answer 1

why not include a laser rangefinder in DSLRs? This would let the camera focus in the worst possible conditions when the standard methods fail.

This is really a question that calls for some speculation, but I can think of a few reasons:

1. Focus points. DSLRs typically have multiple focus points that let the photographer choose which part of the image should be in focus. A laser rangefinder would probably only support one point, so it would be much more limited than what existing AF systems provide.

2. Cost. Sure, the laser diode found in a laser rangefinder might be relatively cheap, but the electronics needed to detect and time the round trip of a pulse of light also come at some cost.

3. Calibration. Existing DSLR AF systems don't really care how far away the subject is, they only care about whether the subject is in or out of focus, and in the latter case which direction to adjust focus. A laser rangefinder measures actual distance, but getting the lens to focus precisely at that distance would require some degree of calibration, and that would need to be repeated for each lens the photographer might use.

4. Need. It sounds like you intend for the rangefinder to work as a backup system, not a replacement for the existing AF technology, but it's not clear that current AF systems fail often enough to require a backup. In cases where there's a problem, other aids (the AF assist light built into many bodies and also speedlights) already help.

5. Optics. Laser rangefinders have their own lenses built-in; it might be tricky to build one into a camera body in such a way that it works reliably with interchangeable lenses, and without interfering with the existing AF system, the reflex mirror, or the image sensor.

6. Marketing. The lasers used in rangefinders may be safe for the eye, but that doesn't mean people will necessarily feel comfortable having one pointed at their eyes.

The answer to most questions of the form Why doesn't product X include feature Y? is that the feature in question doesn't provide enough benefit to justify the cost. The points above are really just some suggestions for reasons that building a laser rangefinder into a DSLR might not make economic sense. Basically, it comes down to adding a bunch of complexity and cost to provide a feature that's not really needed.

Answer 2

In portraits, we usually want to focus on person's eye (hence laser can be detrimental to vision). More importantly, sometimes we want to focus on person's eye when it is not in the dead-on center of the frame.

This means that laser rangefinder will have to either "scan" the field of view constantly, or change position (as we move around AF area using 4-way joystick on modern DSLRs).

Yes, we can "lock AF and recompose", but it is not always desirable. Also using single AF point (effectively) kills ability to autofocus intelligently, for example using face-recognition or for tracking of moving subjects (footbal players).

Either way it is somewhat complicated to implement. Galvo-mirrors used for that usually use a lot of voltage and pretty bulky. Other solutions are not much better either (DMD versus galvos, for example).

Answer 3

• Lasers will be blocked or confused by: fog, rain, windows, dark and specular materials.
• Lasers will lack the advanced abilities of dynamic focus tracking on features.
• Power consumption might go up, I suspect that's a minor point.

Side note: some Graflex rangefinders had a neat feature that would project a red dot from each "eye" of the rangefinder so you could focus to when the dots coincided even in deep darkness. Handy for crime scene shots with flash, I would imagine. Perhaps your original question requires more thinking outside the box from my side for a better answer.