best AF assist gadget??

[RTfuture]

Hey. New to the community. My first post is....

I shoot in VERY Low light conditions with no flash. 

www.RememberTheFuture.co.uk  for examples. It makes AF a nightmare. To the point I don't even bother. I shoot in MF, which also isn't the easiest. I'm wondering if there has been a good AF assist gadget I could use. I need good range, and as less cumbersome as possible. Using my speedlites assist is not an option. For that very reason. I need to stay mobile, and agile! 

Thanks again!

[TCampbell]

What camera body and lens are you using?

The 6D for example, can focus in extremely low light.  Canon claims it's down to -3 EV.  (just the center AF point).

Ev is Exposure value.  The Ev is the sum of the Av + Tv (Av is Aperture value and Tv is Time value).  

The Av and Tv numbers have their values assigned based on the number of "stops" down from the base value.  

Tv:  The base value for shutter speed (Tv) is 1 second.  That means a Tv of 1 second gets a Tv of 0.  As you speed the shutter, the Tv goes up by 1 for each full stop.  So 1/2 sec. = Tv 1 and 1/4 sec = Tv 2, etc.

Av:  The base value for aperture is f/1.0.  An Av of f/1.0 is Av 0.  Each stop down increases the Av by 1.  So f/1.4 = Av 1, f/2 = Av 2, f/2.8 = Av 3, etc.

As an example... an outdoor photo taken in full mid-day Sun is a "Sunny 16" exposure.  That means if you set the aperture to f/16 then the shutter speed is the inverse of the ISO setting.  At ISO 100 the shutter speed is 1/100th.    f/16 is Av 8.  1/100th sec. is Tv 7  (really that's 1/125th since 1/100th isn't an "even" f-stop value).  Ev = Av + Tv.  Since 8 + 7 = 15, A "Sunny 16" exposure is Ev 15.   It's a lot of light.

But Canon boasts the 6D can focus at Ev -3.   If you had an f/1.4 prime lens, that'd be Av 1.  1 second would be Tv 0.  So really we'd have to slow the shutter down by 4 stops...  that would mean using a 15 second exposure at f/1.4 to get the shot (at ISO 100).  That's Ev -3!  That's NOT a lot of light.

Of course you're not really going to shoot a 15 second shot... you'll probably boost the ISO considerably.  Maybe you'll shoot at ISO 6400 (6 stops up from ISO 100).  That would let you soot f/1.4 at 1/4 second (at ISO 6400)  That's still much slower than you'll ever shoot... but that's Ev -3 and the camera can actually focus in lighting that low (a 6D can).  The 5D III is one stop behind... it can focus in light down to Ev -2.  (Only the center point can focus at those low levels.  Other points require more light.)

If you use a Rebel body for comparison (e.g. I looked at T3i, T5i, T6i) then all focus down to Ev -0.5.  Even the mid-level bodies like the 60D and 70D have the same Ev -0.5 lower end of the range.  The 6D can focus in about 1/6th as much light as these cameras.  The camera body you use makes a big difference.

Apart from that, the AF assist beam built into a Canon speedlight (430EX II, 580EX II, 600EX-RT) can emit the red pattern beam to allow the camera to lock focus even if you're in completely darkness and you can disable the flash.

As far as dedicated AF assist gadgets... the only one I've ever heard of is the Yongnuo YN12AF.  It attaches to the hot-shoe and fires the AF assist beam for the camera.  It's not a flash nor is it a flash trigger... it's just a focus beam emitter.   I say "I've heard of" and not "I've seen" because while news of the device came out more than a year ago, I  have yet to find the device on the market anywhere.  Last I read, pricing and availability still haven't been announced, and most companies don't "announce" a device and then not release it for well more than a year.  That makes me wonder if Yongnuo has had a change of plans and maybe they've decided not to release it.

[ScottyP]

I did not know that is what "-3 EV" means.

[RexGig]

The ST-E2 will cast the same pattern of red light upon a subject as a Speedlite, to assist AF, but assuming "MF" refers to manual focus, I do not see much of a benefit. For AF, however, it is beneficial. These can be acquired on the pre-owned market for much less than the new cost.

For shooting in very low light, I have often used a hand-held light. My favorite is the Surefire LX2 Digital Lumamax, as it has good white balance, so can be used to light the scene when I take the shot, if I wish, and has two intensity levels. Keep in mind that if an outside light source is used for focusing, but then switched-off for the shot, the camera must be allowed to re-meter the light, to get proper exposure.

Another useful light source, for focusing, is my Canon 320EX, in constant-on LED mode. It can be placed to cast just a bit of light on the subject, from an angle, using a light stand, an extra tripod, a flash bracket, standing on its supplied foot, or hand-held by by the shooter or an assistant. The 320EX is properly white-balanced, and therefore needs not be switched-off for the shot.