How to improve focusing time when using AF assist light

Discussion in 'Lighting Equipment' started by aonsen, Sep 7, 2010.

  1. I shoot with a Canon 40D and a Sigma ef 500 DG Super flash, mainly using a 50mm f1.4 lens, using the center focus point exclusively.
    Recently I've started shooting in dance clubs and concert events (venues where light is at a premium).
    http://photo.onsendesigns.com/
    The problem I'm having is shooting at really low light conditions, while using the AF assist light to aid focusing results in many missed shots. (The AF assist light is the red light patters the flash puts out, not bursts of the light from the flash itself, just so everyone is clear on what I'm referring to)
    What happens when the AF assist light kicks in that focusing takes up to a second or two (the red AF assist light/pattern flashes a few times, and with each flash, the focusing gets adjusted a little more). This delay (of even a second) is very annoying and results in many lost moments since the shutter didn't get triggered at the moment I pushed the button, since person photographed is dancing (moving) or artist is moving/dancing/singing.
    As I have never owned a canon flash (430 or 580) and I don't know if the AF assist red light also flashes or if it's a continuous light on the canon flash?. This may be just a Sigma flash thing?
    I have tried not using the flash AF assist, and let the camera do the AF in ambient light, but even though the camera's AF indicator light indicates an in-focus image, the final image is less then sharp (when viewed on a computer). When the flash AF assist light is used, images are in perfect focus, which leads me to believe that due to the low light, and low contrast of the images, combined with the low DOF of the 50mm lens wide open at f1.4 (where even a small error in focusing makes a big difference in image sharpness), the AF accuracy is lower.
    What are my options to improve the focusing time, so the delay is reduces in low light conditions?
     
  2. At what aperture are you actually shooting, when the exposure happens? If that 50/1.4 is being used even close to wide open, you have razor-thin depth of field. If you or your subject move even an inch or two, they will fall out of the in-focus plane. So, need to know a little more about those exposures.
     
  3. Most shots are taken at between f2.0 and f1.4, and shutter speeds about 1/80 or 1/100 (not that it matters too much when the flash is used, except to control how much ambient light gets in)
    ISO is usually at 1600
     
  4. So, let's say you're shooting someone who is 8 feet away from you. With that camera body and that lens, used at f/2, you've got less than 6 inches of workable DoF. At f/1.4 and 6 feet away, you've barely got 3 inches. That means that someone who is bobbing their head as they dance, or moving their torso or feet at all ... is going to fall right out of focus. More flash power, and stop down to buy yourself greater DoF.
     
  5. Thanks Matt. The DoF figures are good to remember.
    I'm am I correct in assuming you mean I should not use the AF assist light?
     
  6. No, I'm not even addressing the AF assist light issue. I'm pointing out that many of the out-of-focus results you're seeing may have nothing at all to do with the speed of your AF (assisted or otherswise), but could instead be the very much expected result from shooting with such a wide aperture. You can get a sense for what working distance and aperture will get you what sort of workable DoF by playing with a depth of field calculator like this one. It can really help you with your exposure strategy.
     
  7. That AF assist only works in single shot so, once you lock focus and the subject move you'l get fuzzy pictures. Unfortunately that AF assist won't work in AI Servo, plus, due to low light, your camera is already struggling to get focus on a one shot setting let alone asking it to focus tracking.

    So yeah, what Matt said, increase ISO to the maximum you're comfortable with, to let in ambiance light. Stop down to maybe f/5.6 or f/8. Use M setting on your dial, and let the flash provides most of the light for your foreground, also maybe experiment with flash exp compensation, maybe dial it down by 1 stop or so. I would continue with one shot and the af assist light for now.

    BTW I check your work on your web page, on the pictures shown, looks like you got the exposure fine.
     
  8. If you think the Sigma flash is an issue then buy/rent/borrow a Canon 430EX(i or ii) and see if you notice a difference. I have both versions of the 430 and a Sigma EF 530 Super.... and the light pattern emitted by the Sigma does not cover the two outer focus points on a 50mm lens mounted on the 40D until you get more than about 6 feet away. The pattern is vertical lines as well mostly concentrated for the central focus point. The Canon flashes however cover all of the focus points for the 40D with 50mm lens attached as close as 2 feet, and it emits a grid of horizontal and vertical lines.
    I think you'll find focusing easier with a Canon light attached, but you should do a real world test yourself and not just take my word for it.
     
  9. @A Novisto: Thank you for your input. Shooting at f5.6 or f/8 requires way too much light. Way more light then I can provide on camera. I always bounce my flash either off the floor, or side walls which eats up a lot of light to begin with. Even at f2.8 I sometimes find that the light is not sufficient to illuminate properly some subjects.
    Maybe getting a DSLR body that's a few stops better then the D40 (like a D700?) may help in bumping up the ISO to 6400 and still produce acceptable images.
    @Dan F.: I don't find the fact that the Sigma lens only provides AF assist for the center point a limitation. I mainly use the center point anyway.
    Doing some reading on the net, and I think what I may try (on the cheap) is another form of AF assist that will put out a continuous light beam to allow me to shoot with AI-Servo.
    I am thinking of a LED flashlight/laser pointer (taped to the side of the flash), or even a headlamp, with a red filter on it so the light it puts out is not too intrusive, but sufficient to add enough light for the camera's AF sensor to work reliably and continuously.
     
  10. How far away are these subjects? You might want to see if you can borrow or rent an SB-900, and see if more horsepower helps to solve your problem. At typical people-shooting distances, f/5.6 shouldn't be a big problem.
     
  11. A few additional thoughts: if you're shooting with a flash on a bracket the assist lamp may not line up with the camera's AF sensor. Also, some lenses just focus easier in low light than others. For some reason, my discontinued Canon 20-35mm (approx f/3.5-4.5) does really well in dark rooms.
     
  12. The people are not far. Maybe 5-10 feet. And if I was pointing the flash directly at them, I agree that most flashes will work just fine at f5.6 or f/8.
    The problem is the venue eats up a lot of light when bouncing the light. Most music venues or dance clubs have dark walls/ceilings, so little light reflects.
    I have found that often the floors reflect more light (and are closer to the strobe) but other people can obscure the flash light (outside my control)
     
  13. Adrian, with all due respect I think that the 40D+430EX combo is pretty good with low light (I've shot bounced with 40 foot ceilings at f/4) and switching to Nikon will mean that you'll need to switch your lens and flash as well since they are not interchangeable. If you do I'd suggest getting a Nikon flash over another Sigma.
    If you're bouncing off the floor and you're not getting enough light I don't think that the camera/lens is the problem.
     
  14. Dan, I am not switching to Nikon, rest assured.
    When there is no obstructions (people) walking in front of the flash, or between the subject and flash, I get awesome results. There is more the enough light when I bounce the light off the floor. I actually prefer the look of the light bouncing up from the floor then ceiling reflected light or wall reflected (just my preference).
    Mind you I have not tried shooting at f5.6 of f8 and doing the focusing based on ambient light only. I will give that a try next time I am out.
    The issue of slow AF still remains for those situations when there just isn't enough light to focus with ambient light only (and the AF assist needs to be on). That's why I am also going to pursue the DIY AF assist idea.
     
  15. You have a lot of good info above, but I can add one other thing. For the dancing shots, use wide focal lengths and bone up on DOFs for various focal lengths, subject distances and f stop combinations. Then, use manual, zone focus (very old school). It is absolutely the fastest way of shooting. There is NO focus lag. This is the way I used to shoot the dancing sections of wedding receptions. You will find that even at f2.8, you can still have enough DOF for a good margin of error, if you are good at judging distances. Even now, I still use a variation of it. I focus on something more or less at the same distance I anticipate the action will happen. Then, when it happens, I shoot. No focus lag.
     
  16. Sorry, that was my Nikon-brain suggesting the SB-900. Replace that comment with a suggestion to try More Powerful Strobe X (a Quantum, for example?). Or Canon's heftier model (a 580-whatchamcallit, I think).

    Good tip from Nadine. A shorter focal length (with you working a little closer, of course) not only gets you much deeper DoF even at wider apertures, but it also greatly (the square of distance change, here, remember?) improves the light that's getting delivered. Closer = more power, quite literally exponentially.
     
  17. Also forgot to say that you might try putting the wide angle diffuser down over the flash head, used direct, so that the very wide dispersion of light will bounce of other nearby objects, such as the floor, other people, tables, etc. It will help in general reflectivity and save on flash power, not having to bounce off far objects like the walls and ceilings. You might also just bounce the flash off other, nearby dancers.
     
  18. I'd like to thank all of you that replied to my initial question. I went out last friday and used a lot of your advice, and got some decent shots.
    http://photo.onsendesigns.com/2010/club-neutral-17-sept-2010/
    Most of the shots were taken at f/8 and the flashes I used were only at 1/8 power (I used 2 flashes mounted above the dance floor). Near the night I even experimented with manually focusing (as the AF sensor was not cutting it) only to get some surprising results.
    The light is pretty harsh in most photos since there was no modifier used. Straight strobe. Maybe a small soft box may help smooth out the light, but as a first attempt, I was happy.
     

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