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Problems w DSLR not autofocusing under low vis

Discussion in 'Tips and Techniques' started by kaaralex, Oct 16, 2011.

  1. kaaralex

    kaaralex Solo Diver

    # of Dives: 200 - 499
    Location: Shanghai
    48
    2
    8
    Maybe i m doing something wrong and i'd be very grateful if somebody points me to the right path. But i hv found autofocus on DSLR (in my case 7D with EF-S 10-22mm) to be real menace. Once it becomes dark (depth or night or low vis), when i press the shutter, typically this happens: camera starts focusing, strobe fires a pre-flash, but that is - my guess - too short for camera to get focused, so the camera fires another pre-flash .. and that goes ad nauseam. All creatures from fish to buddies get p...d off and dont want to participate in the photographing activity anymore. You can forget about composition, coz the shot just comes out randomly, if at all. Also, time runs really fast in depth... etc.

    It is even more annoying coz when Her Autofocusy finally decides to bless me with a shot, usually it is sharp and clean, just what u would expect, see below. (Pics not perfect, left strobe being still too much into the picture but due to this issue i was actually happy just when camera fired AT ALL.)

    With macro, i tried to use the assisting beam in my strobes (i use red cover lid on my Inon Z240 so it is red) but it did not work very well. The most important reason is that usually u hv the strobes slightly off the subject in order to eliminate backscatter, so the assisting red light on strobe usually points somewhere else than subject. Which is even more the case of wide angle - the strobes point in completely different direction.

    I also tried to use my UW light, but that created another sort of problems: first, it is quite strong, so it was visible in some shots, second, it created backscatter.

    Maybe i m using wrong autofocus mode? One more thing, i hv the diffuser on the strobes all time, and i got a red cap on the assisting light (the weak light that can be on all time) - i must admit i hv never tried to use the assisting light in white color, but as i said, since usually it points other direction than subject, i m not sure if it would work better than red light... ?

    It is extremely frustrating and really ruins my day down there, that's why I would really appreciate any help with this issue.

    _prelle-4359.jpg _prelle-4361.jpg _prelle-4362.jpg _prelle-4369.jpg
     
  2. danvolker

    danvolker Dive Shop

    # of Dives: I'm a Fish!
    Location: Lake Worth, Florida, United States
    5,884
    3,007
    0
    I shoot with the canon 5d mark II....though 99% video, I am fairly familiar with most of the still issues...
    I believe the auto focus relies on edges and contrast...if light and contrast is low, autofocus can't see an edge, and you wont get a good focus.... since I shoot video, there were many additional reasons to use manual focus instead...My Aquatica housing and ports make this easy, and in any ligt I can see in, I can get a crisp focus.....However, it require the time to point at something the correct distance away ( as far as the subject you intend to shoot), and to hit the magnify X 5 or x 10 ( typically x ten), so that you can really see the focus crisply. For me, most video of big marine life will be me trying to get as close as I can, but this is usually 2 to 5 feet....my dual hid video cave light can handle this distance, and from 3 to 5 feet, the focus is pretty good for much greater distance...but could be off alot if a big Goliath actually runs in to me and the shot is more like 6 inches... :)
    Usually I focus at the beginning of the dive at the bottom, on a distance I want to use, then run with that... If a unique scenario arises, I will pull focus again at the different distance. Anything that works for video like this, will work for stills...Of course, I have to change ISO, aperture, and consider other items when I deccide to pull the trigger on some stills..but focus is already done :)
    This is an example of an area where without manual focus, it would be hard to use autofocus... way-too-much-fun - YouTube I had decided before penetrating into the wreck, how far away I would try to get from the goliaths....I typically shoot in flat horizontal trim, and frog kick along at the pace that keeps me in the right distance range....If I need to get further back, I can use a reverse kick to back up. After you focus, you spend a minute getting dead neutral, if needed....get so a half breath makes the difference of going up or down, so that all of your bouyancy is breathing controled throughout the shooting, and so that you can stay in a mid water ( in the hold of the ship) position like you are being held up by strings....It's actually pretty easy, and this totally avoids silting, which is what happens as soon as a shooter stands on the bottom, or bumps into it accidentally.
     
  3. Lwang

    Lwang Photographer

    # of Dives: 200 - 499
    Location: NYC
    1,189
    86
    48
    Manual focus is the way to go, especially with in such wide angle mode. Then you can calculate your depth of field based on the focus distance and aperture. That was how it was done in the old days.

    But as for autofocusing, DSLRs usually use phase detection that does not rely on contrast edges like contrast detecting autofocus (used by P&S and DLSR in video mode).

    The faster the lens, the easier it is for the camera to perform its focus. f5.6 is the absolute these cameras. Maybe you need a f2.0 fixed focus lens in such situations?

    You can also try to switch to contrast detection focus mode, which might be activated when you switch to live-view mode. Those usually focuses more inferior, but under your situation, it might be different.
     
  4. decidedlyodd

    decidedlyodd DIR Practitioner

    # of Dives: 200 - 499
    Location: Seattle
    172
    31
    28
    Have you considered getting a proper focus light? I use an Inon LE 550W. It's pretty basic (just one level on/off, no special red LED, takes standard AA batteries), but works well for my local conditions. The L&M Sola lights and many in the iTorch line look pretty nice too.

    When I'm not using the light in darker conditions, my 550D has a lot of trouble focusing.
     
  5. craracer

    craracer Solo Diver

    # of Dives: 200 - 499
    Location:
    895
    72
    28
    I had the same problem with my DSLR. I added a Sola 600 and my problem virtually solved. It's an expensive option, but I'm glad I got it. It's a fantastic light, and bright enough that I lit up octopus for other divers to shoot video of.
     
  6. kaaralex

    kaaralex Solo Diver

    # of Dives: 200 - 499
    Location: Shanghai
    48
    2
    8
    BIG thanks for all your suggestions. Manual focus is out of question, i m afraid, first my housing does not enable it and even if it did, focusing would be YET ANOTHER thing to think of ... I think i m already quite task-loaded down there. :)

    As for the focusing lights. I checked LE 550 specs, it says it has 550 of lumen, which is quite a lot. i would suspect that this is going to happen: the beam of focusing light is going to lit the stuff in front of my camera and thus create backscatter? Or am i wrong? What is the typical position of the focusing light? On my housing (Nauticam) there is nowhere else to attach it then to the arm of the strobes anyway...
     
  7. EastEndDiver

    EastEndDiver Captain

    # of Dives: 2,500 - 4,999
    Location: Long Island NY
    1,979
    1,248
    113
    There are focus lights which cut off the instant the flash is fired to avoid burn outs and in your case back scatter.Search out one of those.
     
  8. SeaInFocus

    SeaInFocus Nassau Grouper

    # of Dives: 1,000 - 2,499
    Location: Huntington Beach, CA
    109
    17
    18
    I use a SOLA 600 for my night dives when shooting wide-angle (and macro for that matter) and it works like a charm. When shooting wide-angle with the SOLA, I be sure to set my shutter speed fast enough to not capture any ambient light, and therefore minimize any backscatter. Also, I LOVE the red light function of the SOLA 600. It has allowed me to capture more pics of skittish critters than I can count. Totally worth the investment. I got mine from Bluewater Photo. Great shop with lots of experience and knowledge.

    Cheers,
    Zig
     
  9. Doubler

    Doubler Loggerhead Turtle

    # of Dives:
    Location: Bremerton, WA
    1,606
    316
    83
    Big Blue Focus Light cuts off for 5 seconds automatically with strobe fire. No hot spots, no backscatter due to Focus light. The Sola is a great light but so is the Big Blue and it is cheaper. Either will solve your problem.
     
  10. Warren_L

    Warren_L Instructor, Scuba

    # of Dives: 2,500 - 4,999
    Location: Toronto, Ontario, Canada
    3,104
    55
    48
    One thing that has worked for me with shooting wide angle is to take a shot of something using a hand held light (or something that is already lit) at a distance that you'll be shooting at and then change the focus mode to manual. Since the lens is focused for this distance, switching it to manual keeps the focus set and you can shoot without having the lens autofocus and the shutter fires every time. If you change your camera to subject distance significantly, you will have to re-do the process. This likely won't work too well for macro since you'd have a pretty shallow DOF already and small changes in the distance to your subject can throw your focus off quite easily. But for wide angle, this works quite well. I use this for shooting wrecks quite often, especially when inside a wreck when there's little ambient light.
     

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