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Thread: Focusing in tricky lighting

  1. #1

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    Focusing in tricky lighting

    A lot of what I do seems to include a dark subject with a well-lit background. My D90 has a strong tendency to focus on the background instead of my subject, even when the focus point is completely covered by the person. This is independent of lenses (it even does it with my manual focus lens if I'm trusting the focus indicator). I don't think it would be miscalibrated focus, since it does it with a variety of subject-to-background distances. It seems like the camera just grabs whatever it wants as opposed to just the focus point (I am in single-point mode, no dynamic area or auto selection).

    I know I need to be finding contrast to help the focus system, but does anyone have some more extensive advice?

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    Re: Focusing in tricky lighting

    When manually focusing (but with different camera) I never trust any auto focus indicators and always go by what looks right to my eyes, through the viewfinder.

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    Re: Focusing in tricky lighting

    That doesn't really help me...

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    Re: Focusing in tricky lighting

    It's a very common problem.

    Some ideas ...

    - Use a strong torch to temporarily illuminate the subject until AF is achieved (and then either turn it off and take the shot, or set the lens to MF and "go from there")

    - Use a stopped-down aperture for greater DoF (thus minimising focusing errors) and then calculate focus via distance scales.

    - Use AF assist function of your flash (if you have one)

    As Geoff mentions, never trust AF in these situations -- at least use what you're seeing through the viewfinder for confirmation.

    Hope this helps.

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    Re: Focusing in tricky lighting

    The point you see in the viewfinder is not the point where the camera focuses. If it tends to focus on the background instead of a target that fully fills the outlined point, the actual focusing point covers part of the background, and it finds better contrast there.

    The simplest solution is that you do not trust the size of the focusing point, but estimate a somewhat bigger one and make sure that there is no background within that larger spot.

    A more complicated solution is to find out exactly how large that focusing point is, and where in the viewfinder it is found. It is close to the marking on the viewfinder screen, but not exactly there. It might be somewhat larger, and its position may be different. Say it might be just a little larger and somewhat higher or lower than you expect. It is unlikely that there would be a shift to left or right. When you have located it, you must always remember to have your focusing object cover that spot entirely.

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    Re: Focusing in tricky lighting

    Blake, the camera needs an area of high contrast to focus properly. Solid tones of any color will give it problems which are only multiplied in darkness. In dim light you may find it searching for some time and sometimes never find a suitable point. Depending on the camera and the number of focus points the software may ignore your dimmer center subject in favour for the more contrasty stuff on the side. I only shoot in manual but as an example my camera has 51 sensors and in full auto will search for something it recognizes as being able to focus on. That could be the far left or right. You don't indicate what camera you have but many have similar features. I would suggest you try spot or center weighted focus metering if it's available to you. The light may still be too dark to provide the needed contrast for the sensor normally but forcing it into a confined area may give you a better chance at doing so. If you still can't achieve the focus an alternative is to add enough light to do that. My camera has a small on-board light to do that but sometimes it's not strong enough depending on the distance. If your's doesn't have that then another source will have to be used. When I encounter something like that I use a small flashlight on the subject, press the shutter half way to get the proper focus, switch the lens to manual so it doesn't move and fire away. Your only alternative to more light is to set the distance on the lens using the distance scale. It's not overly accurate between the numbers but by selecting the widest f-stop you can and still hold the camera steady will maximize your DOF to hopefully get the shot.

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    Re: Focusing in tricky lighting

    Another option - calibrate your AE-L/AF-L button as AF-on. Now your camera will focus with your thumb on the button. Then you can recompose and shoot - the shutter button will not try to hunt for focus for you.

    That doesn't help directly, but then you can focus on something at a similar distance to your subject, recompose and shoot. For example, the camera might lock focus on the floor if there's enough contrast (I'm thinking outdoors with tiles/paving maybe reflecting a little light and revealing a little contrast/texture)

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    Re: Focusing in tricky lighting

    You're right that's probably the easiest Phil. Funny, I meter and recompose every day and never thought of that in this case.

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    Re: Focusing in tricky lighting

    Quote Originally Posted by Andrew1 View Post
    You don't indicate what camera you have
    D90

    I would suggest you try spot or center weighted focus metering if it's available to you.
    Are you meaning centre-weighted or spot automatic-exposure metering? Can't say I've heard of "focus metering".

    It's not overly accurate between the numbers but by selecting the widest f-stop you can and still hold the camera steady will maximize your DOF to hopefully get the shot.
    Widest f-stop maximises DoF?

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    Re: Focusing in tricky lighting

    Quote Originally Posted by dubaiphil View Post
    Another option - calibrate your AE-L/AF-L button as AF-on. Now your camera will focus with your thumb on the button. Then you can recompose and shoot - the shutter button will not try to hunt for focus for you.
    I'm guessing that you're meaning CONFIGURE the AE-L/AF-L button? But that aside, I'm still not seeing the wisdom in the advise -- as you say, it's not going to improve the camera's ability to focus in the required area (so focus and recompose will be needed) - and if focus and recompose is needed then why not just continue to do that with the shutter release 1/2 depressed rather than have to press and hold an additional button.

    Or are you suggesting that he disables AF with the shutter release 1/2 down, and uses only the AF-L button for focusing?

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    Re: Focusing in tricky lighting

    Configure/Calibrate/Set Up/whatever!

    If you use the AE-L/AF-L button as AF-ON with the camera in AF-C, then you have two camera focusing modes in one at your fingertips. Hold the thumb down to continue tracking or click with the thumb to attain/fix/set/get focus and recompose to shoot. The shutter release therefore has no effect on focus and the camera will shoot if focus is not attained/set on the selected cross point after you have recomposed. The camera will not fix exposure based on the focus point you have acquired/attained/fixed/set, but meter based on the recomposed scene. It takes a little getting used to, but after I tried it after a walkabout with a pro photog I haven't looked back.

    i.e. - Using single point focus, centre cross point. Focus on object at a similar distance with thumb. Release thumb from AE-L/AF-L. Recompose. Shoot.

    If it's so dark that you cannot attain/fix/set/get focus on the subject directly, stopping down much on a D90 will give you mush - either high ISO mush as 1600 is the furthest you'd realistically want to go or blurred mush due to camera shake or subject movement. Similarly, depending on the lens used you will either not be able to see the distance scales clearly or it won't have them at all.

    I have a D90 and used it a lot. I know it's limitations. I could not shoot the kind of images I wanted with it. Moving to a D700 allowed me to take photographs in darker conditions. Partly this was down to greater high ISO capability, partly to better focusing, and partly to using the technique I've mentioned. Pooh-pooh it if you wish/desire

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    Re: Focusing in tricky lighting

    So a B- for my previous post, grammatically

    But hopefully an A+ for a different technique that might work


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    Re: Focusing in tricky lighting

    Blake, my poor choice of words in my last post obviously caused some confusion and a problem understanding. If you are still having problems drop me a PM. I don't get over on the ferry very often but perhaps a phone call would help clear things up.

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    Re: Focusing in tricky lighting

    It may be best to stick with single point focusing as well, rather than 3D auto area etc. Then you can trust your focus point, because you set it. This, combined with spot metering, will give you the correct exposure for your subject.

    Your backgrounds, if they are bright, will be blown. Such is life with camera technology.

    Another option - learn to balance flash with ambient. Expose for the background/ambient and then fill flash for the subject. Too much fill flash may make your subject look too obviously hit with flash, so you can dial it back with flash exposure comp. You'll ideally be in full manual mode for this. Your D90's light will help potentially help with focus.

    Another option - move your subjects!

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    Re: Focusing in tricky lighting

    Maybe I should clarify, most of my shooting is at events and is 'on the fly.' I understand how the autofocus system works. Sometimes it isn't practical to shine a light or stop down, given the nature of what I'm doing, especially at concerts.

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    Re: Focusing in tricky lighting

    Also, I'd love to upgrade to a better body, but I'm still in the slow process of buying lenses. That being said, having a 24-70 may help (as opposed to my 18-55).

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    Re: Focusing in tricky lighting

    Sounds like you may have difficulty using manual focus in the type of shots you shoot but it should still work out ok. The usual problem with not being able to use manual focus is not carefully setting the dioptre adjustment on the viewfinder. The best way to do that is to use the AF to focus on something flat with detail that fills the view from a shortish distance to minimise the depth of field. Then carefully set the dioptre adjustment. There is a bit of a problem though. Your eyes need to be relaxed looking into the far distance. Young people may focus there eyes at incredibly short distances especially as they are doing there best to set the dioptre adjustment. Older people tend to do the same thing as well. The distance will just be longer but still too close. It's possible to get a feel for what it feels like to relax the eyes by looking "through" say a wall that's 4mtr's or more away. The problem is that our eyes tend to automatically focus on what ever we look at and this can tend to interfere with setting the dioptre adjustment. Later when we use the camera without thinking about it our eyes may focus at a different distance to how we set it. This shouldn't be too much of a problem on a camera providing people remember that it can happen.

    The other problem with using manual focus on some camera such as a Pen that use an electronically coupled focus mechanism is that it's way to slow in manual mode to be of any use other than for finally tweaking the focus setting. On a Pen it feels like a whole turn of the focus ring hardly does anything.

    -

  18. #18

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    Re: Focusing in tricky lighting

    No, the problem is being unable to get the camera to focus on the correct thing.

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    Re: Focusing in tricky lighting

    What if you need glasses?

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    Re: Focusing in tricky lighting

    Quote Originally Posted by blakemcguire View Post
    No, the problem is being unable to get the camera to focus on the correct thing.
    Welcome to the real world of photographic limitations and compromise

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