Multiple Focus Points

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  • #3640
    Profile photo of Peter Robinson
    Peter Robinson
    Keymaster

    Thanks for going to the trouble of doing those exposure tests Ian. It does clarify thing a lot. I had some exposure problems over the weekend. I was photographing a band in Nantwich. The guys were wearing white shirts and the stage had a black background. So an average reading gave me over exposure du to the black background. When I metered on the white shirts (by accident) the picture was underexposed. I got an acceptable exposure by taking a partial reading from  the guys face and locking the exposure. Thanks Ian.

    #3641
    Profile photo of Ian McNab
    Ian McNab
    Keymaster

    Yes, the tests shots illustrate the issues better than any amount of verbal description, don’t they?  Your white-shirts-black-background scenario is exactly equivalent – a real problem to expose well.  Your solution sounds a good compromise;  but I guess you may have to do some local adjustments in post-production?

    #3644
    Profile photo of
    Anonymous

    Well Peter as you know-we were always advised in the old film days to increase exposure more than the meter reading for bright snow or beach scenes.  This is the problem in the situation where you were taking several  varios scenes  and no doubt using and general average meter mode  as we would. The picture you describe is a good example where a “spot meter” reading on the white shirts would have been spot on  as the increased under exposure of the black background would not have mattered. But as with most in the situation  –it would be most un likely  mid shoot to change meter modes.   Look forward to seeing your pics Peter.

    #3664
    Profile photo of Peter Robinson
    Peter Robinson
    Keymaster

    I know we’ve gone of the subject of focus points but exposure is an interesting and important subject. As a real world example of how taking the wrong exposure has spoilt my photos on Saturday I’m posting these to show the effects. I haven’t done any post processing.

    This shot was taken with a partial exposure that covered to much of the black background.This has caused the white shirt to be blown out.

    The partial reading in this one is from the guys head which turned out to be a good compromise.

    In this one I accidently exposed from the guy’s arm so most of the photo is too dark.

    I could probably recover the mishaps in PhotoShop and get all three to look well exposed.

    When they stepped out into the sun the harsh contrasty lighting made the exposure very difficult because of the wide contrast range. It’s much better the photograph shot like this in the shade.

    #3667
    Profile photo of Ian McNab
    Ian McNab
    Keymaster

    Very good demo of the issues, Pete.  If you’ve got a big dynamic range, exposing for something near a mid tone is the safest bet, as you did using the guy’s head in the middle shot.

    Direct bright sunlight can create scenes with a dynamic range of 12+ stops: far greater than any camera can cover in a single exposure.  Any exposure in that situation will be a compromise.  As you say, the solution is to photograph in open shade.

    Yes, good real-life demo.

    #3668
    Profile photo of Ian McNab
    Ian McNab
    Keymaster

    We’ve forgotten to mention Sean McHugh’s ever-wonderful site, Cambridge in Colour.  He has a very clear exposition of camera metering and exposure here.

    And, to get back to the original topic of this thread, he explains digital camera auto-focus here, including discussion of the way modern digital cameras use multiple focus points. (See particularly the section called “Factors affecting autofocus performance”.)  Towards the bottom of the page, in the section “In practice: portraits and other still photos”, he explains the main problem about using the central AF point to focus and recompose. He says

    For portraits, the eye is the best focus point—both because this is a standard and because it has good contrast. Although the central autofocus sensor is usually most sensitive, the most accurate focusing is achieved using the off-center focus points for off-center subjects. If one were to instead use the central AF point to achieve a focus lock (prior to recomposing for an off-center subject), the focus distance will always be behind the actual subject distance—and this error increases for closer subjects. Accurate focus is especially important for portraits because these typically have a shallow depth of field.

    The issue about focus distance versus subject distance is easiest to understand for a flat subject like a piece of board.  Stand immediately in front of the board, point your camera at one corner and lock the focus; and now point your camera at the centre of the board:  the distance from your camera’s sensor to the centre of the board is less than the distance to the corner, so the focus distance will now be slightly behind the centre of the board.  This can make a noticeable difference if the board is close to the camera and you are using a wide aperture, because the depth of field may be very shallow.

    And the problem is worse with a human face because it’s not flat but convex – it bulges out towards you.  So we see portraits with the eye in focus, but the nose out of focus. Solutions?

    1. Don’t use the central focus point: switch to the one that’s on the eye you wish to get sharp. (This is most practical, as we’ve mentioned, if you’re doing a studio shot and the camera is on a tripod.)
     
    or
     
    2. Use a smaller aperture (which is why professionals doing normal advertising or commercial shots of models who are moving don’t go wider than about  f/5.6 and may work at smaller apertures routinely – it ensures everything about the person will be in focus, and it gives the retouchers room for manoeuvre).

    In most ordinary situations, you don’t need to worry about the very slight differences between focus distance and subject distance that focus-and-recompose may occasionally entail, especially if you don’t use your lens’s widest aperture.  (And using an aperture setting two or three stops smaller than you lens’s maximum will usually also ensure better resolution across the whole frame, because many lenses get a bit soft round the edges of the frame at their widest aperture.)

    #3670
    Profile photo of Dennis Crabtree
    Dennis Crabtree
    Participant

    Thanks everyone for a really good read and great demonstrations. I’ve learnt a great deal and now have to practice what I’ve learnt.

    #3798
    Profile photo of Ian McNab
    Ian McNab
    Keymaster

    Just came across a good diagram illustrating the ‘focus & recompose’ geometry:

     

    Light blue line is the center of the DOF (i.e. it’s the plane of focus, which is flat). Solid black line is the actual distance of the subject from the camera. So you focus on the flower, and then recompose so that the camera is pointing straight ahead. The plane of focus is now behind the subject by the distance ‘d’.

    OK, this is important if you are at macro distances, wide open or using long fast lenses wide open and doing focus-critical work. Otherwise, it’s not a problem in practice, as the DoF will almost certainly be bigger than ‘d’. So the subject will still be in focus.  🙂

    (Thanks to a discussion on DPReview Forum for this diagram: http://www.dpreview.com/forums/post/51214788)

     

    #3814
    Profile photo of Peter Robinson
    Peter Robinson
    Keymaster

    Thanks for that Ian. It clearly explains it. I’ve read some forums where photographers complain that their lens AF is faulty because they are focusing behind the subject. This diagram explains why that may be the case. It’s the focusing technique that’s at fault not he lens. While we are discussing sharp images I think it shouldn’t be forgotten to use a fast enough shutter speed. Some people seem to think that because their has a good image stabilizer it will give them a sharp picture. But, of course, it only helps to eliminate camera shake. You still need to use a fast enough shutter speed to freeze a moving subject. Many thanks.

    #3820
    Profile photo of
    Anonymous

    Quite agree Peter–isn”t it amazing   how automating to make focussing easier-for some has made it complicated. To reiterate a point Ian made-to “stop” down a bit to get some depth of field–except in particular circumstances.

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