What AF mode to use?

Discussion in 'Digital Photography' started by adonisadonis, Feb 9, 2011.

  1. adonisadonis macrumors member

    Feb 17, 2009
    Hello everyone

    Newbie question. I recently purchased my first Nikon DSLR (D7000) + sigma 17-50 2.8. I would like to seek your help if what is the most appropriate AF mode (AFA, AFS, AFC) as i'm taking pictures of my kids (3 and 6 y/o) with nice sceneries (Landscape). I know that using aperture of 2.8 will blur background and going to F8 to F11/16 will focus a lot. My question is what AF mode is appropriate to use and where to focus as I want the whole sceneries and kids to be sharp. which is more suitable single point AF or autofocus AF in this scenario.

    Thank you very much.
  2. Ruahrc macrumors 65816

    Jun 9, 2009
    Single-point AF and autofocus AF are not different terms.

    I would highly recommend you read your camera manual for a good explanation of the different AF modes, and when they might be useful.

  3. avro707, Feb 9, 2011
    Last edited: Feb 9, 2011

    avro707 macrumors 6502a

    Dec 13, 2010
    Single-point AF is a different thing to the AF-C (continuous autofocus, which updates as you track something moving).

    Single point AF is looking at the AF points (your camera has 39, if I remember right).

    What you might be thinking of doing is AF lock, say for instance, you'd focused on something, then want to recompose the image while having the original subject stay in focus. This would be a typical thing when taking photos of people outside in nature (nice background, etc).

    Your Nikon camera manual has an excellent section on the operation of autofocus and the different modes - this is your best reference.

    Take a look at this PDF too:

    http://nps.nikonimaging.com/technical_solutions/ (see D3S Technical Guide).

    Even though it is for the top-line D3S, a lot of it will apply to your D7000 as well, since they operate in a very similar manner (as do all Nikon cameras). Page 12 talks about AF modes, and it gives you examples of where you might use more AF points (remember, you have 39 points I think, not 51). Most of the custom selections (like lock on times) you will not be able to do since I don't think those are in the consumer cameras, but the theory and practice transfers over - hopefully it'll bring some clarity for you.
  4. compuwar macrumors 601


    Oct 5, 2006
    Northern/Central VA
    You are mixing three different things. I'd suggest reading your camera manual to gain a better understanding of the issues.

    1 - Depth of field is the amount of the scene which is in apparent focus. This is controlled by the aperture of the lens, which is balanced by the shutter speed and ISO setting to gain a correct exposure. What aperture you can shoot at to get what portion of the image in focus is dependent on both the amount of light and the camera to subject distance, and is gated on the small end by the effect of diffraction, which will be more visible with a higher number of pixels. So, yes- f/2.8 will have a narrower depth of field than f/16- but that's not the sum total of the equation. Most lenses are at their sharpest two stops from wide open, so for an f/2.8 lens that's generally f/5.6- now for lots of subjects that sharpness difference isn't going to be all that visible, especially on a computer on in a small print. However, with moving subjects like kids, you also need a fast enough shutter speed to stop the action and almost *all* natural light photography looks better very early or very late when the light is lower- so you'll have to deal with motion blur which will sometimes make it impossible to shoot at a small aperture like f/11. Flash removes this problem by exposing the main subject for a very short duration, eliminating most motion blur (depending on the exposure and speed of the subject) and can be balanced with ambient light by adjusting the flash power and setting the exposure appropriately. Depth of field calculators can be used to determine the amount of apparent focus you can get at a specific distance for a particular aperture. You can also use a hyperfocal distance calculator to determine the maximum amount of apparent focus for a given situation. Your shutter speed must be high enough to offset any camera movement if you're not using a tripod, monopod or other support structure, as well as any subject movement that you want frozen- for instance a child running, but not so fast that it freezes any movement you don't want frozen, such as wheels on a race car or the propeller of an aircraft. The "rule of thumb" for most lenses is that the shutter speed must be at least one over the focal length to take camera movement into account- that is a 60mm lens requires at least 1/60th of a section and a 500mm lens requires at least 1/500th of a second. I find personally that I can hand-hold at slower speeds, but with a high-resolution body I want a stop *more* shutter speed to be certain of a sharp image- such as 1/125th of a second with a 60mm lens.

    2- The number of focus points used by the camera determines the area the camera uses to determine where to focus, how quickly it can follow a moving subject and which type of focus points are in use. The more points, generally the slower the process- which may be fine for some subjects, but won't be for others. Add in things like 3D tracking and you start to need more CPU horsepower. You also get a chance of focus not being where you'd expect it if there are more interesting high-contrast areas over the larger amount of space with more points. Since modern cameras have relatively good CPUs, and a fair database of things that look like faces, this is something you have to get a feel for on each particular camera. I personally think one of the main differences between Nikon's prosumer and professional DSLRs is CPU horsepower- which tends to explain why D3 owners say they get a little faster AF than D700 owners do. The camera has to find the spot, follow the moving subject and do a database lookup to determine where to focus, all multiple times per second while waiting for the shutter button to be pressed fully.

    3- The AF mode is pretty simple, C is continuous- that is always try to maintain focus. S is servo, focus, then let me re-compose if I want to and so long as I don't take my finger off the half-pressed button or AF button, I can take the shot with my originally selected focus point still in effect. M is manual- I'll set the focus myself.

  5. adonisadonis thread starter macrumors member

    Feb 17, 2009
    Thank you very much for your replies. I really appreciate it. My son and daughter are very cooperative when posing for the camera so i could set my shutter speed in an acceptable speed and because we are going for a holiday mostly i'm going to shoot during the day. If I let them pose for the camera, will it be better to just use single point as oppose to let say 39 autofocus mode and just use aperture of 8 as to make both background and my kids sharp? I'm just worried because if I use single point and focus to one of my kid the other one will not be focused.

    Thanks once again.
  6. compuwar macrumors 601


    Oct 5, 2006
    Northern/Central VA
    The number of AF points really only has an effect on moving subjects and just needs to be decreased if the camera is having trouble tracking a moving target (generally these days just one heading towards or away from you.)

    You can choose aperture or shutter speed, so you can pick stopping movement (camera and subject) or depth of field. Choosing both is not likely to net you a good exposure. I'd recommend getting a copy of Understanding Exposure by Brian Patterson, in the meantime, you can use shutter priority with at least 1/focal length for most things.

  7. LtCarter47 macrumors regular


    Oct 21, 2005
    Roseville, CA

    This book was amazingly helpful to me when I bought my first DSLR with no photography experience.

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