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.
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.
Paul
