Back Button Auto Focus

Source
link

1. Verify that your camera has an AF-On button.  If not, you’ll need to set up the AE/AF-lock button in the custom menus to use it as the AF-On button.  In the Nikon D90, this is custom setting f4.
2. Set the camera’s AF servo mode to Continuous (AF-C).  This is done through the switch next to your lens mount, or via a custom menu setting. D90 users: hold down the AF button on the top of your camera, and turn the Main Command Dial until AF-C is displayed in the top LCD panel.
3. In the camera menus, go to submenu “a” (Autofocus)
4. Set custom setting a1 (Continuous Release Mode) to Release Priority (in the D90, this is already set for you when you choose continuous servo AF mode)
5. Find the custom setting for AF Activation (a5 on the D3s) and set it to AF-On Only. This step is not necessary for the D90 and other cameras, as it is already set up by custom setting f4.

Now you’re all set up and ready to go.

HOW TO USE THE AF-ON TECHNIQUE IN THE FIELD

To emulate single-servo mode (focus/recompose/shoot)

1. Place the active AF point on your subject
2. Press the AF-On button to acquire focus
3. Release the AF-On button to lock focus
4. Recompose and shoot

To focus continuously on a moving subject

1. Place the active AF point on the subject
2. Press the AF-On button
3. Keep the AF-On button pressed to track focus while simultaneously pressing the shutter release
4. Remember to initiate the VR system (if your lens supports it) by half-pressing the shutter button prior to releasing the shutter.  Remember, VR takes about a half-second to stabilize, so you’ll want to anticipate your subject.

Now that you understand how to use the technique, you’ll want to spend some time practicing.  It usually takes about a day of shooting in the field to get used to the new technique.  Once you know how to use it, the AF-On only method of focusing will help you get more “keeper” shots.

White balance

White balance

Where, with film you either shot print film in daylight or with flash indoors, and with slide film, you used daylight or tungsten film, or used compensating filters with these. White balance is Digital’s version-you select a white balance based on the condition of the light you are using, so the camera’s meter allows for white to photograph as white
Auto white balance does a reasonable job most of the time, but it cannot cope in some situations-especially when subject is in deep shadow. You need to know when to use the white balance settings for all situations, and this table will help you understand: ( degrees Kelvin is a means of determining colour temperature)

Sunrise/sunset 2400-3000 Kelvin……………………Use Auto white balance
Tungsten lighting 3200-3500 Kelvin ……………………Use tungsten/Incandescent white balance
Fluorescent lighting 4000 Kelvin …………… Use fluorescent White balance
Early morning/afternoon sun 4000 Kelvin…….Use auto white balance/ --fluorescent white balance
(Magenta filter) to counter greenish cast on skin from foliage if under trees or on really green grass
Noon sun/Sun overhead…5000-6500 Kelvin…………………….use Cloudy White balance 
Flash photography in daylight 5500 Kelvin ………………………Use Flash White balance
Deep shade………………….6500 Kelvin…………………………Use cloudy/shade white balance
Shade in daylight 7500 Kelvin ………………………………......Use shade white balance
Heavy overcast, very dark shade 8000 to 10000 Kelvin………Use shade white balance plus 81a-85c glass filter

To set custom White balance: (Nikon)

Select a neutral coloured object to set your white balance
It’s best to avoid using a white target. The camera prefers grey. If you use white in direct sunlight you'll always get a "No Gd" response from the camera because the Matrix meter is rendering it white instead of grey. Thus the trick is to select the centre-weighted meter when using a white reference in bright light and you'll get "Good" from a camera. Remember to set back to Matrix metering as soon as you're done.


-------------------------------------------
TRICK
WhiteBalance : Tungsten
Set -1 or -2 exposure compensation and use this setting in daylight to simulate night!
Cloudy : best for most shots outdoors in direct sunlight
Custom White Balance :
Set it pointed at something colored or through a colored filter
and your resulting photos will have a color cast opposite the color to which you set it!
Set it on something blue and photos come out yellow,
set it on something purple and the photos come out green.
Point it at something warm and you get cool and vice versa.

DSLR AUTO FOCUS

SOURCE

AF-Area Mode	Nikon Focus Modes
	AF-S Mode	AF-C Mode	AF-A Mode
Note: Not all of the above focus modes may be available on your Nikon DSLR. The new AF-F and other AF-Area video modes are not included in the above chart.
Single-Point AF-Area Mode	Camera acquires focus only once and on the selected single focus point only.	Camera focuses on the selected single focus point only and will reacquire focus if the subject moves.	Camera detects if subject is stationary or moving and will automatically select whether to use AF-S or AF-C. Only one focus point is used in either case.
Dynamic AF-Area Mode	Disabled, works just like Single-Point AF	You choose an initial focus point and once the camera acquires focus on the subject, it will engage the surrounding focus points to track subject movement. The number of surrounding focus points to use can be selected in camera menu.	Camera detects if subject is stationary or moving and will automatically select whether to use AF-S or AF-C.
Dynamic AF-Area with 3D-Tracking	Disabled, works just like Single-Point AF	Instead of using a particular number of surrounding focus points, the 3D-Tracking activates all available focus points and uses color recognition to track subjects. You pick the initial focus point and the camera will track the subject across the frame automatically, letting you recompose the shot without losing focus on the subject.	Camera detects if subject is stationary or moving and will automatically select whether to use AF-S or AF-C.
Auto-Area AF Mode	Camera automatically picks a focus point, depending on what’s in the frame.	Camera automatically picks a focus point on a moving subject and will track the subject in the frame.	Camera detects if subject is stationary or moving and will automatically select whether to use AF-S or AF-C.

Nikon Lens

Source

MANUAL FOCUS NIKKOR LENSES				AUTOFOCUS NIKOR LENSES
NIKON FILM SLR	Pre-AI	AI, AI-S, E	AI-P		F3AF	AF	AF-D	AF-I, AF-S	G	VR	DX
N/F50	NO!	MF4	MF		NO!	AF	AF	MF3	AF	AF5	vg
N/F55	NO!	MF4	MF		NO!	AF	AF	AF	AF	AF5	vg
N/F60	NO!	MF4	MF		NO!	AF	AF	MF3	AF	AF5	vg
N/F65	NO!	MF4	MF		NO!	AF	AF	AF	AF	VR	vg
N/F70	NO!	MF1,2	MF		MF1,2	AF	AF	AF	AF6	AF5	vg
N/F75	NO!	MF4	MF		NO!	AF	AF	AF	AF	VR	vg
N/F80	NO!	MF4	MF		NO!	AF	AF	AF	AF	VR	vg
N90s/F90x	NO!	MF1,2	MF		MF1,2	AF	AF	AF	AF6	AF5	vg
F100	NO!	MF1,2	MF		NO!	AF	AF	AF	AF	VR	vg
FM2n	NO!	MF	MF		MF	MF	MF3	MF	NO	MF 5	vg
FM3A	NO!	MF	MF		MF	MF	MF3	MF3	NO	MF3,5	vg
F3	MF7	MF1	MF		MF	MF	MF3	MF3	NO	MF3,5	vg
N8008s/F801s	NO!	MF1,2,3	MF		MF1, 2	AF	AF3	AF3	AF6	AF3,5	vg
F4	MF1, 2, 7	MF1	MF		AF1	AF	AF3	AF3	AF	AF3,5	vg
F5	NO!	MF1,2	MF		MF 1,2	AF3	AF	AF	AF	VR	vg
F6	MF1,7,8	MF1	MF		NO!	AF2,3	AF	AF	AF	VR	vg
	MANUAL FOCUS NIKKOR LENSES				AUTOFOCUS NIKKOR LENSES
NIKON DSLR	Pre-AI	AI, AI-S, E	AI-P		F3AF	AF	AF-D	AF-I, AF-S	G	VR	DX
D1	NO!	MF1,2	MF		NO!	AF3	AF	AF	AF	VR	DX
D1H/D1X	NO!	MF1,2	MF2		NO!	AF3	AF	AF	AF	VR	DX
D2H/D2Hs	NO!	MF1	MF2		NO!	AF2,3	AF	AF	AF	VR	DX
D2X/D2Xs	NO!	MF1	MF2		NO!	AF2,3	AF	AF	AF	VR	DX
D3	NO!	MF1	MF2		NO!	AF2,3	AF	AF	AF	AF	DXvg
D3x	NO!	MF1	MF2		NO!	AF2,3	AF	AF	AF	AF	DXvg
D3s	NO!	MF1	MF2		NO!	AF2,3	AF	AF	AF	AF	DXvg
D700	NO!	MF1	MF2		NO!	AF2,3	AF	AF	AF	AF	DXvg
D100	NO!	MF4	MF		NO!	AF3	AF	AF	AF	VR	DX
D200	NO!	MF1	MF		NO!	AF3	AF	AF	AF	VR	DX
D300/D300s	NO!	MF1	MF2		NO!	AF3	AF	AF	AF	VR	DX
D70/D70s	NO!	MF4	MF		NO!	AF3	AF	AF	AF	VR	DX
D50	NO!	MF4	MF		NO!	AF3	AF	AF	AF	VR	DX
D40/D40x	NO!	MF4	MF2		NO!	MF3	MF	AF	AF10	VR9	DX
D80	NO!	MF4	MF2		NO!	AF3	AF	AF	AF	VR	DX
D60	NO!	MF4	MF2		NO!	MF2,3	MF	AF	MF10	VR9	DX
D90	NO!	MF4	MF2		NO!	AF2,3	AF	AF	AF	VR9	DX
D5000	NO!	MF4	MF2		NO!	MF2,3	MF	AF	MF10	VR9	DX
D5100	NO!	MF4	MF2		NO!	AF2,3	MF	AF	MF10	VR9	DX
D3000	NO!	MF4	MF2		NO!	MF2,3	MF	AF	MF10	VR9	DX
D3100	NO!	MF4	MF2		NO!	MF2,3	MF	AF	MF10	VR9	DX
D7000	NO!	MF1	MF2		NO!	AF2	AF	AF	AF	VR9	DX
* Always consult your camera manual.

NIKKOR LENS NOMENCLATURE
LENS TYPE	CHARACTERISTICS
Pre-AI	Non-AI Manual Focus Nikon lenses made from 1959 and prior to 1977. Don't have a CPU. All Non-AI lenses have a letter after the word Nikkor, to tell the number of elements in the optical formula. For example, in the Nikkor-P 105mm f/2.5, the P stands for Penta, i.e. five elements. Types: A (chrome filter ring), C (black filter ring) and K (rubber coating)
AI	Manual Focus Nikon lenses, produced from 1977 until mid 80's, introduced Automatic Maximum ApertureIndexing. A mechanism for meter coupling, that is, to inform the meter in the body what is the maximum aperture of the mounted lens. With all black barrel, rubber focusing ring and multicoated elements. Don't have a CPU chip.
AI-S	Manual Focus Nikon lenses, introduced in 1982, with Aperture Indexing Shutter system for meter coupling. Smallest aperture is orange (if not, then the lens is either AI or pre-AI). Most of these lenses have extraordinary optics, like the legendary 105mm f/2.5, available in AI-S version. Don't have a CPU.
E	Manual AI-S Nikon Series E lenses, made for the compact Nikon EM introduced in 1979, starting the use of plastics. The 75-150mm f/3.5 Series E reached mythical stature. Don't have a CPU.
AI-P	Manual AI-S Nikon lenses with a CPU that sends the lens information to the camera body. The latest is the ultracompact Nikkor 45mm f/2.8 P "pancake", made to celebrate the FM3A and proving Nikon's loyalty not only to film enthusiasts but also to manual body users.
F3AF	Auto focus pioneering Nikon lenses introduced in 1983, exclusively for the F3AF.
AF	Auto focus Nikon lenses introduced in 1986. When on Auto bodies, there is no need to use the aperture ring in auto modes. AI-S lenses with a built-in CPU and screw motor for AF operation.
AF-D	Introduced in 1992. AF Nikon lenses with a CPU that also relays distance information to the camera, most useful for ultra-precise TTL flash. Among the first were the 35-70mm f/2.8D AF and 80-200mm f/2.8D ED AF Nikkor.
AF-I	Introduced in 1992. Nikon lenses with a coreless Integrated motor for faster AF in high-end telephoto lenses. The first were the 300mm f/2.8 and the 600mm f/4, both D ED IF AF-I.
AF-S	Introduced in 1996, Nikon AF-D lenses with a "Silent Wave" ultrasonic motor of their own, for fastest AF operation. The first were the 300mm f/2.8, 500mm f/4 and 600mm f/4, all D ED IF AF-S Nikkor.
G	Introduced in 2000. Nikon AF-D lenses without aperture ring. Need to be controlled through the body dials of latest cameras. The first was the 70-300mm f/4-5.6G AF.
VR	Introduced in 2000. Nikon lenses with a Vibration Reduction system allowing for crisp images handheld at very slow shutter speeds. The first was the 80-400mm f/4-5.6D ED VR Zoom Nikkor.
DX	Introduced in 2003. Nikon G lenses designed to just fill the frame of the DX format APS-C sensor size used in Nikon D-Series SLR cameras. The first was the AF-S DX 12-24mm f/4G IF-ED Nikkor.
NOTES
NO!	Definitely do not use, for it may damage the camera body. Also, warranty will be void. However, the F5 body can be modified to accept pre-AI lenses.
NO	It may not damage the camera body but it will not function.
MF	Manual Focus
AF	Auto Focus
1	Only in A (Aperture Priority) or M (Manual) modes. P (Program) or S (Shutter priority) exposure modes will not function.
2	No 3D Matrix Exposure Metering.
3	No distance information through the lens for TTL flash; Nikon speedlights will use their own sensor.
4	M (Manual) exposure mode only. The meter will not operate. Sunny 16 rule or external meter.
5	The VR system will not function. Will MF or AF depending on whether the body is MF or AF
6	Opposite of 1. Will only work in P and S modes, not on A or M.
7	Care must be exercised to flip up the coupling tab on prior to mounting the lens to prevent damage.
8	Requires factory modification of meter coupling lever.
9	The VR system will only function with G VR and D VR lenses
10	Auto Focus is possible with G lenses only if they are also AF-S; G AF lenses will MF only
VR	The VR system will function and all other available functions.
DX	Will function in DX format bodies without vignetting.
vg	Will seriously vignette on a 35mm film or full frame digital camera with FX format, more so at the shortest focal length when a zoom and wide open; all of the G lenses restrictions apply. Will work in cameras offering High Speed Crop function.

FILTERS

The only filters that you really need to produce professional-level images are a warming filter, polarizing filter, and a graduated neutral density filter.

Warming EffectThe filter that I use the most, by far, is the warming filter. It is used primarily in overcast light or in shade to remove the inherent blue or cold light from such scenes. As a result, an amber or warm tonality is added to the scene

Polarizing Filter Effect
Polarizing filters are used primarily to remove glare and can darken a blue sky if used at approximately a 90Þ angle. Here’s the shot on film without a polarizing filter. Notice how flat it is. The colors are actually very vibrant, but the glare is masking, or absorbing,
the color

A polarizer is almost never appropriate for portraiture.

Split Graduated Neutral Density
These filters are amazing in that they make images that are otherwise impossible to capture. These filters lower the contrast range by masking the highlights thus rendering detail in very dark and very bright areas, rather than blowing out the highlights or blackening shadows. The following examples are illustrative of using graduated neutral density filters


UV Filter
The "traditional" visible spectrum runs from red to violet. Red light has the longest wavelength and violet the shortest. Light which has a longer wavelength than red is called infrared, and light which has a shorter wavelength than violet is called ultra violet or UV.

There are basically three color sensitive layers, one sensitive to red light, one to green light and one to blue light. The blue layer not only responds to blue light, but also to UV light, so if there is a lot of UV around the blue sensitive layer gets extra exposure and the final image takes on a blue color.

three filters often used in place of a UV filter (i.e. filters which some photographers keep on the lens at all times as protection). These are the Hoya 81B, the Tiffen 812 and the B+W KR1.5. All three are warming filters in that they shift the color balance towards the red (warm) end of the tonal range.
As you can see all three of the warming filters were effective UV absorbers, as well as slight absorbers in the blue and green regions of the spectrum (which is what makes them warming filters). The polarizer absorbed slightly more in the UV than the visible, though I wouldn't call it an effective UV absorber.

Tiffen 812 . It blocks UV, it protects the lens and it's a warming filter. Some people use an 81B for this, but I slightly prefer the color shift of the 812. Not everyone wants a warming filter, so the clear winner for a neutral filter that really bocks UV is the Tiffen Haze-1, though the Hoya UV should also be pretty effective.

HISTOGRAMS

http://luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml

http://www.slrnikond50blog.com/
You can use the histogram to check if the exposure was correct.
When you review a picture and pull up the histogram, you’ll usually see a pattern that resembles a bell curve. If that curve is squished to the left of the screen, you’ve underexposed – a lot of shadow areas will be completely black. And if the curve is squished to the right of the screen, you’ve over-exposed – a lot of highlights are completely white.
If a portion of the histogram touches the top that means the highlights are blown out.

The common advice is to ‘expose to the right’. That means to expose the shot so that the curve is as far as possible to the right of the box, without actually blowing any highlights.


So take care to adjust the exposure so that the histogram doesn’t touch either of the sides or the top of the histogram doesn’t touch the top of the screen.


Here’s more on the reasoning behind that: http://luminous-landscape.com/tutorials/expose-right.shtml

BOREKH

http://www.daifukusensei.com/?page_id=52

Brenizer Method

Bokeh consists of the out-of-focus highlights seen in photographs. You know, those beautiful little balls of light that just seem to pop off the screen and make a picture look so much sexier? Here’s an example. So it follows that a bokehrama is a panorama — or a series of shots — of bokeh, lots of blur, and a single subject in focus. The best way to describe it is to showcase a terrific example, one from kick-butt wedding photog Ryan Brenizer, from whom I’ve learned this method:
http://www.flickr.com/photos/carpeicthus/3554480740/

Free substitute for Autostitch
http://hugin.sourceforge.net/

http://www.philmalpas.com/pages/artcomp.asp

FOCAL LEnght and Point and Shoot Zoom

The zoom factor like e.g. the 10X you mentioned above is simply the
longest focal lengt divided by the shortest focal length of that lens.
It could be 10mm-100mm or 20mm-200mm or 100mm-1000mm. All of those
lenses would be a 10x zoom.
The zoom factor does not tell you anyting about the magnification (or
tele) factor. That can be expressed in e.g. 8x, too. This is commonly
used in e.g. binoculars where 7x is a common marine magnification, 8x or
10x a common terrestial magnification, and 15x a strong glass for e.g.
bird viewing. For photography however it is much more common to use the
focal length. Problem is that the magnification not only depends on the
focal length but also on the sensor size.

On a full-frame sensor a focal length of 50mm is considered a 'normal
lens'. A 200mm lens would have a 4x magnification, a 1000mm lens a 20x
magnification.
For a DX sensors you have to adjust the magnification factor for the
smaller sensor size by a factor of 1.5 or 1.6, depending on the actual
camera.

Ok, going back to your original question: a 200mm lens on a DX body
(Canon EOS Digital Rebel XTI) will have a magnification(!) factor of
200/50*1.5 = 6x.
And it will have a zoom(!) factor of 1 because is is not a zoom lens to
begin with.

If it were e.g. a 50-200mm lens, then the magnification factor would be
1.5x - 6x and the zoom factor 4x on a DX body.
On a full frame camera this lens would have a magnification factor of
1x-4x and still the same zoom factor of 4x.

Oh, one more note: neither the camera nor the lens determines the image
size. The image size only depends on how you print it.

APERTURE

http://www.digital-slr-guide.com/maximum-aperture.html

1 – Natural Light Photos
No flash with a max aperture of f/2.8
A digital SLR camera sensor needs light to create an image.

There are two ways that it can get the light that it needs: it can keep the shutter open for a long period of time or open the aperture as wide as possible.

Let's say that you hate taking photos with flash, because you know how washed out they look.

You're indoors and your camera tells you that with your aperture set to f/4, your shutter speed will have to be 1/30 of a second. That's not going to be fast enough to take a shot of your child rampaging in the living room.

You're going to have to use the flash, or get the kid to sit VERY still.

If you had a lens with a maximum aperture of f/2.8 (which lets in double the amount of light of f/4) then you can use a shutter speed of 1/90, which means that you can get clear photos without the flash



2 – Fast Shutter SpeedsA fast shutter prevents motion blur
You're taking photos on an overcast day of a flower in the backyard. A strong breeze is blowing, and that flower is moving all over the place.

You set the aperture on your lens as wide as it can go (let's say f/5.6), and check the shutter speed. The camera tells you that you can use a shutter speed of 1/125 of a second.

That's not fast enough to get a clear photo of the flower. You're stuck.

If you had a lens with a wider maximum aperture you could get the shot. With a wider aperture (say f/2.8) your shutter speed could be 1/500 of a second.

Since the wide aperture lets in so much light, the shutter doesn't have to stay open as long.


3 – Reduced Depth of FieldWide aperture = shallow depth of field
Lenses with wide maximum apertures have shallow depth of field.

Without getting into a discussion on depth of field (that could take awhile), I will just refer to the photographic example at right.

This shot was taken with a lens set to f/3.2. Notice that the background is completely out of focus even though the ball in the foreground is clear?

This is shallow depth of field.

Nature and portrait photographers use this all the time to reduce the impact of a busy background.

If you have a lens with a maximum aperture of f/5.6, you can't reduce the depth of field as much as you can with a wider aperture.

CCD Cleaning

http://www.bythom.com/cleaning.htm

If you've got a D3s, D60, D90, D300, D300s, D700, or D5000, regularly use the built-in dust cleaning mechanism. About once every shooting session and after any long period of disuse should be fine. Remember to hold the camera normally (sensor perpendicular to the ground) so that the dust falls to the adhesive strip at the bottom of the sensor chamber designed to catch the dust. Skip to Step 3.
(For those without a built-in shake dust removal system) On a regular basis, use your blower bulb to try to dislodge casual dust from the filter surface (camera should be facing down so that dust dislodged falls out of the camera through the lens mount). Do this in a clean environment and regularly, and you generally will get most of the dust dislodged this way without having to touch the filter.
Use a Sensor Brush whenever you need to remove casual dust the automated cleaning or blower bulb can't dislodge. Follow their procedures exactly; don't take shortcuts. Most of the time, this is enough cleaning to remove the offending dust. If you're traveling, you can use an Arctic Butterfly for this step, but be extremely careful not to touch it to the sides of the sensor mounting frame.
If a Sensor Brush session doesn't remove stubborn dust, use a Sensor Swab wet with Eclipse or Dust Aid Ultra Clean solution. Again, follow the manufacturer's procedures exactly.
If you still have a persistent dust bunny, you'll probably have to really wet a Sensor Swab (being careful not to wet it so much that the cleaning material breaks) and go over that spot again, perhaps with a gentle scrubbing action. This will likely leave streaks. If it does, let the sensor dry and then do another regular Sensor Swab of the sensor. Sometimes you have to do this sequence several times (wet pollens are no fun to remove).

Nikon focus modes

When you set your camera lens to AF (auto focus), digital SLR camera's offer the photographer a choice between interchangeable modes. Auto focus modes can vary between different digital camera brands. In the past, Nikon's two most popular modes were called Continuous Servo AF and single area AF. Nowadays, they are called AF-C (short for auto focus continuous) and AF-S (short for auto focus single). Many Nikon models also have AF-A and MF (manual focus) modes to choose from as well.

What are the differences between AF-C, AF-S, AF-A and MF focus options?
AF-C (AF continuous, sometimes called continuous servo) is good use when photographing moving objects. When your camera is set to AF-C and you focus on a moving subject, for example a dog running towards you, the focus will stay on the animal so long as your shutter button is held half way down. In other words, the camera will keep re-focusing as the animal moves. That is, so long as you keep your shutter button held half way down.

AF-S (AF single, sometimes called single area AF) mode, is good for photographing subjects that don't move, such as flowers or portraits etc. It locks the focus on the non moving object that you want to photograph. You can then recompose the shot and take the photograph.

AF-A is where the camera selects and goes between the last two modes. If it thinks the subject you are photographing is stationary, then it will automatically use AF-S focus mode. If it picks up that the subject you're photographing is a moving subject, then it will automatically use AF-C focus mode. This is also the default camera focus mode, unless you change the setting to AF-C or AF-S.

MF is the manual focus mode. It's unclear why this focus mode is included, because nowadays there is usually an option to switch between auto and manual focus on the side of the actual lens. Maybe it's in case you come across a lens that doesn't give you that option.


Why would photographers change between focus modes?
The default camera setting is AF-A. As mentioned earlier, this mode guesses if the subject you are photographing is stationary or moving, and accordingly sets the focus to either AF-C or AF-S. Photographers don't always trust the camera to pick the correct setting, preferring to make the choice themselves.

http://www.slrphotographyguide.com/camera/nikon-digital-slr/focus-modes.shtml

Metering

Metering means to ensure that u properly expose ur scene. There are 3 kinds of mode (a) matrix metering (b) centre weighted (c) spot metering

(a) matrix metering - normally used in landscape photography to properly expose the whole scene, the camera will determine the light across the whole picture and try to find the best setting to properly expose everything.

(b) centre weighted - normally used in portrait setting or other scenes where there is a main object that occupies abt 70% of the picture center. The camera will try to properly expose the 70% of that picture and "ignore" the background. So for example, if ur object is black and ur background is white, the camera will meter the black object, and will try to adjust itself to bring out the black object's details. In order to do so, it got to expose more. So wat u get is a properly exposed black object against a over exposed background. Remember the background is already white, and because of the need to expose more to bring out the black details, the white background will be overexposed.
So i think u get wat i mean, the camera will only care abt the center 70%.

(c) spot metering - similar to the centre weighted metering, but just that in spot metering, we meter only a small spot eg. a 6mm diameter circle in the picture. This is used for example, when there is a small frog in a bright picture. if u use matrix metering, the camera will evaluate the whole scene and think that the whole picture is too bright, and hence under expose the picture to generate a proper exposure for the whole scene. But in doing so, the small frog may be further underexposed and become dark frog.
in addition, if u use centre weighted metering, remember it assigns abt 70% to the center of the picture, but the frog is only very very small in the picture! hence the end result will also be the camera feels that the scene is too bright and underexpose.
Thus in spot metering, u aim the center crosshair on the frog to meter the frog. This will ensure that the frog is properly exposed and be seen! but of course the background may not be properly exposed

photography 101

http://digital-photography-school.com/photography-1014-exposure-and-stops

D50 Lens

The Nikon D50 employs the Nikon DX Format sensor and Nikon F lens mount design. This maintains seamless compatibility with all AF Nikkor lenses and allows photographers to take full advantage of DX Nikkor lenses designed for Nikon's D-series digital SLR cameras and which are optimized to achieve maximum center-to-edge-to-corner image quality.

Compatible Lenses

1) DX Nikkor: All functions supported

2) Type G or D AF Nikkor: All functions supported

3) Micro Nikkor 85 mm F2.8D: All functions supported except autofocus and some exposure modes

4) Other AF Nikkor: All functions supported except 3D Color Matrix Metering II

5) AI-P Nikkor : All functions supported except 3D Color Matrix Metering II and autofocus

6) Non-CPU : Can be used in exposure mode M, but exposure meter does not function; electronic range finder can be used if maximum aperture is f/5.6 or faster

Note: IX Nikkor lenses can not be used