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Focus Areas
Hi, I'm using a D50 and in order to focus (if I'm not in auto mode) I always use the centre focus area and then recompose. The 4 other focus areas don't seem much use to me (they would rarely be in the right location for me). Am I missing something ? How should you use these extra focus areas..

AL...

Comments (9)

It is important to understand that focus/recompose can change the relationships between the camera lens and the subject, especially when the distance to the subject is small. It pays to use the correct focus point in such circumstances to avoid the fine focus errors that can creep into the image, especially with wide open lenses and the corresponding shallow depth of field..

There are also many metering variables that "pay attention" to the specific focal point. Sometimes you can throw off your meter by using a specific focus point and then moving it for the actual exposure calculations. These concerns change with the specific metering mode in use at the time..

Nothing is enough for the man to whom nothing is enough...

Comment #1

So should I focus using the focus area nearest my subject even if I need to recompose anyway ? How close does the subject have to be for my method to be a problem ? .

AL...

Comment #2

There is no "rule" you can follow. You need to read the manual carefully and understand how the camera combines focus and metering, and then experiment to find out what you can do in various situations..

The truth is that most of the time focus/recompose will work quite well. The original question asked about why there are other focus points. They exist because sometimes focus/recompose creates problems that switching focus points can help solve. Experience and understanding the algorithms of your camera's decision making are the only way to learn when you can use the different techniques..

Nothing is enough for the man to whom nothing is enough...

Comment #3

Arctic Lizard wrote:.

So should I focus using the focus area nearest my subject even if Ineed to recompose anyway ? How close does the subject have to be formy method to be a problem ? .

The answer is not black and white, because it depends on a number of things which aren't the same in each shot. For example, camera used, lense used, subject distance, and aperture all have a significant effect on if your method will work or not..

In general, if you are using a P&S or a DSLR (a DSLR less than $2,000) and a lense with a maximum aperture smaller than or equal to f/2.8, and your subject is greater than 10 feet away, then your method will work. The previous generalization encompasses about 90% of photographers out there..

The easiest way to see the effect on depth of field (DOF, or "focus area" as you describe it) is when taking macro pictures. Put a macro lense on your camera and set it to manual focus and the fastest aperture available (smalled f/number). Focus as close as possible on a subject, then move your entire camera to the left 1/2". You'll see that the DOF changes the area in focus of your subject. It can very easily change the area in focus from the head of a butterfly to the body of the butterfly (without the head being in focus)..

Experimentation on the above will show you how the relationships work. In general, a larger maximum aperture (small f/number) gives shallower DOF. In addition, a longer focal length gives shallower DOF when combined with a comparably small f/number..

Tim'Be the change you wish to see in the world.' -Mahatma Gandhihttp://www.flickr.com/photos/timskis6/..

Comment #4

The D50 manual does not say a lot about the focus areas but what it did say made me think that the extra ones are not much use. Thanks for your replies..

AL...

Comment #5

Http://www.mhohner.de/recompose.php?lang=e.

Http://www.outbackphoto.com/workshop/phototechnique/essay06/essay.html.

Http://www.bobatkins.com/photography/technical/focus_recompose.html.

Above are links to the arguments for and against Focus and Recompose..

They do not address all arguments for more than one focus point:.

1. Many Nikon (and Olympus) cams weight the metering to the lighting conditions under the active focus point (using Spot Metering AND Matrix Metering. It does not tell you the latter in the manual but it is a fact). So focus and recompose can lead to under or over exposure..

2. With any sort of action shot you simply do not have time to focus and recompose. That does not have to mean high speed action. I do a lot of low light stage work. I may be focusing on the main character who is commonly off centre stage but want to fill the frame with other cast to get their reactions to something said or sung. By the time I had focused and recomposed I would have missed the moment..

3. With high speed action you want the camera to track focus for you across the viewfinder. Now that is in the manual try p. 89..

To present the argument in an upside down way you may want to ask if it does not matter why more expensive cams have more focus points. In particulat the Nikon D300 and D3 both have 51 AF points..

But back to a more reasoned approach. To argue against the third link above (which champions focus and recompose):.

A) there is an optimum focus for any object and lens and cam. Then there is an area around that optimum where the image is acceptably sharp. That is your depth of field.

B) autofocus systems are rarely 100% accurate. They miss that optimum focus by a shade but will usually focus within the acceptably sharp field.

C) some lenses back or front focus. (They can be adjusted but put them on a different body and you may have the problem back again).

D) add together the incidence of problems at b) and c) above and your capture rate of good photos can begin to fall. If you work in low light with the lens wide open as I usually do and most of us do some of the time the incidence of less than sharp end product can begin to rise.

For the above reasons it makes sense to me to adopt a technique that will always work rather than use two two different techniques in different situations..

Chris Elliott.

*Nikon* D Eighty + Fifty - Other equipment in Profile.

Http://PlacidoD.Zenfolio.com/..

Comment #6

So, in the case of the off centre character do you select your focus area then take the shot, framed such that the character is under your selected focus area ? .

AL...

Comment #7

Arctic Lizard wrote:.

So, in the case of the off centre character do you select your focusarea then take the shot, framed such that the character is under yourselected focus area ? .

Yes. Exactly. Being in sharpest focus that will often draw the eye to that character quite naturally with others usually in focus but sometime less sharply focussed. Sometimes in PP I will brighten the image of that character a little (or use saturation or whatever) in Capture NX to subtly draw the eye to them but that is not always necessary..

I am usually either photographing the dress rehearsal (or a live concert) so nobody stops for me to get it right!.

At 30ft my AF-D 50mm f/1.4 at f/2 has a depth of fiield of about 8ft. A stage is likely to be deeper than that. Add in a 10% AF error and you may have a problem. (Similarly my 80-200 f/2.8 at f/2.8 at that distance has a DOF of 9 inches so giving a margin for error focus on the nose and the back of the head will be easily OOF).

Use the same 50mm ens for a portrait shot at 8ft f/4 and your DOF is 14 inches. So you only need slight AF error to have problems. At f/2.8 which I would commonly use to render the background out of focus your DOF is 10 inches..

P.S. When shooting portraits you should always focus on the eyes. That is what your eyes are drawn to. So use the camera in portrait orientation and the top sensor to focus on an eye..

P.P.S. If you have not come across it before these tables are an invaluabale resource. I keep a couple of prints offs with me in my camera bag:.

Http://www.dofmaster.com/doftable.html.

Chris Elliott.

*Nikon* D Eighty + Fifty - Other equipment in Profile.

Http://PlacidoD.Zenfolio.com/..

Comment #8

Very helpful. Thanks for your comments and links..

AL...

Comment #9

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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