Multiple focus areas selection in D70

Hello,

I'm trying to understand how to select multiple focus areas in D70.

Manual says that: <P>

1. Set the focus selector lock "." <P>

2. Use multi selector to select the focus area. <P>

3. Set the focus selector lock to "L" to lock that selection. <P>

Now if I use these steps to select one focus area then I'm unable to

select another foucs area because multi selector does not move once I

set the focus selector lock to "L". So I'm left with only one focus

area selected.

Any inputs about how to achieve this? Steps in the user manual are

not much helpful.

Thanks,

- Sanjay Modi

To my knowledge, you cannot select multiple focus areas at the same time. You choose one area where the focus sensor should be active. The steps you quoted from the manual explains just how to select one of the areas and lock the selection so that you do not move it by accident (multi-selector is the name of the four-way switch, up-down-right-left, not that you can use it to select multiple things).

You can set the camera to automatically choose focus area, and in some vari-program modes this is the default behaviour.

What are you trying to accomplish with AF? In other words why isn't selecting one of the 5 AF sensors what you want? The single and contiuous AF selection which apply to single AF sensor modes also behave differently in other AF modes.

I'm not up on the D70 AF modes that choose the sensor for you, but on older bodies they were closest subject priority and a continuous plus dynamic mode where you picked what you wanted in focus using a single sensor by pressing the shutter release part way down and the body would attempt to track focus on that subject even if it moved to another AF sensor area. Check in other areas of the manual for the other D70 AF modes.

I'm learning photography. I was trying to get objects in different planes in to focus so that

they all will appear sharp. I took picture of this flower and only the center appears sharp,

all the surrounding petals are blurry, kind of out of focus. I was under impression that

multi point focus will allow me to get these petals and center into focus and everything will

look sharp. Please let me know even if I'm thinking way too wrong in assuming something like this.<div></div>

Sorry Sanjay, but there is no such thing as "multi point focus" on your D70. You have to achieve what you want by other means. There are two reasons to why the flower petals are not sharp. First you have a very short camera to subject distance, this creates a shallow Depth-of-Field (DOF). Second you probably have quite a large aperture, stopping down will increase the DOF, but at very short subject distances the effect of this is very small.<P>The simple answer will still be to stop down. Take different photos with different apertures. In the P-mode you can turn the command dial to select alternate aperture/shutter settings resulting in the same exposure level. Please note that it might be possible that the shutter speeds will be so long that camera shake will cause unsharpness in your photos. Then you need to use a tripod or some kind of bracing to improve stability. Use the Depth-of-Field preview button located at the little finger of your right hand when holding the D70. This will allow you to see the DOF although the viewfinder will become darker while previewing.<P>You see, focus is really only in one plane, but there is an illusion that the area just in front of this plane and right behind is sharp also. If you do a very large enlargement you will see that this is not true, but close enough for our purposes since our eyes cannot see very small details. By stopping down this area in front of and behind will increase. These are laws of optics. You cannot select multiple focus points to create multiple planes of focus. For more information (quite technical though) have a look at "<A HREF="http://www.photo.net/learn/optics/dofdigital/">Depth of Field and Digital</A>" and "<A HREF="http://www.photo.net/learn/optics/lensFAQ">Lens FAQ</A>" at <A HREF="http://www.photo.net/learn/optics/">http://www.photo.net/learn/optics/</A>. The first one is really about another discussion, the myth of focal lenght multipliers, but it starts out by talking about DOF which you should get a hang of.<P>

That said, I believe that there has been camera bodies with a feature similar to what you have been looking for. A friend once mentioned such a camera. On these you could select two points in the scene using the AF-sensor and the camera tried to set a focus point in the scene and an aperture that gave a depth of field that tried to cover this range inbetween. It sounds to me that it would be slow to use though and when you have gotten the hang of apertures and the DOF preview, this is just unnecessary.<P>

You have some nice tutorials on DOF with illustraded examples at <A HREF="http://www.photozone.de/bindex4.html">PhotoZone.de</A>, but since you have a digital the easiest way would be just to take some photos of the same flower with different settings and see what happens.

Thanks Max for this detail explanation. This problem was due to very small subject to camera distance

and large aperture.

About the focus area, now I understand that there is only one focus area, but then what's the purpose of

providing 5 focus points? In my old SLR camera I used to focus on the subject, then lock the focus and used

to re-frame the picture so that subject can be anywhere in the frame. I understand that this 5 point focus

mechanism provides same functionality, but for me re-framing the picture looks like easier one.

<BLOCKQUOTE><I>About the focus area, now I understand that there is only one focus area, but then what's the purpose of providing 5 focus points? In my old SLR camera I used to focus on the subject, then lock the focus and used to re-frame the picture so that subject can be anywhere in the frame. I understand that this 5 point focus mechanism provides same functionality, but for me re-framing the picture looks like easier one.</I></BLOCKQUOTE>

Well, there are some advantages. The first and most obvious would be that you can select one of the other points (ie non-center) and not having to recompose. On the D70 this has a slight disadvantage though, I'll come back to this.<P>

The second advantage would be that you now can have modes that focus on the closest object in the image by scanning all sensors and deciding which one is closer. You can also have a mode that "tracks" the subject while moving, although I doubt that these two modes are not really very useful with only 5 focus points that cover a limited area of the screen. But I never shoot anything fast moving so I have limited experience with these two modes.<P>

The disadvantage I mentioned. Focus points can either be cross- or line shaped. You see, in order for a focus point to focus it has to have some contrasting element, a line or edge is usually very good. In addition this line or edge should optimally be perpendicular to the focus sensor or at least "cut through" it, if it is parallell it is hard for the sensor to see the contrast difference. With a cross-shaped sensor most edges will "cut through" it some how. On the D70 only the center sensor is cross-shaped. The other ones are line-shaped, which means that they will have a harder time to focus on a general subject. If I recall correctly, the layout of the focus points is:<P>

<PRE>

_

| + |

_

</PRE>

Which means that the ones to the left and right will have harder to lock on to vertical lines than horizontal and the one to the top and bottom will have harder to lock on to horizontal lines than vertical. So by selecting the other focus points the camera sometimes ought to "hunt" for focus more, although I have not conducted any tests so I cannot say for certain. There is a simple explanation of how passive autofocus systems work on <A HREF="http://electronics.howstuffworks.com/autofocus3.htm">HowStuffWorks.com</A>. Imagine that the sensor was vertical in their examples and you will realize why it would be harder for it to lock focus.<P>

Just for comparison, on the D2H we have 11 focus points of which 9 are cross-shaped, covering larger parts of the image area and being more reliable. But that shouldn't be surprising, it is in another class of cameras.<P>

Well, I do just as you did with your old camera. I only have the center focus point active, focus, lock focus (by holding the shutter release halfways) and re-frame/recompose the photo. My kind of shooting allows me to do this.