Trying to understand focus

[drmac]

Need some help understanding focus....

2.8 70-200 lens (set at 2.8) with the center point focus point activated...

If I shoot a picture of a person with the focus point at the chest, will all things at that distance be in focus or does the area of focus "spread out" from the middle?

[John_B]

drmac,
There are a few things that affect the DOF

Check out Online Depth of Field Calculator DOFMaster <-- click to see (http://www.dofmaster.com/dofjs.html) as it might help you figure it out :)

[drmac]

I looked at the link and tried a couple of numbers... I guess my question is more up and down than depth.... If I take a picture of the chest at 2.8 and ten feet, how far up towards the head can I expect to be in focus?

[sandpiper]

If I shoot a picture of a person with the focus point at the chest, will all things at that distance be in focus or does the area of focus "spread out" from the middle?

Yes, all things at that distance (and only that distance) will be in focus.

At any other distance, things will be progressively less in focus as you move further away from the plane of focus. The points nearest to the actual focused point will be 'acceptably sharp' graduating to completely out of focus blurs.

The area where things are 'acceptably sharp' is called the Depth of Field (DoF). This will extend further behind the focused point than in front (up to twice as far although this varies depending on lens, distance etc.).

You can extend or contract the DoF by adjusting aperture. A wide aperture (small number such as f/2.8) will give a shallow DoF, a small aperture (large number such as f/16) will give a deeper DoF with much more of the image acceptably sharp.

Other things that affect DoF are focal length (a longer focal length will have shallower DoF than a short one) and distance from subject (DoF increases as you move further back). Of course, these will generally cancel out when trying to frame a specific image. If you want to move further back to increase DoF, you will need a longer lens to keep your subject framing (although perspective will still change) which will decrease it again. Hence aperture being the primary control for DoF. Also keep in mind that using very small apertures (f/16 and smaller) can cause diffraction softness, particularly with a 'crop' body. You have to balance the need for DoF with the loss of absolute sharpness.

It is also best to focus on a person's eyes, rather than their chest by the way. The eyes are the most important part to be absolutely sharp.

Hope this helps.

[John_B]

I looked at the link and tried a couple of numbers... I guess my question is more up and down than depth.... If I take a picture of the chest at 2.8 and ten feet, how far up towards the head can I expect to be in focus?drmac,
You really don't give enough info but I will try:
If you have a 1.6 (don't know what you are using) sensor and have the lens set at 70mm f/2.8 with the subject 10ft you only have .65ft in focus when the lens is at 200mm you only have .08ft in focus

So, it depends on the focal length and angle because the longer the focal length give less in focus and any angle will change the distance to the subject.

I suggest you put your equipment in your signature line so it can make it a bit easier to know what you have which helps us answer your questions easier :)

[sandpiper]

I looked at the link and tried a couple of numbers... I guess my question is more up and down than depth.... If I take a picture of the chest at 2.8 and ten feet, how far up towards the head can I expect to be in focus?

It's not a matter of up / down. It's a case of distance from the focal plane in the camera ('the sensor'), so you are really looking at forward / back. There will be a slight effect vertically, but only in so far as the angle causes a greater distance between the points (imagine a right angled triangle, the side opposite the right angle will be slightly longer).

The face will usually be slightly forward of the chest and so not pin sharp. However, if they were stood 'chest out and head back', the face could be on the same focal plane as the chest.

You would generally be OK with a good DoF, but shooting wide open you would likely notice the softness in the eyes, if shooting a head and shoulders portrait focused on the chest.

If you were further back, to take a full length shot, then even wide open you may have enough DoF to keep the eyes sharp enough.

As I mentioned in my previous post though, focus on the eyes as slight softness will usually be far less noticeable on the chest.

[jra]

Also, keep in mind, the focal plane is just that...a plane. So objects that lie within that plane of focus will be in focus. For instance, an object to the side of your frame will be physically further from the camera but still be in the focal plane. The focal plane is not circular.

[chauncey]

Kinda like an arc.

[jra]

Kinda like an arc.

Ideally, the focal plane is not like an arc but a flat plane.

[chauncey]

But as you pointed out, the edges of your image are not the same distance from the lens as is the actual focus point.
Granted, we're talking minute differences.

[Farsad]

Hi
for understanding DOF read this article:
Understanding Depth of field (http://fototalk.net/photography-tutorials/understanding-depth-of-field-dof/)
This article have good examples for any Effective factors to Depth of field.

[Wilt]

But as you pointed out, the edges of your image are not the same distance from the lens as is the actual focus point.
Granted, we're talking minute differences.

The Plane of Focus is ideally a flat plane which is ferfectly parallel to the Focal Plane (the sensor). Not all lens designs achieve that flatness of plane.

Yes it is true that as you move from the point of focus that the points along the Plane of Focus are geometrically farther than the point on the focal plane, but the distance between the two parallel planes is what counts usually.

[20droger]

The "field (or plane) of focus" is actually the surface of a spheroid, somewhere between the surface of a sphere and a flat plane. With a simple single-element lens, it is very nearly spherical. As the compensation gets better (and the price increases), the field flattens out. But it approximates a plane only with the very best of lenses.

This effect is more pronounced the closer the point of focus. In fact, that is one of the primary differences between a macro lens and a lens with close focusing and high magnification; a true macro lens has an exceptionally flat field of focus.

[anorphirith]

try aligning glasses one after another on a slope
focus on each one of them
you will understand focus better