Can you help me understand depth of field?

[markubig]

Hi everyone -

Does aperture setting change with the distance of my subject? In other words, if I use f/2.0 for someone standing 2 feet from me, will I get the same result if I use f/2.0 at 8 ft? or do i need to change the aperture setting for that distance?

I have a party tomorrow and I figured I should use the Av mode on my G3, but I am trying to understand when I should change the aperture setting.

Thanks for your help!

~mark

[jwcdds]

As I understand it, aperture is completely independent of distance from object. f/2.0 is the widest your aperture could possibly go on your camera, allowing the most amount of light, hence allowing for a faster shutter speed in lower-light situations. However, your lens is only f/2.0 when it is at full-wide. So if the object is 8 ft away, and you decide to zoom in, the maximum aperture starts to change since extending the lens barrel will result in reducing the amount of light able to hit the sensor.

I'm sure other veterans will have a better way of explaining it, and probably links to boot. But to the best of my knowledge, focal distance is independent and different from aperture.

Depth-of-field (DOF), however, is related to aperture.

[Sam North]

Depth of Field is the area of apparent sharpness in front of and behind the focusing point of the lens. Depth of field does not extend equally either side of the focussing point. Rather it extends twice as far behind your focusing point as it does in front. So to maximise depth of field, focus on a point about one third into the area you want to be sharp.

Although a lens can only focus on one subject at a time, the range of apparent focus, or sharpness, can be extended or reduced by adjusting the size of the hole through which light passes. This hole is known as the lens aperture. If the aperture is large, f2.8 for example, there will be less apparent focus than if the aperture is set to f16.

Many cameras have a special button or custom function setting that will allow you to see the effect of depth of field by closing down the lens to your chosen aperture. However, it’s not always easy to see the effect through the viewfinder.

Wide-angle lenses
A lens’s angle-of-view also affects the range of apparent sharpness. Wider lenses, or wider focal lengths on a zoom lens, will extend the range. A landscape shot, properly captured with a 20mm wide-angle lens set to a small aperture, will appear to be sharp from the foreground to the horizon.

Longer focal lengths
The sharpness range can be greatly reduced with longer focal lengths and larger apertures – an effect that can be used to isolate a subject from a distracting background.

Macro lens
The perceived range of focus with any lens decreases the closer the subject is to the camera. This very shallow range of focus presents unique difficulties when using a lens designed for the photography of small subjects. The camera should be perfectly steady and the subject still.

In a Nutshell
In more simple terms, the aperture setting, the lens’s focal length and the shooting distance, all work together to affect the appearance of sharpness in your images.

[flowe]

Hi markubig

While Sam North posted his reply, I wrote another one. They don't seem to contradict each other, but different wording might help to understand. And there it goes:


Any camera depicts sharply only what is in the focal plane, the plane in the currently set focal distance from the camera. From this plane, sharpness decreases steadily towards the camera and away from it. So in theory, there is not really any DOF.

But it is common practice to agree on a rather arbitrary distance closer than the focal plane and a distance beyond it where a still acceptable sharpness changes to unacceptable. This is not a distinct point, but an arbitrarily selected point on a sliding, tapering degradation. The distance (or range) between the two selected points is defined as DOF.

The DOF depends on many aspects: current focal length and aperture of the lens as well as the focal distance. Then it depends too on your own requirements – casual shots on 4x6in prints or any higher or even professional work. All this goes into known formulas to calculate the DOF.

In the field, there is no easy way to know or even estimate the DOF in any given situation. With the Canon G's – as applies to you and me – there is no way at all to even know the current focal length (apart from min/max) or the focal distance (apart from the crude "manual" scale) – Canon decided not to tell us.

So we have to make do with rules of thumb such as:
- "the DOF gets bigger with smaller apertures (bigger f numbers, eg. f8 ) and/or shorter focal length (towards wide angle)"
- "for smaller DOF use bigger apertures (smaller f numbers, eg. f2) and/or longer focal length (towards tele setting)"
- "one third of DOF lies towards the camera and two thirds beyond the focal distance"
- "for landscapes don't focus on infinity, but on something much closer"

Apart from the DOF, there is the concept of the "Hyperfocal distance" or HFD. This is the focal distance where the DOF begins at half the HFD and reaches to infinity. Or, if you set the focal distance to half the HFD, then the DOF reaches from about a third of the HFD to HFD. But the HFD depends entirely on the DOF concept and the knowledge of the current focal length and aperture.

Digicams are said to have a rather big DOF compared to conventional ones. The theory is diffuse, but apparent sharpness definitely is increased by the customary sharpening of digital pictures, while unsharpened ones as a rule are rather soft.

If you want to know more (or "all") about DOF and HFD, go here (http://homepage.hispeed.ch/flowe/digifoto/hfd_dof.htm). But it's probably more than most people want to know :wink:

flowe :)

[Andy_T]

Only one thing to add ... if you go to the smallest aperture of your camera, the effects of diffraction are going to make the result less sharp.
The resulting suggestion (for all kinds of lenses) is to use 2 f/stops below the smallest aperture for best results.

For the G2, the 'sweet spot' (=possibility to get most of your picture sharp) is normally seen to be around f/5.6.

Best regards,
Andy

[RhoXS]

I typed the following explanation for a friend's kid physics class last spring. I thought it might also answer your question.

When you focus a camera you focus on a certain point, say the tip of someones nose. If we then take a picture it is usually obvious that the persons other facial features are still in focus even though we focused on a point a little in front of their cheeks, eyes, lips, etc. Therefore, even though we focus on a single point, objects behind this point are also in focus. In fact, even objects somewhat closer to the camera will also be in focus. The "Depth of Field" is the distance between the closest object and the farthest object that will be in acceptable focus.

Various factors effect how deep the depth of field really is. There will always be some finite distance in focus in front of and behind the point ficused on but this distance can vary from being very very short (millimeters) to almost infinite. The aperature has a significant effect on this distance. Wide open lenses (e.g. f2) have relatively very short depth of field. Stopped down lenses (e.g. f16) have a relatively very long depth of field. Portrait photographers tend to use large aperatures so the persons face is in focus but the background is blurred.

I hope this helps.

[figueira]

Markubig, I don't know if you were looking only for a "practical terms regarding dof" just think like this:
(Note that I'm using hypothetical numbers here).

If you use f2.0, objects that are between 1meter and 1,5 meter away from the camera will be in focus. If you use f8.0 objects that are between 0.5 meters and 3 meters will be in focus.

I have exagerated, but in practical terms, I believe that is basically it.

Here you have an explanation an photos with example:
http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/950/depth-of-field.html

BTW, Andy gave a big advice, avoid the f extremes of your lenses, and since you have a 420ex with you, light shouldn't be a problem.

Regards,