'Image Size' should be the primary categorization factor.

[ho_co]

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Can someone please define "Depth of Field" and "Depth of Focus" for me...

 

Bill, this is a grave oversimplification at every step of the way, but may help:

 

 

Depth of field relates to the perceived range of sharpness of an image in front of and behind the focused distance. The engraved depth of field scale on your lens represents this.

 

There are literally hundreds of discussions on this forum regarding depth of field, and I won’t go into it here.

 

 

Depth of focus relates to the focus/film plane. If you think of a cone of light rays coming from a longer lens, the edges of the cone are fairly steep. They meet at the focus plane. Extend the cone in your mind’s eye to beyond the focus plane; you’ll have a figure rather like the pattern of an hourglass.

 

Now consider the cone of light coming from a wide-angle lens (non-retrofocal, with exit pupil near the focus plane). Extend this cone mentally beyond the film plane as well. Here you’ve got a much broader hourglass, with the rays at a comparatively lesser angle to the focal plane.

 

The point is that although only one subject plane is in focus on the sensor, the difference in the steepness of the cones of light makes a tremendous difference in the needed tolerance in location of the parts of the optical chain.

 

That is, if you take the cone of light from the wide-angle and move the sensor forward, say, 2 mm from where the light cone is at its narrowest (i.e. in focus), there will be a tremendous difference in the width of the cone that the sensor intercepts: The image will be noticeably and grossly out of focus.

 

If, on the other hand, you take the cone of light from the longer lens and move the sensor forward from its intended location by the same 2 mm, the width of the cone will have changed very little. It should be a point, since that would mean “in focus”; instead, it's a circle--but a very small one. You might need a loupe to see that there’s literally nothing in focus in the image, because the portion of the cone caught by the sensor is so narrow.

 

 

 

Since the X that the light cone makes is narrower with a longer lens, a slight shift of the film plane won’t make much difference in the image.

 

But the X of the light cone of a shorter lens is much broader. With it, the same slight dislocation of the sensor will slide it into a much-changed diameter of the ray cone; the result will immediately show as an out-of focus image.

 

 

This effect (depth of focus) is why tolerances with short-focal-length lenses must be much tighter than with longer ones.

 

 

The same diagrams you've made in your head here will also be helpful in describing depth of field. But you can't talk about depth of field until you've got the focus plane exactly located. In other words, depth of field and depth of focus are closely related and derive from the same optical and mechanical properties of the camera and lens. Discussions of depth of field begin with the assumption that the depth-of-focus matter has been resolved. That's why they're so much more common than discussions of depth of focus.

 

 

Hope that helps a little for a non-technical explanation.

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[billmary]

Thanks Howard,

 

I've read your references and now I don't feel quite so bad about being confused. It seems that part of the problem is a long history of imprecise use of the terms.

 

My take home message from those sites is that Depth of Focus refers to the range (in small units such as mm) of acceptable sharpness inside the camera at the sensor plane, whereas Depth of Field refers to the range (in large units such as inches or feet) of acceptable sharpness outside the camera at the subject area.

 

I wonder if/how you can change the Depth of Focus, and why you would want to? It seems to me the Depth of Focus should be set permanently and properly at the time of manufacture of the camera and lens. (If ANYTHING can be permanently proper is probably a discussion for another forum.)

 

Happy New Year,

 

Bill

[ho_co]

Happy New Year to you as well, Bill!

 

I think you've given a better summary than I did.

[jankap]

A happy new year to you all.

Depth of field is of interest for the photographer.

Depth of focus plays a role for the camera manufacturer, more precisely for the (auto)focusing mechanism.

For a mirrorless camera this is more important, because the light rays for the picture and the focus are mixed up. In case of a mirrorhaving or a rangefinder camera the manufacturer has more freedom to make the focusing more exact and/or faster.

Question, light rays penetrate into the sensor (see the foveon type). Does this play a role in determining the focus (the depth of focus)?

Jan

[kamilsukun]

Happy Holidays...

 

I hope the future brings an upgrade to the photo web sites, allowing the readers to examine the products categorized under the 'image size' as the primary specification of a camera. The first thing even before the sensor size is the size of the image the camera lens combination produce. This factor dictates the rest.

 

We are still in a semi secret world of the specs and almost all web sites are embedding the sensor sizes somewhere in the spec tables. All the cameras must be listed by their sensor sizes to let the reader understand what category of a camera he/she is looking into. The variation of the image quality according to the rest of the factors come after from this basic first thing first.

 

I have no doubt that this is a known reality for most and the photography press deliberately joined the pixel count wagon to ensure the rise and expansion of the industry. Pleasingly nowadays some few web sites do provide the pixel density as an additional specification.

 

It is time now to take this bold step forward. The industry will not be hurt.

 

 

I feel obliged to report the site wide change at dpreview. All cameras are now listed with their precise sensor sizes in millimeters. A giant leap forward to avoid confusions and efforts to decipher cryptic names of sensor categories.

 

Congratulations Mr. Phil Askey, Congratulations dpreview. This is only one of the thousands of positive inputs you have provided so far. You are not the best for no reason.