Here's a second (or third, or something) attempt of making a photo of the beautiful Weissbach (white creek) near Inzell in spring. Depth of field-wise, it's a failure. Stopping down to f/11 with the telezoom lens was not enough to get everything in focus - even in the web version you can see a slight softness at the top of the frame:
|Weissbach (NIKON D700, 1/5s @ ISO 100; f/11, 70mm)|
I need to change my habits and get used to that! Stopping down to f/11 on a crop sensor would give me more depth of field than on the full frame sensor (f/11 x 1.5 crop = f/16.5), and I followed some fierce discussions on the internet (in forums, where else:-) why that is so, and how, and the level on which these discussions were held were rather high, with a lot of laws of optics and whatnot involved. With this post I will try a different approach and explain this as simple as possible (with a terrible lack of illustrations, I must admit).
If you look at the above photo again and think of the part of it that you would see on a camera with a crop sensor at the same focal length (!) it's very easy: the frame is cropped, and you see less of it (for the above photo, it's the equivalent field of view of a 105mm focal length: 70mm x 1.5 crop). The critical area at the top of the frame that is soft and slightly out of focus would simply not be there! Looking at it the other way around, there's simply "more picture" around the original frame of a crop sensor (I want to take my explanation approach from that side).
Here's where the "trying to explain it really really simply" part starts:
To understand what is going on we must understand what "depth of field" really is, and how it happens. Commonly, the "depth of field" is understood as being that part of a frame that has acceptable sharpness. Everything outside this area if more or less blurry (and btw.: how that blurriness looks is called "bokeh"). So that's what depth of field is.
And how does it happen? Strictly speaking, the focal plane is always just that - a plane, but not an area, or a field. There's one single distance, no matter what your aperture is, where everything is perfectly in focus - the focal plane.
But as the light rays pass through the lens, they are "focused" - and how much they are focused depends on the size of the aperture. We could say that the smaller the opening of the aperture is, the more light rays are "squeezed together" so that they form a point instead of blur at the resolution of the medium (sensor, film), and thus create the effect (illusion) of sharpness over a larger area or field. If the aperture is wider open, more of these light rays "wander off" in other directions, they are not "bundled" to form points on the medium, but remain blurry.
The Wikipedia article on Depth of field has a better explanation (of course), quote: "Decreasing the aperture size reduces the size of the blur circles for points not in the focused plane, so that the blurring is imperceptible, and all points are within the DOF."*
End of the "trying to explain it really really simple" part. :)
The first look through the viewfinder of my D700 was enlightening: there's simply "more unsharpness" "surrounding" that area of the frame (from a crop sensor) that is sharp. To squeeze the light rays together so that they reach those outer areas of the full frame sensor (again: because these outer areas are not there on a crop sensor) as "sharp", you have to stop down more.
The question remains: do we really need to know all the background and explanations to make photos? In my opinion: no. Knowing that I need to stop down more on a full frame sensor is enough.