It’s old news by new that full frame sensor cameras like the new Nikon D700, the slightly new D3 and the aging Canon 5D have shallower depth of field and allow for better isolation of the subject than crop (APS, DX, Digital Only) cameras. But why? Why would making the sensor larger create a shallower depth of field?

There is certainly no question that it happens. Behold, more of my quick and dirty “let’s shoot everything with the new camera” shots:

Shallow DOF

More Shallow DOF

Other than the better noise performance, this is one of the biggest reasons people sometimes prefer a full frame camera. I can certainly agree. I really like the new creative possibilities afforded by the larger sensor size.

But why does it happen? I felt the need to get a better answer than “because the sensor is larger.” These are my findings, and could be totally off base.

Following a lead I got on flickr, I set out to read Wikipedia’s articles about DOF and the Circle of Confusion.

“1. To calculate a camera’s depth of field (“DoF”), one needs to know how large a circle of confusion can be considered to be an acceptable focus. The maximum acceptable diameter of such a circle of confusion is known as the maximum permissible circle of confusion, the circle of confusion diameter limit, or the circle of confusion criterion, but is often incorrectly called simply the circle of confusion.

2. Recognizing that real lenses do not focus all rays perfectly under even the best of conditions, the circle of confusion of a lens is a characterization of its optical spot. The term circle of least confusion is often used for the smallest optical spot a lens can make, for example by picking a best focus position that makes a good compromise between the varying effective focal lengths of different lens zones due to spherical or other aberrations. Diffraction effects from wave optics and the finite aperture of a lens can be included in the circle of least confusion, or the term can be applied in pure ray (geometric) optics.”

A circle of confusion, indeed. I guess we can assume that this is the circle of light that hits the sensor. For full frame sensors, this circle is larger than on cropped digital cameras. Easy enough. But it still doesn’t answer the question.

Well, I do know one thing for sure. Longer telephoto focal lengths produce shallower DOF than wide focal lengths. I then found this lovely article.

“As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject. This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors.”

So I think what we have here is that the circle of light hitting the sensor is larger, which removes the crop factor that used to make a 100mm lens into a 150mm lens. So, we have less length and we need to get closer or zoom in more to create the same picture we had in mind. Since we are closer or shooting longer, the effects of shallow DOF start to appear more. I guess. I still feel a bit confused by the whole thing.

On to something I DO understand:

Editability to the Max!

Better noise performance and cleaner images from the D700 don’t just mean that images look better straight out of the camera (they do, but that’s not the point of this post). It also means that editing photos just opened up quite a bit. In the photo above, for example, I was able to process in lightroom so that both the sky and the buildings were clear, exposed properly and noise free. Similar to an HDR or even grad ND filter technique, but without all of the hassle. Since the image has less noise overall and especially in shadows, a lot more can now be done in post without losing image quality. For this image I ended up exposing for the buildings, then applying one of Lightroom 2’s new “fake” Graduated Neutral Density filters and tweaking some other settings. It’s not a very artistic photograph, but it sure shows a lot of potential for D700 RAW image editing.

This entry was posted on Thursday, August 14th, 2008 at 8:44 pm and is filed under Photography. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.