DEPTH OF FIELD, PERSPECTIVE AND SAME FOV WITH TELEPHOTO LENSES

Image content has three major technical components that we are interested in for this article FOV (field of view), depth of field and perspective. These factors are optical in nature and have nothing to do with the digital side of photography.

DEPTH OF FIELD and SHUTTER SPEEDS
Depth of field decreases as you shorten the distance between the lens and subject. This is true for any given focal length lens and aperture. Depth of field is related to lens focal length, focused distance and aperture and has nothing to do with film format size or sensor size. If we must move closer to the subject with our larger sensor equipped cameras and same focal length lens to obtain the same FOV as the 20D, we should expect to see less depth of field for any given aperture used with the larger sensor equipped cameras. If you like an out of focus background, for any given lens focal length, aperture and FOV it will be easier to achieve with the larger sensor equipped cameras. Conversely the 20D with any given FOV, lens focal length and aperture will always have more depth of field.

I'd like to touch on the subject of appropriate hand held shutter speeds. There are some folks who claim the old 1/focal length rule isn't a good starting point for insuring sharp images when hand holding with smaller image sensor cameras. As an example of the current generally accepted rule, to minimize image softness due to camera shake use 1/250 sec as your minimum hand held shutter speed for a 250mm lens.

Some people are advocating 1.6 X focal length as your minimum hand held shutter speed with a 1.6 crop factored camera (1/400 sec for your 250mm lens). The reasoning given for theory is that the 1.6 crop image will ultimately have to be enlarged more than that same FOV image shot with with a full frame camera and therefore, any camera shake induced blur will be more magnified in enlargements. It seems to me that physics would tell us that image blur due to camera shake is related to three factors: the distance from the camera to subject, the lens's focal length (actual magnification power) and the shutter speed used. The full frame DSLR would require a slightly longer focal length lens (more magnification) at the same focused distance to achieve the same FOV or need to be closer to the subject if the same focal length lens was used to get the same FOV. Both of these conditions would theoretically enhance the chance for camera shake blur. In practice, I'd bet it would be a wash. I suspect that other factors such the strength of the shooter, the weight of the lens/camera and just plain old technique would be more important factors in reducing handshake induced image blur.

I will say that all of these digital cameras are producing relatively high resolution images. The big prints that people are now more willing to make with their clean high resolution DSLR images will show off photographic skills (or lack thereof) and lens quality issues in startling fashion. I've found from experience that at my advanced age and decrepitude I can't hand hold my 1Ds Mark II and 400mm lens at much less than 1/800 second if I  expect to get all the resolution that this lens is capable of. That's twice the old rule of thumb of 1/focal length.

PERSPECTIVE
The photographic term perspective is used to describe the relational size differences between objects in the foreground of your photo and objects in the background of your photo. Foreground objects are those parts of the image that are closest to the camera. Background objects are those parts of the image that are furthest from the camera. For any given focal length lens, the closer you get to the nearest part of your subject the more those near objects seem to become larger in proportion to the background objects. As a practical example. A 50mm lens on a full frame DSLR is considered to be equal to a normal human eye FOV, and therefore, normal perspective. If you focused the 50mm lens from eye level on a landscape scene, the objects in the foreground will look to be the proper size in relation to objects in the background. Take that same camera and lens and focus it 10 inches from a person's nose and the resulting photo will have that person's nose looking way bigger than it actually is on their face. You'd get approximately the same FOV with your own eyes if you just walked up to someone and stared at their nose from 10 inches away.

SAME FOV WITH TELEPHOTO LENSES
In the image series below we moved our tripod and 180 lens until we had the same field of view for all three photos. Well, not quite, due to the 20D and 1D Mark II not having a 100% accurate viewfinder, we ended up with about a 5% error between the 20D and 1D, and including the 1Ds Mark II, there's about a 10% spread in the error. Oh how I hate sloppy testing technique.

Less than perfect framing (FOV) aside. I think it is fairly easy to see that with the same FOV and same output resolution pixel density doesn't really effect the amount of detail in the two 8mp camera images. See the 20D and 1D Mark II full resolution crops of the bear's nose below. The only thing that shows up in this 'same field of view' series with an excellent telephoto prime lens is that overall image resolution is the main factor contributing to image quality. The 1Ds Mark II bear's nose crop, even if you factor in for about10% framing error, contains noticeably more detail than the other two cameras. However, the 1Ds Mark II didn't exactly blow the other two cameras out of the water in this series. Sure, it has quite a few more pixels but the 20D and 1D Mark II acquitted themselves rather well in this series.

Since the two 8mp cameras have the same output resolution and this image set has the same FOV, the 20D and 1D Mark II nose crops would have had the same pixel dimensions if I had been able to get the exact same field of view.

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ARTICLE LINKS:
PAGE 1 - INTRODUCTION, TERMINOLOGY EXPLANATIONS AND PIXEL DENSITY WITH TELEPHOTO LENSES
PAGE 2 - DEPTH OF FIELD, PERSPECTIVE AND SAME FOV WITH TELEPHOTO LENSES
PAGE 3 - FOV AND LENS RESOLVING POWER WITH NORMAL TO WIDE ANGLE LENSES
PAGE 4 - DIGITAL NOISE AND DYNAMIC RANGE