Depth of Field Determination
A fairly simple way to determine Depth of Field (DOF) when using an SLR camera is by following some simple steps.? There are three factors which determine how ‘deep’ or ‘shallow’ your photo is rendered.? First is the distance between the focal plane(camera) and the subject.? If you are close to the subject, say a few feet, without other considerations, you will get a low DOF or in lay terms, the background will be blurry.? If you are far away, say across a large room, again other factors aside, you will get high DOF or in lay terms, the background will be clear.? Second is the focal length of your lens.? At shorter lengths (for sake of discussion 20mm-100mm approx) , other factors ignored, you should get deep DOF (clear background).? At longer lengths (for sake of discussion 200mm and up), you should get shallow DOF (blurry background).? Third is aperture (the size of that tiny hole that opens and closes).? If you use an f-stop of f2 or f4 you should get shallow(low) DOF (blurry background).? If you use an f-stop of f16 or f22 you should get deep(high) DOF (clear background).? F stop is the size of that hole and the numbers start from largest opening (f1) to smallest opening(f32 for most SLR lenses).? So those three factors being said, if you combine them, here is what you should get:
- Focal Length (short) + Aperture Wide(f2) + Distance from Subject(close) = Blurred background(low DOF) mostly because of the wide aperture
- Focal Length (short) + Aperture Wide(f2) + Distance from Subject(far) = Blurred background(low DOF) mostly because of the wide aperture
- Focal Length (short) + Aperture Narrow(f32) + Distance from Subject(close) = Clear background(high DOF) mostly from the narrow aperture
- Focal Length (short) + Aperture Narrow(f32) + Distance from Subject(far) = Clear background(high DOF) mostly from the narrow aperture
- Focal Length (long)? + Aperture Wide(f2) + Distance from Subject(far) = Very blurred background(low DOF) because of the long focal length and wide aperture
- Focal Length (long)? + Aperture Wide(f2) + Distance from Subject(close) =not possible, subject cannot be close with this focal length
- Focal Length (long)? + Aperture Narrow(f32) + Distance from Subject(close) =not possible, subject cannot be close with this focal length
- Focal Length (long)? + Aperture Narrow(f32) + Distance from Subject(far) = Fairly clear background(low DOF) mostly from the narrow aperture(the long focal length should lessen clarity)
Of course the in between values here will yield different results but hopefully if I explained well enough you should be able to calculate it approximately.? For example, 22mm lens, f8, subject=close should give you moderate DOF (somewhat clear background).? If you wanted it totally clear, use a smaller f stop(f16,f32,f64).
A few more examples: 300mm, f4,? subject(far) should give you a very blurry background(low DOF). 300mm, f16, subject(far) should yield somewhat clear background(moderate DOF) due to aperture.? See where I am going with this.? In general, mostly you will not determine subject distance.? What you can vary is the aperture and the focal length(possibly with a zoom lens).? In Summary,? to get more DOF(clearer background), use a small aperture and/or short lens.? To get less DOF(blurrier background), use a large aperture and/or long lens.? Typically for portraits, 100mm lens, f2 or f4, subject at moderate distance should yield a very blurred background(low DOF).? Hopefully my experience with these variables will help you to be a better photographer.
