It’s amazing how often I get questioned about depth of field (DoF). Since it’s been frequent in the past weeks, I’ll write about it a little. But first, click here to see the picture above in a larger resolution. It’ll show you how shallow is the DoF in that.
First, a bit of definition. Depth of field is the part of an image that’s acceptably in focus (i.e. sharp, not blurry). Lenses obviously can focus at one single distance, but the decrease in sharpness is gradual as we move away from the focused plane. So, within the DoF, the decrease in sharpness is unnoticeable to our eyes. How you play with DoF varies: sometimes we want everything in focus, so we use a large (deep) DoF; in other situations, we want only a small bit of the image in focus, so we use a small (shallow) DoF.
There’s a series of variables that determines DoF, such as focal distance, lens aperture, subject to camera distance, and sensor/film format. I will start by the most common factor, the lens aperture, which is somehow tied to the focal distance as we will see.
Bottom line for this is: the larger the aperture, the shallower the DoF. The most confusing fact here is that many texts out there (including some badly written camera manuals) say that aperture and f-stops are the same thing. Well, they aren’t – I suppose you suspected that since they have very different names.
In optics, aperture is defined by the ratio between focal distance and the f-stop. So, for example: a 50mm lens at a f-stop of 1.4 has an aperture of 35.71mm. This means that, in practice, your 200mm telephoto lens at a 4.5 f-stop has a greater aperture than your 50mm at f/1.4 (44.44mm vs 35.71mm), hence, producing a shallower DoF.
So that common saying that a 50mm f/1.4 lens is the most adequate for shallow DoF is just popular stupidity — people should pay more attention to their physics classes in high school. On the other hand, fast ultra-wide lenses (like my Tokina 11-16mm f/2.8) have very small aperture (3.93-5.71mm), so you can use these to achieve virtually infinite DoF in almost any conditions.
That last statement raises another common question: how do I get shallow DoF at small focus distance (like 11mm)? Easy, get closer to the subject. For a given focus distance and f-stop, DoF decreases as the distance to the subject decreases.
So, if you wanna get some nice background blur in your shots, use large apertures (by either using a low f-stop or longer focus distance – i.e. telephoto lens) and/or get closer to the subject.
Another thing to consider is something called bokeh. That’s the japanese word for blur and it determines the quality of background blur. In optics, we may quantify that by a series of boring equations to determine something called Circle of Confusion. It’s basically a circle in the image formed by a cone of light rays coming from the lens that isn’t in perfect focus. Example:
I shot this using my old Fuji 28-300mm lens and it’s just a bunch of Christmas lights out of focus, but you get the picture. The quality of bokeh is tightly tied to the quality of spherical aberration correction in a lens.
So, back to the 50mm f/1.4, it’s not because of the DoF they rock, but because of bokeh quality. I can vow for the Canon EF 50mm f/1.4 USM and the Nikon AF NIKKOR 50mm f/1.4D lenses. Both produce very pleasant bokeh. In general, good professional-class lenses produce nice bokeh, but you should always check yourself if that’s what you want before buying a lens. Or, if you can’t, you may check lens reviews on the web for that. I can recommend Ken Rockwell’s website,Photozone.de and DPReview.
Hope it helps! Happy clicking!