I’ve not done a tech post in a while, so it’s about time.
There is an old Photojournalist’s saying: “F8 and be there”.
Basically in our print film days, if you left your camera on F8 you could snap a pic of something happening without worrying about focus, shutter speed etc. and have a reasonable chance of salvaging the important moment in the darkroom, and of course you “have to be there” to capture the moment. We would leave the camera at 1/60 F8 and the lens focused at just under infinity and have a pretty good chance of taking the photo.
This works because of some photographic magic called Depth-of-Field. On most lenses (though modern ones are starting to leave this off), there is a gauge that shows you the distance to where the focal plane of the lens is focused. It can range from a few inches/mm to Infinity (that 8 on its side!). There used to be markers that also gave you a rough idea of how much would be in focus at various F-Stops. Those marks are disappearing like mad these days.
Depth-of-Field or DOF is a measure of how much of a scene will appear to be in focus, in front of and behind the point where the lens is focused. DOF is not an easy concept to master. There are three factors that go into determining how much DOF you have. It’s not an absolute. Focus fades in and out over a distance, it’s not a sudden “I’m in focus. Now I’m not” situation.
There are four important ideas in thinking about DOF. Even seasoned photographers need reminded of these:
- Subject Distance: The closer the subject, the less will be in focus; the further away, the more will be in focus.
- Lens Focal Length: The longer (i.e. more telephoto) the lens, the less will be in focus; the wider angle the lens the more will be in focus.
- Aperture or F-Stop: The larger the lens opening (smaller F-Stop number) the less that will be in focus; the smaller the opening (larger F-Stop number) the more that will be in focus.
- The distance in front of the subject that’s in focus is less than the distance behind the focus point.
So if your subject is 100 feet away and you’re using a wide angle lens, and an F-stop of F16 you pretty much won’t have to focus the camera, yet your hot 70-200 F2.8 lens at 200mm wide open shooting a subject 3 feet away will have almost none of the photo in focus.
Lets look at some examples. There are several online DOF calculators that can help you understand this complex subject. Lets look at a couple of examples using our favorite normal 50mm lens.
Photo at F8 and 145mm makes the background part of the photo!
If our subject is 10 feet away and you’re F-stop is F11, almost 6 feet will be in focus. But your subject isn’t centered in that 5.86 zone, but a little over 2 feet in front of the focus point will appear to be in focus, and around 4 feet behind the focus point will appear sharp.
But keeping the same camera settings, we move our subject/focus point to 5 feet away. Only a mere 1.36 feet will be in focus; about 7 inches in front and 9 inches behind the subject. Now move your subject to 30 feet away. Now over 150 feet will be in focus, 13 feet in front and 140 feet behind. It’s amazing how much distance affects this.
Now let’s look at another example. We will use the same distances, but this time open up to F2.8. With our subject at 10 feet, the depth of field is almost identical to F11 at 5 feet (1.36 feet, though the front and rear distances are slightly different, but close enough). Bring your subject back to 5 feet at F2.8 only 4 total inches will be in focus.
Shoot your subject at 5 feet away at F2.8 with a 200mm lens only 1/4th of an inch will be in focus.
With our gear, our common F-stops run from F2.8 to F11. F5.6 is the middle of this range. Some of our lenses will let us open up to F2.0 or more and of course, we can stop down to F16 or more, but F5.6 is pretty much the middle. So one might think that F5.6 might be a good F-stop to use. But here is the problem…..
At most common distances with “normal” lenses, at F5.6 too much will be in focus to achieve that “defocused” look and blur out background elements; but it doesn’t bring enough depth of field to bring the background into focus, so you’re kind of left in no-mans land, so to speak. It’s a safe F-stop for several reasons. One is shutter speed. If you were to shoot at F8 for F11, it’s going to cost you a couple of shutter speeds. And while you might be getting a nice hand-holdable 1/60th of a second at F5.6, suddenly becomes 1/15th at F11 and your photo is going to be blurry, but now because of camera shake, not focus. Going the other way, open that lens up to F2.8 gets you some nice shutter speed, but your focus now has to be very precise or your photo will appear to be blurry because the focus depth is too little.
So that makes F5.6 sound good to me, you might be saying. Well if your goal is happy snaps of your family and friends, then it is, but when you look at someone’s photos who gets off of F5.6 and works with the extremes of aperture, you ooh and aww going “Why can’t my photos look that good?”. Well, its because they are taking advantage little or a lot of depth of field.
So if you want to work on more creative photography, ban yourself from F5.6.
You may also hear the term Bokeh used when talking about low depth of field photos. This term is frequently misused to mean a photo with very low depth of field where the background is seriouslly blurred out. Bokeh actually means the “Quality of the out of focus highlights”.
Consider this photo:
Here, we have the bokeh effect showing. The background consists of white Christmas lights, but they are so defocused that these point lights have formed large circles. Different lenses will render these out of focus highlights differently. For instance, a mirror lens produces donut shaped highlights. A lens with 6 aperture blades will produce a hex shaped highlight. Lenses with more blades produce rounder highlights. This quality is what Bokeh is all about, not how much of the photo is in or out of focus.