When I posted last week showing photos of how to get eyes in focus, I got a fantastic comment from one of my readers. He agreed to write a post for all of you on Depth of Field that was a tad more technical that my visual way of explaining. Thank you Brendan Byrne for this amazing explanation.
Jodi was kind enough to ask me to write a few words about DOF or depth of field. I hope to present this information in a way that is easy to understand without resorting to crazy mathematics or going back to the chapter about optics in my college physics book. There is a great deal of information on the Internet about DOF, I’ll post some links to interesting sites.
Please keep in mind, I am not a professional photographer, physicist, or mathematician, so I’ve written what I believe is correct, based on 25 years of amateur photography. If anyone has any comments, questions, or criticisms, please email me. Here goes nothing:
I often look at my discarded photos to figure out how I screwed them up. If the problem involved the subject not being sharp enough, it typically will be one of four problems. In this article we’ll be concentrating on the final item.
- Camera shake – Drinking too much Starbucks on the morning of the shoot & aging hands sometimes cause my camera to shake during the exposure. This can often be seen during longer exposures. The rough rule of thumb is that hand held exposures should have shutter speeds faster than 1 / focal distance. For example, on my 55mm lens, I had better be shooting at shutter speeds faster than 1/60 of a second. Possible solutions: Using IS (image stabilization) lens, using faster shutter speeds, or using a tripod will help prevent camera shake issues.
- Moving subject – This can be hard to control, especially during longer exposures. Possible solutions: Using faster shutter speeds. Since there will be less time for the subject to move, there will also be less chance of blur. Using a flash can also help to freeze motion. And of course, you could always tell the subject to keep still (Good luck with that one.)
- Poor quality lens. - I have often heard that if you have to choose between the two, it is better to invest in good quality glass rather than in the camera body. While I’d love to have an L class lens for my Canon, I do try to buy as good a lens as I can afford.
- DOF – Depth of field is the area around a point that is in focus. In theory, the exact focus is possible in only one single point from the lens. This point can be calculated mathematically based upon a number of factors. Luckily, for us humans, our eyes are not quite that fussy, so instead, there is a range of area in front of and behind that focus point that is considered acceptably focused. Let’s look at this closer.
Please keep in mind that the size of the area of acceptable focus is neither a good nor bad thing. In other words, a large DOF is not necessarily a good thing. It all depends on what you are looking for. Photographers will utilize DOF to their advantage and it can be manipulated for artistic reasons.
For example, portrait shots often utilize a much shallower DOF to put the focus on the subject while blurring the rest of the shot.
In landscape shots, on the other hand, a photographer may want the photo to have a large DOF. This will allow a huge area to be in focus, from the foreground to the background.
By the way, I have read somewhere, that people are naturally drawn to photos with a shallow DOF, because it is very similar to the way that our eyes naturally see things. Our eyes work very much like a camera lens. With our vision, we do not see things clearly from close up to infinity in a single look, but instead our eyes adjust to focus on different ranges of distance.
The first photo is an example with a very shallow DOF. I shot these tulips from about 3 feet away at 40mm f/2.8 at 1/160 second. You can see the front tulip is in focus (more or less), while in the background, most notably, the rear tulip is blurred. So despite the fact that the rear tulip is only 4 or 5 inches from the front tulip, the rear tulip is out of the acceptable range of focus.
The photo of the Roman forum is an example of a much deeper DOF. It was shot from about 500 feet away at 33mm f/18 at 1/160 second. In this shot, items are in focus from the foreground to the background.
Why did these acceptable focus ranges occur the way they did in these photos? We will explore the factors that affected the DOF in these pictures.
DOF is affected by a number of factors. Now, I’m not going to give you the formula to calculate the DOF because it is going to make this article needlessly complicated. If anyone is interested in the formulas, please email me and I can send you them. By the way, there is a great website where you can calculate what a given DOF is. http://www.dofmaster.com/dofjs.html
So instead of looking at the math behind it all, I’m going to concentrate on the things that cause DOF to change and show you how you can change manipulate your DOF.
There are four major factors that affect the size of the range of the acceptable focused area: They are:
- Focal Length – The focal setting on your lens. In other words, how zoomed into a subject you are, for example, 20mm on a 17-55mm lens.
- Distance to the Subject – How far it is to the subject that you want in focus.
- Aperture Size – (f/stop) (Size of shutter opening) – For example, f/2.8
- Circle of Confusion – lives up to its name because it a very complicated & confusing factor that is different on all cameras. On the above mentioned website you can select your camera, and it will enter the correct circle of confusion. We won’t look at this because you can’t change it unless you use a different camera.
So, we’ll focus on the first three, because these are things usually within our control.
Focal length – This is how zoomed into a subject you are. DOF is affected greatly by this. It works as follows, the more you are zoomed in, the shallower your DOF will be. For example, if your subject is 20 feet way, and you use a wide angled lens like a 28mm, the area in the acceptable area of focus is much larger than if you use a zoom lens at 135mm. Using the above mentioned website, for this example, at 28mm, the acceptable range of focus runs from 14 feet through 34 feet, whereas if I zoom in to 135mm, the acceptable range of focus runs from 19.7 feet to 20.4 feet. Both these examples, are at f/2.8 on my Canon 40D. At 28mm, the total acceptably focused range is about 20 feet, whereas at 135mm, the acceptable range is less than 1 foot. It is much easier to get the focus right at the wider focal length of 28mm than at the zoomed in length of 135mm.
Distance to the Subject – This is how close your lens is to the subject that you want in focus. DOF works as follows when it comes to distance to the subject. The closer you are to the subject, the shallower the DOF will be. For example, on my 40D at f/2.8 using a 55mm lens, if the subject is 10 feet away, the acceptable range goes from 9.5 feet to 10.6 feet. If the subject is 100 feet away, the acceptable range is from 65 to 218 feet. This is an enormous difference, at 10 feet; the focused area range is about 1 foot, whereas at 100 feet, the focused range is over 150 feet. Once again, focus is easier, when your subject is further away.
Aperture Size – The final element within our control is the aperture size or f-stop. To make matters a bit more confusing, a small f-stop size (like f/1.4) means your aperture is open wide, and a large f-stop number (like f/16) means your aperture is very tiny. The way DOF is affected by aperture is as follows. A small f-stop number (which means the aperture is opened wide) has a shallower DOF than a big f-stop number (where the aperture is tiny). For example, on my big zoom lens set at 300mm, if the f-stop is set to 2.8, and I’m shooting at a subject 100 feet away, the acceptable range runs from 98 feet to 102 feet, but if I use a small f-stop of 16, then the good range goes from 91 to over 111 feet. So, with the lens open wide, the acceptable focus range is about 4 feet, but with the tiny aperture (large f-stop), the good range is about 20 feet. Again, focus is easier, when the f-stop is large (aperture is small).
Now that we have reviewed the three main factors in affecting DOF, let’s take a look at my two previous photo examples, and let’s see why I got the results that I did.
In the first photo with the tulips, the three main factors in the shot were: Photo shot at 40mm, subject at 3 feet, using f/2.8 aperture. Using the calculator, the acceptably focused area range runs from 2.9 to 3.08 feet. This is a total range of .18 feet or about 2 inches. The distance from the front to the back tulips was about 4 or 5 inches, so therefore the back tulip is out of the acceptable range and therefore, very blurry.
In the second photo in Rome, the three main factors were: Photo shot at 33mm, subject at about 500 feet, using f/18 aperture. Using the calculator, the acceptable focused range actually runs from 10.3 feet to Infinity. That is why, the whole photo is in sharp focus. So even if the moon was in my photo, it would be sharp too.
So what does this all mean for you? Should we only shoot distant subjects with wide angled lenses at large f-stops? Obviously not, we want to be able to compose photos using DOF in a way that best works for the look that we are trying for. We need to keep in mind what affects DOF, and learn how best to utilize it to achieve our goal.
When Distance to Subject Increases (subject gets further away), DOF increases
When Focal Length Increases (when we zoom in), DOF decreases
When Aperture Size Increases (f stop number gets smaller), DOF decreases
Good Luck & Happy shooting!
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