Here's the transcript from Podcast number 65, originally released on Dec 2, 2006. Use this audio player if you'd prefer to listen: [audio src="http://www.podtrac.com/pts/redirect.mp3?http://www.bokeaji.com/podcasts/mbpp_ep065.mp3" format="mp3"]
Hyper-focal Distance
The hyper-focal distance is the distance at which a particular focal length and aperture combination can be focused at, to render everything from half of that distance to infinity in focus. This is especially useful to know about when we’re shooting landscapes for example, when we want sharp focus throughout the image, from the nearest to the further object. Here’s a couple of quick example of a focal length and aperture combinations and the resulting hyper-focal distance, before we get into the details. First, if we shoot with a 50mm lens at F16 aperture, the hyper-focal distance is 4.74m or 15.54 feet. That means if we focus on a point 15 and a half feet from our camera, everything from almost half of that distance, or just under 8 feet to infinity will be in focus. In metric that would be a hyper-focal distance of 4.74m to focus on, rendering everything from 2.37m to infinity in focus.
The wider the lens, the closer the hyper-focal distance becomes, so here’s another example. Let’s use 28mm. If we again use F16 at 28mm focal length, the hyper-focal distance is 1.5m or almost 5 feet. If we focus at this point, everything from 2.46 feet or 0.75m to infinity will be in focus. So you can see that the wider the lens, or put another way, the shorter the focal length you are shooting at, the closer the hyper-focal distance and near focus distance gets.
We should also note that telephoto lenses are not really suited to shooting using the hyper-focal distance to get subjects from the foreground to infinity in focus, as the physics make it impractical and even impossible to do this after certain focal lengths. For example, even if we go back to F16 for the aperture, but set the focal length to 200mm the hyper-focal distance becomes 75m or 247 feet. This gives us a near focus distance of 125 feet or 38m which is really not good if we want the foreground subjects in focus.
Circle of Confusion
One thing that we cannot ignore when calculating the hyper-focal distance is the Circle of Confusion. Now, a detailed explanation of the Circle of Confusion would make this week’s episode way too long, but here’s a quick explanation.
The size of the Circle of Confusion for any camera or film size or sensor size is the size of the smallest spot that will show up as just a dot with no recognizable height or width when printed on 8x10” paper and viewed from 2 to 3 feet away. So by a spot, I mean just a tiny little dot that you will not be able to make out as having any shape. If it was to get any bigger, it would start to look like a tiny circle, and that’s too big. Now of course, if you print the same image out at larger than 8x10, or view it from closer or further away, then the results will change, and this is why there is so much debate over how to accurately calculate the Circle of Confusion. Of course the spot is only attainable when sharply focused. As soon as the focus moves to something other than the spot it becomes slightly blurred. This is what gives us the bokeh, or the out of focus areas of an image. The circle of confusion becomes larger as it moves closer to or further away from the film plane. Only parts of the image at exactly the point where the light converges being focused on the film plane are totally sharp.
I like to think of this as the spot of a magnifying glass. If you’ve ever taken a magnifying glass outside on a sunny day and dropped a piece of paper on the ground, then concentrated the light of the magnifying glass to burn the paper, you’ll know what I mean. When you place the magnifying glass over the paper initially, the spot of light will be large and faint, but as you move it closer to the paper, the spot will become smaller and brighter, more concentrated. There’ll be a point when the spot becomes the smallest it’s going to be, and this is the point where the paper usually goes black and bursts into flames to the amazement of you and all your friends, but if you move the magnifying glass even closer, to the paper, the circle starts to get larger and fainter again. At the point at which the spot is the smallest it can be, that could be thought of as the smallest circle of confusion for that magnifying glass. Everything outside of that is just varying degrees of bokeh.
So how big is this spot on a 35mm camera? Although it varies by equipment, common sizes I’ve heard of are between 0.025 to 0.030mm, most schools of thought and many calculators use 0.030mm as the default setting for a 35mm film or full-size sensor DSLR. For digital cameras with a focal length multiplier of 1.6, 0.019mm is recommended, and for 1.5X cameras 0.020mm is recommended. There’s actually a pretty extensive table that I got these sizes from at dofmaster.com.
Calculating Hyper-focal Distance Mathematically
Remember that the size of the circle of confusion is based on an image being printed out at 8x10 and viewed from 2 to 3 feet. So it follows that if you intend to print out much larger, you would actually need to calculate your hyper-focal distances differently, but if you start going down that root you’ll probably spend more time calculating your distances than actually shooting. I’d recommend just sticking with the sizes I just mentioned or the one for your camera in the chart on DOFMaster.com for your calculations. I’ll talk in a moment about some useful tools to use as quick look-up guides in the field, but first let’s talk briefly about the actual calculation. Once you know the size of the Circle of Confusion you want to work with, to calculate the hyper focal distance, you have to divide the focal length to the power of two by the aperture multiplied by the diameter of the circle of confusion. Firstly, let’s say we’re going to be shooting at 35mm, 35 to the power of two is 1,225. If we are working with a circle of confusion of 0.030mm, and an aperture of F16, we need to multiply 16 by 0.030, which gives us 0.48. Now if divide 1,225 by 0.48 we get 2,552 and some tiny fraction below the decimal point. As this is currently millimeters, if we divide by 1000 we get 2.55 meters, or 8.37 feet, which is the hyper-focal distance.
Tools to Calculate Hyper-focal Distance
Although this is not a difficult calculation if you have a calculator with you, in the field when we’re trying to get the shot, if you are going to shoot using the hyper-focal distance, then you really want something a little easier. I have actually recently bought a new cell phone (This is outdated. I have since gotten an iPhone, and have new tools, which I mention in my Podcast and will blog about later), which is a Windows Pocket PC based PDA, so I’ve installed a piece of software from a person called Jonathan Sachs, who has kindly made a number of incredibly useful application for the Pocket PC, three of which I use a lot and would like to quickly mention are Ephemeris, which is a utility to tell you the phases of the moon, and the sun and moon rise times based on the city you select from a pull-down. This is incredibly useful, not to mention fun to play with. Another is Expose 1.0, which lists exposure guides for various light sources, and the option of applying a polarizer or ND filter to the calculation as well as changing the ISO, Aperture and Shutter speed etc. With the meters in cameras being so good these days, I don’t think I’d use this in the field, but again, it’s fun to play with. Finally, the program that is really relevant for today’s topic is DOF 1.0. As the name suggests this is a Depth of Field calculator, but it also displays the hyper-focal distance and near and far focus distances for any focal length and aperture combination. This also allows you to change the resolution, which I find is pretty much the same thing as the Circle of Confusion in this utility. The hyper-focal distance is a constant once you’ve selected your focal length, F stop but there’s also a pull-down to set the distance at which you intend to focus, and the near and far focus distances are calculated from this. Once you set the focus distance to anything equal to or further than the hyper-focal distance though, the far focus indicator changes to say “infinity”. I’ll put a link to Jonathon’s download page for these applications in the show notes too. There are other utilities that you can download from the page too, which are really very useful.
[caption id="attachment_736" align="alignright" width="433" caption="Fuji & Flowers"]
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Practical Use
One of the reasons I bought a Pocket PC based cell phone was because I wanted to use this DOF tool in the field. I used it for the first time recently when shooting Mount Fuji with some flowers in the foreground that I wanted to also get in focus. If fact, let’s take a look at the shot so that I can explain what I did. It is image number 1156. Basically, I wanted the pampas grass at the edge of the lake and as many of the flowers in the foreground in focus as possible. I don’t like to stop down less than F16 as the image starts to soften up after that, so I dialed in 50mm into the focal length field, and 16 into the F stop field. I could then see that the hyper-focal distance was just over 4 meters. This means that if I focused on the pampas grass, which was about 5 meters from where I was standing, of course the pampas was going to be in focus, but also everything from 2.25 meters to infinity was also going to be in focus.
Another option which was raised a while back in the forum, especially if you don’t have a PDA, is a card that can be printed by a piece of software available from dofmaster.com. I’ll put a link in the show notes as a reminder, but basically there are a few applications to make it easy to print out both Depth of Field charts and Hyper-focal distance charts. These can be printed on card or paper that you may want to laminate to stop it getting all messed up in your camera bag, and although I’ve not tried this myself, I’m sure they make a very easy look-up option while in the field. You can also change the various parameters like the size of the circle of confusion and the widest and smallest aperture, and the shortest and longest focal length, among others.
Of course, you’re not always going to have a subject that you want to focus on, so although I actually moved back from the scene we just looked at to get my 5 or so meters distance, and focused on the pampas, what you can do is just use the distance legend on your lens. Most lenses have them, so you can use that to set the lens to the hyper-focal distance and forget about it for that scene. In fact, even if you can roughly guess your focusing distances, I’d suggest that you check the scale on your lens to make sure it’s accurate. And remember to switch your lens or camera to manual focus mode or when you press the shutter button it will re-focus on the scene, likely moving the focus away from the hyper-focal distance point at which you manually focused.
I also want to briefly reiterate, because I don’t think I stressed this much earlier, that the Hyper-focal Distance is not the nearest point of the image that will be in focus. The calculation is actually a little more complicated, but basically roughly half of the Hyper-focal Distance is the near focus distance. For example if you focus on a Hyper-focal Distance of 10 meters or about 30 feet, the near focus will be half that, which is 5 meters or 15 feet. Everything from that point, out past the Hyper-focal Distance to infinity will be in focus.
Podcast show-notes:
The software for easy printing of the DOFMaster Hyperfocal Chart can be downloaded from here: http://www.dofmaster.com/
There's also a table at DOFMaster.com with the Circle of Confusion sizes for most digital cameras. This is necessary to calculate the Hyperfocal Distance with most tools if you don't just go with the default for 35mm cameras, which is 0.030mm. See here: http://www.dofmaster.com/digital_coc.html
Here is where you can find Jonathan Sach's DOF 1.0, Ephemeris 1.0 and Expose 1.0: http://home.comcast.net/~jonsachs/
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