Photo Notes A place to talk about making images.

This image uses short depth of field to give the image a sense of three-dimensionality.

When I left off last time I had just suggested the concept of depth of field, and we will start with that this in this blog. The real problem is that the language is confusing here: what does “depth of field” mean? Obviously we aren’t talking about how much acreage you might have planted in corn. What we are taking about is how much of your image, closer and further from the lens, is in sharp focus. So if I want an image that will concentrate the eye on just part of the face, of a subject, I would say that I need a very narrow depth of field. In order to get this I will use a wide aperture (small number, say 2) that will let in a lot of light through the lens. This will create an isolated zone of focus. How low will depend on the focal length of the lens. For instance f2.8 is extremely short depth of field on a 200mm lens while it provides significant depth of field on an 18mm lens.

The focal length of the lens is one of the factors that affects the depth of field. If you use a long lens you see more details in the subject than you would with a short lens at the same distance. The need to use a smaller aperture (bigger number like 16) to achieve sufficient depth of field is related to this increase in detail. If you shoot with a 28mm lens at a distance of 10 feet from the subject many of the details of the subject are no more than specs, so you wouldn’t expect to see detail in them. If you shoot the same subject, from the same distance with a 150mm lens many of the parts of the subject that were specs in the 28mm shot will have visible detail, and thus need to be sharp. For this reason you can expect to need more depth of field with a longer lens. I should point out that the 150mm lens will see much less of the subject.

Another factor with depth of field is visualizing what will be sharp and how the soft areas will appear. This is covered by a rule that is usually referred to as the one third, two thirds rule. It is also know as the Scheimpflug rule, but since this name is hard to pronounce it doesn’t get used much. Regardless of what you call it the idea is that as you stop down the lens 1/3rd of the extra depth of field exists in front of the point you focus on and 2/3rds of the additional sharp focus area is behind the focus point. Think about this, if you want a lot of stuff in focus, don’t focus on the front of the image, but midway through the image. You will use the depth of field you have, at any give aperture, more effectively. Of course if you don’t have enough depth of field, for an image that needs it, the point of focus won’t save you; you need to use a smaller aperture.

This shot uses long depth of field to keep the whole room in focus.

This shot uses short depth of field to bring the viewers' eye to the soup.

There is no other subject in teaching photography that is at the same time so basic and so important as understanding aperture and still so confusing and hard to explain. I want to start with a simple comparison to shutter speed: as the shutter speed number increases the amount of light let in by the shutter decreases. This is because the number is a fraction, and we are talking about the bottom number of the fraction, called the denominator. The number we use to discuss the aperture is also the bottom number of a fraction and as this number gets bigger the amount of light transmitted by the lens gets smaller. So for instance f16 lets less light reach the sensor than f8. Two stops less light, but we will get to that soon.

The aperture is a hole, which can be varied in size, in the middle of the lens. It blocks some of the light coming through the lens. The aperture number is actually the focal length of the lens divided by the width of the lens. So if you have a simple one-element lens that is one inch wide and has a focal length of 8 inches the lens aperture would be f8. If the lens had a one inch diameter and had a focal length of 4 inches that the aperture would be f4. Please don’t make me do this in metric, the relationships works but the math is more annoying. That wasn’t so bad was it? Here’s where it gets tricky, if you want the aperture to increase the light coming through the lens by one stop you have to double the area of hole in the middle of the lens. If you double the diameter (distance across the lens) you will increase the area of the circle to 4 times the originals size, or two stops. Instead of doubling the diameter of the lens you have to divide by the √2 (square root of two) which is approximately 1.4. This is similar to converting a focal length from your sensor size to what it would be in full frame 35mm. So 8÷1.4=5.714, which we refer to as f5.6. If you want to decrease the amount of light reaching the sensor by one stop you need to multiply by 1.4, remember the number must be smaller to let in more light. So 8 X 1.4=11.2, which we refer to as f11. It is important to remember that since we are working with fractions, things get turned around.

Please forgive me for all of this, but I didn’t make it up. As you may find it difficult to multiply by 1.4 in your head you may want some help on how to manipulate and use these numbers. There are a number of important applications that I will probably blog about later. In the mean time you might want to try to remember the full stops: 1, 1.4, 2, 2.8, 4, 5.6, 11, 16 and 22. You may notice that every other number is double; this helps to memorize the numbers.

This shot uses long depth of field to keep the control panel and the room in focus

I also want to mention that small numbers isolate focus and large numbers increase the area. This is called depth of field. I wanted to mention it so that I can put pictures into this blog.
I am going to write about using the aperture to control the depth of field very soon, but I thought this was enough confusion for one week. People often tell me that they don’t need to know these things, and that is certainly true. But I want to control what happens in my pictures; I want to make pictures rather than just take pictures. If I understand how my camera records light I will, inevitably, have more control over my pictures.

A 1/20th of a second, paned with the horse.

I talked about the shutter and what it does, a couple of weeks ago. This time I am going to discuss aspects of how to use it.

Since DSLR cameras are so small and light that we often hand hold them, they are designed to be used this way some of the time. Unfortunately too often we use them at longer shutter speeds than we can hold steady. This causes blurred pictures. Many years ago Kodak did a study on blurred photos and fond that this is the biggest reason for blurred images. I think that many people have more trouble with this now than when we used film cameras. There are a couple of reasons for this; first digital SLR cameras are heavier than film cameras were. Second since the sensors are smaller than film lenses bring subjects closer than they did with film cameras. For instance a 50mm lens has about a 45º angle of view on a full frame camera and about a 30º angle of view on a Canon D30. Not only does this mean that the subject appears to be closer to the camera, but it also means that camera shake is increased. You can see this for yourself if you want to do a simple experiment. Cut a small hole in a piece of paper, say a circle about an inch in diameter. Now if you hold the paper close to your face and look through it you will be able to keep it pretty steady. If you hold the paper a couple of feet from your face it will be a lot tougher to keep steady. A telephoto lens is like the piece of paper a few feet from your eye, tough to keep steady. There use to be a rule of thumb that you could hand hold 1/the focal length of a lens. So if you were using a 250mm lens you could hand hold a 1/250th of a second. If you were using a 60mm lens you could hand hold a 1/60th of a second. Basically this is now wrong. Unless you have a camera or lens with image stabilization you should not try to hand hold anything less than 1/twice the focal length of the lens. So if you have a 250mm lens don’t hand hold anything less than 1/500th.

There are a couple of ways around this short of image stabilization. Everyone knows about tripods, the only problem is that you have to carry them. By the way, if you are using a lightweight tripod you might want to use the self-timer to trigger the camera, this will reduce vibration. You can also get a monopod, which is like tripod that is missing two legs, this will reduce shake on shots that are not so long. Finally you can build a chain-pod. This costs almost nothing, weighs a couple of ounces and takes up less room than a 35mm film can. Chain-pods are he simplest and best piece of photo equipment you can build. It works like a monopod. To build it drill a small hole in 1/2 inch 1/4X20 (that is a thread size) thumbscrew. Attach about 6 feet of chain to the hole (more if you are really tall). Next put a nut onto the thumbscrew and position it so that the screw can’t go too deep into you tripod socket and glue the nut in place. To use attach the thumbscrew to the base of your camera drop the chain and step on it. Now pull up against the chain. Steady!

This shows how you build a chain pod.

fast shtter speed

So I discussed what a stop is in photography. This week we’ll talk about the shutter; among other things we’ll see how the shutter is controlled in stops. But first a quick explanation of the shutter: in all DSLR cameras and many of the fixed lens cameras the shutter is a physical object that moves over the sensor. It is important to remember this, since it explains some of the limitations of the shutter. There are two curtains on a shutter, one uncovers the sensor and the second covers it up again. Both must move at exactly the same speed for the entire time they are moving over the sensor or your exposure will be uneven. It is really quite amazing how well shutters work, considering the difficulty of the job.

Each full shutter speed is separated by one stop from the shutter speed on either side. So if you have a one second shutter speed the speed with one stop less light will be 1/2 second and the speed with one stop more light will be 2 seconds. That makes sense, since as I said last week one stop more light is double the amount of light you had and one stop less is half the amount of light you had. The same principal applies at the higher shutter speeds: one stop more light than a 1/250th of a second is a 1/125th of a second, and one stop less light is 1/500th of a second. Modern shutters also have 1/2 stop intervals so you would actually see speeds in this order 1/125, 1/180, 1/250, 1/350, 1/500. There might be an advantage in memorizing these numbers, but I don’t know what it would be. It is pretty easy to double or halve a number is you need to know what the full stop change will be.

The change in the shutter speed affects the way the camera sees time. A short shutter speed, say a 1/250th of a second freezes time and a long shutter speed, say a second blurs time together. So for shooting sports you almost always want a high shutter speed to capture the action. If you are shooting waterfalls, you may want to shoot a different shutter speeds in order to get the look you want. Soon I’ll post information about hand holding shutter speeds.

Long shutter speed