How to Select a Lens for Your Camera Part 3: Depth of Field and Cameras with Small Sensors

To loop back around to our early observations in Part 1 of this series, cameras with small sensors, even professional models that create beautiful images, have a difficult time of creating shallow Depth of Field. To illustrate, let's take a look at a professional camera.

This is the JVC GY-HM170. It is a camcorder that has a flexible lens system like our SLRs, so it qualifies for this exercise. The big difference is that it has a 1/3-inch sensor, rather than the 4/3-inch or full frame sensors that we've previously explored. The 1/3-inch sensor has a 4.8x crop factor when compared to 35mm, whereas the Micro 4/3 had a 2x crop factor. To put this into perspective, look at the drawing below:

Let's return to our discussion about field of view. Remember how we can increase our distance to increase the field of view (viewing angle)? And, do you remember what happens to depth of field the farther away from the subject the camera is moved? It compresses. And what was our observation about the image from small sensor cameras?

Compressed depth of field.

This is the wrinkle of sensor size on depth of field. Like the Angle of View, the Depth of Field is affected by the size of the sensor. 

In the case of this JVC camera, the computation looks something like this:

50mm lens (reference)

Aperture f/1.4

Crop factor 4.8

50mm / 4.8 = 10.4mm. There are 10mm and 11mm lenses out there that will fit the 1/3 inch sensor, so the correct Field of View isn't hard to achieve. However, opening the aperture from f/1.4 by nearly 5 stops isn't possible. The lowest possible f-number is 0.7, and I don't think I've seen more than 3 of these floating around. From a practical standpoint, the fastest available lens in this format will be f/0.95. This is one stop faster/more open than f/1.4.  Therefore, from a practical standpoint, the lowest f-number in 35mm that can be reproduced by a 1/3 inch camera is f/5.6. So, f/0.95 on a 1/3 inch camera is approximately the same as f/5.6 on the 35mm camera. This can be calculated by simply counting five stops down from f/0.95:

0.95    1.4     2     2.8     4     5.6     8     11     16     22

An f/stop calculator can be found here.

Let's use the reference 50mm lens on a 35mm camera combination to visualize how shallow a depth of field the 1/3 inch camera is capable of, regardless of field of view. Since we are able to achieve f/0.95 on a 1/3-inch camera, we need to count down 5 stops on that 50mm lens. F/5.6. There isn't a lot of focal separation there. Since we know that the shallowest focal area occurs when the subject is closest to the camera, let's look at another chart to see what we can achieve:

1/3" Sensor 10mm Lens 1.5-foot Subject Distance

The chart indicates that we have focal separation of 1.8 inches. That's basically 3x the depth of our first experiment with the GH2. So, shallow Depth of Field is achievable with the small sensor camera, but the requirements to achieve the effect are much more demanding.

With these concepts and tools, you should be able to determine which lens to use to achieve the look you want, or even to determine whether or not the look is possible.

Key references that should help you with your calculations:
Crop Factor Calculator
Depth of Field Calculator
F/stop Calculator

I welcome your comments and questions.

How to Select a Lens for Your Camera Part 2: Understanding f/stop, Depth of Field, and Aperture

How to Select a Lens for Your Camera Part 2: Understanding f/stop, Depth of Field, and Aperture


In our last lesson... 
we explored Field of View. By now, if you see a shot from a 35mm camera and you know the lens, you should be able to determine what kind of lens will give you the same field of view on your camera. Do you still have the note card with your camera's 50mm equivalent lens written on it? If so, you're going to need it again here. If not, it's a good time to refresh your memory on how to make that field of view calculation.

I'm going to now switch from the 5D MkII to the Panasonic GH2 for our test bed. This is simply because the GH2 is the camera I have at my disposal.

F/stop, Depth of Field, and Aperture

The actual part of the camera we are going to explore is the Aperture Ring, which is typically at the rear of the lens, and is marked in f/stops.

The f/stop scale looks something like this:

1.4     2     2.8     4     5.6     8     11     16     22     

In this example, f/1.4 is the widest that the aperture can open, lets in the most light, and provides the shallowest area of critical focus.

F/22 is the smallest that the aperture will close on this lens. It lets in the least amount of light, and provides the deepest area of critical focus.

Each full stop up the range results in a reduction of light by half. So, f/2 let in half the amount of light as  f/1.4. F/22 let in 1/256 the light as f/1.4.

Right now, this might seem like just a bit of math. So, we're going to look at how it can be applied.

When we see close up shots of a character in a movie, the background is often blurred. This helps us mentally and emotionally separate the character from the surroundings. Not only does it "feel" like how a movie should feel, it is a strong storytelling tool. This is also a contributing factor to why most video has a "non filmic" look and doesn't draw us in as readily as film. Due to the small sensor on the camera and the limited adjustability of the lens, everything in the shot is in focus. There is no focal separation.

This image is taken from a GH2 with a 50mm lens at f/1.4. By now, hopefully you will recognize that this is equivalent to 100mm on a full frame camera. In the following shots, notice the labels on the tags…they indicate the distance of the tags from the camera, and will help demonstrate the limits of the depth of field at f/1.4, which, as I've already mentioned, represents the aperture at its most open position.

Notice that as I focus on objects further away, more of the focusable area becomes longer.

Finally, at the end of our run at 3.5 feet away, enough detail of the 3 foot mark is visible for us to make out its shape.

The tighter objects are clustered in terms of depth, and the further away from the camera they are, the sharper they will be in focus. This is because, the closer to the camera an object is, the shorter the area of sharp focus will be. The farther away an object is, the deeper the area of critical focus is.

The graphics below further illustrate this point. Look how the subject distance, which is the number to the upper right of each of these graphics, affects the total depth of field, which is the area between the two vertical lines in the illustration at the bottom of each of the graphics:

This demonstrates why, as focus is adjusted toward infinity, all of the markers are visible, whereas in the first photo, focused at 1.5 feet, only the first tag was discernable. 

The following images demonstrate the same focal points, but at a different f/stop. I've adjusted the aperture ring to f/4, which closes down the aperture by 3 full stops.  With the aperture closed to f/4.0, notice how the near and far extremes of the focal depth is affected:

So, to better illustrate the point, look at the difference between f/1.4 and f/4.0 at the same focal length:

To further explore this concept, you can practice with this Depth of Field calculator.

So....

Objects closer to the camera have greater focal separation. 

Objects farther from the camera have little focal separation.

Increasing the f/stop decreases focal separation.

In keeping with our understanding that we need to double our distance to subject  or use wider lens, we also need to open up the iris twice as much (one stop). The next stop down from f/1.4 is f/0.95. That's a big piece of glass!

Voigtlander Nokton f/0.95 25mm Lens

With these concepts and calculators, you should be able to determine which lens on your camera will best accomplish a specific 35mm look. However, there is a deeper level to this f/stop business, and it becomes visible as we get into cameras with smaller sensors. We will cover this topic briefly in our next lesson.

Lens Selection Part 3: Depth of Field and Cameras with Small Sensors

How to Select a Lens for Your Camera Part 1: Field of View

Field of View

Field of view is the area that the camera sees. If you were to draw a line from the center of the camera lens to each of the edges of your photo, you would have the outline of a cone. That cone is your field of view.

To understand the impact of sensor size on field of view, look at the full frame image below that was taken with an 85mm lens at f/1.2:

Now, if we were to set up a GH2 (or similar camera with a 4/3-inch sensor) with the same lens and aperture setting as the full frame camera, its field of view would be in the area defined by the red box:

So, the GH2 image would look like this:

If we were used to taking photos with a 35mm camera, this image is framed much tighter than we would expect from this lens. The cause of this narrowing effect is what we call crop factor.

Crop Factor

Since the 4/3” sensor is about 18mm wide, and the full frame sensor is 36mm wide, the M43 sensor is capturing a field of view that is ½ that of the full frame sensor. Therefore, our 85mm lens visually becomes the equivalent of 170mm on the GH2.  This is important to remember. The narrower the field of view, the higher the focal length. The GH2, with it's sensor size half as wide as a 35mm camera, ends up with an image equivalent that is twice as long. Therefore, if we want to capture the same field of view on the GH2 as we did on the 5D MkII, we would need to use a lens that is half the focal length of a lens on a 35mm camera, which leaves us with the focal length of 42.5mm.

The key takeaway here is to know the crop factor of your camera, which is easy to find online. Here is one such calculator.

To summarize, to match the field of view of a 35mm camera, such as the 5D MkII, you must divide the focal length by 2. That will be the desired focal length in Micro 4/3. See the table below to illustrate this idea:

35mm to GH2 Lens Conversion Table

If on a 35mm camera you use…	…on a GH2, you should use
25mm	12mm
35mm	17.5mm
50mm	25mm
85mm	42.5mm
100mm	50mm

Now that we understand the basic mechanics of Field of View, and how to select a lens that matches the FoV characteristics of our 35mm reference lens, it's time to move on to Depth of Field.

Activity
Go online and research your camera. Find the sensor size and use the calculator located here. Write down your crop factor value. Also jot down the lens that would match the angle of view of a 50mm lens on a 35mm camera. This will aid you in your future lens selection exercises:

Reference focal length (Fr)
Crop factor (c)
Target focal length (Ft)
Fr / c = Ft

Next, we will explore the impact of Depth of Field.

How to Select a Lens for Your Camera Part 2: Understanding f/stop, Depth of Field, and Aperture

How to Select a Lens for Your Camera: Introduction

Important note: Since these lessons discuss the attributes of the 35mm look, we I will use the terms "35mm," "full frame," and "5D Mk II" interchangeably. Each of these reference the 35mm format, or a camera that uses this format.

The Film Look

A topic that keeps coming up in forums and blogs, and was the motivator of the entire 35mm adapter cottage industry, is the "look" of 35mm film. This has been a holy grail of composition for at least a decade. For the last few years, this has been attainable with the Canon 5D Mark II camera and its full frame sensor.

However, filmmakers are finding other tools that suit their needs in different ways, such as the flexibility of the Canon 7D, or the image fidelity of the Panasonic GH2. Neither of these cameras have a full frame sensor. In the choice between the "35mm look" and the capabilities of the camera, something has to give.

That “something” is the combination of Field of View and Depth of Field.

Next section:
How to Select a Lens for Your Camera Part 1: Field of View