Student’s Guide
Choosing a Camera Cell Phone or Reflex Camera
Olivier Tardif-Paradis
Mathieu Riopel
Cégep Garneau
Photo : Zack Vega.
Photo : Nicolas Esposito.
APP/Guide de l’élève
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Choosing a Camera
Background
Since its invention, the smart phone has been playing a larger
and larger role in our lives. The functions of these devices and
the efficiency of their electronic components are evolving every
year. Now almost all these phones have a camera, an agenda, a
GPS, a gaming console and more. This new technology has such
an impact on our lives that many people would have trouble
getting through a day without it.
But don’t we overestimate the capacity of some phone functions?
For example, lots of people use their smart phones to capture the
magic moments of a trip or special occasion. It’s so handy! The
device is small, light and pretty robust, and it lets you share the
photos easily as soon as they’re taken. So why do professional
photographers bother to buy high-priced, more cumbersome, less
versatile cameras to take photos? Why pay more for what seems,
at first glance, to offer less? Are people comparing the same
functions? Is the quality of the photos taken by these two types of
devices comparable?
This is the question we are asking: When it comes to taking photos, is the price difference between a
professional camera and a cell phone worth it? To make an informed decision on this, you will analyse
and compare the physical features of a professional camera, the Canon EOS, with those of a smart
phone, the iPhone 4s.
Since a camera is a tool with a complex operation that draws on a panoply of physics principles, we
suggest that you focus on certain specific phenomena. Analysing these principles may help you not only
understand this subject more clearly, but also take better photos!
Table 1: Characteristics of the Canon EOS 5D and the iPhone 4s
Name of device
Canon EOS 5D
iPhone 4s
Size of photos
12.8 megapixels
8 megapixels
Pixel diameter
8.2 µm
1.75 µm
Weight
810 g without lens
140 g
Price
$3000 in 2005
$450 in 2013
f-number
ƒ/1.4
ƒ/2.4
Focal distance
70 mm
3.85 mm
Lens coating
MgF2
No
Polarizing filter
Yes
No
Infrared filter
Yes
Yes
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That Magenta Hue
Have you ever noticed that a camera lens can give off magenta reflections, even if the ambient light is
white? Why magenta, specifically? It is true that the lens has been coated with a very thin coat of
magnesium fluoride (MgF2). You might therefore think that this
coating is magenta. But MgF2 is transparent and completely
colourless! Why bother to coat a camera lens with a transparent
coating?
Could there be some connection between the magenta colour of the
reflections and the transparent coating of MgF2? Spreading a layer of
MgF2 must be done carefully and it increases production costs. Why
take so much trouble to add this thin coating to the glass of the lens?
The manufacturers say that thanks to the MgF2 antireflective coating,
the transmission of light is excellent. What does that mean? What is
the connection between the colour magenta and improved light
transmission?
Photo : Rainer Knäpper.
License : artlibre.
Here are a few questions that will help you understand the physics behind thin antireflective coatings.
1) How do antireflective coatings reduce the quantity of light reflected by a camera lens? At the same
time, explain the connection between the phenomenon of interference and antireflective coatings.
2) Find the criteria that influence the total phase difference between the ray reflected on the surface of
the coating and the ray reflected on the surface of the lens. Do all light reflections on any interface
always have the same effect on the phase of the reflected wave in comparison to the incident wave?
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3) Why do we talk about antireflective coatings when the point is for the camera to optimize the
transmission of light? What is the connection between an antireflective film and the transmission of
light?
Did You Know?
The human eye is generally most sensitive to rays with a wavelength in the vicinity of 555 nm, which
corresponds to the colour green.1
4) Why is the reflection from lenses treated with antireflective coating magenta?
5) Determine the thickness of the thinnest possible antireflective layer of MgF 2 on a camera lens.
1
http://fr.wikipedia.org/wiki/Spectre_visible#cite_note-CIE-1
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Is Smaller Always Better?
The current trend in technology is to constantly reduce the size of
the objects we use, to make them more portable. What about in
photography? Is it beneficial to reduce the size of a camera lens?
It is true that a bigger lens allows more light to enter the optical
pickup. This is useful if you want photos with a very short exposure
time. Could there be other advantages to having a big lens?
How does the size of the lens affect the quality of the image?
Photo : Carlos Delgado.
1) A community of amateur photographers created a very interesting website that offers very accessible
explanations for a number of physics concepts related to the clarity of images and the size of camera
lenses.
http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm
Enter the parameters of the Canon EOS 5D (see appendix) and do the following test:
 VISUAL EXAMPLE : APERTURE VS. PIXEL SIZE;
 Test different sizes of lenses.
What is the effect of the pixel size on the size of the Airy disk?
2) Describe the connection between an Airy disk and the Rayleigh criterion. Hint: In your explanation,
refer to the phenomenon of diffraction.
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3) An important setting in photography is the f-number (N). What parameters does its value depend on?
Hint: Look for information in Wikipedia.
4) Using the focal distance of the lens and the f-number of the camera provided in Table 1, calculate
the biggest possible aperture opening for the Canon EOS 5D and the iPhone 4s.
Did You Know?
“The diameter of a hair changes based on age, genetic factors and ethnic origin, ranging from 40 to
100 micrometres, or 0.04 to 0.1 mm.”2
5) To what distance would we be able to distinguish a hair 100 micrometres in diameter in photos taken
by a Canon EOS 5D and an iPhone 4s?
2
http://fr.wikipedia.org/wiki/Cheveu
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What Exactly Is It Filtering?
It might seem, therefore, that photo filters
are no longer really very useful. This is not
the opinion of professional photographers,
though. It seems that some filters create
effects that could never be produced by
software. These filters influence the very
nature of the light by changing it before it
strikes the detector.
Photo : L. Briho.
Some photo filters placed in front of a
camera lens can create interesting visual
effects. Today, however, we can create lots
of effects by using software to touch up
photos after they are taken.
With polarizing filter
Without polarizing filter
There are several types of filters, but how do they work? What do they filter? Here are some questions
that will help you understand the physics behind photographic filters.
1) How does a polarizing filter create photos showing skies that are bluer than reality? Your answer
should include a few lines of text and a diagram.
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2) How does a polarizing filter create photos with no reflection from snow, glass or water? Your answer
should include a few lines of text and a diagram.
3) The iPhone 4s has an infrared filter. Without this filter, the camera would be able to detect infrared
light, like the light emitted by a TV remote control.
Did You Know?
Here is a fun test to check whether your camera can detect infrared light:
In the dark, take a photo or video of the light emitted by your TV remote (at one end of the device). You
will have to press on a command button for the light to come on. If the camera can detect infrared light,
you should see a white or pinkish light on your camera screen.
a) Does an infrared filter function on the same principle as a polarizing filter?
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b) If not, explain how an infrared filter works. Your answer should include a few lines of text and a
diagram.
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Three-step Cycle
List all the relevant information you gathered when you read the problem. Based on this information, state
what you need to know to solve the problem. As you discover new information, you should summarize
and update the relevant information you have gathered and ask new questions.
List the Following:
What We Know
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To Determine
Summary
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Conclusion
Now it is time to answer the following question:
When it comes to taking photos, is the price difference between a professional camera and a cell
phone worth it?
In your answer, draw on the features of the Canon EOS 5D and the iPhone 4s and the physics concepts
you have learned. Present arguments defending your position by discussing the various things you
discovered in this activity. To help you make your decision, complete the table below.
Feature
Which of the
two cameras is
superior?
Why?
How much more
should the superior
camera cost for this
feature?
Weight
of device
f-number
Lens treatment
Use of filters
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