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Magazine Chapter 5: Q44E (page 1214)
A wildlife photographer uses a moderate telephoto lens of focal length 135 mm and maximum aperture f/4.00 to photograph a bear that is 11.5 m away. Assume the wavelength is 550 nm. (a) What is the width of the smallest feature on the bear that this lens can resolve if it is opened to its maximum aperture? (b) If, to gain depth of field, the photographer stops the lens down to f/22.0, what would be the width of the smallest resolvable feature on the bear?
Short Answer
- The smallest bear feature that the lens can resolve using the maximum aperture is
- The smallest bear feature that the lens can resolve using the smaller f/22.0 aperture is
Step by step solution
Limit of resolution
The resolving power of an optical instrument, also known as the limit of resolution of an object, is a property that quantifies the minimum angular separation of two objects that an instrument can resolve.
Here;
D is the aperture diameter
y is the minimum width resolved by the lens
s is the distance of the objects from the lens
is wavelength
Aperture
The aperture is a numerical description of the size of the opening in the lens through which light can pass.
Here;
f is the focal length
D is the aperture diameter
Width of the smallest feature
(a) Aperture diameter by rearranging the equation;
Limit of resolution;
by isolating variable y on one side of the expression, the expression for the minimum width that the lens can resolve, 
On putting the values;
Hence, the smallest bear feature that the lens can resolve using the maximum aperture is
The width of the smallest resolvable feature on the bear
- Lowering the aperture by f/22.0;
The value of the diameter of the camera鈥檚 aperture
Limit of resolution;
Therefore, the data is;
Hence, the smallest bear feature that the lens can resolve using the smaller f/22.0 aperture 
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