Quiz

From one camera position on the same sensor, what does a longer focal length do?

It records a narrower angle. Perspective changes when the camera position changes.

You put the same 50 mm lens on an APS-C camera. What happens to the lens's focal length?

Crop factor changes the recorded field of view, not the lens's focal length.

A 50 mm lens set to f/2 has an entrance pupil about how wide?

F-number = focal length ÷ entrance pupil diameter, so 50 ÷ 2 = 25 mm.

What does optical stabilisation primarily correct?

It shifts a lens group to counter camera shake; moving subjects still need a suitable shutter speed.

To photograph a tall building with straight verticals using a shift lens, what should you do?

Shift moves the frame within a larger image circle while the camera stays level.

A lens focuses close, but you want a tiny insect large on the sensor. Which specification should you check?

Maximum magnification tells you how large the subject projects onto the sensor; 1:1 means life size on the sensor.

What is the main difference between a fisheye and a rectilinear ultra-wide lens?

Both can cover a wide view, but their projections render straight lines differently.

You open the aperture from f/4 to f/2. What happens to the background blur disk?

Twice the opening diameter spreads out-of-focus rays twice as far, so the blur disk is about twice as big.

Starting from 50 mm, f/4 and a person 2 m away, which single change blurred the background most in the chapter's example?

Halving the distance made the blur disk 2.7× bigger, more than opening up two stops (2×).

What is the circle of confusion?

Anything that lands on the sensor as a circle smaller than that limit looks sharp. This book uses 0.03 mm for full frame.

At f/2.8 the back row of your group is soft. What's the best first fix?

Stopping down widens the sharp zone. With a 35 mm lens at 4 m, f/5.6 covers 2.6 to 8.8 m.

You stop down from f/1.8 to f/5.6 and background lights turn into hexagons. Why?

A blur disk is an image of the aperture. Wide open it's round; stopped down, the blades make a polygon. Rounded blades help.

At f/1.4 you focus on the eye with the centre point, then turn the camera to recompose. What goes wrong?

At 50 mm, f/1.4 and 1 m, only about 3 cm is sharp. Move the focus point onto the eye or use eye detection instead.

Near the edges of the frame, bokeh disks look like cat's eyes. What causes it?

Light reaching the corners passes the lens at an angle and the barrel clips one side of the disk.

A 50 mm lens on a Micro Four Thirds camera frames like which lens on full frame?

Multiply by the crop factor: 50 × 2 = 100 mm.

Does a 50 mm lens become a 75 mm lens on APS-C?

The lens doesn't change. The smaller sensor records only the middle, so it frames like a 75 mm lens would on full frame.

Which full-frame setup gives the same framing and background blur as 33 mm f/1.8 on APS-C (1.5×)?

Multiply both the focal length and the f-number by the crop factor: 33 × 1.5 ≈ 50, 1.8 × 1.5 = 2.7.

APS-C at f/1.8 and full frame at f/1.8, same shutter speed and ISO. How does the brightness compare?

Exposure follows the f-number. Equivalence is about framing and depth of field, not exposure settings.

Why does a smaller sensor need a smaller sharpness limit (c)?

More enlargement means the blur on the sensor must be smaller to look equally sharp: about 0.03 mm ÷ crop factor.

A phone's main camera: 6.9 mm at f/1.8 with a 3.5× crop. What does its background blur look like on full frame?

1.8 × 3.5 ≈ 6.3. At normal distances almost everything is sharp, which is why phones fake the blur in software.

Which is a giveaway that background blur was faked by portrait mode?

Hair, glasses and gaps between fingers are hard to separate, so software blur often looks cut out there. Real blur also changes gradually with distance.

The hyperfocal distance is 10 m and you focus there. What looks sharp?

Focused at H, everything from H/2 to infinity is acceptably sharp.

A 50 mm lens at f/8 on full frame, with c = 0.03 mm. What's the hyperfocal distance?

50² ÷ (8 × 0.03) + 50 = 2500 ÷ 0.24 + 50 ≈ 10,467 mm ≈ 10.5 m.

You stop down from f/8 to f/16. What happens to H?

f/8 to f/16 is two stops, and N is in the denominator, so H roughly halves.

You zoom from 24 mm to 48 mm at the same f-number. Roughly what happens to H?

The focal length is squared in the formula, so doubling it moves H about four times farther.

Where is the sharpest point when you focus at the hyperfocal distance?

The sharpest point is always the focus point. Hyperfocal gives the deepest sharp zone that still includes infinity.

You focus at infinity instead of at H. What do you lose?

Focused at infinity, the near limit sits at about H instead of H/2.

You're not sure of the exact distance. Which way should you err?

Short of H, infinity turns soft. A little beyond costs only a little near sharpness.

A flower 35 cm from a 24 mm lens and mountains on the horizon both need to be sharp. Best approach?

At f/8, H/2 is about 1.2 m. f/22 gets closer but diffraction softens everything. Stacking keeps the sharpest aperture.

Why can't even a perfect lens focus a point of light to a point?

Diffraction spreads light passing through an opening. The smaller the opening, the more it spreads.

What does the size of the Airy disk depend on?

Airy disk ≈ 2.44 × wavelength × f-number, whatever the lens or camera.

About how large is the Airy disk at f/16 in green light?

1.34 µm per f-number × 16 ≈ 21.5 µm.

Viewed at 100%, which camera shows diffraction softness at a wider aperture?

45 MP on the same sensor size means smaller pixels, so the Airy disk spans two pixels sooner.

For equally framed pictures, Micro Four Thirds at f/8 shows about as much diffraction as full frame at…

The smaller sensor is enlarged twice as much, so multiply by the crop factor: 8 × 2 = 16.

"f/22 gives the sharpest landscape." Is that right?

Compare the whole picture at the size you'll use. f/8 to f/11 at hyperfocal covers most landscapes.

Where should you start looking for your lens's sharpest aperture?

Aberrations can blur wide open and diffraction takes over when stopped far down. Test your own lens; 2 to 3 stops down is a common starting point.