# Lenses, Field of View and Depth of Field

Photography I · Photographic Language and the Taking of the Image · https://tryals.app/learn/photography-i/lenses-field-of-view-and-depth-of-field

## Focal Length and Angle of View

Focal length only has meaning relative to sensor size. Angle of view depends on both dimensions.

For a standard 24 × 36 mm frame with a 43 mm diagonal:

| Focal length | Diagonal angle of view on 24 × 36 mm |
|---|---|
| 20 mm | about 94 degrees |
| 28 mm | about 75 degrees |
| 50 mm | about 47 degrees |
| 135 mm | about 18 degrees |

A **normal** lens has a focal length close to the frame diagonal. This is why 50 mm is standard on 35 mm film.

### Depth of Field

Only one plane is truly in focus. Points outside it blur into discs. They appear sharp while smaller than the **circle of confusion** (typically 0.03 mm for 35 mm format).

Four factors control depth of field:

* **Aperture.** Smaller apertures increase depth.
* **Focal length.** Shorter lenses yield greater depth.
* **Subject distance.** Greater distance increases depth.
* **How big the print is.** Larger prints demand tighter tolerances.

### Hyperfocal Distance

Focus at the hyperfocal distance to keep everything sharp from half that distance to infinity.

Hyperfocal distances (0.03 mm circle of confusion, 24 × 36 mm frame):

| Focal length | f/5.6 | f/8 | f/11 | f/16 |
|---|---|---|---|---|
| 28 mm | 4.7 m | 3.3 m | 2.4 m | 1.7 m |
| 50 mm | 14.9 m | 10.5 m | 7.6 m | 5.3 m |
| 100 mm | 59.6 m | 41.8 m | 30.4 m | 20.9 m |

For instance, set a 28 mm lens to f/11 and Focus at 2.4 m. Everything from 1.2 m to infinity remains acceptably sharp.

> **Common pitfall:** believing longer lenses inherently possess less depth of field. At matched image scale and aperture, a 200 mm lens matches a 50 mm lens.

## Practice questions

7 of this lesson's 14 practice questions, with answers. The full set is in the app.

### 1. A landscape photographer with a 35 mm lens wants everything from a few metres away to the horizon acceptably sharp. How does the hyperfocal distance tell them where to focus?

A. Focusing at the hyperfocal distance keeps only objects beyond that distance acceptably sharp
B. It is the closest focus distance at which the lens eliminates optical diffraction completely
C. Focusing at the hyperfocal distance centres depth of field equally before and behind the subject
D. Focusing at the hyperfocal distance keeps half that distance to infinity acceptably sharp

**Answer:** D. Focusing at the hyperfocal distance keeps half that distance to infinity acceptably sharp

**Why:** Focusing at the hyperfocal distance is the setting that reaches infinity while giving away as little near sharpness as possible: everything from half that distance outward is inside the circle of confusion. Focusing at infinity instead throws away the entire near half of the available range for no gain at all.

Page: https://tryals.app/practice/photography-i/lenses-field-of-view-and-depth-of-field/a-landscape-photographer-with-a-35-mm-lens-wants-everything-from-a

### 2. A 35 mm lens is set to f/8 on the 24 by 36 mm format, with a circle of confusion of 0.03 mm. What is its hyperfocal distance, in metres, to one decimal place?

**Answer:** 5.1 (within ±0.3)

**Why:** **About 5.1 metres.** 1225 divided by 0.24 is 5104 mm, and adding the focal length gives 5139 mm. Focused there, everything from about 2.6 metres to infinity is sharp by the 0.03 mm convention, which is why a 35 mm lens at f/8 was the standard pre-set for reportage.

Page: https://tryals.app/practice/photography-i/lenses-field-of-view-and-depth-of-field/a-35-mm-lens-is-set-to-f-8-on-the-24-by-36-mm-format-with-a-circle

### 3. Which of these increase the depth of field in a photograph?

A. Closing the aperture from f/4 to f/11
B. Making a much larger print from the same negative
C. Stepping back from the subject
D. Changing from an 85 mm lens to a 28 mm lens at the same aperture and distance

**Answer:** A. Closing the aperture from f/4 to f/11; C. Stepping back from the subject; D. Changing from an 85 mm lens to a 28 mm lens at the same aperture and distance

**Why:** The fourth goes the other way and is the one people forget. Enlarging more magnifies the discs of confusion along with everything else, so the acceptable disc on the negative shrinks and the depth of field shrinks with it. Depth of field is a property of the viewing as much as of the taking.

Page: https://tryals.app/practice/photography-i/lenses-field-of-view-and-depth-of-field/which-of-these-increase-the-depth-of-field-in-a-photograph

### 4. Sort each focal length by the class it belongs to on the 24 by 36 mm frame.

**Answer:**

- Wide-angle: 20 mm, 28 mm
- Normal: 50 mm, 45 mm
- Long: 85 mm, 200 mm

**Why:** The classes are defined against the frame diagonal, which is why the same 50 mm lens is normal on 35 mm film, distinctly wide on 6 × 6 and extremely wide on 4 × 5 inch. A focal length has no character of its own until a format is named beside it.

Page: https://tryals.app/practice/photography-i/lenses-field-of-view-and-depth-of-field/sort-each-focal-length-by-the-class-it-belongs-to-on-the-24-by-36-mm

### 5. Arrange the steps of pre-setting a lens for street work by the hyperfocal method in the order they are carried out.

**Answer:**

1. Decide the nearest distance that must come out sharp
2. Double it, since the near limit lies at half the hyperfocal distance
3. Find the aperture whose hyperfocal distance for this lens matches that figure
4. Set the focus ring to that hyperfocal distance and leave it there
5. Check that the shutter speed still available at that aperture is usable

**Why:** The last step is where the method usually fails. A near limit of one metre on a 35 mm lens demands a very small aperture, and in dull light the shutter speed that goes with it is too long to hold, so the honest answer is often to accept a nearer limit of two metres rather than to pretend the exposure works.

Page: https://tryals.app/practice/photography-i/lenses-field-of-view-and-depth-of-field/arrange-the-steps-of-pre-setting-a-lens-for-street-work-by-the

### 6. A lens is described as 'normal' relative to the format diagonal, not because of an absolute optical constant. If a 50 mm optic is transferred from a 24 × 36 mm film body to a much smaller sensor, what follows for its role?

A. It widens its field of view to capture more of the overall scene
B. It retains standard perspective because its focal length is fixed
C. It functions as a telephoto optic due to a narrower angle of view
D. It increases total depth of field across the reduced active frame

**Answer:** C. It functions as a telephoto optic due to a narrower angle of view

**Why:** A focal length is only 'normal' when close to the format diagonal (~43 mm for 35 mm film). On smaller sensors, that same focal length covers a narrower angle of view, acting as a telephoto lens rather than a normal or wide lens.

Page: https://tryals.app/practice/photography-i/lenses-field-of-view-and-depth-of-field/a-lens-is-described-as-normal-relative-to-the-format-diagonal-not

### 7. The 24 by 36 mm frame is the reference for calling a 50 mm lens normal. What is the diagonal of that frame, in millimetres, to the nearest whole number?

**Answer:** 43 (within ±1)

**Why:** **43 mm.** The square root of 576 plus 1296 is 43.27. A normal lens is conventionally one whose focal length is near the diagonal, which puts 50 mm slightly on the long side of normal, the reason a 45 mm lens looks more like unaided vision than the standard 50 does.

Page: https://tryals.app/practice/photography-i/lenses-field-of-view-and-depth-of-field/the-24-by-36-mm-frame-is-the-reference-for-calling-a-50-mm-lens