Choosing a focal length for interiors
Two lenses cover about 95 per cent of Barbara Brown's interior work: a 24mm shift lens and a 35mm prime, both on full frame. The pair is not a compromise between wide and standard. A 24mm lens covers 73.7 degrees horizontally and a 35mm lens covers 54.4 degrees, and the 19.3 degree difference changes both the working distance and how large the foreground reads against the back wall. The equivalent pair on APS-C at 1.5x is 16mm and 23mm; on Micro Four Thirds at 2x the pair is 12mm and 17.5mm.
Does a wide lens stretch perspective, or does the tripod position?
Perspective comes from camera position alone, and focal length only crops. A 24mm frame and a 35mm frame shot from the same tripod position show identical near to far relationships, and cropping the 24mm file to the 35mm angle of view yields the same picture at lower resolution: 45.1 megapixels becomes 21.2 megapixels, or 5620 by 3770 pixels.
Walking backwards is what actually happens on a job, which is why the two focal lengths feel different. Framing the full width of a 4.0m wall needs the camera at 2.67m with a 24mm lens and at 3.89m with a 35mm lens, because the required distance equals the wall width times the focal length divided by the 36mm sensor width.
Those two distances rewrite the size relationships in the room. A sofa arm standing 1.5m in front of the back wall sits 1.17m from a 24mm camera and 2.39m from a 35mm camera. The distance ratio to the wall is 2.28 at 24mm and 1.63 at 35mm, so the sofa arm renders 40 per cent larger against the wall in the 24mm frame. The lens distorted nothing; the tripod moved 1.22m.
What angle of view do 24mm and 35mm cover in a real room?
A 24mm lens on full frame covers 73.7 degrees horizontally and 53.1 degrees vertically, while a 35mm lens covers 54.4 degrees and 37.8 degrees. In a 4.2m by 3.6m living room a corner mounted 24mm takes in both the window wall and the fireplace wall, which is more information than most viewers read at once.
The same corner at 35mm shows the fireplace wall plus a third of the window wall, and the frame gains a subject. Vertical coverage follows the same ratio: a 24mm lens reaches 1.25m up a wall standing 1.25m away, while a 35mm lens reaches 0.86m from that spot.
| Focal length | APS-C equivalent | Horizontal angle | Distance to frame a 4.0m wall | Corner elongation |
|---|---|---|---|---|
| 16mm | 11mm | 96.7 degrees | 1.78m | 68 per cent |
| 21mm | 14mm | 81.2 degrees | 2.33m | 44 per cent |
| 24mm | 16mm | 73.7 degrees | 2.67m | 35 per cent |
| 28mm | 19mm | 65.5 degrees | 3.11m | 26 per cent |
| 35mm | 23mm | 54.4 degrees | 3.89m | 18 per cent |
| 50mm | 33mm | 39.6 degrees | 5.56m | 9 per cent |
Barbara Brown works to one distance rule after several hundred rooms: less than 3m of clear floor behind the tripod calls for 24mm, and 3.5m or more calls for 35mm. A typical Bristol bathroom offers 2.2m of run, which forces 24mm on almost every bathroom frame, and a hallway rarely offers more than 1.5m.
Below 21mm a room photograph starts to lie. A 16mm lens covers 96.7 degrees and needs only 1.78m to hold a 4.0m wall, so a 16mm frame in a 3.0m bedroom renders a 1.4m double bed as a narrow slab against a distant wall. Barbara Brown owns a 17mm and fits the 17mm perhaps twice a year, for a stairwell or a plant room where no other lens reaches.
How much does a rectilinear lens stretch objects at the frame edge?
Rectilinear projection holds straight lines straight by stretching everything away from the optical axis. An object at field angle theta images with its tangential dimension multiplied by 1 divided by cosine theta, and its radial dimension by 1 divided by cosine squared theta, so a sphere becomes an ellipse with an aspect ratio of 1 over cos theta.
The corner of a 24mm full frame image sits at a field angle of 42.0 degrees, where 1 divided by cos 42.0 equals 1.35. A globe pendant at that corner therefore images 35 per cent longer on one axis than the same pendant on the optical axis. The corner of a 35mm frame sits at 31.7 degrees, giving 18 per cent, and the corner of a 16mm frame sits at 53.5 degrees, giving 68 per cent.
Round objects and human faces show the stretch first. A drum lampshade near the corner of a 24mm frame reads as an oval, a globe pendant reads as an egg, and a 26cm dinner plate reads as a wide ellipse. Flat rectangular objects such as cupboard doors, rugs and picture frames stretch by the same percentages without looking wrong, because a rectangle stretched along one axis is still a rectangle.
Composition fixes the stretch faster than software. Keeping spheres and faces inside the middle 70 per cent of a 24mm frame caps the field angle at 32.3 degrees and the worst elongation at 18 per cent, down from 35 per cent at the corner. Moving a lampshade from 1.8m off centre to 1.4m off centre in a frame shot from 2.67m takes five seconds and cuts the elongation from 21 per cent to 13 per cent.
Which focal length shows a room at its true size?
A 35mm lens on full frame reports room proportions most honestly, because a 35mm frame places the foreground only 1.63 times nearer the camera than the back wall against 2.28 times at 24mm. Estate agency briefs push the other way, since a 96.7 degree frame appears to show more property than a 54.4 degree frame of the same room.
The exaggeration lives in the near to far ratio rather than in the wall measurements. A 24mm frame measures a 3.6m bedroom wall as 3.6m, correctly, while rendering the bed in the foreground 40 per cent larger against that wall than a 35mm frame would. Buyers who viewed the property then read the whole set of eight images as unreliable.
Furniture makers want the opposite treatment. A 35mm frame keeps a 2.1m sofa reading as a 2.1m sofa, with arm thickness and 60cm cushion depth in proportion. For a product led shoot Barbara Brown goes to 50mm, which covers 39.6 degrees and needs 5.56m of run, and accepts that under half the room appears in the frame.
What aperture and shutter speed suit each focal length?
Interior frames run at ISO 100 on a tripod with shutter speeds between 1/8 second and 4 seconds, and the aperture is chosen for depth of field rather than for sharpness. A 24mm lens at f/11 focused at 1.8m holds everything from 0.9m to infinity acceptably sharp at a 0.030mm circle of confusion.
| Lens and aperture | Focus distance | Near limit | Far limit | Shutter at EV 7 |
|---|---|---|---|---|
| 24mm at f/8 | 2.4m | 1.2m | infinity | 1/2 second |
| 24mm at f/11 | 1.8m | 0.9m | infinity | 1 second |
| 35mm at f/8 | 3.5m | 2.1m | 11.2m | 1/2 second |
| 35mm at f/11 | 3.7m | 1.9m | infinity | 1 second |
| 50mm at f/11 | 4.0m | 2.6m | 8.5m | 1 second |
Stopping down past f/11 buys depth of field and pays in acuity. The Airy disk measures 2.44 times the wavelength times the f number, so at 550 nanometres a disk spans 10.7 micrometres at f/8 and 14.8 micrometres at f/11, against a 4.4 micrometre pixel pitch on a 45 megapixel full frame sensor. Barbara Brown stops at f/13 and never sets f/16 for a room frame.
Two focal lengths, one levelled tripod and a shift movement of 8mm cover the great majority of interior briefs. The remaining frames want a longer lens for details rather than a wider one for rooms. Reading the light, described in the note on overcast daylight, decides more of the final file than any choice on this list.