Prime or zoom?

Learning goal

You will be able to weigh the difference between prime and zoom lenses in terms of light, sharpness, size and way of working, and choose the one that suits your own work. You will explain that a prime lens has a single focal length and a zoom lens a range; you will read how the maximum aperture can change with focal length on zoom lenses; you will calculate how many stops of light a prime lens's wider aperture gains in a dim place; you will see how a prime lens forcing you to walk changes your way of looking; and you will know how in dim light this gain in light keeps the detail in the face on every skin.

The question

One evening, you are photographing an author's book signing in a dim bookstore. With the zoom lens in your hand you come closer to the author from far away; but as you extend the end of the lens, the camera raises the ISO and the photos come out grainy, with the author's face smoothed. The photographer next to you, on the other hand, comes a few steps closer to the author's table with a small lens that can't zoom, and takes much cleaner frames in the same light.

Why is a lens that can't zoom so loved? Next to a zoom that can build any frame by turning a ring, what advantage can a piece of glass whose frame doesn't change have? In this lesson we will compare prime and zoom lenses in terms of light, sharpness, portability and how they make the photographer work.

Context

We photographed the musician in the square once from inside the crowd and once from the café; each lens family put the viewer somewhere else. A zoom lens gathers several of these families in a single body: a 24–70 zoom stretches from wide angle to short telephoto. A prime lens, on the other hand, has only one focal length: such as 35, 50 or 85 millimeters.

This difference isn't only about convenience. A lens's internal design also decides how much light it can gather, how big and heavy it will be, and its price. Choosing a lens means making a trade-off between these values. Which trade-off is right is decided by what you photograph and your way of working.

Simple explanation

How does the aperture change on zoom lenses?

On many zoom lenses two aperture numbers are written, such as 18–55 f/3.5–5.6. This shows that the lens's maximum aperture changes with focal length; at the widest angle it can open to f/3.5, while at the longest focal length it can only open to f/5.6. Lenses like this are called variable-aperture zooms. When you turn the zoom ring and move to telephoto, you reduce the light entering the lens without noticing, and the camera makes up for it by raising the ISO or the time.

On some zoom lenses, on the other hand, a single aperture number is written, such as 24–70 f/2.8. These are constant-aperture zooms that keep the same maximum aperture across the whole range. They are usually bigger, heavier and more expensive, because bigger glass is needed to give a wide opening at the long focal length too.

Why can a prime lens be brighter?

A lens designed for a single focal length can have a simpler internal structure than a zoom that has to cover a range. This simplicity makes a wide maximum aperture possible in a smaller and cheaper body. Values such as f/2 or f/1.8 are common on prime lenses.

You can see with stop calculations how much light this difference brings. Each step from f/5.6 to f/4, from there to f/2.8 and to f/2, is one stop; between f/2 and f/5.6 there are three stops, that is, eight times the light. In the dim bookstore, in light where the zoom at f/5.6 asks for ISO 6400, an f/2 prime lens can shoot at ISO 800 with the same shutter speed. A wide aperture also narrows the slice of sharpness and softens the background more; this can be a gain, or a limitation that needs care in a group photo.

How does a prime lens make you work?

With a lens that can't zoom, you walk to change the frame: you come closer, move away, shift to one side. This small effort changes your way of looking too. With a zoom it is easy to turn the ring and build a frame from where you stand; with a prime lens, you have to decide where to stand in every frame. Many photographers say this necessity pushes them to walk around the scene more carefully and choose their place more consciously.

When you work with a single focal length for a while, you start to build the frame that lens will see in your eye before raising the camera. When approaching a scene on the street, knowing where the frame will stretch from and to lets you shoot without missing the moment. Don't forget, too, that changing the frame by walking doesn't give the same result as zooming: when you change place, the size of objects relative to each other changes too. We will talk about this in detail in the next lesson.

What does a zoom gain you, and how much does sharpness matter?

A zoom lens's biggest gain is flexibility. In situations where you can't get up from your place in a theater or a stand, where moments flow fast at a wedding or a news event, or where you have to make do with one lens on a trip, a zoom lets you go from wide angle to telephoto without changing lenses. Not changing lenses also reduces the chance of dust getting onto the sensor.

It is often said that prime lenses are usually sharper. A simple design can make this easier, but today's good zoom lenses can be very sharp too; the difference in sharpness between two lenses is often less important than sharpness falling in the right place, a steady hand and the light. In a dim place, on the other hand, a prime lens's gain in light shows directly: it means lower ISO, less smoothing to reduce grain, and more detail in the face. This gain especially makes it easier to keep detail in dark skin left in shadow; on light skin, too, it prevents the texture of the skin from being erased artificially.

Visual evidence

You go back to the bookstore. The author, a dark-skinned woman with gray hair, is signing books at a table; the only light is the yellow lamps on the ceiling. When you open the zoom to 70 millimeters and shoot from far away, the lens stays at f/5.6 and the camera goes up to ISO 6400. When you magnify it, the shaded side of her face is a blotchy gray surface; the strands of her hair have blended into each other.

You put on the 50-millimeter f/2 prime lens and come a few steps closer to the author's table. With the same shutter speed, you can shoot at ISO 800. This time, in the shaded part of her cheek, the warm brown of her skin and fine lines can be made out, and the knuckles of the fingers holding the pen and the ink on the book's page are sharp. The bookshelves in the background have softened with the wide aperture. The same evening, the same lamp; the difference was made by the light the lens gathered and the few steps you took.

Common mistakes

Summary

A prime lens has a single focal length, a zoom lens a range; on many zooms the maximum aperture gets smaller toward telephoto, while constant-aperture zooms are bigger and more expensive. Thanks to their simple structure, prime lenses can offer a wide aperture in a small body: the three stops between f/2 and f/5.6 mean bringing ISO down to an eighth in a dim place. A prime lens forces you to walk and to choose your place consciously, while a zoom gives flexibility; in dim light, a prime lens's gain in light makes it easier to keep the detail in the face on every skin.

Mini assignment

In this assignment you will tie your way of looking to a single focal length for a week, and compare it with a zoom.

1. If you have a zoom lens, set it to a single focal length, for example 35 or 50 millimeters, and shoot for a day without touching the ring.
2. Photograph the same scene once using the zoom and once only by walking; note what changed between the frames.
3. In a dim indoor place, shoot with the zoom first at its shortest end, then at its longest; note the aperture and ISO the camera chose.
4. In dim light, photograph a face at the lowest possible ISO, with a bright lens or the zoom's widest aperture, and look at the detail in the shadow.

Check yourself

• On the day you spent without touching the ring, did you start building the frame in your eye before raising the camera?
• When you opened the zoom toward telephoto, did you notice the camera raising the ISO?
• In the face you photographed in dim light, is the detail in the shadow kept, regardless of skin tone?

Glossary

Mini quiz

1. What does 18–55 f/3.5–5.6 written on a lens describe?

• A) That the lens is two separate lenses
• B) That the maximum aperture is f/3.5 at the wide angle and f/5.6 at the longest focal length
• C) That the lens only works between f/3.5 and f/5.6
• D) That the lens is constant-aperture

2. How many stops of light difference are there between f/2 and f/5.6?

• A) One stop
• B) Two stops
• C) Five stops
• D) Three stops

3. What effect can the way a prime lens makes the photographer work have?

• A) It forces you to walk to change the frame and makes your choice of place conscious
• B) It never makes the photographer move
• C) It changes the angle of view by itself
• D) It sets the exposure automatically

4. In a dim place, why does a bright prime lens help keep the detail of a dark-skinned face in shadow?

• A) Because it shows the skin lighter
• B) Because it corrects the white balance
• C) Because it gathers more light and allows shooting with lower ISO and less smoothing
• D) Because it darkens the background

Answers

1. B — Two aperture numbers show that the maximum aperture is different at the two ends of the zoom range.
2. D — There are three steps between f/2, f/2.8, f/4 and f/5.6; each step is one stop, eight times the light in total.
3. A — With a lens that can't zoom you build the frame with your feet; this requires deciding where to stand in every frame.
4. C — A wide aperture lowers ISO; when grain and the smoothing done to erase it drop, the detail in the shadow stays.

Summary card

• Prime: a single focal length; zoom: a range.
• A zoom with two aperture numbers loses light toward telephoto.
• Between f/2 and f/5.6: three stops, eight times the light.
• A prime lens makes you walk; a zoom gives flexibility.
• In dim light a bright lens means low ISO; on every skin, the detail in the shadow is kept.

Bridge to the next lesson

In the bookstore, instead of extending the end of the zoom, you walked to the author's table. Both frames showed the author's face at the same size, but there was a difference: as you came closer, the shelves behind started to look smaller and farther away compared with the face. It is often said that a long lens shows a face in a more pleasing way; yet the real source of this effect isn't the lens. The next lesson talks about perspective and compression, that is, what really decides the size of objects relative to each other.