Lenses for Photography Explained, for Beginners

The Ebay Answer

Before we get into the details of lenses, if you’re
reading this to figure out what lens to get for product
photography for Ebay, the answer is:

The 18-55mm kit lens is fine. As long as the lower number is around 18, like 14 or 20, you should be OK.

The lower number is the “wide angle” part of the lens. You want a low number.

On to the complex info.

How to Read This Document

You should read this and then read a similar tutorial coming from the more traditional perspective of a general photographer. This tutorial was written to be different, to give you what I think of as a more abstract, but simpler understanding of lenses.

Angle of View, Wide Angle, Normal, and Telephoto Lenses

Look at this picture.

The picture shows two lenses, and the angled lines show the “angle of view”.

The lens on the left has a wide angle of view, is called a “wide angle lens”. The lens on the right has a narrow angle of view, and is called a “telephoto” because it can focus on a distant object.

On the kit lens with the numbers 18-55mm, the 18 is the wide angle, and the 55 is the upper limit, in the case of 55, it’s called a “normal” lens. If it goes higher it could be a “telephoto” lens.

Now, you might be thinking: the wide angle looks like 90 degrees! The telephoto looks like 30 degrees.

If we identified lenses based on the angles, we’d call them a “90 degree lens” or a “40 degree lens”. However, we do not identify lenses this way, even if would make a lot of sense.

Instead, we identify lenese by “focal length”.

Focal Length and its Relation to Angle of View

Look at this picture. The drawings show a sensor, a lense, and rays that show the angle of view. It shows how a longer focal length corresponds to a narrower angle of view:

The dark line at the bottom represents the sensor, or a piece of film in the camera.

The first lens is a wide angle lens. The second is a narrower angle, maybe a “regular” lens. The third is a “telephoto” lens.

Notice that the lens is positioned farther away from the sensor as the angle of view narrows.

The distance from the sensor to the lens is the focal length.

The focal length does not refer to how far away you’re focusing. It refers to the distance from the lens to the sensor — and what that determines is the angle of view.

This is why a 20mm lens is called “wide angle”.

Now, the “telephoto” may be a 100mm lens, but there are also lenses called “macro” lenses that also have 100mm. Telephotos focus on distant objects, and macros focus on very nearby objects, like bugs, coins, and food. What’s up?

The 100mm just defines the angle of view. The “telephoto” or “macro” describes how the lens was designed to focus on distant or near objects.

Sensor Size and its Relation to Angle of View

Different cameras have different sensor sizes. Big cameras have bigger sensors, and small cameras have smaller sensors. Phone cameras have tiny sensors.

Note that all the lenses have the same angle of view.

As the sensor gets smaller, the focal length gets smaller.

So, that’s how they get cameras into phones: tiny sensors, and a tiny lens, and a very short focal length.

The 35mm Equivalent Focal Length

Did you know that the iPhone camera has a 29mm lens?

29mm. That’s almost 3 centimeters, and it’s more than 1 inch.

How is that even possible? The iPhone is only around 8mm thick!

That’s because lenses are described using a unit called “35mm equivalent”. What the heck does that mean?

The standard SLR camera since the 1960s has been a 35mm camera, which uses a 35mm piece of film. When DSLRs came out, they used a few different sensors sizes: some were 35mm, but others were 25mm (aka, APS-C), and there are still other sizes.
To avoid confusion, and cater to the existing market of SLR photographers, the camera companies continued to use “XXmm” to describe the angle, but the value would be focal length of the lens if the camera it was attached to had the 35mm sensor.

That’s what’s meant by “35mm equivalent.” They’re saying, “this lens would have a focal length of 29mm if your camera were a 35mm SLR.”

So that 18mm DSLR lens for a camera with an APS-C sensor… has a shorter actual focal length.

The iPhone’s sensor is around 4mm wide.

The iPhone camera lens’ actual focal length is probably around 3mm. It’s still called “29mm” though.

Look at this picture again:

These lenses would all have the same measurement, using the “35mm equivalent” unit of measure. Their actual focal lengths would vary considerably.

I hope this explanation helped. If you want to be alerted to new articles about cameras, ebay, or anything similar to what’s on the site, subscribe.