What is a focal length of a lens?

• It is the length in MM between the optical center of the lens to your camera's sensor when the lens is focused at infinity.
• Determines how much of the world in front of your camera can you fit in your sensor.
• Determines how much of the world you will be able to see from your current position.
• Not a description of how long or short a lens is physically.

What is the optical center of a camera's lens?

• The point where all light rays intersect inside the lens.

How does focal length relate to what you see in a camera?

• Determines a lens' FOV and thus how much the subject will be magnified or reduced from a position.
• Shorter focal length = wider FOV
• Longer focal length = narrow FOV

When positioned at a particular point and you want to take a wider image of a building, you will:

1. Take multiple images of the building an combine them via photoshop.
2. Move farther away.
3. Use a lens with a shorter focal length.

What is crop sensor in photography? A camera sensor smaller than full frame (most cameras in the world use this).

Three most popular categories of camera sensors:

1. Full frame
2. APS-C
3. Micro 4/3

To achieve the same composition shot using a full frame lens, a photographer using a camera with a smaller sensor should do either of two things:

1. Mount a lens with a shorter focal length.
2. Move back farther.

Why is a 50 mm lens mounted on an APS-C camera similar to an 80 mm lens?

• equivalent focal length
• But it should be called equivalent field of view. Because the 50 mm lens did not change its focal length when mounted on an APS-C camera. What changes is the crop and the equivalent FOV.
• A 50 mm lens on an APS-C camera is similar to an 80 mm lens FOV on a full frame camera.
• Field-of-view Crop Factor (FOVCF)

A 50 mm lens on a Micro 4/3 camera would have a similar FOV to a 100 mm lens on a full frame camera.

A crop sensor camera will be tighter (narrow) and getting ultra wide angles will have more distortion.

Digital lenses are designed for APS-C. They are lighter, contain less glass, and are less expensive.

Why would one choose a crop over a full frame camera?

Prompts

What is a focal length of a camera lens? :: Length between the lens' optical center and the camera sensor.

The focal length of a lens is measured in mm.

What is the camera lens' optical center? :: Point where all light rays intersect in the lens.

Shorter focal length = wider FOV.

Longer focal length = narrow FOV.

When positioned at a particular point and you want to take a wider image of a building, you could:
1.
2. Move farther away.
3. Use a lens with a shorter focal length.
?
Take multiple images of the building and combine them via Photoshop.

When positioned at a particular point and you want to take a wider image of a building, you could:

1. Take multiple images of the building an combine them via Photoshop.
3. Use a lens with a shorter focal length.
   ?
   Move farther away.

When positioned at a particular point and you want to take a wider image of a building, you could:

1. Take multiple images of the building and combine them via Photoshop.
2. Move farther away.

?
Use a lens with a shorter focal length.

When positioned at a particular point and you want to take a wider image of a building, you could:
?

1. Take multiple images of the building an combine them via Photoshop.
2. Move farther away.
3. Use a lens with a shorter focal length.

What is a prime lens in photography? :: Lens with a fixed focal length

What is a zoom lens in photography? :: Lens with variable focal length

What is a crop sensor in photography? :: A camera sensor smaller than full frame.

Three most popular categories of camera sensors. :: full frame, APS-C, Micro 4/3

Nikon calls full frame FX.

Nikon calls APS-C DX.

Mount the same lens in different cameras with different camera sensor sizes and you will come up with different images.

Why is the camera image rectangular when the camera lens is circular? :: The camera sensor is rectangular.

Why do APS-C and Micro 4/3 camera sensors produce cropped images compared to full frame when using the same lens? :: They're smaller, so they produce less of the image projected by the lens.

APS-C and Micro 4/3 see less of the world than full frame.

To achieve the same composition shot using a full frame lens, a photographer using a camera with a smaller sensor could either move back further or mount a lens with a shorter focal length.

APS-C has an FOVCF of 1.6x for Canon.

APS-C has an FOVCF of 1.5x for Nikon.

Micro 4/3 has a 2x FOVCF.

A 50 mm lens on a Micro 4/3 camera would have a similar FOV to a 100 mm lens on a full frame camera.

Canon EFS and Nikon DX lenses are designed for digital cameras.

References

What Every Photographer Should Know About Lenses: Focal Length and Field-of-View. Directed by Envato Tuts+, 2014. YouTube, https://www.youtube.com/watch?v=1kAVGQvYGEw.