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4 - Depth-of-field & Sensor Size

The size of the recording format affects depth-of-field.

Smaller formats have more depth-of-field.

The smaller size of most sensors increases depth-of-field compared to 35mm film, given the parameters discussed below (see Confusion and More Confusion).

The increased depth-of-field is both good and bad.

Good

The increased depth-of-field of most digital cameras makes it easier to:

• Photograph landscapes in which everything is sharp.

• Do macro photography, in which depth-of-field is critical because you're so close to the subject.

Bad

The increased depth-of-field of digital cameras makes it harder to blur near backgrounds, such as for portraits.

Why Does Sensor Size

Affect Depth-of-field?

The smaller focal length of the digital camera, needed when making a comparison to a 35mm film camera, increases the depth-of-field.

When comparing cameras with different size sensors, the camera with the smaller sensor must have a smaller focal length (or needs to be further away from the subject).

This is necessary to keep what's "in" each photograph, the field-of-view or magnification, the same (see Confusion and More Confusion).

The smaller focal length also increases the depth-of-field of the camera with the smaller sensor.

Let's compare the depth-of-field of a 35mm-film camera, with a 50mm lens, with a Nikon DSLR.

In order to keep the field-of-view (what's "in" the photograph) the same, the focal length of the Nikon DSLR lens must be 33mm, a lens factor of 1.5.

1.5 x 33mm = 50mm

Decreasing the focal length by a factor of 1.5, with a Nikon DSLR, increases the depth-of-field by about 1.5.

Drilling further into the explanation, the Nikon DSLR lens at 33mm has a lens opening of 4.13mm.

33mm ÷ f/8 = 4.13mm

The 50mm lens on 35mm film camera has a lens opening of 6.25mm.

50mm ÷ f/8 = 6.25mm

The Nikon DSLR, with a smaller lens opening, is going to have more depth-of-field, than the 35mm film camera.

Nikon DSLR Lens

At 33mm & f/8

35mm Film

Camera Lens

At 50mm & f/8

4.13 m

m

6.25 mm

4.13mm is a smaller lens opening, so there's more depth-of-field.

You can see this in the table below.

The figures are also given for a Canon Rebel DSLR, which has a lens factor of 1.6.

The depth-of-field increases by about 1.6 for the Canon Rebel DSLR.

Camera

Focal

Length

Lens

Opening

Distance

to

Subject

Size

Of the

Lens

Opening

Total

Depth-of

-field

35mm Film 50 mm f/8 5 Feet 6.25 mm 1.4 Feet
Nikon DSLR 33 mm f/8 5 Feet 4.13 mm 2.3 Feet
Canon Rebel DSLR 31 mm f/8 5 Feet 3.88 mm 2.5 Feet

What if we moved the camera back, rather than changing the focal length, to keep the field-of-view the same?

Because the camera is further away, there's more depth-of-field.

Here's a chart of the lens openings and focal lengths needed to keep the depth-of-field constant across these formats.

Camera Focal Length Lens Opening
8 x 10" Film 455 mm f/72
35mm Film 50 mm f/8
Nikon DSLR 33 mm f/5.3
Canon Rebel DSLR 31 mm f/5
4/3" Sensor 25 mm f/4
2/3" Sensor 13 mm f/2
1/8" Sensor 10mm f/1.7

Visual Proof

There's nothing like having a photograph explain something photographic.

Rik Littlefield photographed a still life using the same field-of-view, and the same lens opening, but with two different formats.

He used a Canon Digital Rebel 300D and a Kodak DC4800.

The sensor of the Canon camera is larger, and thus, has less depth-of-field, than the Kodak camera.

In order to achieve the same depth-of-field as the Kodak camera (smaller sensor), the Canon (larger sensor) had to be set to f/13.

Compare his photographs taken at f/4 with the above cameras.

Go to Comparison Photographs.

Most photographers can stop reading here.

Debate

There's debate about whether format affects depth-of-field.

The proof that it does is seen in photographs, such as those by Rik Littlefield above.

But, the photographs must be taken under certain conditions.

You can't have more than one variable.

If there's more than one variable, then a debate results where one person is arguing "apples" and the other person is arguing "oranges."

Confusion

When comparing the amount of depth-of-field of two formats, the photographs must have the same field-of-view or magnification (focal length ÷ object distance).

If the field-of-view is the same, then you're comparing apples to apples.

A 35mm-film photograph taken with a 50mm lens, has a wider field-of-view, than does a photograph taken using a 50mm lens on most DSLRs (digital SLR).

So, if a comparison is made between a 35mm-film camera with a 50mm lens, the focal length of the DSLR must be 33mm (1.5 lens factor, Nikon) or 31mm (1.6 lens factor, most Canons).

Because equivalent focal lengths are used, giving the same field-of-view, the only variable is format size.

More Confusion

Besides having the same field-of-view (magnification), these factors also apply to the discussion.

• The subject is the same distance from both cameras.

• The lens opening number is the same for both cameras.

• The circle-of-confusion used to determine depth-of-field for each format is the same fraction of the camera's format sizes (such as 1/1440).

That is, the out-of-focus blur that is considered to acceptably sharp, the depth-of-field, is judged using prints of the same size, and viewed from the same distance.