Focal Length and Aperture Equivalence by Sensor Size

What is the full frame equivalent of an APS-C lens? What does a medium format lens, a premium compact or a smartphone camera correspond to in 35mm full frame terms? In photography, comparing focal lengths is not enough: sensor size changes the angle of view, the framing and the apparent depth of field.

Does a 23mm f/2 lens on APS-C really give the same rendering as a 35mm f/2 lens on full frame? Does a 90mm f/3.5 lens on 6x7 medium format produce the same depth of field as a 50mm f/3.5 lens on 35mm full frame? These equivalences are often misunderstood, yet they are essential when comparing lenses used on smartphones, premium compact cameras, APS-C, full frame, medium format or large format systems.

Focal length, angle of view and equivalent aperture calculator

Use the calculator below to compare a focal length and aperture between two sensor formats. It separates two notions that are often confused:

• the angle of view or equivalent framing, which corresponds to the equivalent focal length in 35mm full frame terms;
• the equivalent depth of field, which corresponds to the aperture giving a comparable amount of background blur.

Why is 35mm full frame used as the reference?

In this calculator, the most common reference format is 24x36 mm, also known as full frame. This choice is not arbitrary. It corresponds to the historical format derived from 35mm film, popularized by Oskar Barnack with the Leica, then adopted for decades by most camera manufacturers using 35mm film.

The 24x36 format became a major photographic standard because it offered an exceptional balance between image quality, compact equipment and ease of use. Leica, Nikon, Canon, Pentax, Minolta, Olympus, Contax, Zeiss Ikon, Voigtländer and Konica all contributed, each in their own way, to making 35mm film a universal format for reportage, travel, family photography, professional photography and fine art photography.

In the digital era, the term Full Frame refers to a sensor that roughly matches the dimensions of 24x36 mm film. This format therefore naturally serves as a reference when comparing focal lengths and depth of field across different systems: smartphones, premium compact cameras, Micro Four Thirds, APS-C, full frame, digital medium format, 645, 6x6, 6x7, 4x5 inch view cameras or 8x10 inch large format cameras.

What is an equivalent focal length?

The focal length written on a lens is a real physical value. A 23mm lens remains a 23mm lens, whether it is used on an APS-C camera, a Micro Four Thirds camera or a compact camera. What changes is the angle of view obtained according to the size of the sensor placed behind the lens.

A smaller sensor crops the image. This is why a 23mm lens mounted on an APS-C camera gives framing close to a 35mm lens on full frame. This is called an equivalent focal length. This equivalence makes it easier to compare lenses across different formats.

In this calculator, focal length equivalence is based on the diagonal angle of view. This is the most common method for comparing two sensor formats, because it brings very different systems back to a shared reference, the 24x36 mm full frame format.

The diagonal angle of view is therefore a useful reference for understanding the correspondence between two focal lengths. For example, a 35mm lens on 24x36 full frame gives a diagonal angle of view of about 63°. A focal length giving a comparable diagonal angle on another format will produce similar framing, but not necessarily an image that feels exactly the same.

This correspondence remains approximate when formats do not share the same image ratio. Photographic formats are generally named after the dimensions of the sensitive area, whether it is the film gate or the sensor: 24x36 mm, Micro Four Thirds, 6x6, 6x7, 4x5 inch, 8x10 inch, and so on. But these formats do not all have the same proportions. Full frame 24x36 has a 3:2 ratio, Micro Four Thirds has a 4:3 ratio, 6x6 is square, 6x7 is a more elongated rectangle, while 4x5 and 8x10 formats use yet other image ratios. Two lenses can therefore offer a comparable diagonal angle of view without distributing the field in exactly the same way horizontally and vertically.

The diagonal angle should therefore not be interpreted as an absolute equivalence of rendering. A 35mm lens on 24x36 full frame and a focal length giving a similar diagonal angle in 6x6 will not produce exactly the same image: the first one fits into a 3:2 rectangle, while the second one fits into a square. The diagonal angle makes comparison easier, but the final composition also depends on the image ratio.

The formula is simple:

equivalent focal length = real focal length × conversion factor

For Nikon, Fuji or Sony APS-C, the conversion factor is generally close to 1.5. A 23mm lens used on APS-C therefore gives framing equivalent to about 35mm on full frame.

23mm × 1.5 = 34.5mm

What is an equivalent aperture?

Equivalent aperture is used to compare depth of field, not exposure. This is the most important point to understand. When comparing two different formats, comparing focal lengths is not enough. Aperture also has to be compared using the conversion factor.

The formula used to compare depth of field is the following:

equivalent aperture = real aperture × conversion factor

In this example, the conversion factor between Nikon, Fuji or Sony APS-C and full frame is approximately 1.5:

f/2 × 1.5 = f/3

A 23mm f/2 lens on APS-C therefore gives framing close to a 35mm lens on full frame, but a depth of field comparable to about f/3 on full frame.

23mm f/2 APS-C ≈ 35mm f/3 full frame

This equivalence does not mean that a 35mm full frame lens should necessarily be set to f/3. It is mainly useful for understanding the limitation of the smaller format: a 23mm f/2 lens on APS-C will not naturally produce the same shallow depth of field as a 35mm f/1.8 or f/1.4 lens used wide open on full frame. Full frame therefore retains an advantage when looking for very blurred backgrounds, strong subject separation or shallow depth of field with equipment that remains relatively compact.

Exposure aperture does not change

f/2 remains f/2 for exposure. This is a frequent source of confusion. If you shoot at f/2, 1/250 s and ISO 400, the exposure does not directly depend on sensor size. An aperture of f/2 lets in an amount of light corresponding to f/2.

It is therefore important to distinguish the aperture used to expose the image from the equivalent aperture used to compare rendering. If the calculator indicates that a 23mm f/2 lens on APS-C is approximately equivalent to a 35mm f/3 lens on full frame, it does not mean that a full frame photographer should stop a 35mm lens down to f/3. It simply means that the 23mm f/2 APS-C lens produces a depth of field comparable to a 35mm f/3 lens on full frame. With a 35mm f/1.8 or f/1.4 lens, full frame will allow a blurrier background.

However, depth of field rendering changes with sensor size, the real focal length used and the focusing distance. This is why two cameras can display the same aperture, for example f/1.8, while producing very different images in terms of background blur.

Conversely, medium and large formats can produce a very shallow depth of field with apertures that may look modest on paper. An f/4, f/5.6 or f/8 lens on medium format or large format can produce blur rendering comparable to a much faster lens on 24x36. However, since the real aperture remains less bright, you will often need to compensate with a longer exposure time, higher ISO sensitivity or more light.

Medium format, 6x6, 6x7 and full frame rendering

The calculation becomes especially interesting when comparing full frame with the larger historical formats of film photography. The 6x6 format popularized by Hasselblad, Rolleiflex or Bronica, the 6x7 format associated with Mamiya, Pentax or Fuji, the 645 format used by Mamiya, Pentax, Contax or Phase One, and 4x5 inch or 8x10 inch view cameras from Linhof, Sinar or Toyo all produce renderings that are not easy to compare intuitively with 24x36 full frame.

For example, a 90mm f/3.5 lens on 6x7 medium format does not give the same rendering at all as a 50mm f/3.5 lens on full frame. Since the 6x7 format is much larger than 24x36, its conversion factor toward full frame is below 1. A 90mm lens on 6x7 corresponds approximately to a 43mm lens on full frame in terms of framing.

90mm on 6x7 ≈ 43mm on full frame

For depth of field, an aperture of f/3.5 on 6x7 corresponds approximately to f/1.7 on full frame.

90mm f/3.5 on 6x7 ≈ 43mm f/1.7 on full frame

This is one of the reasons why medium format images can have such a distinctive look: a wide field of view, natural perspective, strong subject separation and a smooth transition between sharpness and blur. Medium format and large format can therefore produce shallow depth of field with longer focal lengths and larger sensitive surfaces, even when the displayed aperture looks relatively modest compared with full frame standards.

The practical trade-off is exposure. These real apertures, f/3.5, f/4, f/5.6 or f/8 depending on the system, transmit less light than an f/1.4 or f/1.8 lens on full frame. To make the image, you may therefore need to use a longer exposure time, raise the ISO or work with more light. This is one of the reasons why 24x36 full frame often remains an excellent compromise between shallow depth of field, fast lenses, compact equipment and ease of use.

Smartphones, compact cameras and aperture marketing

Be careful with flattering aperture values advertised on smartphones and compact cameras. When a manufacturer announces an f/1.7 or f/1.8 lens, that value is true for exposure. But it does not mean that the image will have the same rendering as a 35mm f/1.8 or 50mm f/1.8 lens on full frame.

On a smartphone, the sensor diagonal remains far smaller than that of a 24x36 full frame sensor. To obtain an angle of view equivalent to 24mm, 26mm or 28mm, the real lens uses a very short focal length. This short focal length naturally increases depth of field. The visible background blur on a smartphone is therefore often enhanced by software processing, Portrait mode or computational depth simulation.

The same reasoning applies to premium compact cameras. A Leica D-Lux, a Canon PowerShot G7 X or a Sony RX100 may advertise very attractive apertures such as f/1.7, f/1.8 or f/2.8. These apertures are real for exposure, but their depth of field rendering remains tied to sensor size.

A premium compact such as the Leica D-Lux 8 can therefore be very bright for exposure, convenient for travel and excellent in image quality, without naturally producing the depth of field rendering of a 35mm f/1.8 or f/1.4 lens on full frame. The calculator is useful precisely because it helps visualize this gap between advertised aperture, equivalent angle of view and the real rendering of blur.

Examples of equivalences

Here are a few simple examples to understand how rendering differs between formats:

These values remain practical approximations, but they help clarify the expected rendering. Focusing distance, the chosen circle of confusion, the actual crop of the image and software processing can also influence the perceived background blur.

Further reading: this article compares lenses, focal lengths and depth of field across formats. If you are more interested in image detail and resolution, read my analysis of digital sensors vs film resolution, with a focus on full frame, medium format and the real potential of scanned film.

Key takeaways

Comparing lenses across formats requires distinguishing three notions: real focal length, equivalent focal length and equivalent aperture. The real focal length remains the one written on the lens. The equivalent focal length is used to compare framing with 24x36 full frame. The equivalent aperture is used to compare depth of field, but it does not change exposure.

A 23mm f/2 lens on APS-C can therefore frame like a 35mm lens on full frame, while giving a depth of field close to a 35mm f/3 lens. This means it will not naturally produce the same blur as a 35mm f/1.8 or f/1.4 lens used wide open on full frame. Conversely, a 90mm f/3.5 lens on 6x7 can frame like a 43mm lens on full frame, with a depth of field close to a 43mm f/1.7 lens, which explains part of the distinctive medium format look.

This calculator helps visualize these differences and avoid the frequent confusion created by technical specifications, marketing equivalences or overly quick comparisons between cameras, smartphones, compact cameras, full frame, medium format and large format systems.

In practice, 24x36 full frame often remains an excellent compromise: it allows shallow depth of field with fast lenses, while keeping the equipment more compact, more versatile and easier to use than medium or large format.

FAQ on focal length and aperture equivalence

Does an APS-C lens actually change focal length?

No. A 23mm lens always remains a 23mm lens. What changes is the angle of view obtained according to the sensor size. On APS-C, a 23mm lens frames approximately like a 35mm lens on full frame.

Why do we talk about equivalent aperture?

Equivalent aperture is used to compare depth of field between different formats. It does not change exposure. f/2 remains f/2 for light transmission, but its background blur rendering may correspond to a different aperture on full frame.

Is a 23mm f/2 APS-C lens equivalent to a 35mm f/2 full frame lens?

Not exactly. It frames approximately like a 35mm lens on full frame, but its depth of field is closer to that of a 35mm f/3 lens on full frame. A 35mm f/1.8 or f/1.4 lens used wide open on full frame will therefore produce stronger blur.

Why is 24x36 full frame used as the reference?

The 24x36 full frame format corresponds to the dimensions of 35mm film popularized by Leica. It became a historical, then digital, reference for comparing focal lengths, angles of view and depth of field.

Does f/1.8 on a smartphone produce the same blur as f/1.8 on full frame?

No. The f/1.8 aperture is real for exposure, but the small sensor and very short real focal length of a smartphone naturally produce much greater depth of field. The visible background blur is often enhanced by software processing.

Why can medium and large formats produce strong blur with modest apertures?

Because the sensitive surface is much larger. An f/4, f/5.6 or f/8 lens on medium or large format can produce a depth of field comparable to a very fast lens on 24x36. However, the real aperture transmits less light, which may require a longer exposure time, higher ISO settings or more lighting.

Are equivalences perfectly accurate between 24x36, 6x6, 6x7, 4x5 and 8x10?

No. They are practical approximations. The calculation is generally based on the diagonal angle of view, while different formats do not all share the same image ratio. A square 6x6 format, a rectangular 24x36 format, a 4x5 or an 8x10 may have a comparable diagonal angle without distributing the field in exactly the same way.

About the author

Sebastien Desnoulez is a photographer, author and image maker based in Paris. His work spans architectural photography, landscape photography and travel photography, with particular attention to composition, lines, light, blur and visual accidents. Trained in photography in the mid-1980s, he covered Formula 1 and reported from around the world before developing a fine art photography practice built around the tension between graphic rigour and visual instability. He also shares his technical experience through practical articles for passionate photographers, drawing on a strong visual culture acquired in both film and digital photography.