From full-frame to APS-C:
What Really Changes and Why

Switching from a full-frame (FX) sensor to an APS-C sensor is common—but often misunderstood. Let's begin by correcting a frequent misconception: Going from one sensor size to another does not change the focal length of a lens. How could it? The focal length of a prime lens, or the range of focal lengths of a zoom lens, is the result of the physical design of the lens. It is defined as the distance between the optical center of a lens and the plane (i.e., film or sensor) on which the lens, focused at infinity, produces a sharp image. Clearly, this is not something that can change with film or sensor size! Whatever you choose to mount the lens on, including no camera at all, the focal length, or range of focal lengths, does not change the slightest bit. The 50 mm lens that I use with a full-frame film-based camera is just that and will not miraculously become a 75 mm lens just because I attach it to an APS-C camera.

As a consequence, magnification does not change either. Lenses of identical focal length produce images of the same size at equal subject distance. For example, let's say one takes a picture with a full-frame camera and the subject comes out to be 15mm tall on the sensor. If one now attaches the same lens to an APS-C camera and takes the same shot from the same distance, the subject will still measure 15 mm on the APS-C sensor. The difference is that with the full-frame camera, one has also captured 9 millimeters of surrounding space (since the frame is 24mm tall), whereas with the APS-C sensor, one has just about used the entire height of the sensor, which measures 15.6 mm (this is shown in the illustration below, where the subject is a sailboat; with the APS-C sensor, the boat takes up almost the entire height of the frame, whereas the full-size sensor captures more space around the boat. The size of the boat on the sensor, however, does not change.

boat

But it does feel as if we have more reach with a given lens when we go to a smaller sensor, so something clearly changes. If it's neither the focal length nor the magnification, what is it? Before we can answer this question, we must quickly introduce a concept that is key to discussing this topic:

The Crop Factor

The crop factor is the ratio that describes how much smaller (or larger) a camera's sensor is compared to a traditional 35mm film frame (also known as "full-frame"). It is calculated by dividing the diagonal measurement of a 35mm full-frame sensor (43.3 mm) by the diagonal of the camera's sensor. APS-C sensors from most camera manufacturers (Nikon, Fujifilm, Sony, Pentax...) measure 23.5 × 15.6 mm. The exception is Canon that uses a slightly smaller sensor size of 22.2 × 14.8 mm.

diagonal

Thus, the diagonal of a normal (non-Canon) APS-C frame measures 28.2 mm, so we get a crop factor of 43.3 ÷ 28.2, i.e., roughly 1.5 (for Canon APS-C cameras, the crop factor is 1.6, and for Micro-Four-Thirds, it is 2). So what does this do for us? It helps us understand how the field of view of a lens changes when used on cameras with different sensor sizes. For example, a lens on a camera with a crop factor greater than 1 will produce a narrower field of view than it would on a full-frame camera, making the image appear more "zoomed in." We can use the crop factor to find the field of view equivalent of a lens on a sensor. For example, if we use a 50 mm lens made for full-frame on an APS-C sensor camera with a crop factor of 1.5, we get the same field of view as if we used a 75 mm lens on the full-frame camera. We get this result simply by multiplying the focal length of the lens (50 mm) with the crop factor (1.5).

Why is it called a crop factor? Because when a lens designed for a full-frame camera is used on an APS-C camera, the resulting image on the sensor is the same one would obtain if one selected the central portion of the 35 mm frame and applied a Crop command in Photoshop or any other image manipulation program. By now, it is clear what the answer to the above question is: what changes is neither the focal length nor the magnification, but

The Angle of View

The angle of view describes the angular extent of a scene that a camera can capture through a given lens. It is expressed in degrees and determines how much of the scene appears in the photograph. The angle of view depends on both the focal length of the lens and the size of the camera's image sensor or film. A shorter focal length (wide-angle lens) results in a wider angle of view, while a longer focal length (telephoto lens) produces a narrower angle of view. It is now obvious why many people believe that they gain magnification by going to a smaller sensor: it's because the angle of view definitely gets narrower, but the apparent magnification is due to the fact tact that the image is cropped to only a subset of what a full-frame image would be. Similarly, if you imagine printing both the APS-C frame and the full-frame rectangles in the above illustration on, say, 6" × 4" photo paper, the boat on the APS-C print would come out larger, but that is not due to the lens; rather, it is because the smaller APS-C area is enlarged to the same size as the larger full-frame area and it is during this enlargement process that that the magnification occurs.

So is that it or does anything else change when we go from full-frame to APS-C sensors? One of the other things we need to be aware of is

The Effective Aperture

Effective aperture refers to the apparent aperture value adjusted for sensor size, affecting the depth of field and background blur—though not exposure. On an APS-C sensor, it's calculated by multiplying the f-number of the lens by the crop factor, making images appear as if they were shot at a smaller aperture (i.e., a larger aperture number) compared to full-frame. So, for example, when I take photos with my Fujifilm 27mm prime lens at its maximum aperture of ƒ/2.8, I get the same depth of field as if I were shooting in full-frame with an aperture of 4.2—assuming the framing and subject distance are the same.

This is both good and bad news: good if you want much depth of field, but bad if you want to isolate your subject by creating a great deal of background blur (bokeh). It's just something to be aware of, particularly when moving from full-frame to APS-C as I did some time ago. Incidentally, this is how smartphones get their very large depth of field and have to resort to software to achieve background blur. My iPhone 16 Pro Max has a crop factor of 3.7, so even shooting at its maximum aperture of ƒ/1.78, it is like taking pictures with a full-frame camera set to aperture 6.5!

ISO: Sensitivity and Noise

ISO is not only a measure of a camera sensor's sensitivity to light; it is also a signal amplification level. While exposure itself does not require adjusting ISO between sensor formats if aperture and shutter speed are unchanged, the sensor size impacts the noise performance. This is because smaller sensors gather less total light, which can impact image quality. To achieve the same noise performance as a full-frame camera, the APS-C sensor would need to gather more light (via a slower shutter or lower ISO), but this adjustment depends on many factors and is not as simple as adjusting by the crop factor squared, as one occasionally reads. When one increases ISO, one amplifies the sensor's signal to brighten the image, and this also amplifies noise. A useful rule of thumb is that as far as noise is concerned, a full-size sensor outperforms an APS-C sensor by about 1 stop. As an example, let's say we shoot at ISO 1600 on a full-frame camera. If we now use an APS-C camera and wish to preserve similar noise characteristics, we might set the ISO to 800. Of course, this is an approximation because noise performance depends on many real-world factors, such as sensor design and pixel pitch.

Summary

This wraps up the changes we can expect when we move from a camera with a full-frame sensor to one with an APS-C sensor: a narrower angle of view at the same focal length, an increased depth of field and therefore reduced background blur, and increased noise level at the same ISO value. All these are a direct result of the fact that APS-C sensors are smaller than full-frame sensors. So what are the advantages and disadvantages of APS-C cameras when compared to cameras with full-frame sensors?

Advantages

Smaller and lighter. Indeed, that is the main reason that made me switch to the smaller format. The camera bodies are lighter, and the lenses even more so. This page, published over five years ago, describes my downsizing.
Telephoto reach. As explained above, this is due to the fact that the crop factor narrows the angle of view. I have a Fujifilm 55–200 mm lens; if I wanted to have the same angles of view on a full-frame camera, i would need to use an 83–300 mm lens, which would be quite a bit heavier.
More affordable. Camera bodies and compatible lenses are usually less expensive compared to full-frame systems. Because Fujifilm does not make cameras with full-frame sensors today, I shall use Nikon lenses as an example. The Nikon AF-S DX NIKKOR 18–200 mm f/3.5-5.6G ED VR II Lens (which I owned and used on my D300 APS-C camera) weighs 1.23 pounds and retails for $647 at B&H in July of 2025; the Nikon AF-S NIKKOR 28–300mm f/3.5-5.6G ED VR Lens (which I owned and used on my Nikon D750 full-frame camera) has the same angles of view, weighs 1.76 pounds and retails for $950 (also at B&H in July of 2025).

Disadvantages

Image Quality. Larger sensors capture more light, resulting in improved dynamic range, more detail, and less noise, particularly in low light conditions. While this is true, it is less and less relevant because sensor technology has advanced substantially, as have the noise reduction and detail recovery capabilities of the software that demosaics raw files into viewable images. While differences may be visible at the pixel level, it is today possible to shoot with APS-C cameras at ISO levels that would have been impossible with full-frame cameras a few short years ago.
Limited wide-angle. This is the counterpart to the telephoto reach advantage mentioned above: the crop factor makes it harder to achieve true wide-angle shots, which can be a disadvantage for landscape, architecture, or real estate photography. While true, it is worth mentioning that there are rectilinear wide-angle lenses that, at 9mm (13.5 mm equivalent, offer a 113° angle of view, and a 4.5 mm (6.75 mm equivalent) fisheye lens with an angle of view of 180°.
More difficult bokeh. Because the crop factor introduces an effective aperture that increases the depth of field, it is harder to isolate subjects by throwing the background out of focus. The effect can still be achieved by using a longer lens, getting closer to the subject, or placing the subject further away from the background, but it is undoubtedly not as easy as with a full-size sensor.

Ultimately, every photographer has to evaluate the advantages and disadvantages of moving from full-frame to APS-C. A great deal depends on the type of photography one practices, how much weight one is willing to carry, and how much money one wants to put into photography. I moved to Fujifilm APS-C in September 2019, and I never looked back. Certainly, I haven't noticed that the image quality in my photos has gotten worse; still, the above changes exist, and it is useful to keep them in mind, especially when one is used to a full-frame sensor camera.

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