What is the Crop Factor?

Due to the smaller sensor size between APS-C and full frame, an effect similar to the "cropping" of an image results. Of course, the angle of view changes, similar to the digital zoom of small compact cameras or smartphones - the image appears cropped. This effect is called the “Crop Factor”. This has already been mentioned in another blog article and actually this triviality would not need to be addressed further if there were not so many misinformation and myths surrounding the crop factor. The wrong myths regarding the crop factor will be cleared up in this article.

Do I have to consider the Crop Factor?

If you own a full frame camera and an APS-C camera as well, then this might be sometimes the case. Only if you use exactly the same lens, but you are changing the camera and therefore the sensor sizes behind the lens, then it might be useful to take the crop-factor into account for getting an idea of how the image angle will be on the other camera.

If you just own one camera or all of your cameras have the same sensor size, then you will not need to calculate the crop factor at all. Even if you are using Full Frame Lenses on APS-C camera bodies, the crop factor does not matter. Do not get confused by it!

The Crop factor is similar to cropping an image – No more, no less

If a photo would be taken with a high-resolution full frame camera (e.g. Nikon D850) and being then cropped in post, so that approx. 17% would be lost towards the edge of the picture, how would the cropped photo look like? The final result would be absolutely identical to a similar photo of an APS-C camera (e.g. Nikon D500), using the same lens!

This is because a smaller sensor size is equivalent to the cropping of an image. The cropped image of a full-frame camera would not differ from the original image of an APS-C camera, when using the same lens. Neither in background blur, depth of field, perspective, nor in brightness or image detail a difference would be visible. The photo would be absolutely identical. If even two cameras with the same pixel pitches were compared, as it is the case with the D500 and D850, the cropped full-frame image would even have the same resolution and image quality as the original APS-C camera photo. By the way, all Nikon DSLR full frame models can be put into the crop mode, so that the images are then directly saved smaller. This allows Nikon APS-C lenses to be used on the full frame cameras and the results look exactly the same as those of an original APS-C camera, even though the camera used has a larger sensor. The crop factor is nothing else than the effect that is also achieved when cropping a photo; no more and no less! In the following, some further myths concerning the crop factor in particular will be discussed:

Can full frame lenses be used on APS-C cameras and does the focal length of the lens have to be converted or calculated?

In short: Yes! Full frame lenses can be used on APS-C cameras without any problems. And no, nothing has to be converted nor calculated.

The question above already contains several false assumptions. The first one is that APS-C lenses are only for APS-C cameras and accordingly it would be the same for the 35mm full frame format. That's not so. For example, Canon itself states that all EF lenses (full frame) are compatible with all EOS cameras (including APS-C). It's the same with Nikon. Nikon even recommends some FX lenses (full frame) explicitly for certain DX models (APS-C). In this respect, the choice of lenses for APS-C cameras is anything but small, although there are not many lenses produced exclusively for the APS-C format. The second assumption is that the focal length of the lens should mysteriously change as soon as there is a smaller sensor behind it. At least this is communicated in a similar way in numerous forums or social media groups. But this is not the case either. The focal length of a lens is a fixed physical property and depends on the construction of the lens groups. Furthermore, the lens does not know how big the sensor is that is located just underneath it. But why are some lenses sold expensively as full frame lenses, while other lenses, declared as APS-C compatible, are often cheaper. What's the difference?

Do not be confused by the crop factor: The focal length on the lens does not need to be converted. A 50 mm lens always remains a 50 mm lens, completely independent of which camera it is screwed onto. However, it is also correct that on small compact cameras the indication "24 mm equivalent" may be given, although the focal length of the lens is only 6 mm. This is probably where the myth of the changing focal length comes from. The word "equivalent" after the focal length is decisive. It says that the actual focal length is different, but the image section would be comparable to a 24 mm lens on the 35 mm format (full frame). With APS-C lenses, however, there is usually no equivalence indication. The focal length information on the lens corresponds to the actual and physical focal length. So 50 millimeters are always 50 millimeters on “real” cameras…

The only difference between a full frame lens and an APS-C optimized lens is the size of the projected image circle. The image circle corresponds to the area that a lens can illuminate on the sensor side. The diameter of the image circle should therefore correspond at least to the diagonal of the sensor. The camera sensor must lie completely within the image circle of the lens. However, to project a larger image circle requires a more complex optical setup, and not only the image circle but also the lens becomes larger, heavier and more expensive. The reason why there are special APS-C lenses is that the manufacturers want to sell additional lighter and much cheaper lenses in the amateur & consumer sector.

If the image circle diameter is larger than the diagonal of the sensor, this is not a problem, so full frame lenses (FX) can be used in combination with smaller APS-C or crop image sensors. If, on the other hand, the image circle is much smaller than the image sensor, the latter cannot be fully illuminated and the image corners appear much darker in the final result. On the other hand, it is therefore not recommended to use APS-C lenses on full-frame cameras, as the strong vignetting is accompanied by a loss of quality. The only exception is when the strong vignette is not disturbing or desired. This is the case with circular fisheye lenses on full frame, for example, when a circular image is required for interactive 360° VR tour or panoramic photos. But this is admittedly a very rare special case. Virtual 360 degree tours can be created e.g. with a Nikon D850 full frame cameraand a modified Nikkor DX 10.5 mm f2.8 fisheye lens, which was actually designed for the smaller crop cameras.

Using an APS-C lens on a full frame camera will lead to poorer results due to edge shading (vignetting). With Canon it can also happen that the EF-S lenses (APS-C) do not fit on the body of an EOS full frame camera. With Nikon, however, this is not a problem, although there is usually no reason to do so. The above mentioned example of 360° VR tours, with the DX fisheye lens, is probably the only reasonable exception in the field of panorama photography. A full frame lens has a much larger image circle, so it can be used on an APS-C camera without any problems. When photographing with an APS-C camera, the final photograph will not show whether the lens was originally designed for full frame or for APS-C. On one and the same camera, a lens with a certain focal length always looks the same. The image section does not change, completely independent of whether it is an APS-C or a full frame lens with the same focal length.

A picture taken with an APS-C lens, such as the AF-S DX Nikkor 35 mm f/1.8G, on a Nikon D5600 crop camera, for example, cannot be distinguished from a photo taken with a combination of a full-frame lens, such as the Nikkor AF-S 35 mm f/1.8G ED, and the same camera. A 35 mm on APS-C corresponds to a 50 mm on full frame from the angle of view.

The size of the image circle is the only difference between full frame and APS-C lenses. Thus, APS-C lenses can be built slightly smaller and cheaper. In summary, an APS-C lens on a full frame camera will result in dark corners of the image, and vice versa, a full frame lens on an APS-C camera can be used without any problems. However, full frame lenses are often larger, heavier and more expensive. This is at least true for zoom lenses and fast ultra-wide-angle lenses; fixed focal lengths in the medium range, on the other hand, do not necessarily have to be larger or much more expensive. The very small and flat "pancake lenses" for full frame cameras are a good example of this.

APS-C lenses are specially designed for smaller image sensors and therefore have a smaller image circle, so they are usually smaller, lighter and also cheaper at its own. If only one camera is used and several lenses are available, the crop factor is not important. In the end, 50 mm always remains 50 mm. The correct focal length is also always indicated on the lens. Only small compact cameras, mobile phones or other toys give a converted focal length in mm equivalent. Mostly these are cameras where the lenses cannot be changed anyway.

When is the crop factor applied?

If the lenses are changed on a camera system, the crop factor therefore plays absolutely no role. When using full frame lenses on APS-C cameras, the focal length does not need to be calculated whatsoever. The crop factor is only applied as soon as the camera systems are replaced.

A 50 mm lens works differently on an APS-C camera than on a full frame camera. This is due to the fact that the image circle remains the same but the sensor size is different. The word "Crop" stands for cropping. Because the image sensor in an APS-C camera is smaller, the image appears cropped and the cropping is different. This effect is similar to the digital zoom. The crop factor between APS-C and full frame is about 1.5 to 1.6 (with Canon). The field of view of a 50mm lens on an APS-C camera corresponds to the field of view of a 75 mm lens on a full frame camera. But this does not change the fact that a 50 mm lens always remains a 50 mm lens. This conversion is only interesting for those who often switch between APS-C and full frame cameras in order to be able to compare the resulting image angles. If a small room is photographed with a 15 mm wide-angle lens on a full frame camera, it can be estimated that for the same scenario with an APS-C camera a lens with a focal length of 10 mm would be necessary to get everything in the picture. For all those who do not switch between different camera systems, the conversion using the crop factor is irrelevant.

Does the crop factor change the aperture?

No, but indirectly the depth of field. In theory, the crop factor does not do this, but in practice it does... The crop factor or the sensor size alone does not change the depth of field. Provided the same lens is used and the distance to the subject remains the same. However, the sensor size changes the image section, but not the depth of field. Ultimately, the crop factor only describes the virtual crop, no more and no less.

In practice, however, the narrower image section results in either a greater distance from the subject to ensure that it fits completely into the picture again, or a shorter focal length with poorer blur-potential is chosen. It is therefore necessary to move away from the subject to compensate for the zoom effect of the smaller sensor or to use a wider lens. In the end it is these factors that influence the depth of field: The distance to the subject the different focal length and aperture value. The crop factor thus indirectly results in a poorer background-blur-potential of smaller sensors, because these are inevitably used differently.

In order to be able to compare the different blur potentials better, the crop factor can also be applied to the aperture value. And it is from this context that the misconception arises that the crop factor has an influence on the light intensity, in sense of the aperture value. But this is not the case. Once the crop factor is applied to the aperture value, this refers solely to the depth of field when using equivalent focal lengths to calculate what aperture would theoretically be required for a comparable image with the same depth of field. This does not mean that the aperture actually changes, because it does not.

The crop factor from APS-C to full frame is 1.5, so a 55 mm lens on APS-C is approximately equivalent to an 80 mm focal length on full frame. If the 80 mm full frame lens has an aperture of f1.8, the 55 mm APS-C lens would need to have an aperture of f1.2 to achieve a comparable depth effect

80mm f1.8 / 1.5 ~ 55mm f1.2

The picture will not look exactly like on a full frame camera, but the depth effect should be at least somewhat comparable. The calculated value only means that if the approximate image look of an 80 mm f1.8 lens on a full frame camera is to be reproduced on an APS-C camera, a 55 mm f1.2 lens will be necessary. A widespread misconception, however, is that a 55 mm f1.2 full frame lens "transforms" accordingly on an APS-C camera into an 80 mm f1.8 lens. That is definitely not the case. Only the image section changes and also the cropping potential would correspond to an f1.8 aperture on full frame, since in practice a larger distance to the subject must be maintained. But the light intensity or exposure would not change. A 1.2 aperture always remains a 1.2 aperture. For example, a 16-70 mm f2.8 lens on an APS-C camera would resemble a 24-105 mm f4.0 lens on a full frame camera in terms of image look. But it would not be the same. If, due to the greater distance to the subject, a depth of field is achieved that resembles an f1.8 aperture, it is still an f1.2 aperture. The lens does not let less light through as the sensor behind it becomes smaller. For this reason, the crop factor must not be applied to either the light intensities or the ISO value. The exposure does not change. Only the image section is cropped, which forces us to increase the distance to the subject to get it into the picture. That's all that happens.

Do large full frame sensors capture 1.5x times more light?

Although this is only indirectly related to the crop factor, this question is related to the latter and is also based on a false assumption. Since the crop factor has no influence on the light intensity or on the ISO value, why should it affect the light incidence? Well, it does not.

The crop factor refers to the image diagonal. If at all, the total sensor surface should be much more decisive than its diagonal length. Compared to an APS-C sensor, a full-frame sensor has an area 2.33 times larger than an APS-C sensor, onto which light can shine. So a full-frame sensor captures more than twice as much light and is therefore better suited for night shots because the images become brighter? Unfortunately, this is also not the case. Canada is also much larger than Scotland and captures more light, yet it is not necessarily brighter in Canada. The exposure is not related to the entire area, but per unit area! This is also the reason why modern light meters do not offer the possibility to adjust the sensor size. What is usually (though not always) true is that full frame sensors have larger individual pixels due to the lower pixel pitch. Larger pixels, however, are less susceptible to image noise, but do not produce brighter images.

If we leave a large bowl and a small glass outside in the rain for a few minutes, the water level will be about the same for both (see exposure). If, on the other hand, we place a lot of small glasses in our garden, there will sometimes be big differences between the individual glasses (noise). If we take a lot of large bowls instead, they will be filled more evenly, as they tend to approach an average value (cf. lower noise). But as there are much less bowls than glasses that fit into our garden due to lack of space, the total amount of water collected would be identical for both attempts. If we have the possibility to extend the experiment to the neighbouring plot (larger sensor), we would get more bowls under (higher resolution), but the water level (exposure) would not be higher in each bowl.

Full frame cameras therefore usually have larger individual pixels due to the lower pixel density per square millimetre. As a result, these usually start to generate noise later and are therefore often better suited for high ISO values and dark situations. But they do not produce brighter images. Therefore, it makes no sense to apply a crop factor to the ISO value between APS-C cameras and full frame cameras. How wrong this assumption is, becomes clear when comparing the cameras Nikon D500 (APS-C) and Canon 5DS-R (full frame). Here, the small APS-C camera performs much better in the dark, at high ISO settings, because, among other things, it has a lower pixel pitch. This has therefore nothing to do with the sensor size directly. Larger sensors do not produce brighter images. Furthermore, this connection has nothing to do with the crop factor, which only changes the image detail.

 Do not be confused by the crop factor