Crop factor is not something that was introduced with the digital cameras. It has existed for decades, since the inception of photography in fact. One would think that crop factor would be a well-understood topic by now, but it does not seem to be that way. In fact, even though I've shot film (35mm), I found myself constantly confused by the discussions about the crop-factor. Of course, digital also added a few of its authentic factors to the discussion, leading to even more confusion. In this post, I will try to summarize my views, and attempt to contribute some clarity on the topic.
What is crop factor anyway?
Let's start with the definition of the crop factor so we're on the same page with regards to what it is that we are talking about.
As the art and technology of photography developed, the 35mm film has become widely recognized as the "standard" format. I cannot say for sure why that is, but perhaps it has to do with the economics of using this format: the 35mm gear is relatively inexpensive compared to the gear used for larger formats, and yet it offers more imaging surface area than the smaller formats. Especially in the film era, 35mm was also preferred for its speed of deployment and portability, and that may have simply translated to habit. At any rate, today the 35mm (the so-called "full-frame"), is the reference format based on which the crop factor is calculated. It leaves one to wonder whether we should talk about crop factor when comparing the full-frame cameras to their larger format brethren, but the convention has already been established, so we might as well stick to it.
Crop factor is the ratio between the diagonal of the sensor or film to the diagonal of the 35mm film.
The diagonal of the 35mm film/sensor around 43.1mm.
Let's say we are calculating the crop factor of the Fujifilm's APS-C X-Trans sensor. According to the X-T3 specs, the X-Trans sensor size is 23.5mm×15.6mm. The diagonal is about 28.2mm. Thus, the crop factor of the X-Trans sensor is around 1.52.
You can find a table with all sensor formats and their respective crop factors on Wikipedia.
Crop factor is a crop factor
If you're wondering what the word "crop" means in "crop factor", it means the first thing that comes to most people's minds: cropping the image. That's all it is. Nothing more and nothing less.
Here are two images shot with a full-frame DSLR, Nikon D700, and an APS-C Fujifilm X-T3 using the exact same lens, the Nikkor 80-200mm F4 at the same focal length of 80mm and the same aperture of F5.6. The shots were taken on a tripod from the exact same distance.
The second image (Fujifilm) appears "closer", because, compared to the full-frame image, it is cropped. Here's a crop from the full-frame image that roughly matches the crop sensor image.
Apart from slight differences in exposure and perspective (Nikon is mounted a bit higher on the tripod due to the body size), there is not much difference between them. In other words, the only thing the crop sensor did was literally crop the image.
Effect on focal length
You have probably heard of the term "full-frame-equivalent field of view." Some people say "full-frame-equivalent focal length", and it means the same thing. Someone may say that "field of view" is not the same as "focal length", but that is only true if we are talking about the physical focal length of the lenses. If we are talking about the image output, those terms are equivalent (as you will see later).
As we have seen in the previous example images, the crop-sensor makes the subject appear closer. In other words, the crop-sensor reduces the field of view. This makes sense since crop sensor cropped the image.
You already know that I've shot the two images with the exact same lens at the exact same focal length (80mm). Crop factor does not affect the physical focal length of the lens (or any of its other optical qualities) in any way. However, since the Fujifilm camera has an APS-C sensor with the crop factor of around 1.52, the image was "magnified" by roughly 1.52×, making it look as if it were shot with a 120mm lens. But the lens is still 80mm. The only thing that changed is the field of view.
Let's take a look at an image shot with a real 120mm on a full-frame sensor and compare it to the image show at 80mm on APS-C. It's only just roughly 120mm as I don't have the 120mm marking on the lens.
On close inspection, we see that I didn't do a great job of camera positioning in the two shots, but let's ignore that. :-) What we see that there is a pretty good match between the two images. There is a slight difference in the depth of field, and we'll get to that later.
To summarize, if you use a crop sensor, the same lens will behave as if you are shooting with a longer lens. If you want to achieve the same field of view as a longer lens on a full-frame sensor, you can just crop the image, and you will get the same effect.
One way in which we can take advantage of the crop factor is to use it to get the field of view of a much longer focal length using a shorter lens. For example, Micro-fourt-thirds (M43) cameras, with their 2× crop can get images that are the equivalent of a 800mm lens on a full-frame body using a 400mm lens. A 400mm lens is much easier to make, lighter, and easier to effectively stabilize. This is why M43 cameras are popular among the sports and wildlife photographers. The APS-C offers a good trade-off between a boost in focal length, and sensor size for shallower depth of field at the equivalent field of view, and this is why many camera manufacturers offer pro crop sensor bodies in addition to the full frame ones.
Effect on depth of field
The depth of field is quite a controversial topic when it comes to the full-frame vs crop sensor arguments. However, as we've already determined, the crop factor literally only crops the image, so there is no logical reason why it would affect the depth of field.
The following two images are taken under the exact same conditions as before: same lens, same focal length, same aperture. Except, this time we are shooting at F4 instead of F5.6.
If you look at part of the table surface that is in focus, you get some idea of the depth of field. You can also see the level of blurriness of the figurine in the back.
In the full-frame image, the figurine in the back looks a bit sharper because it is substantially smaller in the frame (and perhaps because the image has higher contrast). But as before, this is just an illusion created by the crop and nothing else.
As before, let's crop the full-frame image to the exact field of view of the crop sensor and compare the image.
There's not much difference to write home about. There's a slight difference in "sharpness" of the highlight areas but this is more due to the difference in the exposure and overall contrast of the image than anything else. There is no decisive difference that would say that a crop sensor has a different depth of field.
So why is there an argument about crop sensors and depth of field? Let's delve deeper.
One way to look at it is that in order to achieve the same subject framing as the full-frame with the same focal length, the crop sensor camera needs to move back a little, and therefore it will have more depth of field (less bokeh).
So let's try that. I will keep the same setup but move the crop sensor camera back physically in order to match the framing of the full frame image. Of course, with the change of camera position, perspective also changes slightly, so I aimed to make the relative position and size of the main subject (the figurines) roughly similar and did not worry too much about the background.
Now we can more clearly see that the figure in the back (as well as the background details) are better defined in the crop-sensor version of the image. In plain English, there's "less bokeh" in the crop sensor image. This is as expected because we changed the distance to the subject which directly affect the depth of field and background blur.
Another way to look at this is that if we want to keep the same distance to the subject and the same framing, we need to go wider. Because the crop sensor makes the image look narrower, we need to compensate for this. In this case, we shot everything with an 80mm lens, so due to the crop factor of 1.52, we need to use a 50mm(-ish) lens to achieve about the same field of view (technically, it's 52, but I don't have such a lens on me).
The following images compare the original shot from the full frame camera shot at 80mm to the crop camera shot with a 50mm lens, using the same F4 aperture as before.
Because of a different focal length, the perspective is also different, and we can see that the figurine in the back is better-defined than in the crop sensor image compared to the full-frame one (in other words, less bokeh).
If we want the same amount of background blur as we had in the 80mm version, we now need to open up the aperture. The following images are shot at both the different focal length and different aperture to achieve a similar appearance.
In conclusion, while the crop factor does not directly influence the depth of field in any way, our artistic choices when using different size sensors may indirectly do so as we may change the distance or focal length based on the subject.
Lenses with wider apertures are more difficult to make. Also, because the maximum aperture is roughly a ratio between the lens diameter and its focal length, the wider the maximum aperture, the bigger the lens must be. This is why it is easier to achieve shallower depth of field with a full frame camera. If you ask wedding and portrait photographers, they will usually tell you that they prefer the larger sensor for the shallow depth of field. The difference is probably not as dramatic as with focal lengths, but it counts where it counts.
Shallow depth of field is not always desirable, though. For example, in macro and wildlife, more depth of field is often desirable. This is why a crop factor can be a great asset for photographers practicing in those genres.
Effect on sensor noise
There is a popuilar theory revolving around the sensor size and "light-gathering." The theory is essentially this: more surface area, more light. On the surface (pun intended), this appears to make sense to a lot of people. However, this is not how it works in reality.
It is true that a larger sensor / film can capture more light. But then this light is also distributed to the surface of the medium, so we end up talking about light per unit surface, not light per overall surface.
All cameras yield the same exact exposure when using the same exact settings. ISO 200, F2.8, 1/2000s gives the same exposure on any sensor size / film format (save for a few exceptions). If the light-gathering theory were correct, we would expect the larger sensor to be more sensitive to the ligth ("fills up with light quicker" so to speak), and would therefore require less exposure time or lower ISO to achieve the same appearance, which is not the case. This is why light meters have never had, and will never have, a separate setting for film format or sensor size.
My Seconic light meter has no setting for film size or crop factor but works exactly the same way for 35mm film cameras, 6x6 medium format cameras, and my Nikon DSLR.
On the other hand, it is true that most crop sensor cameras will have higher noise and/or more pronounced noise-removal artifacts compared to the full-frame cameras at higher ISO settings. This is usually due to a lower pixel pitch. You can only cram so many pixels onto a sensor before it is too much. Most crop sensor cameras have pretty much maxed out with the existing technologies while the full frame sensors still have some head-room, not to mention the medium format sensors.
Pixel pitch is the distance between adjacent photo sites on the sensor surface. In lay terms, how crowded the sensor is. All other things being equal, lower pixel pitch will typically result in more sensor noise. Of course, this is strictly when all other things are equal, which is usually not the case. Cameras employ different sensor technologies, even different pixel shapes, so it is hard to make an apples to apples comparison and derive a useful conclusion.
For example, Fujifilm X-T3 has a pixel pitch of 3.74µm (micrometers), whereas Nikon D700 has a pixel pitch of whopping 8.42µm. In theory, D700 would have much cleaner images, but it doesn't because it's about a decade older.
The Digital Camera Database contains extensive camera-related information, and, among other things, information about the pixel pitch of the respective sensors.
Effect on dynamic range
Another fallacy that is spread among photographers is that larger sensors have more dynamic range. Similar to sensor noise, this has more to do with the sensor technology than the physical dimensions.
For instance, while full-frame Leica Q, with its formidable 14+ stops of dynamic range does lead in some rankings, the APS-C Leica CL will blow many full-frame cameras out of the water with its equally impressive 13.6 stops. Meanwhile a full-frame Sony A7 III only has 10.3 stops of dynamic range, and the medium format Fujifilm GFX 10.8 stops.
As you can see, there are no rules when it comes to the dynamic range. Some cameras simply have more and some less.
Effect on macro magnification
You may sometimes see "35mm equivalent magnification" listed in the lens specs. This is especially common with lenses made for crop-sensor cameras.
Similar to focal length, the physical magnification will not change with the crop factor. The 1:1 magnification is still 1:1 magnification. What the "35mm equivalent magnification" means is that the 1:1 magnification on a crop sensor will render a subject that fills more of the frame than on a full-frame sensor at the same magnification due to cropping.
Because of the apparent size of the subject relative to the frame, crop factor can be a great asset for macro and close-up photography. You will either get the subject to fill more of the frame, or be able to increase the working distance and achieve the same relative size of the subject to the frame compared to a full-frame sensor.
I hope that I was able to clarify a few things about the crop factor and how it relates to the image output. While it is useful to make direct comparisons in order to understand some of the online discussions or make purchasing decisions, this is not how we normally shoot.
When we photograph, we don't think about how the framing would be done on a larger sensor and translate that to whatever camera we are using. Instead, we look at the subject and the background and make a decision based on that.
Of course, some aspects of the crop-factor are useful to know. Especially the effect on the focal length, which is the most significant one anyway. For the most part, though, the camera sensor's crop factor is not going to be a factor in our final work. Those that appreciate your work will not ask you what the crop factor of the image is.
And lastly let's not forget that cropping your images in post is also a crop.