DX vs. FX Formats : What does it really mean?
The question about what “DX” and “FX” formats really means and how it applies to digital camera sensors and lenses can be a topic that is difficult to understand for those who are new (and even not so new) to digital photography. I was also puzzled by these concepts when I first heard people refer to the “crop ratio” of a DX format camera compared to 35mm standard; how a lens’s magnification was “increased” with DX format cameras compared to full frame, etc. There are plenty of very good explanations of these topics on the internet, probably many better than the one you are reading. But this is my way of thinking about it and hopefully after reading a few explanations like this one, you will also understand. I have tried to simplify the explanation and concepts for all levels of photographer who might be reading this, so if some of the more technical aspects are not precise, please remember this.
First of all, I think it is important to understand the concept that camera sensors are compared to the 35mm format film format (also called “full frame”). In digital world, sensors with the same size as these full frame film cameras are referred to as having a FX format sensor. By definition, FX sensors are the same size as the full frame 35mm film camera had, which measures 36 x 24mm. You can see a representation of this sensor size on the diagram below that I found on the internet. So to recap, the terms “35mm”, “full frame” , and “FX” all refer to pretty much the same thing as far as this review is concerned, and for the purpose of this explanation I will use them interchangably. Currently, full frame sensors are found in FX format “professional level” cameras like the Nikon D3s, Canon EOS-1D Mark IV, or Sony Alpha 900. (ie. they have sensors that measures ~36 x 24mm)
Now it’s also important to understand that there is another smaller digital sensor size used in many, if not most, digital SLRs today called “DX format”. These DX format sensors are sometimes referred to as “APS sensors” which stands for Advanced Photo System. Regardless what you call them, the DX format sensors are defined as having a size of 24 x 16mm. Any of you math wizards who are reading this might have notived that 24 multipled by 1.5 equals 36 (24 x 1.5 = 36). So, basically the full frame (FX) sensor is 1.5 times bigger than the DX format sensor. This is where you get the 1.5x conversion factor that people refer to when discussing FX and DX format sensors (though you also see 1.6x or others multipliers depending on the exact sensor dimensions). In this case size does matter 🙂 Most of today’s popular entry level and semi-professional cameras have DX sensors including the Nikon D300s and Canon 7D.
Below is a nice scale diagram from Art of the Image.com ( http://www.artoftheimage.com/ ) that shows the relative sensor sizes of both Nikon and Canon cameras. You can check out their youtube video on this subject at: http://www.youtube.com/user/ArtoftheImage#p/u/48/j2vLJDswULk
One point I would like to make before going on is that all pixels are not created equal. It always bugs me when someone who knows nothing about photography ask how many megapixels my D300 has (12MP by the way) and then replies, “oh my point-and-shoot digital camera has more megapixels than that.” Clearly these peope don’t understand the basics of digital camera sensors, but you will after reading this. Most digital camera sensors are made up a continuous series of photoreceptors that measure the intensity of light and then convert this measurement into a color. Details about the color filters that allow only certain wavelengths of light to be measured and how these filters are arranged on the sensor, which allows it to extrapolate the surrounding color intensity is was beyond the scope of this reiew. But there are plenty of nice in depth reiews on how camera sensors work on the internet. Here is a nice diagram to give you an idea without a length explanation..
Anyway, back to the concept of megapixels…A digital sensor with 12 megapixels has 12 million photoreceptors that are then represented by 12 million pixels, usually described in their horizontal and vertical axis (for example 4288 x 2848 is the number of pixels in the width x height of the sensor). Though both a digital point-and-shoot camera and digital SLR with 12 megapixels have the same number of photoreceptors, these sensors are far from being the same! The SIZE of each photoreceptor (pixel) is vastly different between a point-and-shot, DX, and FX sensor. For example, a point-and-shoot camera has a sensor around 8 x 6mm (or smaller) in size, comparent to a digital SLR DX format sensor of 24 x 16mm, or a full frame sensor of 36 x 24mm. The individual photosensors on the point-and-shoot camera is much smaller in order to be “squeezed” all 12 million onto such a small sensor. the photoreceptors get bigger as the size of the sensor increases. The bigger the photoreceptors are, the more sensitive they are to light (I think of it like buckets in the rain. A point-and-shoot camera has small open buckets that will catch less rain that the larger FX sensor that has large open barrels to collect rain. This is sort of the same for photoreceptors collecting photons of light.) This is one major reason why larger sensor cameras perform better in low light situations with less noise caused by increasing the ISO.
Now that we have covered DX and FX camera sensors, how about the camera lenses? This is actually pretty simple. DX format lenses are designed internally to project the image onto an area the size of a DX sensor (24 x 16mm). FX (full frame) format lenses have a different internal design that allow them to project the image onto a larger are the size of a full frame (FX) sensor (36 x 24mm). This is not to say that you can only use DX lens on a DX format cameras and FX lens on a full frame cameras, because this is not so. In fact, in general (with some exceptions) either format lens can be used on either DX or FX cameras. The resulting image will differ in each of these circumstance however, but we are getting ahead of ourselves and there will be more time to explain this concept later. Take a look at the diagram below to get a better idea of what I was saying in this paragraph. Notice that the FX lens has a larger circular area which represents the image that the lens is sending to the FX sensor represented by the rectangle. The DX circle is smaller and represents a smaller portion of the image that is conveyed to the DX sensor represented by the rectangle.
Before I go any further though, it is important to understand that a lens’s focal distance does NOT actually change regardless if it is DX or FX format or shot on DX or FX sensor cameras. Again, repeat after me… “a lens’s focal length (which is printed on the side of the lens) does NOT change based on the type camera body you put it on.” For example, a 60mm DX lens is still a 60mm lens, even if it on a FX body. Now you might be saying to yourself…”hey wait a minute, you’re wrong. I was told that a 60mm lens would become a 90mm lens on an FX camera body because of the 1.5x conversion factor thingy”. Well you would be half correct if you thought this…half wrong because the lens is still “technically” a 60mm lens, and half correct because the image that a 60mm DX lens would produce on a FX sensor camera would be the same as that of a 90mm FX lens shot on a full frame (FX or 35mm) sensor camera. This is a result of the 1.5x conversion factor from the size difference between the DX and FX sensors. Still with me? If not, take time to read over this last part one more time. Then take a look at the photos below to hopefully, walk you through the logic.
The photo below is from a FX lens paired with a full frame (FX) camera sensor. Just for the sake of the example, let’s say it is a 60mm FX lens. Because both the lens and sensor are FX format, you are seeing the entire scene that was projected and captured on the 36 x 24mm sensor.
Compared to the photo above, the photo below. Obviously the photo below is a “closer crop” of the above photo. There are a couple of scenerios that could be true to give us this crop related to the DX/FX discussion (ie. excluding the fact that I could have walked closer to the mountain 🙂 Anyway, the photograph below could have been taken with a 90mm FX lens on a FX camera to give a closer crop of the mountain at 90mm. However, the other possibility is that the photo below was taken with a 60mm FX format lens on a DX sensor camera. This latter sceneria with the DX camera sensor would result in an image that would be 1.5 times “cropped”, and thus the same as the 90mm FX lens on the FX sensor camera (remember 60 x 1.5 = 90). Read on to discover why this occurs.
Here is how it works. See the image below. The yellow outer line represents the full frame (FX) camera sensor measuring 36 x 24mm. The inner red box represents the size of a DX camera sensor which is 24 x16mm, which is 1.5x smaller than the full frame (I know it says 1.6x but it’s basically the same). As mentioned before, a FX format lens is designed to project the entire scene onto an area the size of a FX sensor (36 x 24mm). The FX lens will do this regardless if the camera has a FX (36 x24mm) or DX (24 x16mm) sensor. In other words, the image that is projected onto a DX camera sensor by a FX lens is actually too big for the sensor. As a result, all of the image information outside of the 24 x 16mm area of the DX sensor is lost (gone forever), but the central portion of the scene is recorded on DX sensor. This results in an image that appears cropped by a magnitude of 1.5 times, but without the loss of the file size that would result from actually cropping the full frame photo in post-processing. Below are some more scenerios to further explain…
FX lens on a FX camera sensor or DX lens on DX sensor:
When you use a FX lens with a FX camera sensor (ie. full frame on full frame) or DX lens with a DX camera sensor, things stay pretty straight forward. In this situation the lens is designed to project the scene onto either the area of a FX or DX camera sensor, respectively. So, your 60mm lens would give you the results you would expect for a 60mm lens.
FX lens on a DX camera sensor:
The next scenerio is probably the most common that people run into. You have a FX lens and shoot it on your DX format camera. For example, I do this often when I use my FX format Nikon 70-200mm f2.8 lens on my DX format Nikon D300 camera. This is where the 1.5x conversion comes into play. In this situation, you have a full frame lens (FX) which is designed to project the entire scene over the size of a full frame (FX) sensor (36 x 24mm). However, your camera has a 1.5x smaller DX sized sensor. As a result only the information (or portion of the scene) within the center is actually recorded by the sensor. The remaining image information on the periphery of the DX sensor is lost. Look at the photos below to get a better idea of this concept. As described the FX lens would project this entire scene to what it “thinks” is a FX sensor. However only the central portion of the scene in the red box is recorded to the sensor and the remaining peripheral image is not recorded. The resulting photo from this combination would look like the second photo below, where your 60mm FX lens results in an image the same as a 90mm lens.
Based on this, it is easy to understand that many nature photographer often prefer a DX format camera paired with a FX lens to give their lens “additional reach” without loosing megapixels from cropping. I mention this because the same image could be aquired by cropping a photo during postprocessing by 1.5x times that was taken with an FX lens on FX camera. But, this causes you to reduce the file size of your final photo. For example, if your DX format camera is 12 MP, then your final image is 12MP. On the other hand, if your FX camera is also 12 MP and you crop the full frame photo to match the 1.5x conversion, then you have lost all the peripheral image and are left with a smaller file (~8MP). If you plan to print large, this could be a problem. It’s also worth mentioning that it makes sense that landscape photographers would prefer full frame (FX) lenses on FX camera sensors in order to get the entire scene in the photo, verus having it “cropped” by a DX format sensor.
A DX lens on a FX camera sensor:
The last scenerio is probably less common. This would be if you have a DX format lens like the Nikon 18-200mm and use it on a professional full frame (FX) camera like the Nikon D3s. So now, you have a lens that is designed to project only the central portion of the scene onto a smaller DX sensor. However, we are now talking about a camera with a larger full frame sensor. The resulting image from this pairing is represented by the photo below. Now only a DX sized image is being transmitted to the center of the larger FX sensor. No parts of the scene from outside the DX area (24 x 16mm) is being transmitted to the periphery of the FX sensor. Fortunatley, most FX cameras have a “DX format” mode and often will automatically recognize that you are using a DX format lens. One negative to this DX format mode is a loss in total megapixels of the final image since it is only using information from the central portion of the sensor. Here is a specific example…the Nikon D3s is normally 12.1 megapixel camera when shooting full frame (FX) In DX format mode on the D3s, photos are only 5.1 megapixels (also why it shoots faster frames per second in DX mode).
The final photo from the full frame camera “DX Format Mode” would look like the one below, which is only the central portion of the scene. Though it would look similar to the FX lens on DX camera photo above, it is a smaller file size because it is only from the central portion of the FX sensor. This is in contrast with the FX lens on DX camera photo which would still be the entire file size since it was taken from the entire DX sensor.
Well I hope that makes this topic at least a little more clear to some… Feel free to leave comments or feedback.
Also, I must admit that I shoot exclusively with DX format cameras at the time of writing this, so the photo examples were all taken with a DX camera (Nikon D300). Photos were cropped to give them the correct proportions for this review, but all of the concepts are still true. I put this “disclaimer” at the end so as to not confuse anyone before reading. I am currently waiting to see what line of cameras Nikon will come out with next before deciding if FX is right for my needs.
Thanks for reading