If you are new to photography, you may be confused by the number of different sensor sizes you can choose from.
Why are there so many different types? In what way do they differ? How does the sensor size affect the image and its quality? Which one should I choose?
These are all legitimate questions, and in this article, I will try to explain the difference between Full Frame, APS-C, and Micro Four Thirds sensors and answer them simply and directly, without going too deep into the technical aspects.
Overview Of the Different Sensors: Full Frame, APS-C, Micro 4/3 And smaller
Sensor Size
Since digital sensors became the standard in photography, a plethora of formats were developed, and nowadays, there are many different sensors to choose from.
Luckily, the rule of thumb when considering sensors of different formats is rather simple: the larger, the better.
Small sensors (up to 1”-type)
Sensors smaller than a 1”-type sensor, are usually reserved for camera phones and entry-level compact cameras.
Prosumer compact and bridge cameras, such as the Sony RX100 and RX10 series, often use a 1”-type sensor or larger, which ensures higher image quality, better low light performances and lower digital noise than the smaller sensors.
Medium size sensors: Four Thirds and Micro Four Thirds Formats
In the game of “the larger, the better,” Olympus was playing the role of the outsider.
While competitors like Canon and Nikon were using large sensors for their DSLR cameras, Olympus set with the four thirds (FT) format, which is larger than 1”-type sensors but slightly smaller than APS-C sensors.
More recently, together with Panasonic, Olympus developed a new standard, called Micro Four Thirds (MFT), for their new mirrorless cameras.
Note that the MFT sensor has the same size as the FT.
And now, once again, with Panasonic moving towards full-frame mirrorless cameras, Olympus will likely return to play in the game as the outsider, sticking to their MFT standard.
Large Sensors: APS-C And Full Frame Formats
DSLR cameras traditionally aim at professionals, and so they have always used some of the largest sensors, namely the APS-C and the Full Frame (FF) formats.
These were the largest sensors on the market until the rising of the digital medium format, but costs are prohibitive for the general public as well as many pros.
How Small is… Small? The Crop Factor
The sensor of reference is the Full Frame. This sensor has the same size as the frame of 35mm roll film, and measures 36 x 24 mm.
With the exception of digital medium format sensors, all other digital sensors on the market are smaller than Full Frame.
One way to compare the relative dimensions of different sensors is with the Crop Factor, CF.
Commonly, the CF is defined as the ratio between the diagonal of a 35mm frame (the Full Frame) and that of the sensor you are considering:
CFsensor=DFF / Dsensor,
Where D is the diagonal of the sensor.
To clarify this concept, let’s do an example. As digital sensors are rectangular in shape, with sides named A and B, it is easy to calculate the diagonal, D:
D2=A2+B2
For the FF sensor, A and B measure 36 and 24 mm, respectively. The diagonal, DFF, is therefore 43.27 mm. An MFT sensor measures 17.3 (A) x 13 (B) mm, and its diagonal DMFT is 21.65 mm.
By definition, the MFT sensor has a CF = DFF / DMFT = 2.
In the same way we can calculate the CF for the most common sensor types:
- Full Frame: CF = 1
- Canon APS-C: CF = 1.6
- Nikon, Pentax, Sony and Sigma APS-C: CF = 1.5
- Panasonic and Olympus MFT: CF = 2
- 1″-type: CF = 2.72
- 1/3″: CF = 7.7
Finally, the ratio between sensors areas is equal to the CF squared.
Sensor Size And Real Life Photography: What Changes?
Costs
As I said, when it comes to choosing a sensor format, the larger the better. But this does not come for free.
High-quality sensors are expensive to make, and larger they are, more they cost: if you want a FF camera, be prepared to pay a rather steep price.
Field Of View
The Field Of View, FoV, is one of the obvious differences when considering between sensors of different sizes.
As a rule of thumb, for a given focal length and distance to the subject, the smaller the sensor, the smaller the FoV.
How much smaller the FoV depends on the crop factor, which is also known as focal length multiplier.
Suppose you are using a 50mm on a micro four-thirds camera. Because MFT sensors have a CF=2, the FoV is equivalent to that obtained by using a 100mm lens on a full-frame camera.
For astrophotography, you can use the desktop version of the free software Stellarium to check the FoV for a particular target given from a specific combination of camera sensors and astro telescopes.
Depth Of Field
The size of the sensor also affects the depth of field, DoF, i.e., how much of the image looks in-focus after you have focused on your target.
This is more important in portraiture than in astrophotography. As when shooting portraits, you need a good separation between the model and the background.
As a rule of thumb, for a given combination of a camera lens, aperture and frame filling, the smaller the sensor used, the larger the DoF.
Sensor Resolution and Optical resolution.
Light on a digital sensor is recorded using pixels, small bits of the sensors that are sensitive to light.
How many pixels a sensor has, defines the sensor resolution, a quantity often expressed in megapixels.
For a given sensor size and resolution, the smaller the sensors, the smaller the pixels are.
And, for a given lens and pixel size, the smaller the pixels, the greater the optical resolution of your setup, meaning it can capture finer details.
This is particularly important when trying to match a camera to a telescope for your astrophotography setup.
A handy calculator is this one from Astronomy Tools: as you see in the comparison below, the FF camera Canon 6D has pixels much larger than those in my MFT camera Olympus OM-D EM-5 Mk ii.
But the optical resolution, the amount of sky a single pixel can see, changes from 2.25”/pixels of the canon to a better 1.29”/pixels for my Olympus.
Quality of Image In Low Light
I know what you are thinking: if small sensors with high resolutions give a narrower FoV, a larger DoF and better optical resolution for a given lens than large sensors, why the larger, the better?
For a given sensor resolution, a FF sensor has larger pixels than smaller ones, and this translates into better image quality, low noise, low light performances and larger dynamic range (the range of brightness values in a scene the sensor can record).
For many, those qualities are more important than, say, the mere optical resolution for planetary imaging.
Lenses
Lenses are related to the sensor size as they need to provide a circular image that is larger than the sensor.
This is why lenses for full-frame cameras are larger in comparison from those built for, say, micro four-thirds cameras.
And here is where the myth about mirrorless cameras are much more portable than the DSLR ones goes out the window.
The myth was (and is) true when mirrorless cameras were using micro four-thirds sensors or smaller. But, in today’s market, many mirrorless cameras are full-frame cameras and pretty much require the same bulkier lenses than the DSLR counterparts.
Lenses designed for full-frame cameras can be used on MFT and APS-C cameras, but the opposite is not true (or advisable).
On the other hand, lenses designed for cameras using a sensor smaller than Full Frame cannot fully illuminate the larger full-frame sensor. Thus they will create strong vignetting.
Does The Sensor Size Make A Difference With Astrophotography?
Now that you know what changes depending on the sensor size, how does the sensor size affect your astrophotography?
Do I need a full-frame camera for astrophotography?
Because of their superior low light performances and lower noise, full-frame cameras are very good for astrophotography.
The large pixels do a great job of collecting light.
So, if you have a full-frame camera, use it: you will not be disappointed.
Should I upgrade from MFT And APS C To Full-Frame?
If you are thinking of upgrading your MFT or APS-C camera to a Full Frame camera, you can of course do that, but for astrophotography, the overall improvement may not be worth the money of the upgrade.
Smaller sensors have worse low light performances than FF ones, but this is true if we compare sensors of similar ages.
As technology progresses, your APS-C camera may perform better than a 5 years older full-frame camera, so be careful when shopping for a second hand camera.
If you are using a small sensor and you are into starry landscapes, because of the crop factor, you may find it difficult to have a field of view that is large enough. In this case, upgrading to a camera with a larger sensor can also improve this aspect.
On the other hand, small sensors often have pixels smaller than those in a FF camera. For a given lens, a setup based on a small sensor camera has a better optical resolution, meaning you can record more fine details.
Then, consider that a MFT camera, while having a smaller sensor than a full-frame, also has smaller lenses: lightness and compactness of the system can compensate for slightly worse performances with respect to a FF camera.
And this is particularly true if you are on a star tracker, as they have a relatively small maximum payload.
Finally, if the reasons to upgrade are to get better astrophotography images, consider that no DSLR beats a cooled sensor, CCD, monochrome astro camera.
Once again, you have to find a compromise that works better for you.
Is There A Place In Astrophotography For Small Sensor Cameras?
Yes, there is.
This is often the case of planetary astro cameras.
These cameras have small sensors, to narrow the field of view and increase the depth of field, and high sensor resolution, to have small pixels.
The combination of small sensors with small pixels allows for a great optical resolution when a long focal telescope is used for planetary astrophotography.
Video Explaining The Different Sensor Sizes
Here is a quick and easy video discussing how sensor size affects your photography and videography.
Conclusion
Sensor size does matter. FF cameras have better low light performances, lower noise, and generally greater image quality than APS-C and MFT cameras of comparable age.
When it comes to astrophotography, though, things are not so simple.
Larger sensor cameras with their bulky and heavy lenses can reduce the tracking performance of your star tracker.
Also, if you are a traveler astrophotographer, you may value the size and portability of the equipment at the cost of having slightly noisier images.
And if you are after Moon and planetary astrophotography, you want to use small sensor planetary cameras rather than full-frame ones.
In photography, even more so in astrophotography, more than absolute performances, you need to compromise and get the package that works best for you.