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Which Camera Sensor Size is Best for Astrophotography?

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Related course: Milky Way Mastery

Among photographers, the discussion about camera sensor size and what implications it has for their photography is always a hot topic. In this brief article, I will discuss the role of camera sensor size in photography, with a particular focus on astrophotography.
This topic can become very technical very quickly and because many aspects of sensors, other than its size, come into play when considering the overall performance of a sensor, I will give here only some general guidelines.
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Physical Size of Common Sensor Types

Historically, the camera sensor size of reference is that of the old 35mm film.
Rather than using the sensor physical dimensions, when discussing and comparing the different sensor types a more commonly used parameter is the so called crop factor, CP, which is much easier to remember.
The CP is the ratio between the size of a full frame sensor (length and width) and that of the sensor under discussion.
Here is the crop factor for the most common sensor types:

  • Full Frame: CP = 1
  • Canon APS-C: CP = 1.6
  • Nikon, Pentax, Sony and Sigma APS-C: CP = 1.5
  • Panasonic and Olympus MFT: CP = 2
  • 1″-type: CP = 2.72
  • 1/3″; CP  = 7.7

The scheme below allows you to visualise the meaning of the crop factor by comparing the areas of the different sensor types.

Diagram explaing camera sensor size comparison between the most common types of digital sensors and the 35 mm full frame sensor.
Sensor size comparison between the most common types of digital sensors and the 35 mm full frame sensor.

What You Need to Know About Field of View and Depth of Field

The camera sensor size, together with the lens focal length and aperture, distance to subject, etc. will affect your images in several ways, the most obvious of which is the field of view.
In order to avoid confusion when illustrating how the sensor size will affect your image, we will put ourselves in specific conditions, but in all cases we will assume that the focal length and aperture will remain the same for all cases.

Field of View

You can find a detailed discussion of depth of field and field of view here. For this article though, all you need to know is in the diagram below.

diagram explaining depth of field and field of view - crop sensor vs full frame camera
When photographing the same subject from the same distance with the same focal length, crop camera sensors gives a smaller FoV.

In practice, it is easier to talk in terms of equivalent focal lengths, EFL. This is the focal length needed to give the same field of view using a full frame sensor.
For example, a 50mm lens on Canon APS-C camera will give a field of view equivalent to 80 mm lens on a full frame camera.
The calculation is simple: EFL=FL*CP
In short, the smaller the sensor, the narrower the field of view is and the longer the EFL.
For more on focal length, check out this article.
You don’t have to be a math whizz to figure this out in the field. There are many online calculators you can use when planning a shoot, or smartphone apps you can use on the go.
These come in handy when creating panoramas from stitching together multiple images.

Depth of Field

Depth of Field (DoF) is related to the focus/out of focus transition in the image.
Technically, when you focus, you are focusing on a single plane. Whatever is in front of or behind said plane is out of focus.
In reality, things are a bit different. We can still see some objects that are in front or behind the subject as being ‘in focus’, even if technically they’re not.
This is because of depth of field. The DoF represents the area in your frame whose contents we perceive to be in focus.
Check out this diagram to understand how sensor size affects depth of field, when focal length, aperture and frame coverage stay unchanged. The only variable here is the sensor size.

When using the same focal length and aperture, in order to fill the frame in the same way a full frame camera does, the photographer using the crop camera has to move farther from the subject. This increases the DoF.

In practice this means that with a crop sensor it can be more difficult to obtain a nice bokeh.
This is quite important in portraiture where the aim is to isolate the model from the background.

The Role of Sensor Size for Astrophotography

As mentioned many time in our astrophotography related articles, starry landscapes, star trails, planetary and deep space photography are all part of the genre. And the sensor size can impact on those kinds of photography in different ways.

The Role of Field of View in Astrophotography

Field of view is obviously as important in astrophotography as in any other kinds of photography.
If you are after starry landscapes, you may find it difficult to go wide enough to frame the entire landscape. Or to frame both the foreground and Polaris, so as to have nice circular star trails.

Nocurnal landscape over water, creative motion blur star trails above - camera sensor size for astrophotography
Despite using my Samyang 7.5 f/3.5 fisheye lens on MFT camera, I was not able to get Polaris in the frame.

On MFT cameras, for example, a common focal length for wide angle lenses is 14 mm (or 12mm). This corresponds to a not very wide 28 (24) mm on full frame.
To go wider, you have to buy rather expensive zoom lenses in the 7-18 mm range. You can get a fisheye lens instead, such as the Samyang 7.5 f/3.5 fisheye lenses.
On the other hand, for planetary and deep sky photography, a cropped sensor can help to close in on your target better than with a full frame camera.
A rather inexpensive 200 mm on an MFT camera will give the same FoV of a more expensive, bulky and heavy 400 mm on a full frame camera.

Diptych deep space photos comparing M42 seen with a 200mm lens on full frame (top) and MFT (bottom) cameras. camera sensor size for astrophotography
M42 seen with a 200mm lens on full frame (top) and MFT (bottom) cameras. (FoV simulated with Stellarium)

The advantage in using cropped sensor camera is that you can easily close in on your target with a relatively compact and lightweight photographic system.
This will make it easier (and cheaper) to work with tracking mounts to follow star movements across the night sky.
This is perhaps the main reason why planetary cameras have small 1/3″ sensors. On my telescope with focal length 1250mm, when photographed with my MFT camera (EFL 2500mm), Jupiter is still a bright, featureless dot surrounded by four smaller dots.
These are its four largest moons, the Galilean moons: Io, Ganymede, Europa, Callisto.

Jupiter seen with a Skymax 90/1250 on MFT camera - sensor size
Jupiter seen with my Skymax 90/1250 on MFT camera (FoV simulated with Stellarium).

In contrast, when using my ZWO ASI 224MC planetary camera with 1/3″, because of the CP=7.7,  the EFL is a whopping 9625mm.
Jupiter reveals its colours, bands and the great red spot.

Jupiter seen with the Skymax 90/1250 and ZWO ASI 224MC camera (1/3" sensor type).
Jupiter seen with the Skymax 90/1250 and ZWO ASI 224MC camera (1/3″ sensor type). (FoV simulated with Stellarium).

Depth of Field and Astrophotography

Depth of field is not something to worry about in astrophotography. The moon, planets, nebulae and stars are so far away that even if you’re shooting at f/1.2, they will be all in focus.
In practice, for example, both the Moon, 300,000km away, and Andromeda, 2.5 million light years away, will both be in focus.
The only possibility where DoF can play a role is in starry landscapes and star trails. That is, if you are very close to a foreground element.
If that is the case, you can take a photo for the foreground and then move to photograph the sky. You can combine the two images later on in Photoshop.

Pixel Size

It is often said that full frame sensors have better low light performance. This is because, usually, they have larger pixels than smaller sensors.
In practice, we can say that for a given aperture and exposure time, larger pixels will collect more photos. This means less noise, and a cleaner and more detailed image.
Pixel size is also related to the sensor dynamic range. The larger the pixels, the wider the dynamic range. You’ll be able to better record highly contrasted scenes.
In astrophotography, this is useful to retain colours in the bright stars or galaxy core, while recording the faintest details.

Image Resolution

Not to be confused with the resolution in pixels, these depend on the sensor size (or pixel size) and the focal lens used.
In astrophotography, sizes of stars, nebulae and galaxies are expressed as apparent angular size. The image resolution is expressed as arcseconds/px (also indicated as “/px). This will give you a measure of how big the portion of sky you will record on each pixel is.
In practice, the part of the sky that is recorded in a pixel will have no details in it.
The message here is that a low resolution (large “/px) will result in a smoother, but less detailed image. And a high image resolution (small “/px) will give you crispy and detailed images.
The drawback of high resolution setups is that poor seeing (i.e. poor visual conditions for the sky) will null the benefit of such high resolution. The amount of details you can see are limited by the seeing conditions.
Also, tracking errors will be much more visible in the form of elongated stars.

Screenshot of an Image resolution calculator where the image resolution is compared with seeing conditions.
Image resolution calculator where the image resolution is compared with seeing conditions.

Best Cameras For Astrophotography

With the argument presented above in mind, we can say that the best cameras for astrophotography are:

  • APS-C or FF cameras for starry landscape and star trails. It is easy to get a large enough FoV and they have somewhat better low light performances and greater dynamic range.
  • For planetary work, a small sensor planetary camera allows to close in on the target thanks to the small FoV.
  • Deep Sky Photography can benefit from a cropped sensor, like that in MFT cameras. It will keep your photographic setup compact and lightweight. This will allow you to use lighter (and cheaper) tracking mounts.
  • On the other hand, if you photograph with a telescope, then MFT have a too narrow FoV and a full frame camera will be best.

Having said that, remember that these are very general guidelines. Any kind of modern camera will give you good results, once you understand the limitations and how to work around them.

The Milky way at the Zenith. Camera sensor size for astrophotography
The Milky Way at the Zenith. Sony RX10, 28mm, fixed tripod. Andromeda Galaxy is visible as a small cloud near the bottom-left part of the frame.

Conclusion

This article is only the tip of the iceberg. Things such has quantum efficiency, read noise, full well, and so on, are other factors that determine the sensor’s capabilities, particularly for astrophotography.
But this a good broad discussion of how sensor size can affect your photography. And how to take advantage of various sensor sizes for better astrophotography.
Finally, remember that sensors are electronic devices. Their performance can vary dramatically with age. A full frame sensor that is a few years old might perform worse than a brand new APS-C or MFT sensor.
Want to know more about sensor size? Check out our new post about equivalence in photography next!

 

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