Full Frame vs Crop Sensor: Is Full Frame Worth The Extra Cost?
Full frame, crop sensor DX, APS-C, FX, full frame equivalent These are terms that get thrown around a lot when it comes to digital cameras and lenses. And rightfully, it can also be a source of confusion for novice or intermediate-level photographers who don’t know what they mean or how it affects their photography. In this article, I’ll attempt to introduce these concepts in simple terms and how they can affect your images when their applied to astrophotography.
Let’s simplify the terminology
In order to understand what’s what, we first need these terms.
On 35mm film, the imaging area was 36mm (wide) by 24mm (high). There really wasn’t much more to it. You got a 35mm SLR and went about your business of taking photos. There were many options available in terms of features the camera offered like auto-focus and such, but you had 1 image size when it came to an SLR. Nowadays, we find there are far more choices available, but when speaking about DSLRs and in some case, mirrorless cameras, there are 2 basic categories that cover the vast majority of all cameras used by astrophotographers – full frame and crop sensor (the latter also known as APS-C [Advanced Photo System type-C]). Although implemented differently by different camera manufacturers, the concept is the same. These sensor sizes are based on 35mm film camera. Crop sensors come in various physical sizes but most offer crop factors of 1.5 or 1.6x.
Full Frame is the equivalent of 35mm film producing an image with a 3:2 aspect ratio. The physical sensor size is 36 x 24mm, the same size as a 35mm film cell. This is the base standard for all DSLR cameras. Nikon refers to their full frame sensor size as FX.
Crop sensor, or APS-C offers smaller sensor sizes that are a subset of the full 35mm sensor size, or a “crop” of that. The physical sensor size is smaller than a full frame (1/1.5 or 0.67x for 1.5 crop factor, 1/1.6 or 0.625x for 1.6 crop factor), but retains the same 3:2 aspect ratio of their full frame big brothers. Nikon refers to their crop sensor size as DX.
The term “full frame equivalent” is used for lenses used on APS-C cameras. The smaller sensor size affects the magnification and field of view you get from a particular focal length compared to a full frame. This will be explained in greater detail further in this article.
There are other sensor sizes like APS-H, Micro 4/3 and Medium Format. Dedicated CCD and CMOS astro-cameras come with various sensor sizes and formats as well, but for the sake of this article, we’ll be sticking with the sensor formats used by the majority of DSLRs and mirrorless cameras that are popular for astrophotography; namely, Canon and Nikon DSLRs and Sony and Fuji mirrorless.
Why does any of this matter?
So now that we have the terminology simplified, we can get to the meat of this subject. For any examples, I’ll be using a hypothetical 24 MP sensor in the various formats as my example, as this seems to be a common size many current model DSLRs are produced in today, even in entry-level models. Keep in mind that a sensor’s MP (Megapixel) count really means nothing in this case. We’re comparing different size sensors with the same pixel count in all cases unless specified.
The size of your sensor determines 2 things – how much light it can capture, and how wide your field of view will be using the same lens. The sensor itself is covered in “pixels”. The individual light collectors on your sensor chip are called photosites. A 24 MP sensor will have 24 million colour photosites which collect the light focused on them by the lens. Like the sensor itself, the size of the photosites matter. On a full frame sensor, the individual photosites are larger to fill up the larger physical dimensions of the chip, therefore gather more light. And inversely, fitting 24 million photosites on a smaller physical chip requires making each individual photosite smaller.
One of the advantages of full frame sensors is their lower noise than crop sensors. This is because photosites will generate heat when actively collecting light. Larger photosites and larger sensors means that they’re able to dissipate heat better whereas the smaller, mode densely packed photosites on the smaller chip are more sensitive to heat. Sensor heat is the biggest contributor to digital noise when using high ISO (gain) settings or doing long exposures – the 2 things that we do most in astrophotography. This is the reason why full frame cameras will have better noise tolerance and better low light performance than a crop sensor camera with an equivalent pixel count sensor.
The second affect of sensor size is field of view or viewing angle. This is where the aforementioned term “full frame equivalent” comes into play. With an equivalent lens (a 20mm, for example), a full frame sensor will produce a wider field of view. Depending on the crop factor of the sensor, the magnification will be increased by the crop factor of the sensor. In the case of Nikon and many other brands with a crop factor of 1.5, the full frame equivalent will be 30mm (20mm x 1.5). On a Canon ASP-C sensor, the crop factor is 1.6x. So the lens will give the full frame equivalent of 32mm (20mm x 1.6). The image at Figure 1 below will show how the different fields of view vary with sensor size. But the basic thing to keep in mind is that the higher the crop factor, the narrower your field of view and the higher magnification you will get from the same optics.
Figures 2-4 below display the different in the field of view in the same image when rendered at the different crop values. The difference in size between the 2 APS-C sensors is noticeable, although not drastically. However, the difference between the APS-C images and the full frame is quite remarkable. A lot of the field of view is lost. This can be compensated for on an APS-C camera with a wider lens. In this case, a lens of about 8mm would have to be used in order to produce the same wide field of view the 13mm lens produced on the full frame camera
You said pixel count doesn’t matter. Then why is this a selling point for cameras?
Pixel count does matter, but not the way people tend to think it does. High pixel count cameras are often touted as being better, but that’s not the case. A high quality, full frame 10MP camera will produce images orders of magnitude better than a low end 24 MP APS-C will. In terms of camera performance for low light and high ISO, ironically fewer pixels are better, as the individual photosites are larger on the lower pixel count. With all other things being equal (sensor technology, processor, camera features, etc), a 24 MP full frame sensor will have better low light performance and be more noise-tolerant than a 36 MP full frame sensor, as the higher resolution sensor will have smaller, more densely packed photosites. With a bit of simple math, you’ll find out that the 36 MP full frame sensor has the same pixel size and density as a 16MP ASP-C camera in order to pack that many of them onto the same sensor. The trade-off will be the high MP count sensor will be able to resolve finer details in the scene being photographed than the lower pixel count sensor.
Whether that trade-off is worthwhile for you really depends on you and what your target audience is. If you regularly print large poster size images, submit your images to agencies that require a certain pixel count, or use smaller cropped subsections of your images, the higher pixel count is likely the better option as you’ll be able to resolve finer detail. If you photos are for print or online publication or you produce smaller prints (under 20 wide), then the lower pixel count and better performance are likely the best choice.
Do I need a full frame camera for astrophotography?
If only there were a simple answer to this question. The debate on this issue rages on constantly. It’s difficult to get a consensus on which is better. Undoubtedly, your photography in general will benefit from a full frame sensor. Wider vistas, better low light performance and cleaner images are the obvious benefits. But there are pros and cons, just as using an APS-C camera.
For astrophotography using a telescope or a long zoom lens on a tracker, an APS-C sensor is often preferable. The smaller sensor means you’ll be able to get extra magnification and a tighter field of view. On small galaxies, planetary nebulae and globular clusters, you’ll get a larger image (1.5 or 1.6x larger) and be able to resolve finer detail than a full frame camera with an equivalent pixel count. The narrower field of view also means you’ll experience less vignetting than you would using a larger sensor. Many APS-C cameras are smaller and lighter than their full frame counterparts. The payload you have on your mount is always an issue for astrophotography, so being able to shave off some extra weight is always helpful and it will generally make your rig easier to balance.
And there’s also the cost benefits that need to be considered. Outside of a few high end models, APS-C cameras are significantly less expensive than full frame. Their lenses require significantly less glass to produce, so they also tend to be significantly cheaper as well. You can often find top shelf wide aperture crop format lenses for the same price as less capable mid range full frame lenses. On a side note, you can also use full frame lenses on a crop sensor camera, so if you plan on eventually getting a full frame camera, you can start buying your full frame lenses and using them with your crop sensor camera.
On the flip side, large galaxies like the Andromeda Galaxy or large clusters like the Pleiades or Double Cluster in Perseus may not fit into your field of view when your camera is paired with a higher focal length telescope, requiring you to do a mosaic in order to get the full object in one final image. When shooting widefield landscape images like Milky Way or aurora, you’ll have a narrower field of view and not be able to expose your images as long as with a full frame on equivalent lenses (see the Rule of 500 in my How To Shoot The Milky Way And Night Sky With A DSLR Camera article for more information). Although as mentioned above, this last point can always be remedied by using a shorter focal length lens.
For widefield landscape astrophotography, it’s hard to beat a full frame camera. The superior low light sensitivity and more robust noise of full frame sensors mean you get cleaner, brighter images. Your use through a telescope will also benefit from the wider field of view and better noise tolerance, particularly if shooting long, guided exposures. A series of long (5+ minute) exposures at ISO 1600 will reveal details you won’t be able to capture as easily with an APS-C sensor without adding significantly more integration time at lower ISO settings to maintain the same low noise levels.
However, on many telescopes, the vignetting you experience may be quite significant. As an example, the vignetting I experience using my my Nikon D750 on my 8 Meade LX90 optical tube with a 0.63x focal reducer is so bad that I only get a usable image area the same size I would if I were using an APS-C camera. This can be partly correct by shooting flat frames, but is still an annoyance. But this really depends on the telescope used. When I use the D750 on my ED80 refractor, the vignetting is negligible. Your results will vary.
Another disadvantage of shooting deep sky through a telescope with a full frame sensor is shooting smaller objects such as planetary nebulae, globular clusters and small, distant galaxies. Your targets will be tiny in your final image compared to APS-C. Unless you’re looking for a wide field of view, you’ll have to crop your images to get them larger, which will give you a final result with less fine detail than if shot with an equivalent APS-C camera. This, however, is an area where high pixel count (like 36MP) sensors can help significantly. They won’t perform as well in terms of noise and sensitivity, but they’ll be able to resolve finer details that lower pixel count sensors won’t.
Which is right for me?
As with most things in life, budget will often be the single biggest determining factor that guides your purchase. While it may be nice to rock the latest and greatest Nikon D5 or Canon 1D