Let's talk about "reach", which I'll loosely define as the number of pixels remaining in the desired composition, rendering a subject large when the image is viewed at 100% resolution. Longer focal lengths are the ideal method for accomplishing "reach" with any camera, but the term "reach" is usually reserved for when the longest available lens focal length is not long enough for the desired composition.
This article evolved from its conception, and with crop factors, focal lengths, extenders (teleconverters), pixel density, and reach in the discussion, I take on the challenge of omitting confusion.
To get started, I'll share the original purpose behind this article. A friend is on a mission to obtain sharper, higher-resolution images of distant birds. He is using an EOS R5, currently Canon's highest resolution mirrorless camera, with a Canon EF 600mm f/4L IS III USM Lens and EF 1.4x III Extender mounted.
The primary question is, will the Canon EOS R7 and 600mm F4 lens provide better final image quality than the R5, 600mm F4 lens, and 1.4x extender combination? I'll answer that specific question while providing more widely relevant information.
An important clarification is that an APS-C imaging sensor's smaller size does not provide more "reach" than a full-frame imaging sensor. Instead, it crops away a portion of the image circle. While the cropping provides a significant 1.6x narrower angle of view, simulating better reach in the viewfinder, a higher pixel density on the imaging sensor is what provides reach. That most APS-C imaging sensors have a higher pixel density than the full-frame camera models means that photographers touting APS-C cameras as having a reach advantage are usually right, regardless of why they think that.
Mounting a 1.4x extender behind the lens on a full-frame camera makes up for much, but not all, of the full-frame vs. APS-C angle of view difference just discussed. That comment addresses the final framing available but not the reach. When focal length limited, the max available framing is often less important as cropping is likely still necessary. Again, for this article's primary purpose, we are looking for the option that affords the most reach.
For reach, high pixel density is paramount.
A simple way to measure pixel density is to view the pixel size spec. For example, the EOS R5 has a 4.39µm pixel size, and the Canon EOS R7's spec is 3.20µm. Modern image sensors are gapless, so a smaller pixel size correlates to a higher pixel density, and in this example, the R7 has a significantly higher pixel density than the R5.
Interesting is that the R7 pixels are 1.37x smaller than the R5 pixels — creating R7 reach nearly equivalent to that of the R5 with a 1.4x mounted behind the same focal length. The full-frame camera will provide a wider angle of view (1.4x vs. 1.6x) and will provide more pixels overall (45 MP vs. 32.5 MP). Still, when the images are zoomed in to a 100% pixel level view, individual subjects are contained in approximately the same number of pixels from both options. Crop both images to the same composition within the APS-C angle of view, and the images will be nearly identical — assuming that the pixels from both solutions have equivalent quality.
Pixel-level image quality can vary from factors that include low pass filter strength (or lack of this filter), processing applied to the base RAW image, etc.
Another important clarification is that global statements about extender performance must be carefully crafted, as every extender model performs differently with every lens it is mounted behind. Magnifying the image circle of a lens that barely out resolves the imaging sensor may push it past that resolving point, resulting in some amount of blur imparted in an image.
I've long wanted to create an exhaustive extender comparison, but that means testing every camera, lens, and extender combination available, an unrealistic endeavor that is sure to have results impossible to describe concisely. Testing only the lens manufacturer's latest extender models with each lens test provides relevant, valuable results.
When comparing reach, the pixel-level image quality matters, and the R7 and R10 are both excellent in this regard. Always true is that extenders magnify lens aberrations. However, so do higher density imaging sensors.
Another universal truth is that 1.4x extenders reduce the maximum aperture, the focal length to entrance pupil diameter ratio, by one stop (and a two-stop reduction comes with 2x extenders). That one stop is approaching the difference in the amount of light captured by an APS-C sensor vs. the full-frame variant — before any cropping.
Extenders can impact geometric distortion. For example, the Canon RF 1.4x Extender introduces modest barrel distortion. Barrel distortion magnifies the details in the center of the frame more than those in the periphery. In that case, is the 1.4x rating is from the center of the frame (with the periphery magnification something less), or is the magnification rating an average over the entire frame?
Extenders impact AF performance.
"While it’s apparently less than was the case with EF-mount tele extenders and AF, there’s a designed-in reduction in actual AF drive speed of a lens with extenders mounted. This isn’t a design flaw, but rather a feature to ensure consistent AF, and ability for the AF drive to stop at the precise point of sharpest detected focus. Obviously, there’s also the issue of light loss with extenders, and while modern R-series cameras can technically AF at effective max apertures down to f/22, it’s clear that any modern AF system performs better with more light hitting the AF sensor, or image sensor in the case of mirrorless cameras." [Rudy Winston, Canon USA]
As a generalization, smaller pixels create a lower signal-to-noise ratio (SNR) and show more noise visible at a 100% resolution view. While that aspect does not distinguish between full-frame and APS-C imaging sensor sizes, APS-C imaging sensors often have higher pixel densities. This means that 100% view comparisons typically show full-frame models outperforming APS-C models.
Because of its larger size, a full-frame format (35mm) imaging sensor captures over a stop more light than an APS-C format sensor, with equivalent output size reflecting that difference in noise levels.
If cropping the full-frame image to the APS-C angle of view or narrower, the sensor size advantage evaporates, and in that case, f/4 is twice as wide as the f/5.6 max aperture of the 600 F4 + 1.4x combination.
Higher density imaging sensors show the effects of diffraction more readily, with slight effects beginning to show at about f/5.2 for the R7. However, those photographing long-distance subjects with long telephoto lenses likely want the widest aperture available, avoiding diffraction issues.
Should I get a higher pixel density camera or a longer focal length lens is a legitimate question. When getting to the long telephoto focal lengths, with reach as a goal, the camera option may be smaller, lighter, and less expensive.
A variation of that decision and the specific comparison investigated by this article is: should I get an EOS R7 or a 1.4x extender for a full-frame camera? Both options meet the same need.
When focal length limited with the highest resolution full-frame camera model, moving to a longer focal length lens with equal or better image quality is the ideal solution. However, such a lens is not always available, and it may be extremely expensive if it is — potentially far more costly than the R7.
In the case of the Canon RF 600mm F4 L IS USM Lens, Canon offers longer lenses, but with integrated 2x extenders, the Canon RF 800mm F5.6 L IS USM and Canon RF 1200mm F8 L IS USM Lenses do not have equivalent image quality and they are extremely expensive.
Let's take a look at a single APS-C vs. full-frame plus 1.4x comparison.
Note that I am testing the RF versions of the 600 F4 and 1.4x as they are what I currently have. The RF 600mm F4 and EF 600mm F4 III lenses have the same optics, and the RF 1.4x has only a slight optical advantage over the EF 1.4x III.
The images below were processed identically to the samples in the image quality test tool, with the low contrast neutral picture style and a very low sharpening value selected. However, the target was photographed at the same distance for both cameras and framed from farther than the standard framing distance.
Here is the R7 vs. R5 resolution comparison using the proper chart framing.
Back to the promised test images:
The R7 image appears to have very slightly better resolution, and the R5 result's details are slightly larger, though I doubt these slight differences will be noticed in real-world images. The R5 image has more pixels and a modestly wider angle of view, but the R5 + 1.4x and the R7 have about the same reach.
When composed and cropped identically, the background blur created by 600mm f/4 should be similar to that of the 840mm f/5.6. At the same APS-C or wider angles of view, the R7 should take some high ISO noise advantage from the wider aperture enabling a lower ISO setting, and the R7 should avoid the (minor) AF performance penalty imparted by the extender.
Obtaining a sharp image requires all subject details to remain within the indivdual pixels capturing them during the entire exposure. In other words, motion blur is created by subject details crossing into adjacent pixels while the shutter is open. As imaging sensor pixel density increases, so does the shutter speed required to avoid camera and subject motion blur. The image brightness effect from increasing the shutter speed will often be offset by increasing the ISO setting, which increases noise.
However, increasing the focal length has the same effect. So in the end, the option with the most reach will have the highest shutter speed requirement.
Sometimes the camera settings required for a situation include a shutter speed sufficient for stopping motion at the lowest-noise ISO setting, making this point irrelevant.
Did you notice the diffraction softness showing in the R7 f/8 result vs. f/5.6?
Usually, a lens produces better image quality in the center of the image circle than in the periphery. APS-C imaging sensors utilize only the optimal center of the imaging circle. However, extenders magnify the center of the image circle, also utilizing the sweet spot. Thus, both options avoid the worst aberrations.
Here is a periphery comparison from the test described above:
These results tell a story similar to the first results.
Hopefully, the mix of information presented in this article was helpful. A conclusion from this discussion is that the Canon EOS R7 (or another high-density APS-C format camera) is a viable alternative to a 1.4x extender on a full-frame model when significant cropping (APS-C angle of view or smaller) will be required. That's just in case you needed an excuse to get this high-performance camera.