Some possible reasons, arranged in the likely order of influence, for the lack of clarity in the example photo:
1) The optical limits of your lens. The EF 100-300mm f/4.5-5.6 was released as a budget telephoto zoom lens in 1990 at the dawn of the EOS era. Compared to the current EF-S 55-250mm f/4-5.6 STM, at the longest focal lengths and widest apertures there's a significant difference in sharpness.
35mm film is much less demanding of a lens in terms of resolution than modern digital sensors such as the one in your 20MP 70D. From an answer to a question about the difference between "digital lenses" and "film lenses"¹:
Although not universally the case, most lenses designed and introduced during the digital age are better than their older film era counterparts, especially in the consumer and mid grade sectors. Manufacturers of the top tier lenses have also been forced to introduce newer versions of old classics. The new consumer lenses may not be as good as the old "L" glass (but sometimes they get close), but they are much better than yesterdays consumer lenses. Especially zoom lenses which have benefited tremendously from computer aided design and modeling. What used to take weeks or even months to test by making a physical prototype can now be accomplished in a few hours using supercomputer simulation.
Users of digital cameras tend to expect more out of their lenses due to primarily two factors:
- Digital sensors are perfectly flat. Film isn't. Some of the most expensive film cameras actually had mechanisms that created a vacuum behind the film to aid it in laying as flat as possible while being exposed. Even then, with color film the emulsion layer for each color was at a slightly different depth. So if focus was perfect for one color, it would be slightly off for the other two!
- Pixel peeping has raised expectations to a ridiculous level. Take a 20MP image and display it at 100% (1 pixel per screen pixel) on an ≈23 inch HD (1920x1080) monitor and the magnification is equivalent to printing at 56x37 inches! No one expected a 35mm consumer grade lens to be perfect at 56x37! But a lot of folks now seem to.
2) Shooting a very dim object that is moving across the frame. One second is far too long to expose the moon using a 300mm focal length without a tracking mount if one is going to critically look at the image at 100% magnification. At 100%, it is easy to see the trails of the two bright stars in your example photo. The moon is also blurred by approximately the same amount of movement.² The moon is not normally a dim object, so we usually do not need to worry about our shutter times being too slow. Even though we usually shoot it at night, the moon's surface is being directly illuminated by the sun. At ISO 100 and f/8, we would normally expose the moon for about 1/125-1/250 second. But during a total eclipse, when the earth blocks the sun's direct light from illuminating the moon, the moon's surface gets a LOT darker.³ The earth still rotates at the same rate underneath the sky. The reduced brightness pushes us into a very tight corner regarding how to collect enough light for a usable image without the apparent motion of the moon making it blurry. The most obvious solution is to use a wider aperture - if one is available. But even moving from, say, f/8 to f/2.8 only gains us three of the thirteen-plus stop difference between a full moon and totality. Going from 1/250 second to 1/15 second only gains another four stops and at 300mm we are already going to start seeing motion blur when pixel peeping. At that point we're still about 3-6 stops dimmer than when the moon is full. Going from ISO 100 to ISO 1600 gets us back in the ballpark, but we have given up a lot in terms of clarity due to:
- The much slower shutter time allows some motion blur
- The wider aperture (most lenses are sharper stopped down than when used wide open)
- The higher noise associated with using higher amplification (ISO) to make up for less light entering the camera, and the resulting noise reduction we use.
3) Atmospheric interference. If you were shooting from the location indicated in your user profile, the moon was fairly low on the horizon at the time. Just as the sun is much more distorted by the earth's atmosphere at sunrise and sunset than when it is high in the sky, so is the moon. Not only is the light having to travel further at an angle through the ocean of air surrounding our planet, but the temperature differentials near the terminator (the line between daylight and dark) tend to increase atmospheric turbulence in the times around dawn and dusk.
4) Letting the camera make all of the decisions about how to process the raw data from the sensor. This is particularly the case with a dim object, such as the moon during totality, that is moving across the frame. This limits our exposure time. Most great moon photos (when it is not in the earth's shadow) you see are saved in a raw file format and post-processed to fine tune the contrast between darker and lighter areas on the surface of the moon. Color temperature and white balance adjustments, sharpening, and in some cases even digitally applied color filters, can bring out the contrast between different areas of the moon. This is even more critical when the photo in question is taken during a total eclipse.
5) The noise reduction applied to using ISO 1000 with a Canon EOS camera. I'm a Canon shooter because, overall, Canon works for what I do. Every system, though, has advantages and disadvantages. One of the things where Canon falls a little short is in the way their cameras handle the "partial" stop ISO settings. For a comprehensive look at how Canon cameras handle the "partial stop" ISO settings and why using the "+1/3 stop" ISO settings (such as ISO , 250, 500, 1000, 2000, etc.) can make your photos noisier than other ISO settings that are even higher, please see Is it really better to shoot at full-stop ISOs?. The amount of NR the camera applies to ISO 1000 by default will reduce the detail in the image.
¹ Back near the beginning of the consumer digital SLR era, APS-C only lenses were often marketed as "digital" lenses.
² The moon moves roughly 1/2° less per hour than the stars as viewed from the earth's surface. That also happens to be approximately the moon's angular size in the sky. So for a one second exposure, the moon would move across the frame 1/3600 of its own diameter less than the nearby stars would during the same exposure.
³ This article from Space.com says anywhere from 10,000 to 100,000 times dimmer, depending on the earth's atmospheric conditions. That's between 13 and 17 stops darker than a full moon!