Shooting for the Stars: Canon EF 100mm USM



Yes, my friend! You read that correctly; it’s possible to use the Canon EF 100mm macro lens for astrophotography. I haven’t used this lens too much for night sky work, but from my little experience, the Canon EF 100mm USM macro lens is certainly viable.

The lens is an F2.8—fast enough to capture starlight without the process being too much of a pain in the ass. However, do to its focal length (100mm), unguided exposures need to be kept relatively short compared to a wide-angle lens.


Rough Exposure Guide


  • 14-24mm = 30sec.

  • 50mm = 10sec.

  • 100mm = 5sec.


To complicate things further; your camera’s sensor plays a role. A full-frame sensor are able to take a longer, unguided exposure without star trails becoming evident compared to a crop-sensor. For example, a 100mm lens is equivalent to a 160mm on my Canon Rebel XSi (Nikon has a 1.5x crop factor).


100 x 1.6 = 160


If you’re using a camera with a crop-sensor, reduce exposure times by a few seconds according to the reference figures I provided above.

Everyone has different tolerances when it comes to star trails. Obviously, the best case scenario would be not to have any at all, however if you’re shooing from the hip, so to speak, you decide when star trails become visually unacceptable. If you plan on stacking images, do your best not to trail by keeping exposures as short as possible.

You have to be careful when focusing the Canon EF 100mm lens! On the barrel is a symbol that’s shaped like an “L”, align that symbol with the white line printed underneath, otherwise you’ll focus past infinity…and no one likes that. If you still have issues, pick out a bright star or planet, and manually eyeball the focus to the best of your ability.


The Canon EF 100mm macro lens offers a 24° field of view. To put that in perspective; it’s possible to fit Orion’s Belt, and perhaps Rigel within the frame at the time. This is a decent lens if you want to target specific objects like the Pleiades or Orion Nebula if you don’t own a telescope that is able to do the job.


This image is of the red giant star Betelgeuse located in the constellation of Orion. You can read about Betelgeuse in more detail—here.

Image Specs:

  • Exposure 10 seconds

  • ISO 1600

  • F2.8


As you can see; an untracked, 10 second exposure is enough for the Canon EF 100mm USM to cause star trails. It’s minor, but certainly perceptible.


The same goes with this uncropped image of M42: star trails are perceptible, especially around the edges of the frame.

Image Specs:

  • Exposure 10 Seconds

  • ISO 1600

  • Aperture F/2.8


To minimize star trails without a tracking system, cut down the exposure to about 5 seconds with a crop sensor. Originally when I took these images, I purposely exposed for an extra five seconds to collect those yummy, dim photons emitted from that cosmic cradle known as the Orion Nebula. You can see it below Orion’s belt as a wispy, pinkish gas.

Orion Nebula

In this cropped image, star trails are much more discernible.

Next, let’s take a gander at an exposure of the Banded Giant—Jupiter.


Image Specs:

  • Exposure 5 Seconds

  • ISO 1600

  • Aperture F2.8


Compared to the other images that were exposed unguided for 10 seconds; star trails are much less discernible.

Chromatic aberration is kept at a minimal with the Canon EF 100mm USM macro lens—especially compared to Canon’s EF 50mm macro lens. Personally, I thought it was going to be much worse due to the longer focal length, but the over-all optical construction (along with special coatings) reduces the optical deformity, but it’s still noticeable.


Published by FlyTrapMan

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