Is it possible to draw a bead on celestial targets with a standard Canon zoom lens?
It’s All In The Numbers
Infiltrating the night sky can be accomplished with many types of lenses: kits, primes, zooms, macros—if there’s a piece of glass wedged onto a barrel—then it could be used to image stars. Canon’s EF 75-300mm lens just so happens to have a piece of glass that fits snugly within a telescopic barrel, which means the lens can be used for astronomical purposes.
** The Stats **
Aperture: Max (f/4-5.6) Min (f/32-45)
Focal Length: 75-300mm/120-480mm (APS-C 1.6x)
Angle of View: 32º 11′ – 8° 15′
Image Stabilization: None
Filter Thread: 58mm
Weight: 1.6 lb (480 g)
Price: $199.99 (as of 2016)
A lens like this is well suited for snipping high-ranking celestial officers, especially if it’s equipped on a crop sensor camera (1.6x). The extra boost in focal length makes a substantial difference: 480mm is equal to a short tube refractor, however, the camera lens will be much smaller than most typical refractor objective lenses, which means it will not be able to collect as much light.
Canon’s EF 75-300mm zoom kit lens is made out of a lightweight plastic, enabling the everyday solider to carry it out into the field, with minimal effort.
The manual focus ring glides, as well as the zooming mechanism. But that doesn’t mean this lens won’t backfire! Be careful…or you could be left fending for yourself within the night sky trenches.
Although the lens is able to change focal lengths, it can’t do so without changing the aperture, which means the lens can only open to f/4-5.6 (depending on the focal length chosen). A 300mm f/5.6 camera lens is not ideal for astrophotography, and it will produce dim star fields within the frame.
Read, Aim, Focus!
Zoom lenses can focus starlight, but they can also focus past infinity…which is never a good thing. Focusing past infinity causes stars to bloat into hazy blobs—if you’re not careful—then this lens will backfire. Every zoom lens on the market comes with this optical hindrance, so it’s your duty to make sure focus is precise.
Untracked exposure times will vary depending on the focal length being used.
When the focus and exposure times are properly dialed in, then the Canon EF 75-300mm zoom lens will accomplish the mission, and you’ll be granted a chance to
live photograph another night.
Anything less will result in absolute termination.
Secrets, Techniques & Tricks
Basic training is almost over, but before you depart on your first mission, it’s imperative that you learn a few tactics.
Focus on a bright object (planet or star)
Locate the brightest object in your local night sky, and then focus the lens before trying to image objects on your celestial hit list.
Camera viewfinders are not designed for night time use. Most stars in the night sky won’t be visible through a typical camera viewfinder, which means only the brightest objects will be easily visible.
Use Camera’s LCD Screen
Focusing starlight through the camera viewfinder is a needless pain in the ass. So don’t do it. Instead: turn on your camera’s LCD screen (or LiveView).
Set ISO to 1600 or above (6400 max)
Open Camera Aperture
Locate Bright Object Within LCD Screen
Magnify image 5x
Magnify image 10x
This focusing method will ensure a predictable level of precision, especially when imaging dim objects that can’t be seen through the camera viewfinder. Atmospheric turbulence becomes apparent the more the image is magnified—keep that in mind while you try to obtain the best possible focus.
** Approximate Exposure Chart For Untracked Images **
75mm = 8-10 seconds
100mm = 5 Seconds
300mm = 3 Seconds
A Closer Inspection
Lightweight and light cost usually comes with a price, and that price is degraded optics. The inexpensive glass will exhibit chromatic aberration, especially around bright objects, such as Jupiter or Sirius. But that shouldn’t convince night sky soldiers to never carry this lens as a backup, nor shall its hindrances persuade individuals from never using this lens.
Bright object may cause flares or reflections.
Jupiter and Sirius tend to cause lens flares, but so can Venus, and any other similarly bright object.
Take note of the subtle reflection above the cropped image of Sirius (click on image to enlarge). Chromatic aberration is also (purplish halo) apparent.
The overall optical quality is comparable to many other standard issue Canon lenses. The field of view is also sharper toward the middle of the frame, and then gradually softens toward the corners.
You can see both optical defects within the cropped image of Orion’s Belt.
Each bright star of Orion’s Belt (Alnitak, Alnilam, Mintaka) expresses a distinct coma. The blueish nature of the background stars is caused by a modified white balance.
Stars also appear less sharp toward the edges of the frame, however, many short tube refractors also have a degree of field curvature (unless corrected: field flatterer).
All of the negative aspects of Canon’s EF 75-300mm is not nessarily caused by poor optics, but that doesn’t mean the glass is made from high caliber material, either. A spherical piece of glass will always exhibit optical flaws—it’s just a matter of severity.
If you happen to own this lens and are interested in photographing celestial objects, then there’s no reason why you can’t snipe a space nebula tonight (assuming the clouds don’t give you the middle finger).