5 Important Checks Before Buying a Lens for Astrophotography
Not a day goes by without an email or DM from somebody asking me about gear. Should I buy this lens or that lens? What do you think of that lens? Is the extra £400 worth the extra 2/3 stops advantage in aperture?
The issue is that there are so many personal factors that go into such a decision that I usually can’t be of help. Is this person a full-time professional or a light hobbyist? How much money do they earn? Will their spouse kill them if they buy another new lens?
But at the very least I can offer some advice on how to check if a lens is good for astrophotography before even buying it. Then the final decision is up to you.
Check 1: Aberrations
So first up we need to check what aberrations the lens suffers - imperfections in the way a lens focuses light onto the sensor, resulting in unwanted artefacts. There’s a few different types, most of them we don’t want, but one of them can actually be desirable.
The most important in my opinion are coma and astigmatism and for that we’re gunna head to LensTip.com. If they’ve reviewed the lens you’re interested in jump to Chapter 7. Coma, Astigmatism and Bokeh.
Here they take a photo of a small red diode allowing you to see some of the lenses aberrations.
So coma can make the stars in your images look like comets.
And astigmatism can add wings and tails to your stars, which we obviously don’t want.
We want round-point stars, not swarms of seagulls.
It’s also worth noting that these aberrations are always worse in the corners of the frames and do improve when you stop the lens down. So if you’re looking at an f/1.4 lens it probably sucks at f/1.4 but might be OK at f/2.
The reason this test is so important in my opinion is because it can’t really be fixed in post-production unless you crop out the funky stars from your images.
You can then jump to Chapter 5 to quickly check on the chromatic aberration, also known as colour fringing, which can add a coloured ring around the stars and usually affects the entire frame, not just the stars in the corner, however, this aberration is pretty much fixable in Lightroom’s Lens Correction tab, so a bad performance in this test for me isn’t the end of the world.
At the bottom of the same page page they look at spherical aberration but this can actually be a desirable aberration in astrophotography because it can add a glow to the bright stars which can look nice. Some people made a lovely filter to do just that before they got screwed by the parent company. RIP Starglow filter. Anywho.
Lenstip sadly hasn’t reviewed every lens but they’ve done a lot and they’re pretty consistent in their reviews.
Two lens reviewers I enjoy in particular are Philip Reeve who has a great website and Dustin Abbott who has a website and YouTube channel. If a lens is suitable for Milky Way photography they usually try to include some tests on starry night skies which is great. Reeve will always test a lens on some distant city lights if not the stars but be careful here because city lights will exaggerate coma and astigmatism a bit more strongly than tiny stars, but it can give you an idea of what to expect.
Check 2: Vignetting
Check number 2 is vignetting which is also a lens aberration. Vignetting is the darkening of the edges of the frame and sadly in astrophotography we’re fighting vignetting at the boss level as well because we want to use a lenses widest aperture to try and collect as much light as possible, but this is exactly where a lens exhibits its worst vignetting.
The issue here is that the edges of the frame are darker because less light is being collected in that area of the sensor and this results in more noise in your images which we don’t want because it obscures detail.
Again, you can check the vignetting on lenstip.com but sadly, it’s unavoidable because of the wide apertures you will be using so you will have to learn how to deal with it to some degree. But when deciding between two lenses, if one has muuuuch stronger vignetting than the other, it could be a factor in choosing the other lens instead. Some f/2.8 lenses can feel like f/3.5 lenses because of heavy vignetting.
Check 3: Price
Check number 3 is probably going to be the most significant factor for 90% of you and that’s the price and things are not always as you’d expect.
There’s a classic example from 7 or 8 years ago when there wasn’t really much choice for a Milky Way photography out there. One fast wide-angle option was the Canon 14mm f/2.8 which launched for $1800. But it had pretty strong vignetting issues and the coma and astigmatism turned all the stars in the corners of the frame into a swarm of millenium falcons. It was a mess.
Then there was the no frills, manual focus, manual aperture Samyang 14mm f/2.8, sold under the brand name Rokinon in the US. This lens launched for $300, a sixth of the price of the Canon. But the vignetting wasn’t as harsh and the coma and astigmatism were handled probably 6 times better as well.
So the price of a lens doesn’t always dictate it’s performance for astrophotography and that holds true to this day. Quite often lenses will be expensive because they have fancy auto-focus systems, which you don’t need in astrophotography.
Some companies are worse than others, Samyang, known as Rokinon in the US, are known for pretty bad lens variation and even Sony have a pretty notorious reputation although they’re perhaps judged slightly more harshly due to the prices of their lenses and rightfully so.
But cheap brand, expensive brand, doesn’t matter, make sure you buy from a reputable source.
Check 4: Example Images
Now before buying a lens it’s nice to look through some example images. It’s a way of fantasising what you’re going to be able to do if you finally make that purchase. But you have to be cautious because 90% of the final image is going to be editing and post-processing and for example, if you’re trying to asses coma and astigmatism, you never know if an image has been cropped to hide the bad looking stars.
But either way it’s another form of reassurance before burning a hole in your wallet.
Social media is probably the worst place to look as they don’t work very well as search engines and images will also be compressed to death.
So there’s two places I recommend hunting.
The first is AstroBin, the internet’s home for sharing astrophotography. Just head on over and search for the lens your interested in and hopefully you’ll find a bunch of images. Quite often the images are high resolution and high quality. Sometimes you can even zoom in to full size depending on how the photographer has uploaded the image. It’s just a fantastic resource and community. You could even comment on the images asking the owner what they think of the lens and you know you’re going to get an answer from someone who does astrophotography which is great.
The second source I recommend is flickr where again you can search for the lens your interested in, but this time it helps to add Milky Way or Aurora after the lenses name in your search because unlike Astro Bin, Flickr is home to all genres of photography. Sometimes you can find groups for images captured with a particular lens and then you can search inside that group with astrophotography terms to dry and dig out some images but always be sure to check the exif or details in the description to make sure it actually was captured with the lens you’re looking for.
Check 5: Seek RAWs
The fifth and final step before the bonus tip is to try and get your hands on some RAW files which is easier said than done.
If you have a friend who has the lens then great you can ask them. Otherwise you could approach the photographers you find on Astro Bin or Flickr as mentioned in the previous step.
I’ve always wanted to create a website, like an online database that has RAW files from as many lenses as possible with sample images of just the night sky but aside from finding the time to set it up and maintain there’s going to be storage costs because RAW files are pretty large in terms of megabytes and I’m not sure how well the website could be monetised. But if you have any thoughts on this idea let me know in the comments below or get in touch if you’d like to help set something like this up, especially if you have good expertise in web design and hosting for such an idea.
Anyway the bonus step.
Bonus Step: Try Before You Buy
All the of the steps we’ve gone through thus far involve you never actually physically holding the lens in person before buying it. But what if you could actually try before you buy. Well there’s three ways you can, the last one being a little more risky than the other two.
Firstly, you could buy a lens and return it if the performance is inadequate. This is going to massively depend on the retailer you bought the lens from so be sure to read their returns policy first and buy from a reputable source. You're also going to have to be very careful not to mark or damage the lens in any way otherwise the return won’t be accepted.
You don’t necessarily have to buy new because MPB does have a 14-day return policy on their used lenses.
Secondly, depending on where you live there may be lens rental companies. For a fraction of the costs of the lens itself you can rent it for a day, 3 days or even a week. That will give you a chance to test the lens and see if it meets all your needs.
Lastly, and this one is a little more risky, but you can buy a lens from MPB and sell it back to them later. It’s obviously going to cost you some money, but let’s say you wanted to go aurora chasing for a month, this approach would end up costing you way less than renting a lens for a month.
The risk here is that MPB’s buy back price could drop over the time you use the lens, so it may end up costing you more than you calculated.
Or you may lose or damage the lens whilst using it which could either render it worthless or at the least decrease its value more than you were expecting. This is of course why I have all my camera gear insured with Photoguard.
Now you have all the tools to objectively assess a lens before buying it but there’s still one big decision you need to make which I haven’t covered in this video. Do you get a prime lens or a zoom lens? And to answer that question you should check out this article because there’s a lot of important considerations when it comes to this question and astrophotography.