I thought I’d use this post to show how your lens’s focal length can significantly impact your shots of the Milky Way.  First up, I should say that I’m not going to go into depth into the physics of focal length – if you’re not sure about what it is, then check out this post on understanding focal length from expert photography. But, for the purposes of this post I’ll just say that I’m comparing a wide angle lens (Samyang 14mm f/2.8 ED AS IF UMC) with a medium focal length lens (Sony Sonnar FE 55mm t* f/1.8 ZA).  Both lenses were used on my Sony Alpha A7ii body and shot on the same night.  It was quite a bright night because I had a first quarter moon behind me but there was still plenty to see in the Milky Way and its structure.  Ideally, I’d use two lenses that are identical in every way except focal length (aperture, build etc) but that’s just not possible in this case.

Why go wide?

A wider lens will have a much wider angle of view. That means you can capture more of the surrounding area in a single shot.  This makes it invaluable when you want to try and take in as much of the scene as possible with one click.  It saves you time and means you can move on to do something else.  However, a wide angle lens does distort your image – the Samyang lens is a great lens for low light and capturing a huge amount of the scene at once.  HOWEVER, capturing more the scene does mean you lose some of the details in the scene.  You’re essentially trying to shove a lot more information in the scene into the same space on your sensor.  What ends up happening is losing some of the definition of the scene.  Couple that with the natural distortion you get from a wider angle lens and you start to get a bit of a skewed image that lacks the same level of detail that you might get from a lens that isn’t as wide.

As a comparison, here’s a single frame straight off the camera of the Milky Way’s core from both the 55mm and 14mm lens:

Single Frame of the Milky Way

Already, you can see a big difference between the two.  The wider lens captures the Large and Small Magellanic Clouds and far more of the Milky Way, but you get far more detail from the 55mm lens.  If I were looking for meteorites, I’d definitely go with the 14mm as you’re more likely to capture something in a wider angle of view, compared to the 55mm (which I caught one in by blind luck!).

But there’s more to focal length than detail alone.

Focal Length and the Rule of 600

One thing new astrophotographers come up against is how to get crisp star shots without any star trails.  The Earth is always rotating – although we may not see it happening, it’s happening. A simple rule of thumb to avoid star trailing it to use the ‘Rule of 600’.  It’s a simple (but not flawless) heuristic to help you identify how long you can leave your shutter open (here’s my go to blog about the Rule of 600). With wider angle lens takes in more of the allows you to leave the shutter open for longer and avoid star trailing.  With my 14mm lens on my Full Frame body I can leave the shutter open for 4x longer and capture far more light before I get star trailing according to the Rule of 600 – I should note here that I almost never leave my shutter open for the full amount I am able to according the Rule of 600 – I prefer the crisper shot associated with a shorter exposure.  So, even though you don’t get as much detail, you can get more light onto the sensor with a wider angle lens, which can make a huge difference on a very dark night.  But, what difference does focal length have on panoramas?

Panoramas with Different Focal Lengths

I am almost never happy with a single frame of the Milky Way.  It’s so big and it’s so beautiful that I want as much of it as possible in my final picture.  But it’s almost impossible to do this in one shot, so taking multiple shots and stitching them together to make a Mosaic Style Panorama is the best way to get more of the view.  When you have a wider angle lens you don’t need to take as many photos because each shot captures more of the sky.  With a more zoomed in lens, you need to take a lot more photos to get the same scene.  How that translated on the night – when I was using my 14mm lens I needed to take 9 photos in total to get most of the scene I wanted (3×3 panorama).  However, to get roughly the same scene, I needed to take a total of 25 shots with my 55mm lens (5×5 panorama).  When each shot takes about 20 seconds with the 14mm lens and 10 seconds with the 55mm lens you’re using up far more time and far more of your battery (a concern for people running with the Sony A7ii body!) with your zoomed in lens.

Taking the photos is also more complicated with a tighter shot – I try and overlap each photo by at least 25% to ensure my stitching software works when I get back home.  This is easy to do with a wide angle lens because there’s more reference points on the LCD or through the viewfinder to help you set up your next shot.  I can spot a tree and keep that in frame or spot the LMC and keep that in frame etc.  With a tighter shot you don’t have that and you have to do you best to keep plenty of overlap (without too much, wasting time).  It takes practice and there have been plenty of occasions where I’ve come home and there are gaps in my panoramas where I haven’t overalapped the scene enough.  If money wasn’t an option, I’d grab a Gigapan Epic Pro and let that do all the work for me, but when doing panoramas manually it is much easier with a wide angle lens.  Do keep in mind that if your lens has a lot of distortion then you may need more overlap between shots to make sure that the final image isn’t too distorted.

Stitching Panoramas with Different Focal Lengths

Something you may not consider until you have to do it yourself is how much processing power you may need to stitch a big panorama together.  I have a decent computer that can handle most of the games I play but if you don’t have a decent computer you may struggle to put together a lot of photos at once, especially if you don’t have some decent stitching software.  I can highly recommend both MS ICE and Hugin as decent free options for stitching panoramas.  I’ve recently bought PT Gui and I love it for complex stitching – very easy to use and very easy to adapt as needed.  However, as you can see, the complexity of the stitching between the 55mm and 14mm shot is quite significant, as shown below (you can also see how I suck at lining up shots with some all over the place!):

Stitching Complexity with Different Focal Lengths

Now, take into account that the total size of my files being stitched with the 55mm panorama is 3.5GB compared with the 14mm panorama which was less than half the size at 1.6GB you can see why some people aren’t able to carry out stitching with multiple files, like you need with a tighter lens, like the 55mm.

Focal Lengths, Panoramas and Noise

One of the key benefits associated with using a lens like a 55mm is that you end up with a much larger image.  Once I’d cropped my images to exclude any unneeded aspects of the scene my 55mm lens yielded a panorama that was a massive 13,560×15,134 pixels! Compare that to the measly 14mm that only gave me a panorama that was 4,302×5,018 pixels in size (which is plenty for most applications, to be honest!).  But, one tip that I learnt from the amazing astrophotographer, Rob Dickinson, is that when you have such a huge file when you can reduce its size to something more manageable you naturally reduce any noise you have in the image.  Definitely an added advantage compared to using various software solutions to reducing the noise in a shot of the Milky Way.

UPDATE: Another bonus tip from RJD – when using a tighter focal length is that you can go beyond the normal ‘Rule of 600’ time – when you have a huge file you can shrink it down to a more manageable size – this will eliminate any star trails you may have picked up from going for a longer exposure than that dictated by the Rule of 600.  Thanks Rob!

Focal Length and Final Imaging

Finally, let’s look at the final images after editing.  Straight away you can see more detail and more punch in the shot taken with the 55mm lens.  For all its issues with the size of the files, the complexity of shooting the panorama, the time it takes, and the processing power needed to throw it all together, I still prefer the final result.  It is crisper, more detailed and the colours are easier to see.  The bright moon does mean that a lot of my 14mm shots get affect more from the moon’s glow behind me, which is not as much of an issue on a truly dark night.  But, if you’re shooting in a light polluted area you’ll naturally get more light creeping into your image from a wider angled lens, washing out your picture a bit more.  You also don’t get the detail in any astronomical bodies you want to shoot (ie, the LMC and SMC are far smaller in the Samyang panorama).

Final Images

Final Summary

All in all, focal length and shooting the Milky Way makes a huge difference to how a photo is taken, how it is processed, how it is stitched together and how the final image turns out.  I thought I’d summarise it all in the table below.  I’d love to hear your thoughts on this post and whether you agree/disagree with me and my analysis.  Don’t forget to subscribe and share for future posts 🙂

Summary Table

 

4 thoughts on “How Focal Length affects your Milky Way Shots

  1. On the same time required to capture a 25 mosaic you could have stacked e.g. 4 images for every 14mm shot stacked them to improve image quality before making a panorama. I believe the square root of 4 stacked images equals on stop improvement. Could you please comment on this approach.

    • You’re absolutely right that you can take more frames but generally you need quite a few frames for the layering/stacking to be effective (e.g. I’m processing about 150 frames at the moment from last night) – this is not to say that 2-3 more won’t help, but it just wouldn’t offer the huge differences – it also doesn’t give you the other benefits from stitching, like a higher resolution file, a pic that isolates light pollution etc. I would experiment with both, if I were you, and see which one suits you best 🙂

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