Lens testing for coma with a laser star
Check lens coma with artificial laser star
Show lens coma and diffraction spikes
When testing wide lenses, one of the questions regularly asked, is what the lens is like for coma.
Using a wide lens for star photos means you’re likely using it at its widest aperture. Since you probably would like to see stars appearing as small points, coma matters.
Keith looks at a quick hack using a green laser pointer to test lens coma.
SAFETY FIRST – never look directly into the laser beam and be sure to use a diffuser as shown below
Looking at lens coma
Designing wide angle lenses is not a trivial job, and any optical designer has a range of distortions and aberrations to control and trade off against each other.
I’m an architectural and commercial photographer so I want a lens that renders straight lines as fairly straight and has low levels of chromatic aberrations. I can fix some issues with software, but I’d prefer a good lens to start with. I’d also like low vignetting and sharpness across the field.
That’s fine for me, since I’ll invariably stop down any lens I’m using by several stops to maximise depth of field.
Photographing stars can easily show up different problems. All lenses trail off in performance when wide open, but wide open lets most light into the camera.
When I’m looking at new lenses, this is one reason why I try and get some star photos to add to the mix.
Two problems arise though…
1) I live in a city, so need to travel to somewhere dark. Preferably when the Moon isn’t out.
2) I live in the UK so it’s often cloudy
I need artificial stars…
Modifying a green laser pointer
A green laser pointer looks a good starting point. Green is easier to see and is in the area of the spectrum where you find optimal lens performance.
If I simply shine the laser on the wall in my lounge, it makes for a far too big (and bright) a spot of light.
Shining the light through a pinhole gives me a very small source of light. However, interference speckles make it vary in brightness depending on viewing angle. I also want to be sure that the brightness is at a safe level to look at.
First up, I’ll cover the laser opening with a diffuse bit of paper.
Two layers of paper give a bright green spot.
Next, I take a bit of aluminium foil, and poke a small hole in it with a sewing needle.
Small holes are easier to make if you put the foil on a surface you can use to resist the needle going too far through.
After a couple of attempts, I’ve a hole no bigger than 0.2mm
Wrap the foil over the tip of the laser pointer, so that the pinhole is over the diffuse light spot.
How to hold down the power button for the pointer? A loop of paper is fine and it also lets me hold the whole assembly in a small clamp.
Here’s the whole thing sitting on my piano. The nearby battery is from my earlier attempt with a green LED. Right idea, but just not bright enough.
You don’t even have to point the laser towards your camera to get a bright ‘star’.
I’ve set the camera about 6 metres away from the light. The lens is an Irix 15mm f/2.4, attached to a 50MP Canon 5Ds
All I need to do is point the lens in whatever direction is needed to put the light into whatever part of the field of view I want to test.
I can use live view to ensure optimal focus. With a 15mm lens the difference between infinity focus and 6m is very tiny, so I’m quite confident that I’ll see similar results to using real stars.
A quick move of the camera means I can test other parts of the field.
Here’s a shot taken with the Irix 15mm lens at f/2.8 (click to enlarge)
Here’s a 100% crop from the 50MP image. With this exposure, you’re seeing the amount of coma I’d expect for a fairly bright star in any image, at the corners.
Before you dismiss the lens, this performance is not dissimilar to my Canon EF11-24 F4L – a lens that costs a -lot- more than the Irix.
But wait there’s more
You can obviously take shots with the ‘star’ at different points in the frame, and at different apertures, to give an idea of how your star shots may look.
The amount of coma should be similar at each corner.
It’s a good way of seeing if the performance of your lens is biased to one side (misaligned).
Put the light in the centre of the field and see how the aperture blades of your lens produce diffraction spikes at different settings.
This is the result of the 6 blade aperture in an old Carl Zeiss (Jena) 135mm f/3.5 M42 screw fit lens at f/8 (see also using the CZ135/3.5 with an adapter)
Here’s the 12 blade aperture at f11 of the Tamron ‘Twin Tele’ 135mm f/4.5 lens from 1959 (see it tested on a Canon 5Ds)
The pinhole gives me a ‘star’ subtending an angle of under 10 seconds of arc. Not a perfect point source, but with wide angle lenses more than good enough to show coma.
Remember to keep the exposure relatively short, since even the tiny point source is very bright.
DON’T FORGET to use the diffuser under the foil and never stare into the beam of the laser pointer.
Any questions/comments – feel free to ask below.
All articles and reviews are listed on our main Articles and Reviews page, or use the search box at the top of any page. Experimental items, hacks and how-to articles are all listed in the Photo-hacks category Some specific articles that may be of interest:
- Using old lenses on your DSLR
- The 1Ds digital pinhole SLR camera A Canon 1Ds pinhole camera, making a 50mm 'standard' pinhole and a 200mm zoom version - results are compared to a lens some £1400 more expensive.
- Canon View Camera An adapter ($20) to use an old MPP 5x4 view camera with a Canon 1Ds. Article shows details of construction and just what it can be used for. Could be adapted for any DSLR and many old large format cameras.
More of Keith's articles/reviews (Google's picks to match this page)
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