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Some numbers are indicative and may be adapted to your need, like the focal length to use or the number of images to take.
Setting the Camera
Use the Live View and set the camera so to have the brightest possible image: some camera do adapt the Live View when changing the exposure settings. Try using the Bulb mode or the slowest shutter speed allowed in Manual mode. Rise your ISO enough to see a bright image.
Use the widest aperture you can use to get the maximum of light into the camera.
Use the magnify manual focus assisting tool to magnify 10x the brightest star you can see in the live view. Note that because of star movement, depending on your focal length and magnification you many need to reframe often, during the focusing process.
If you have a zoom lens, you may be tempted to zoom all in to better see the stars in your live view, focus and then zoom out to the desired focal length.
Well, don't. Many zoom, in fact, suffer from a slight focus shift when changing focal length, which often is enough to throw your stars out of focus.
What NOT to look for
Even if you have the infinite mark or focus hard stop, don't trust them.
What to look for
If you are eyeballing the best focus, tell tale signs you are getting to it are:
- The stars shrink to their minimum size and start to get big again once you passed the true focus;
- More stars will come into view: these are the small, faintest ones you didn't see before as they were focus blurred too much;
- Chromatic aberration, in the form of purple fringing around bright star, is reduced to a minimum when perfect focus is achieve. If you pass this point, the fringe will change colour, going, ,for example, from purple-ish to green-ish.
Using Focusing Tools
If you are using a telescope, you may have heard about Bahtinov masks.
These are great tools to achieve perfect focus fast and easily: pop the mask on your scope and look for the diffraction spikes.
You will have three spikes, two of them crossing over the star and one will not. By focusing, you will bring this spike to overlap the crossing point of the other two spikes: when you have that, your focus is good.
The Kitchen Sieve Method
With photographic lenses, particularly for wide angles and small diameter lenses, I could not find a proper Bathinov mask.
What I found is that popping a kitchen sieve in front the lens will create spikes that get more and more contrasted and sharp when you getting closer to the true focus.
How to Use The Kitchen Sieve
The procedure is very simple: get a sieve of proper diameter, if you can, and just pop it on the lens. If you have a sieve that is too small to stay in the lens, you can hold it in front of it and almost in contact with the lens.
Then looks for diffraction spikes to appear, get contrasted and sharp.
I found the kitchen sieve method to be fast and reliable, even if it is less accurate than using a bathinov masks, as you have still to eyeball the optimal contrast and sharpness of the spikes.
How To Achieve Even Better Focus
It is worthy to focus with the sieve in place and take a test shot after removing the sieve. Now watch if you have a too strong chromatic aberration: if you have it, use the live view and gently improve your focus to minimising the CA.
This correction should be minimal, so operate the focus ring gently and slowly. Once you get used to this method, you will focus properly in few seconds.
Tape That Ring Down
Once you are happy with your focus, use a piece of gaffer tape to gently tape in position the focus ring, so that it will not move accidentally. If you are installing an heater strip on the lens to avoid dew formation on it, before committing to imaging, take a test shot and verify a last time your focus is spot on.
Adjust Your Camera Settings
Before start imaging, remember to set your camera properly, with the wanted shutter speed, lens aperture and ISO values.