Focus problem with 10-18mm for astrophotography

Vic Vinegar

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Jan 30, 2015
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I have a canon 10-18 I want to use with my Sony a6000. I went out tonight to get photos of the stars but I can't figure out how to get it to focus on the stars. I have to use manual focus since it's on an off brand camera (I used it on my 40D but I had the same problem). So I put the thing in 10mm mode and then turn the focus all the way to the right and the stars are too blurry. Same thing happens when I turn the focus to the left. I looked online and there are no tutorials for using the lens for astrophotography. The display on my camera is too small to see whether or not the stars are crisp.

Any advice for getting sharp focus with this thing?
Is the moon visible? Focus on the furthest thing you can find; it'll probably go to infinity.
Just to clarify...when you say it's blurry when you turn it to the right and the left, do you mean when you turn it all the way to the right and all the way to the left, or are you including every microscopic adjustment in between?

When I take pictures of the moon (which aren't good...I don't have the right lens for it) I take dozens and dozens of pictures, changing the focus a tiny bit every time. I try to get it looking as sharp as possible on the screen. But there isn't a way to really see if I got it sharp enough until I get the pictures on a computer. Even zooming in on the previews doesn't give me enough information.

Also, what are your other settings? If your shutter speed is too slow, you might actually be see the movement of the earth.
I use live view to focus on the moon/stars. Zoom in with live view and manually focus. Not sure what live view is called on other platforms but it's using the LCD instead of the viewfinder for Canon.
Stars do not provide enough light for the auto focus module in the camera to work properly.

You can do as suggested and during the day auto focus on something more than 1/2 mile away and then turn off AF and don't touch the focus ring on the lens.
Or, if the moon is up AF on the moon, but if the moon is up you'll get a lot more air glow in your astrophotographs.

When I use my telescope as a lens I use a Bahtinov mask to focus with precision.
I don't think a Bahtinov mask small enough to fit the front of an 10-18 mm DSLR lens would let in enough light to work.
DIY Bahnitov mask
astrojargon - Bahtinov Focusing Mask Generator: Overview
How to Make Bahtinov Mask, PDF Templates - DeepSky Watch
If your using a Canon lens on a Sony camera, you've got an adapter between the mounts. These are practically never exactly right in length, they should be the rear flange distance of the lens minus that of the body (44-18=26mm for EF lenses on Sony E mount). Cheaper adapters are likely to have greater errors here.
Usually they are made fractionally too short allowing the lens to focus 'beyond infinity', those that are too long can't focus to infinity. Short focal lengths are affected by this change in flange distance.
My Sony A to µ4/3 adapter was 0.25mm short when I measured it, which meant the 10mm lens I got it for had to have the focus dial turned right down to 0.26m before distant objects where roughly in focus. The focus ring only goes as far as 0.24m!
I'd strongly suggest investigating your focus in daylight first, stars are difficult enough to focus on with a native lens. Go outside & check to see if any distance is in focus, and how that changes as you turn the focusing ring.
With my adapter I've added a shim, behind the lens mount, to increase the length to closer to the correct distance but results are still not impressive. I rather suspect such short focal length lenses aren't practical to adapt. :(
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Focus has to be performed manually. As Keith points out, you won't get auto-focus to work on the stars because their not bright enough for the camera.

If the moon is in the sky, you can use that to focus. But the moon generates light pollution which washes out what you'll be able to see in the night sky -- the best images are taken when the moon isn't in the sky (so you wouldn't be able to use the moon.)

Normally you'll need to (a) switch on live view, (b) set a very high over-exposure (deliberately), (c) zoom in the live-view to the maximum amount possible (e.g. the 10x level if that's what your camera supports) and carefully inspect the quality of focus on a bright star (and use a star... if there are planets in the sky a star will provide more accurate focus.)

I use something called a Bahtinov focusing mask. It works like a lens cap (or thread-on filter) and it attaches to the front of the lens. It has slots cut in it. It's designed so that as you focus a star, the star will throw "diffraction spikes". You'll see a pair of spikes in the shape of an "X" with a third spike that goes through the X. When all three sets of spikes converge at the same center point, the image is perfectly focused.

Here's an example: Bahtinov mask - Wikipedia, the free encyclopedia

I use a mask made by Gerd Neumann (that's the brand) and this attach just like a lens filter. But the Gerd Neumann masks are a bit pricey. A much more affordable option are the Farpoint brand masks -- just a piece of cut plastic that presses in to the threads on one of your existing lens filters -- so if you have a clear or UV filter you can attach the Farpoint mask to your UV filter and thread it on to the lens. You can also use website to generate, download, and print your own template if you want to cut your own mask (and that's free).

Sometimes ... especially with an ultra wide lens which really minimizes the stars... it's difficult to do a live focus and I have to take a multi-second image to evaluate and tweak focus.

This sometimes takes a while to nail the focus. I HIGHLY recommend you not rush through this... if it takes 15 minutes of effort just to focus, it will be worthwhile in the results that you get after you spend your time taking lots of long exposure images (you'd be disappointed if you spent all night and the images are all soft.)

One other thing -- it was implied but maybe maybe not obviously stated -- is that if anything in the night sky is focused, then everything in the night sky is focused. That means if the area of the sky doesn't have anything bright enough to achieve focus... just point your camera in an area where you can more easily achieve focus.

Lastly, don't forget to return your camera to your intended exposure settings (instead of max exposure) and remove the focusing mask (if you used one) before starting to image.

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