New guy with dumb question

[jc.dempsey]

Hey there I'm Jack. I picked up my first camera a month or 2 ago, a Canon 70d, and I now have 2 lenses. A tamron 18-270 f3.5 that I purchased with the camera and a canon EFS 10-18 f4.5-5.6.

I've been really interested in astrophotography. After a lot of trial and error and research, I've begun to take some half decent pictures.

My question is this: I understand that it's best to focus to infinity for stars, but the canon EFS doesn't have any markings on the focus ring, since it is just an encoder for the electronic focus. How would I go about focusing the lens for stars?

Any help would be much appreciated.


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[Buckster]

One method, which I use, is to focus on something during the day that is at "infinite" distance, and then mark the lens myself with a silver Sharpie. That makes it easy to deal with at night using a flashlight.

 

[jc.dempsey]

Would infinite be considered as far away as possible?


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[Buckster]

Would infinite be considered as far away as possible?

Not if you're standing in a closet, it wouldn't.

But yeah, that's the idea - focus on something VERY far away. Clouds work great.

 

[jc.dempsey]

Here's a question. The focus ring on the canon spins freely when it's in autofocus, or when the camera is off. Wouldn't that make the line I could make useless?


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[Buckster]

Here's a question. The focus ring on the canon spins freely when it's in autofocus, or when the camera is off. Wouldn't that make the line I could make useless?

I don't personally have or use the 10-18 (I was thinking of a couple other lenses that I've done that with), so yeah, that's a problem that will require a different solution. Hopefully, someone else will have a good suggestion for you.

If you're really into astrophotography, it's probably worth looking into lenses that are better suited for that pursuit.

 

[jc.dempsey]

Thanks for the help! I've only just started and I know I have a lot of learning and money spending ahead of me.


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[Pejacre]

Might work using zoomed-in live view.

 

[jaomul]

Try focus on moon if it's out

 

[jc.dempsey]

Might work using zoomed-in live view.

Oh I didn't think about live view. I'll have to give that a try when I can find somewhere with minimal light pollution.

 

[TCampbell]

A tamron 18-270 f3.5 that I purchased with the camera and a canon EFS 10-18 f4.5-5.6. I've been really interested in astrophotography.

Astrophotography can be very challenging (not as easy as one might guess.)

Since the Earth is spinning, the stars appear to move slowly from east to west. At very low focal lengths (ultra-wide angle lens) it doesn't seem like much. It long focal lengths, it's surprising how fast things are drifting through the field of view.

For your 70D camera, assuming a tripod which is NOT tracking the sky as it moves, divide 375 by the focal length of the lens. The result is the number of seconds of exposure you can do before the stars no longer appear "round" (they start to elongate and grow a tail due to the rotation of the Earth.)

This is much easier with very wide angle lenses. And since you aren't working with very much light, a lens with an extremely low focal ratio is helpful.

After a lot of trial and error and research, I've begun to take some half decent pictures. My question is this: I understand that it's best to focus to infinity for stars, but the canon EFS doesn't have any markings on the focus ring, since it is just an encoder for the electronic focus. How would I go about focusing the lens for stars? Any help would be much appreciated.

Do not use the focus distance markings on the lens. You won't get good focus that way. While there is an "infinity" mark on the some lenses that do indicate focus distance, the mark looks like an "L" with the bottom portion extended. This is because the focus point for visible light is different than the focus point for infrared imaging (e.g. in film camera you can get IR sensitive film -- for digital cameras we can't just do that easily but there are photographers who modify their cameras for infrared photography and the camera lenses are designed to work for either visible vs. IR.)

In any case, if you simply focus the lens by looking at the position marked as "infinity" you'll almost certainly miss optimal focus (unless you get REALLY lucky.)

Instead, switch on "Live View" mode, crank the exposure settings. Set the dial to "Manual" mode, minimize the Aperture value (lowest value your lens will let you use), set the shutter speed to 30 seconds, and set the ISO to max (whatever your camera allows.) This is NOT the setting you'll use to take your image (you'd get a terrible image) but since Canon cameras provide exposure simulation when using "Live View" mode, this will amplify the brightness of the stars and make them easier to see. Focus the camera on a bright star until you "think" it's looking good. (you are not done yet.)

Now use the magnifier button to magnify your live-view image to 10x size and re-check that star again... tweaking focus as good as you can.

If you've got a computer connected (this is easier to do with a laptop outside and the USB cable tethered to the camera) take a few short images and download and inspect the image for sharpness. The images will be horribly noisy (since you are at max ISO). You only care about refining focus at this point -- don't worry about the noise.

Take your time... if you rush the focus, you'll get images on the back of your camera that "look" good on the back of the camera... then you'll download them into the computer and start kicking yourself for missing focus (it's very hard to tell if you missed focus on that tiny LCD screen.)

NOW... run the exposure settings down to something more reasonable (e.g. ISO 3200 perhaps, but this depends on the lens focal ratio and the shutter speed you can manage -- and the shutter speed is based on the focal length of the lens.) and then begin capturing the images.

Once ANYTHING in the sky is focused, then EVERYTHING in the sky is focused.

If the camera is on a tracker mount (e.g. Losmandy StarLapse tracking head, an AstrTrac tracking head, a Vixen Polarie tracking head, or an iOptron SkyTracker tracking head -- and the tracking head is "polar aligned" (so it's rotation axis is pointed at the north star) then you can take much longer exposures even with a long focal length lens.

 

[KmH]

For all intents and purposes, focusing a consumer grade telephoto camera lens in daylight on something 1/2 of a mile or further away will focus your lens to 'infinity'.
As focal length gets shorter, the distance from the camera 'infinity' focus is achieved gets closer and closer to the camera.

If your camera has it, use Live View, you can 'zoom in' and check focus better.

So it's rotation axis is pointed at Earth's north rotation axis. The north star - Polaris - is close to, but not at the Earth's north rotation axis.
For some less than critical work, using Polaris might be sufficient.
Many polar alignment aids have concentric circles that allow offsetting Polaris according to the mount location and the time of day so the camera mount is precisely aligned on the Earth's rotational axis, and not on the north star - Polaris.

Scroll down to page 9 of the iOptron SkyTracker Instruction Manual to see the visual aid iOptron uses.

 

[jc.dempsey]

That was very informative. Thank you for the help. Now I just have to wait for the rain to stop.

 

[TCampbell]

So it's rotation axis is pointed at Earth's north rotation axis. The north star - Polaris - is close to, but not at the Earth's north rotation axis.
For some less than critical work, using Polaris might be sufficient.
Many polar alignment aids have concentric circles that allow offsetting Polaris according to the mount location and the time of day so the camera mount is precisely aligned on the Earth's rotational axis, and not on the north star - Polaris.

Polaris is about 2/3º away from the pole (it's actually getting closer... by the end of the century it will be about 1/2º, but that's as close as it gets and then it starts moving away again.)

In low focal length lenses, the 2/3º won't be a problem. It is a problem at very high focal lengths (my SCT is has a focal length of about 3500mm -- at which point that 2/3º is a not nearly close enough for tracking purposes. The tracking error becomes a problem rather quickly if left uncorrected.)

A 10mm focal length lens on an APS-C sensor camera has an angle of view of over 100º (diagonally). The stars stars will start to elongate if they appear to drift more than about 1/6th of a degree at this focal length. But even if centered on Polaris (not true north) it will be a long time before that happens.

Stars move across the sky at 15 arc-seconds per second (0º0'15"/sec) if that star is located at 0º declination (0º means the star is located above Earth's equator.) Stars with positive declinations (northern hemisphere stars) or negative declinations (southern hemisphere stars) appear to move more slowly. Multiply the 15 arc-second speed by the cosine of the star's declination to get the actual speed at which it will appear to move.

With a 10mm lens, you'd probably have to image for about 2 hours before the polar alignment error would cause enough drift to be a noticeable problem. But at higher focal lengths, say 100mm... you'd might notice the problem if imaging longer than 12 minutes. That's still not a problem because a single 12 minute exposure would not look good (unless you were in perfect dark sky conditions.) But a 300mm... lens... that might be a problem in just over 4 minutes and it's entirely possible that you might want to shoot an exposure that long if using a high-ish aperture and a low-ish ISO.

 

[wfooshee]

On another note, just cranking the focus as far is it will go on the distance scale is not necessarily infinity. Most of my lenses will focus "past" infinity, whether it's a mechanical or electronic focus ring. Looking at the moon, for example, it's not focused when I crank the lens over against the stop; I have to back it off just a smidge. I suppose it's to cover a manufacturing tolerance to make sure the lens CAN reach infinity, but that's just ignorant me making a guess., but anyway, it makes finding the max distance focus position more difficult than you'd think it ought to be, especially at night with nothing out there but the stars you want to shoot.