Infinity Focus

When we focus for normal daytime photography, the camera lens moves the optical elements inside of it to focus on the object we have the camera pointed at. For close objects, the lens must move out away from the camera's sensor a little bit. This is done internally in most lenses.

The infinity mark on most autofocus lenses is not accurate enough to use for critical focus for astrophotography.

When we focus on an object that is far away, the lens moves the optical elements closer to the camera's sensor. Depending on the focal length of the lens, at some point, far away objects are considered at the lens' "infinity" setting. Obviously, they are not infinitely far away, but as far as the lens is concerned, it doesn't have to focus in anymore, and the objects will be in focus.

The Sun, Moon, stars and planets are so far away, they are effectively at an infinite distance as far as a camera lens or telescope is concerned. In the old days of manual focus lenses, most lenses were made to mechanically stop their inward focus position at the infinity focus, so it was easy to focus them for the stars - all you had to do was turn the focus all the way to the infinity mark on the focus scale on the lens.

These days however, most autofocus lenses are intentionally made to focus past the infinity mark. This is done to protect the autofocus mechanism when the lens is seeking focus, and, with longer telephoto lenses, to compensate for changes in focus due to temperature changes.

Unfortunately, this can make focusing an autofocus camera lens a little bit tricky.

Autofocus

You can try to use autofocus with your lens. It might work if you have a very bright object like the Moon. You can also try it on the very brightest stars or planets. The trick here is to get the star exactly on one of the tiny focusing squares in the viewfinder. Make sure you activate only that square for autofocus. Read your camera manual to learn how to do this. Once you have achieved autofocus, turn autofocus off with the switch on the lens or on the camera body. You don't want the lens seeking focus the next time you press the shutter button.

Many inexpensive camera lenses are made primarily to be autofocused. Frequently on these lenses the manual focusing ring is very small. It may also be at the very front of the lens, and the filter and lens hood may attach directly to it. In this case, it is very easy to accidentally move the ring after you have focused it. You can try taping it down, but these focusing rings are usually so incredibly touchy that if you even touch it you will throw the focus off.

If you can tape the focus down without moving the focus, as you can on many other lenses, then you should do this. There are other ways around these problems that we will discuss below.

Another problem you may run into with taping down the focus ring is that many zoom lenses are not parfocal. Parfocal means the focus stays the same throughout the zoom range. Most zoom lenses need to be re-focused for each different zoom setting. So taping the focus down while the lens is at, say, a wide setting, won't work if you zoom out later. You will need to remove the tape and focus again.

The Autofocus Button

Most DSLR cameras as set up by default to autofocus the lens when you press the shutter button half-way down. This is great for normal daytime photography, but not so great for astrophotography.

If you manage to successfully autofocus on something bright like the Moon, and then you move the camera to shoot a faint deep-sky object, the next time you press the shutter button to take the picture, the camera will try to autofocus again. It won't find anything to focus on, and it may seek a new focus, and move the focus off of your previously successful infinity focus. And, if the camera is set to autofocus and it can't detect that something is in focus, it won't fire at all.

The autofocus function can be switched from the shutter button to the AE lock button.

Autofocus on the lens can be turned off and set to manual focus.

There are two ways to get around these problems.

1. Move the autofocus function from the shutter release button on the front of the camera to the AE lock button on the back of the camera. This is done with custom function IV-9-1 on the Canon 1000D (Digital Rebel XS). Read your manual to find out how to do it with your camera.

   The normal default setting is for the AE Lock button to lock the auto-exposure. It is one of the buttons that normally fall under your thumb as you hold the camera. By moving autofocus to the AE Lock button, the camera will only try to autofocus when the AE Lock button is pressed. The shutter release button on the front of the camera will now only open the shutter.

2. Turn autofocus off on the lens and switch the lens to manual focus. Note that when you do this, it is easy to bump the focus ring on the lens, so it is best to tape it down after you have focused on infinity.

We can use these two settings in a combination of ways. For example, if we have an object that is bright enough to autofocus on, such as the Moon, we can press the AE lock button and use autofocus. Once focus is achieved, don't touch the AE lock button again.

If we don't have an object that bright enough to use autofocus, we can turn autofocus off on the lens and manually focus with Live View. Then when we have achieved focus, we can turn autofocus button back on, and the focus position will be locked on the lens, and as long as we don't press the AE lock button, the focus won't change.

Live View

Basics of Live View Focusing Use manual exposure and manual focus. Use ISO 1600, bulb shutter-speed, and a wide aperture. Use a bright star or planet to focus on. Get close to focus before you use Live View! Zoom in to 10x to focus.

There are a several important things you need to know to use Live View for focusing.

First of all, realize that Live View is not that sensitive. It's really made for bright scenes encountered in normal daytime photography. You're not ever going to be able to see a nebula or galaxy on Live view, but it definitely can be used for astrophotography by focusing on stars. How faint of a star that you will be able to focus on will depend on the aperture and focal ratio of the lens, and the camera settings.

Second, note that most modern lenses focus past infinity. You can not just rack them all the way to the infinity mark and expect critical focus.

Procedure for Using Live View for Focusing

1. Turn on exposure simulation in Live View if your camera has this option. Read your manual to learn how to do this.

2. Set the camera to Manual exposure and the Bulb setting for long exposures and use ISO 1600. The brightness of the image for focusing is related to the camera settings. Higher ISOs and longer shutter speeds produce the brightest star images.

3. Turn off autofocus on the lens. Set it to manual focus. You can try to use autofocus with longer fast lenses, but I like the feedback on actually being able to see the star in focus in Live View. This is coming off the sensor itself, so this is what is going to be recorded in your image. It doesn't get any more accurate than that. So turn autofocus off on the lens and use Live View.

4. Use a bright star. If there isn't a bright star in the field you want to shoot, first move the scope to a bright star, focus, and then move back to your deep-sky object. If you have a refractor, or SCT or scope where the image comes out the tail end, it helps to pick a bright star that is not too high up in the sky so you can see the Live View on the LCD screen on the back of the camera in a more convenient position than if it was overhead.

5. Use your fastest aperture if you are using an f/2.8 or slower camera lens. For example, with an 18mm to 55mm f/3.5 to f/5.6 zoom lens, set the lens to f/3.5 for focusing if you want to shoot at 18mm. If you want to shoot at 55mm, set it to f/5.6, the fastest aperture available at that focal length.

   Note that most zoom lenses are not parfocal. That means when you zoom, you have to refocus. You can't focus at 55mm and zoom out to 18mm and shoot. The stars won't be in focus.

   If you are using a fast f/1.4 or f/1.8 lens, stop it down to f/2.8 for focusing to get rid of the worst optical aberrations. You may want to stop it down even more for your actual long-exposure astrophotos to clean up most of the rest of the aberrations. But focus at f/2.8.

   If you are using a telescope, your focal ratio will be fixed.

6. You have to be close to focus to start with, or you won't see anything on the Live View screen. This is one of most common mistakes in using live view. Set the camera to manual focus and put the focus ring to the infinity mark. If you have a lens that doesn't have a focus scale, just eyeball it through the viewfinder and get as close to focus as you can.

   For the rough focusing by eyeball, don't forget to adjust the diopter on the camera first. You do this in the daytime by taking the lens off the camera and focusing the diopter on the little focus squares on the ground glass. This focus will be different if you use glasses, so if you focus the diopter in the daytime with your glasses on, be sure to keep them on at night when you try to focus the scope or camera lens.

7. Put the star close to the center of the field. This will help when you zoom in later to focus.

8. Turn on Live View. Read your camera manual to find out how. Play around with it in the daytime to familiarize yourself with how it works (you'll have to use a short shutter speed in the daytime however).

   If you start out close to focus, you should be able to see a bright star at 1x. Then zoom in to 10x to critically focus. If the star is not exactly in the center of the field, you can move the zoom box around with the controls on the back of the camera to center it. You can also move the box once you are zoomed in.

9. Focus by going through the point of best focus and back again several times so that you know what the point of best focus looks like. There may be some play or wobble in the lens. Get used to it.

10. Lock the focus down without moving the focus. This is easier said than done! I use a piece of masking tape on the 18-55mm. But you have to be extremely careful you don't move the focus. I watch the star on live view as I press the tape down to make sure it doesn't move.

    In the real world, inexpensive lenses, especially like Canon's 18-55mm f/3.5 to f/5.6 zoom lens, and Canon's 50mm f/1.8 lens, have extremely sensitive focusing rings. Since these lenses are made out of plastic, they have some play in them too. They are very touchy. Once you have them focused you have to be incredibly careful not to move them. Any time you touch the focus ring, the focus has a chance to move. So if you can't tape it down, don't touch it! Don't try to attach or remove any lens hoods, filters or anti-dewers once you have focused either.

11. Take a test exposure. Once you have focused, it's a good idea to take a test exposure and examine the image. Zoom in to 100 percent enlargement and check out the stars to make sure they are in focus and that you haven't accidentally moved the focus ring on the camera lens.

Aperture and Live View

The faintness of a star that you will be able to see on Live view is a function of the ISO setting, the shutter speed, the aperture and f/stop.

You will be able to focus on fainter stars with a larger aperture. But it is with wide-angle lenses where aperture really comes into play.

When you get down into wide-angle lens' focal lengths, the true aperture gets tiny.

Even if an 18mm lens seems to have a 2 or 3 inch hunk of glass in the front, that is not really the size of the aperture. An 18mm lens' true aperture at f/3.5 is only about 5mm. That is not very much. That size aperture is just not going to collect a lot of light. The big lens on the front is to get you a wide angle of view, but it is not the true aperture of the lens like on a telescope.

You can easily calculate the true aperture by simply using the formula aperture = focal length / focal ratio. So with our example of a lens with 18mm of focal length and a focal ratio of f/3.5, aperture = 18mm / 3.5. Therefore the true aperture = 5.1428mm.

The moral of this story is use the brightest object in the sky to focus wide-angle lenses with Live View.

Don't expect to be able to see virtually anything through the viewfinder visually with an 18mm lens. They are not easy to aim exactly, but you can adopt a red dot finder to the hot shoe and aim it at the center of the field you want to shot, and then take test shots to see if you are framed correctly.

If you have a large aperture scope and use a very bright object to focus on, you may get more accurate results by dialing down the shutter speed to reduce the brightness of the object on Live View.

Other Notes

Long fast telephoto lenses, like a 300mm f/2.8, and telescopes are sensitive to focus changes due to temperature. As the temperature falls, metal in the lenses contracts and the focus shifts. The faster the optical system, the shallower the depth of focus, and the more critical the focus. If the temperature is falling during the night, it is a good idea to re-focus. This is usually not as critical with wide-angle lenses.

Bahtinov Mask

A Bahtinov mask is a simple disk with a particular pattern of holes in it. It was invented by Pavel Bahtinov, an amateur astrophotographer. This disc is placed in front of the camera lens. When the lens is pointed at a star, it causes a diffraction pattern that makes it easy to judge when focus is achieved.

It is simple to cut-out the pattern in a piece of construction paper or cardboard.

You'll need to make a custom Bahtinov mask for the particular size of your lens, but this is easy to do. See: http://astrojargon.net/MaskGen.aspx for an online mask generator.

If you have Live View, you can then just view the image of a bright star with the mask on. The Bahtinov mask makes a diffraction pattern in the form of an "x" with a line through it. The line moves as focus is changed. When the lens is focused accurately at infinity on a star, the line will exactly bisect the center of the "x".

Trial and Error

If you have a camera that doesn't have Live View, and the Moon is not up to autofocus on, you can just take a test exposure and vary the focus a little bit and try to find correct focus by a process of trail and error.

A Bahtinov mask will make this process easy. Simply focus as best you can by eye. Then take a test exposure and examine the image on the LCD on the back of the camera at high magnification. It will be easy to tell when the image is in focus when the line bisects the "x" caused by the pattern of the Bahtinov mask.

Daytime Focus for Night

You can focus the lens on something far away during the daytime, and then turn off autofocus. If you don't touch the focusing mechanism, you can then use the lens at night on the stars without having to worry about re-focusing.

The distance of the object you focus on in the daytime will depend on the focal length of the lens. For longer focal length lenses, the object has to be farther away.

Focusing Camera Lenses - The Bottom Line Live View is the best, and easiest, way to focus a camera lens. If you don't have a camera with Live View, then test shots with a Bahtinov mask are the next best method of focusing.