Padlock Focus Technique

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While I was in Valley of the Gods Utah, the forecast was for clear sky overnight, so I thought I would give night photography a try. So thinking I would need to have an idea as to where infinity was, I took my camera out during the day focused on a distant cliff and with a skinny sliver of electrical tape I marked where the infinity loop crossed on my lens. I was now ready for night photography.

That night I set up my camera, pointed towards the Milky Way and, with my 14-24mm set to manual focus and the infinity centered on my mark, I set my aperture to f/2.8, ISO to 6400 and a shutter speed of about 20 seconds, and took a shot.  To my surprise when I looked at my image in live-view, it was blurry.

Back at home, I did some research, and found that there were three established ways to focus a camera for night star photography. 1. Set up your camera during daylight and focus then, and leave the camera set-up until you are ready to shoot. 2. Make a mark on the lens over the infinity mark and align to that before shooting. 3. Use live-view and focus on the stars.

Number one—focusing during daylight—seemed unrealistic as it required you to hang around several hours waiting for the night sky, and since the camera is mounted on a tripod and pre-focused, you can’t even use the camera.

Number two—making a mark over infinity—I had already tried without too much success. I suspect it has something to do with parallax (the position of an object appears to differ when viewed from different positions) there was definite space between my mark and the focus scale under the glass window. I suppose I could make this work as long as the focus alignment was done from the same position every-time, before composing for the shot.

Number three—using live view—looked promising, as I do this all the time during daylight landscape photography. In this scenario, you zoom in on a star and focus until the star appears as a point. 

On my next trip to Zion, I decided to give night star photography another try. I set up my camera and zoomed into to find a star. But my camera was too dark, so I decided to decrease the shutter speed  until I started to see stars and then try to zoom into one of them. The problem is that as I decreased the shutter speed, I increased the noise. As I started to see the stars, the noise made it difficult to determine what was noise and what was a star. I decided that stars where white and the noise has some color. This would actually work, but time-consuming trying to distinguish between sensor noise and star. 

I think I found a better way to focus but first we need to understand how an Nikkor AF-S lens works.

If you have ever played with a Nikkor AF-S lens, you may have noticed that the focus ring does not stop when you hit either end of the focusing range. What happens is the gear between the focus ring and the lens decouples and focusing stops as you continue turning the focus ring in the same direction. Stop, and reverse the direction of the focus ring, and the gears immediately engage and focus resumes until you hit the other end of the focus range. The moment you change direction of rotation (no meter where you are) focus will resume. What we need to do is establish a common stopping point to change directions to arrive at the infinity focus point.

Knowing this I was able to develop what I am calling the “Padlock Focus Technique” like a padlock which you have to rotate your combination dial several time before stoping at the first number (the stop-mark) then reverse direction to arrive at the next number in the combination (the focus-mark). Albeit this is only a two digit combination, so you can forgo the “pass the first number” bit.2016-03-31-22.34.16-Stack

Things you will need:

  • Camera with Live View
  • AF-S Lens
  • Tripod
  • Colored Electrical Tape
  • Exacto Knife
  • Straight Edge
  • Cutting Surface
  • Clear Skies
  • Moon During Daylight Hours

You will be placing three marks (color electrical tape) on your lens one inside the groove of the focus-ring, and two short marks (the stop-mark and the focus-mark) along the out-slide edge of the focus-ring which you will be aligning the focus-ring to. The focus-mark and the stop-mark will be very close together—within a quarter of an inch of each other.

Step 1: Determine a good spot on the lens to place the padlock focus stop-mark. Although the first stop spot is arbitrary, it is a good idea to choose a place where you can see the mark, and align with it, in both the horizontal and vertical camera orientations.

Step 2: Cut 3 skinny slivers of the electrical tape. The thickness should be just small enough to sit inside the focus ring grooves. Two short ones (focus-mark and the stop-mark) that will be placed just out-side the focus ring, and a longer one one that will be recessed into one of the grooves on the focus ring.

Step 3: Place the long strip inside one of the groves on the focus-ring, it does not matter which one any-one will do. Trim to tape to the height of the focus ring, making it easy to identify, and easy to align with the outer marks.

Step 4: Place the stop-mark, where you decided in step one along the out-slide of the focus ring and in a position where it will be easy to align to the one on the focus ring.

Step 5: Place camera on a tripod, and point to moon. If using a zoom set it to the smallest focal-length, set f-stop to widest setting (the smallest number 1.4, 2.8).

Step 6: If your camera is configured to focus when you press the shutter button (not using back-button focus) turn camera to manual focus.

Step 7: Turn on live-view and zoom into the moon as much as posable.

Step 8: Turn focus ring one complete revolution clock-wise and then on the second revolution align with the with the stop-mark.

Step 9: Slowly turn the focus ring in a counter-clockwise direction to focus on the moon. You can  fine-tune the focus by moving in both clockwise and counter-clockwise directions as long as you don’t cross the stop-mark.

Step 10: Once focused, place the focus-mark on the lens, aligning with the focus ring mark.

Step 11: Defocus, and follow the “Using the Padlock Focus Method” below to test, and familiarize yourself with the method.

You’re done!

Using the Padlock Focus method, with your newly marked lens:2016-03-31-22.34.16-Stack

  • Set up you camera and align to the night sky and foreground you intend to photograph.
  • Set your aperture to its smallest focal length and widest aperture.
  • If your not using back-button focus, set your focus to manual.
  • Rotate the focus ring in a clock-wise direction one complete revolution,
  • On your second pass align to the stop-mark (passing the focus-mark).
  • Slowly rotate counter-clockwise to align with the focus-mark.

You are now set to infinity.

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