Check my thinking on hyperfocus and night photography

[Jesse17]

I'm learning to use the PhotoPills app on my phone. I have it set to my D7000 camera, and 35mm lens. It shows my hyperfocus for f11 as being 17'11". Since I don't carry a tape measure with me, is there any reason to not just pace off 20' to set my focus point, assuming that moving the beginning of my acceptable DOF from 9' to 10' won't miss something I want in the foreground?

Other than older lenses with focus distance printed on the barrel, is there a better way to set your hyperfocus point in the field? I'm thinking of night photography, which means large apertures. f1.8 puts my hyperfocus point out at something like 112', so just giving it extra distance to be sure I'm in the right neighborhood gets more difficult, especially if I have to light an object to get a focus on it.

Thoughts? Suggestions? Thanks!

 

[Derrel]

It's slightly better sometimes to focus wayyyyy far out on night skies, and not 112 feet. You could even go all the way to Infinity, and be okay, except on close-in subjects, which might appear a little bit less sharp than you might want.

A bit of experimentation might be useful for you to learn/understand this concept fully, as it applies to you and your gear. The hyperfocal distance is what you are calling "hyperfocus". The thing is, in DAYLIGHT, what you often want is the best,sharpest,clearest focus on the most-important subject: at times, in say a landscape, that might be trees at 100 feet distant; we EXPECT the background, miles away, to show less information than something a mere 100 feet from the lens. Sometimes, the mid-ground, from 200 feet to a mile, is the most-desirable place to have the sharpest,clearest focus.

Hyperfocal focusing works well at long range and on big, broad scenes, but it works less-well I am convinced, when the MOST-important stuff is close-in.

On stars at night? You'll be pretty good at long distance focus, but I would tend to favor more close to an Infinity focus than to a 112 foot focus for night time star shot, unless of course, there are close-in features you must have sharp.

For example, the 35mm lens set to f/11 and at 17 feet: that will NOT be quite as sharp on long-distance stuff as will a focus distance set at 500 feet. "Acceptable sharpness" is what hyperfocal focusing and depth of field is all about. There is only ONE most-sharp distance, and that is the actual focused upon distance. If you want to count every star, I would never focus at 17 feet. That's TOO close for an Infinity focused distance night sky.

 

[TCampbell]

I've used two techniques to focus stars.

First... keep in mind that everything in space is effectively at "infinity" for a camera. If you can focus on anything, you're focused on everything.

This means that if I can find a bright star ... or the moon... you can point your camera at that for purposes of focus even if that's not in the part of the sky you plan for your shot.

Use a tripod, point it at the bright star, switch to live view, then zoom the live-view to the maximum settings possible. If your camera supports "exposure simulation" in live-view (Canon & Sony have this on pretty much all cameras; Nikon on some cameras) then you can crank up the ISO and shutter duration even though those aren't the exposure settings you plan to use for your shot. This amplifies the image making it easier to see the stars to focus.

Then... rotate the focus ring to the infinity mark (which wont be accurate focus but it will quickly get you into the ballpark. If the camera is well out of focus the stars will be so blurred that you'll see absolutely nothing at all. If you adjust to the infinity mark then you'll be close enough that you should see fuzzy dots on the viewfinder). Next slowly adjust focus while watching live-view to try to bring the stars to a small pin-point.

This takes time. You may fuss with this for several minutes, but it's worth it. I've rushed through the process, confident that I was focused well enough. Shots looked great on that tiny screen on the back of the camera. Then I get home, import the images, look at them on a 27" monitor and ... they're all just a tiny bit soft and you're frustrated and kicking yourself for being rushed.



The other method requires buying a gadget. You can pick up something called a "SharpStar" focusing screen by Lonely Speck.

See: SharpStar2 Precision Focusing Tool by Lonely Speck – Lonely Speck

This clear piece of resin has grooves etched in it which cause the stars to throw off diffraction spikes in three different directions. When you look at the live-view image, you'll see the spikes probably don't converge at a common center point. That means you're not in focus. But when all three diffraction spikes converge at a common center point, you've nailed focus.

The size of the diffraction spikes depends on how bright the star is and the lens. Sometimes the spikes are too small to be confident that you've nailed focus. If in doubt... just take a short exposure to confirm (e.g. take a 5 or 10 second exposure). These longer exposure times will result in much bigger spikes in your shot then you can see in live-view and can help you confirm that you've nailed focus.

BTW, the focusing mask is in the form of the square "slide in" type filters and requires a filter holder such as a Lee, Cokin, Formatt-Hitech, etc. etc.). If you've never used a filter holder before, you actually need two parts... one is the filter holder itself, but these are generic and you ALSO add an adapter ring for whatever lens size (filter thread diameter) you have. E.g. if your lens uses 77mm filters, then you'd need the holder plus a 77mm adapter ring. You could also just hold the mask flat in front of your lens as you adjust focus (a lot of ultra-wide lenses don't actually have filter threads because they have a non-removable hood.)


Having nailed focus... point the camera at whatever you want to shoot. Don't forget to change the exposure settings back to sane values (so you don't end up with an excessively noisy or blown out shot). Don't forget to remove the focusing screen (not that I've EVER made that mistake <whistles innocently>) and take your perfectly focused shot.