Preview focus bracketing for perfect star focus
Posted: 12.08.2013 20:08
Hi Stas,
Sometimes, in very dark environment, it's impossible to focus because there's no light or very few contrast. That's especially the case with astrophotography. Focusing stars is a real problem.
Setting the focus ring to infinity doesn't always work, because many lenses goes past infinity. The other trick is to focus on the moon, but if there's moon usually the sky is too bright for stars…
Liveview is completely in the dark, in the OVF you barely see anything.
But remote could help. Here's how.
Ask user to grossly set his focus to the nearest estimated distances + some extra distance distance, to make sure the true focus point is less far than his setting. For stars that would mean at the farthest focussing distance allowed by the focus ring. Farthest because we won't damage lens trying to go past what's possible.
Now, as the user for an estimated number of steps (big, medium, or small. Steps size settable). Let's say 10
Then Remote will do this
Take first picture and preview it, ask user to delimit (by zooming, or squaring off) the area of the scene feature the user want's to focus on.
Then remote will do the following.
Step back 1 step. Take preview, compare new area with first frame area. Decide if new one is sharper (take that area, compress it to jpeg, see if new weight is bigger than before could be a method).
Then repeat, then repeat.
When current area is less sharp than previous area. Then that means that previous steps was the sharpest.
So set the focus to previous step's focus.
If choosed steps size was to the smallest one. Repeat process using smaller step size between the previous and next frame of the last sharpest image.
Basically the idea is do emulate contrast focusing from the live view, but with actual images, in situation live view isn't able to work.
A less elaborate method could be semi auto. The user would go in focus bracketing panel, and each time he presses a arrow button it'd take a picture, and display the selected area at 200%
P.S : That method could also be used to know if the camera missed one step (as I read in your forum that's sometimes the case).
Sometimes, in very dark environment, it's impossible to focus because there's no light or very few contrast. That's especially the case with astrophotography. Focusing stars is a real problem.
Setting the focus ring to infinity doesn't always work, because many lenses goes past infinity. The other trick is to focus on the moon, but if there's moon usually the sky is too bright for stars…
Liveview is completely in the dark, in the OVF you barely see anything.
But remote could help. Here's how.
Ask user to grossly set his focus to the nearest estimated distances + some extra distance distance, to make sure the true focus point is less far than his setting. For stars that would mean at the farthest focussing distance allowed by the focus ring. Farthest because we won't damage lens trying to go past what's possible.
Now, as the user for an estimated number of steps (big, medium, or small. Steps size settable). Let's say 10
Then Remote will do this
Take first picture and preview it, ask user to delimit (by zooming, or squaring off) the area of the scene feature the user want's to focus on.
Then remote will do the following.
Step back 1 step. Take preview, compare new area with first frame area. Decide if new one is sharper (take that area, compress it to jpeg, see if new weight is bigger than before could be a method).
Then repeat, then repeat.
When current area is less sharp than previous area. Then that means that previous steps was the sharpest.
So set the focus to previous step's focus.
If choosed steps size was to the smallest one. Repeat process using smaller step size between the previous and next frame of the last sharpest image.
Basically the idea is do emulate contrast focusing from the live view, but with actual images, in situation live view isn't able to work.
A less elaborate method could be semi auto. The user would go in focus bracketing panel, and each time he presses a arrow button it'd take a picture, and display the selected area at 200%
P.S : That method could also be used to know if the camera missed one step (as I read in your forum that's sometimes the case).