Focus question and advice please!

Started 2 months ago | Questions thread
Erik Kaffehr
Erik Kaffehr Senior Member • Posts: 2,781
Re: CoCs per picture height, some reflections

JimKasson wrote:

Erik Kaffehr wrote:

Notes:

  • Distance scale is logarithmic. Left side = 1m, right side = 1000m focus point 30 m
  • Diffraction is shown as red circle and defocus (CoC) is shown as green circle corresponding images added. These calculations are approximate and use 4.8 microns.
  • The grid shown is the pixel spacing.

Jim's plots show some interesting things. One is that stopping down can handle background well. Jim focused at 30 m and that is near hyperfocal distance for CoC = 0.010 mm and f/16. The diffusion spot will be around 0.012 mm from focus point. Stopping down further will not help background sharpness.

The parts near the camera, that is a few meters, can never be really sharp.

The problem with plotting CoC diameters is that small changes near the focal point aren't very visible.If we instead plot the number of CoCs per sensor height, we can see what's going on near focus well. This also allows us to compare DOF details with various sensor sizes.

The number of blur circles per picture height is roughly comparable to MTF50 in lines/ph, or twice MTF50 in cycles/ph. Anything below 2000 or so is visibly soft. Anything below 1000 or so is quite soft.

Jim

Hi Jim,

It would be interesting to see the same plots with a lesser lens. Ok, I know that all GFX lenses are great, so a lesser lens may be hard to find in the GFX family.

Another thought is that you have around 6300 blur circles/ PH that would correspond to

6300 * 6300 * 44 / 33 -> 52 Mega Blur Circles, I think I have seen that number before.

f/16  would come in around 2800, that 2800 * 2800 * 4/3 -> 10.5 MBC.

I have understood that F 16 can be detrimental, depending on POV.

Best regards

Erik

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Erik Kaffehr
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