Focus question and advice please!

Started 2 months ago | Questions thread
Erik Kaffehr
Erik Kaffehr Senior Member • Posts: 2,772
Re: Focus question and advice please!
3

mtakeda wrote:

Erik Kaffehr wrote:

mtakeda wrote:

10um means 0.100mm?

No, 10 um means 0.01 mm.

The pixel size on the GFX is 5.3 um (micron) that is 0.0053 mm. Choosing allowable CoC to be around twice the pixel size may be reasonable.

Now, using 0.010 mm as allowable CoC would yield a hyperfocal distance of 23 m, or 74 feet.

So, how do you focus at 23 m / 74'? The laser distance meter Jim suggests using may be helpful. You would try to find a rock that is 23 m from the camera and focus on that.

Diffraction will soften the image at small apertures.

That said, if you look at an image at actual pixels on a 2k 24" monitor, it would correspond to a 64"x48" print. If you have a 4K monitor it would be looking at a 32"x24" print.

Best regards

Erik

Thank you. Then focusing the object at 23m away will always give me the depth of field to the infinity? How about the nearest distance in focus. Sorry for repeated question as the subject drives me crazy! Thank you for kind help.

Hi,

Best focus will be at 23 m but infinity will be in pretty sharp focus. Sharpness will be limited by diffraction.

Closest distance within the CoC criteria is half the hyperfocal distance, that is 11.5 m.

Keep in mind that CoC is just a criteria, sharpness drops, we just say that 0.01 mm is good enough.

But, all that depends on how large you print and how close you look.

180 PPI is often mentioned as limit for an excellent print. That corresponds to 180 / 25.4 -> 7 pixels per mm. If you print A3, the 44x33 mm frame needs to be enlarged about 10 times. So, you would need 70 pixels per mm on the sensor. 1/70 -> 0.014 mm. So anything beyond say 10 meters would be pretty sharp in that A3 print.

Would you focus at infinity and still using Jim Kasson's suggestion of 0.01 mm CoC, the near limit would be 23 m.

Best regards

Erik

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Erik Kaffehr
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