Can Pentax make A Decent Lens? Ricoh have not fixed QC issues...

Started 7 months ago | Discussions thread
BobORama
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Re: Please use better experiment protocol
In reply to Trevor G, 7 months ago

Trevor G wrote:

Except that you are wrong, very wrong.

Fo the first time ever I used a DOF Calculator. Here is what it says for an APS-C sensor with a 50mm lens at f1.8 :

Subject distance 110 m
Depth of field Near limit 42.8 m

Far limit Infinity

Total Infinite

In front of subject 67.2 m

Behind subject Infinite

Hyperfocal distance 70.2 m

Circle of confusion 0.02 mm

Your lens is fine, its your expectations that are off center.

I think you might belong to the Circle of Confusion.

The problem is that you are expecting "critical" sharpness and thats not what zone focus gets you.     The CoC from your use of the calculator is 4 pixels in diameter, meaning its blurry as when pixel peeping.

Would you consider a 4 pixel diameter blur "acceptable" focus?   If so, then the left and right side are acceptably focused - what are you complaining about?   So when we are in the pixel peeping realm, we have moved on to a CoC which is significantly smaller.

Even when in the zone, there is a whispy thin zone where things are tack sharp on the digital sensor. Tack sharp images require a CoC much smaller than the size of the individual sensor elements on the camera.  In fact, the CoC can be no larger than half the diameter of the sensor elements,  actually ( .707 ^ 2, which just happens to be ~ 0.5 ) which are VERY small.   Perhaps 0.0025mm on a K-5.    When you look for the zone of tack sharp focus, you have only a few meters in the front.   And since your left side is critically sharp, the right side will be less so.

Now if you wanted a 4x6" print, that 4px blur would be "good enough." That's what the calculator is telling you.  In which case BOTH sides of your photo are in focus.

Fact is, you were shooting towards the left and critical focus was on the left, so the rest of the image will be OOF increasing as you get towards the right.   And yes, in this instance, a few meters would matter in this case.

 BobORama's gear list:BobORama's gear list
Pentax K-5 Sigma 10mm F2.8 EX DC HSM Diagonal Fisheye Pentax smc DA 18-55mm F3.5-5.6 AL WR +8 more
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