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

Started 5 months ago | Discussions thread
Trevor G
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Re: Please use better experiment protocol
In reply to BobORama, 5 months ago

BobORama wrote:

Trevor G wrote:

jf_tea wrote:

shallow DOF at f/1.8

subject distance at left vs. right is significantly different.

Not a very good guess, jf. I'm sorry I didn't post the full image as well so that you could see the layout.

Samples shown earlier cropped from each side of this image...

Please use better experiment protocol.

Hmmm...now that you can see what I did, do you still feel the same way?

Yes.   Thanks for posting this.   Look at the central lamp post, you have adjusted you camera so that the lamp post is perfectly aligned with the window behind it.   And the vertical center line of the photo is aligned with the lamp post.    The upper and lower roof line of the colonnade ( the alternating cream and burgandy roof ) is in perfect horizontal alignment down to a pixel level.   So we have our coordinate system.

But the building is actually skewed with the left side further back than the right.

LOL.  By how much?  Remember we are 110m away from the front of the building.

Would you mind calculating the DOF at that distance, please?

And what does this say about the many hundreds of photos I have taken of this building which don't show OOF on one side, some of them with Pentax cameras and lenses?

You can see this very clearly looking at the angle of the seams in the slanted cream-burgandy roof.     IF the building were parallel with the focal plane of the camera, AND the center of the image were centered on the vertical axis of symmetry of the building, the slanted roof lines would be symmetric from right to left.   They are not.   And they suggests the left side is father back.   So my diagram explains your observation entirely.

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. 

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Cheers
Trevor G
Silkypix tutorials at: http://photo.computerwyse.com

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