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

Started May 18, 2014 | Discussions thread
DAVID MANZE Veteran Member • Posts: 5,266
Re: Mighty Mike made an interesting observation!

awaldram wrote:



Mighty Mike made an interesting observation when he posted that the tested DA 50 1.8 was slightly decentered on SLR gear according to their sharpness graphs, resolution is shown across the frame by a graphic representation marked in blur units, what I hadn't noticed was that this shows and decentering very clearly. I've never really seen any comments by the author about lenses centering qualities, obviously he lets the graphs do the talking. Checking through a selection of tested lenses many appear to have slight centering problems.

Here's a link to the Pentax DA 18-135mm test;

Just click on the blur chart to enlarge, from there you have access to focal length and aperture changes.

All lens every made are d-centered that’s a given, Its if the amount of de-centering is field relevant .

From the graph its looks more like field curvature than d-centered and the authore does comment on the lens alignment " this is matched with some fairly decent central performance"

It is nothing but uncorrected field curvature and probably won't be field relevant unless your into brick walls

Even the worst focal length (24mm) 9 blur unit Vs 13 blur units will not be field relevant @f4 (everything is just blurred)

Looking at the focus footprint @f4-f8 I there is no element alignment issue and is purely the uncorrected field curvature your seeing

this is what d-centered looks like

And indeed the author does comment on the poor alignment.

Hi Awaldram,

From what I understand from Photozone and Lenstip is that all focusing is done manually so  "as  to avoid" problems with curvature and AF errors and is stated as such,  PZ also states it takes the better side of the image to produce it's resolution charts, I'm not sure but I suspect SLRgear do the same (MF wise) otherwise some lenses with a strong field of curvature would produce poor edge and corner resolution. PZ also comments on decentered alignment often qualifying it's conclusion with "if you can find a copy".

But that is an aside, my point merely was that, SLR gear is one of the few sites to produce charts that cover the whole area of the frame and that can show optical short comings wherever they may arise from.

Dave's clichés

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