[MJSfoto1956]

Really? How different sensor can change plane of sharpness of lense? Easiest explanation is that there was need to microadjust af in case of K3.

the answer is: they are not measuring "sharpness" -- they are measuring acutance:

From Wikipedia: "In photography, acutance is the edge contrast of an image. Acutance is related to the amplitude of the derivative of brightness with respect to space."

Michael

[bossa]

There's definitely something fishy going on when they test these lenses on Pentax cameras. The following screen grab where the 35/1.4 "A" was tested on K-5 and D7000 show completely different MTF curves, both S&M (pardon),

I don't buy that explanation about getting the most out of a sensor (These two cameras have the same sensor, right?).

These sensors don't have micro-lenses that act differently toward the edge of the sensor, and I am assuming the lens is of a telecentric design, so it's doubtful the sensors would effect the outcome as much as is shown (unless they had the previously said microlenses). Sigma also offers a lens mount conversion service, which doesn't change any optical elements to my knowledge, so I doubt any major optical tweaking takes place when designing for "normal" sensors. (that service may be a con as they may just swap out your lens instead of converting it)

[bossa]

Adding in a Canon 70D it's easy to see the MTF curves are similar to the Pentax apart from being a tad "sharper".

All lenses seem to have variations though.. I wonder if it is just Sample variation at work.

[MJSfoto1956]

Adding in a Canon 70D it's easy to see the MTF curves are similar to the Pentax apart from being a tad "sharper". All lenses seem to have variations though.. I wonder if it is just Sample variation at work.

what's up with that Nikon aperture shown in the middle of the "compare 3" image? It is showing f/4??? Can't be right.

M

[bossa]

what's up with that Nikon aperture shown in the middle of the "compare 3" image? It is showing f/4??? Can't be right.

M

an accident.

Sorry, it's corrected.

[MJSfoto1956]

well, my experience is that lenses + sensors don't always perform the way you expect them to. Here's a test. The following image shows three "sharpness/CA" tests for the SAME LENS ON THE SAME BODY (Sigma 18-35mm on Nikon D7100). If I give you four different f/stops to choose from can you guess which of the three f-stops illustrated (A, B, C) is which? Your choices are:

• f/2
• f/4
• f/8
• f/11

Have fun choosing!!! (btw, there is lots to learn from this exercise. One of which there is no such thing as an "optimal" f-stop)

[bossa]

Before I go and have a look at the page I'll guesstimate:

A = f/2
B = f/8 or f/11
C = f/5.6

My choices are related to the sensor and diffraction limits more than the lens.

After looking on the site I see that I am wrong on all counts. (well, 1/2 right on one anyway)

---

Last edited by bossa; 01-10-2014 at 09:01 PM.

[MJSfoto1956]

After looking on the site I see that I am wrong on all counts. (well, 1/2 right on one anyway)

LOL! there is a lot to learn! I am fascinated by the "trends" the data show. One might quibble with a single result, but over all the trends are quite revealing. And not entirely what I expected. For example, for certain kinds of subjects, this lens might be better to shoot slightly below its so-called diffraction limit since at f/11 it delivers a remarkable even sharpness center to edge. Who would have guessed that?

Michael

[bossa]

LOL! there is a lot to learn! I am fascinated by the "trends" the data show. One might quibble with a single result, but over all the trends are quite revealing. And not entirely what I expected. For example, for certain kinds of subjects, this lens might be better to shoot slightly below its so-called diffraction limit since at f/11 it delivers a remarkable even sharpness center to edge. Who would have guessed that?

Michael

Exactly!