[fg-one]

I would like to ask for your thoughts on a phenomenon I noticed when using a Vivitar 28mm f2.0 close focus lens (Komine made). At f5.6, the sharpness at infinity is higher in the middle of the frame than on the edges. However, on less distant features, the middle of the frame is noticeably softer than the edges. Is it what is called field curvature? Is it a lens defect or the mere consequence of the lens design? Could it be the result of a lens design compromise to get the "close focus" option in an f2 compact lens? Although this is not a very common lens, I was wondering whether someone else had noticed this, or perhaps with another lens. Here is an illustration: the whole frame then 1:1 crops of left, middle and right areas.

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[stevebrot]

Is it what is called field curvature?

Possibly, though it is also possible that the lens is simply not as sharp in the corners. The way to tell the difference is to bracket focus to the corners at relatively close distance (say 5 feet) using a flat surface as a target (sensor parallel to the target). If the corners go sharp with the center soft, there is field curvature. This may be done by making actual exposures or in live view though magnified live view is better for the task. If your lens has field curvature, it is most likely a deficiency of design. Note that "flat field" was a selling point for wide-angle lenses at the time and lack of such was not unusual for 28mm lenses.


Steve

[fg-one]

Note that "flat field" was a selling point for wide-angle lenses at the time and lack of such was not unusual for 28mm lenses.

Thank you Steve. Maybe this is why I get more consistent results from the Pentax-M 28mm f3.5, for landscapes.

[osv]

Thank you Steve. Maybe this is why I get more consistent results from the Pentax-M 28mm f3.5, for landscapes.

that's my default 28mm landscape lens on the a7r, it's a winner on ff.

we really need to see the full-size ooc jpeg shot, nearfield crops aren't enough, but my guess is that both you and steve are right about it being a classic field curvature issue.

that doesn't make it a bad piece of glass; it could be very usable for centered objects, and it may also be useful for getting clean focus on foreground anchor objects in a landscape shot, just try stopping down to f/8, see what happens.

the good news is that the sides appear to be equally sharp, so the lens may not be decentered, i'd run some more tests on it.

here is another example of fairly mild field curvature, and being able to focus around it... blow up the photo to 100%: 24mm camera lens shootout on the Sony a7R third round Konica 24mm

[fg-one]

that doesn't make it a bad piece of glass; it could be very usable for centered objects, and it may also be useful for getting clean focus on foreground anchor objects in a landscape shot, just try stopping down to f/8, see what happens.
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Thank you osv. In fact, I quite like this lens for close up work. Here is an example:

[Arjay Bee]

I have a copy of this lens and the first thing I noticed about it was the double imaging given to straight edges in out of focus areas. Quite unique.

[fg-one]

I have a copy of this lens and the first thing I noticed about it was the double imaging given to straight edges in out of focus areas. Quite unique.

Very nice Arjay Bee. Your pics are quite unique too. Amazing how a 28mm FOV renders these butterflies in their habitat far better than would a true macro shot.

[fwcetus]

I quite like this lens for close up work. Here is an example:

And here's a couple from close up (the second one very close up), wide open at f/2 -

[Arjay Bee]

Very nice Arjay Bee. Your pics are quite unique too.

Thank you for that. These skippers were quite distracted by one another and allowed me to get to the close focus point if I moved slowly.
In a fit of generosity I lent this lens and a SMC K55 1.8 to my son to enjoy - haven't seen them again for several years. He had my DL2 and then bought himself the K30 Stormtrooper. He tends to use the DA18-135 all the time.

[fg-one]

And here's a couple from close up (the second one very close up), wide open at f/2 -

I would have never guessed the second pic had been shot at f2. There is no halo and the contrast is good. I tend to use a macro lens for such close up takes but you have shown here that the 28mm Vivitar could do it just as well. Something to remember.

---------- Post added 11-14-15 at 08:33 AM ----------

Thank you for that. These skippers were quite distracted by one another and allowed me to get to the close focus point if I moved slowly.
In a fit of generosity I lent this lens and a SMC K55 1.8 to my son to enjoy - haven't seen them again for several years. He had my DL2 and then bought himself the K30 Stormtrooper. He tends to use the DA18-135 all the time.

You should ask your son how your lenses are doing. You never know. They might come back to safer hands. I have the K55 as well. A beautiful lens. I use it indoors in A mode. I find it better than any 50mm wide open.

[fwcetus]

I would have never guessed the second pic had been shot at f2. There is no halo and the contrast is good. I tend to use a macro lens for such close up takes but you have shown here that the 28mm Vivitar could do it just as well. Something to remember.

Well, such a lens is not a substitute for a true macro lens. To get that second pic required very close proximity to the subject -- much easier to get the "same" shot with a typical 50mm, 90mm, or 100mm macro lens, for example. (However, I would expect that a true macro might not have rendered the image in the same way.) A true macro lens would also have a flatter field, too - important for many subjects. However, I was a bit surprised at the results from the CF feature.

Not unlike my comments just above, there are a zillion so-called "macro zooms" out there (Pentax and 3rd-party) that allow focusing reasonably close, but not necessarily with true macro lens IQ, of course. However, there is something to be said for convenience sometimes.

Of course, then there is the VS1 90-180/4.5 Flat Field Zoom, which is certainly a true macro zoom (unless you define "macro" to include 1:1 "magnification"). [But I digress...]