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08-09-2008, 12:48 PM   #1
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Pentax 28-70mm f2.8 Star - TEST LENS

i did a test with a Pentax 28-70mm f2.8 Star (+ pentax k10d) and now i want your opinion, please! (i want know if this lens is good)

TEST:

i did 4 photos: Jpeg, Aperture-Priority; exposure multi-segment; AF in center; 100 iso.

when you see the photos, please pay attention about 4 "important points" (green points):
1 - radio antenna
2 - headlight
3 - official badge
4 - number plate

there is also a "red point" and it is the point of Autofocus.




QUESTIONS:

- i find photo in f2.8 a little soft. is it normal?

- i always used exposure multi-segment but i find different exposures in the photos! why? is it normal?


HERE YOU CAN DOWNLOAD PHOTOS:
(each photo 2,5 mb)

PHOTO A

PHOTO A
f2.8
1/2000


PHOTO B
f4
1/1000


PHOTO C
f8
1/250


PHOTO D
f16
1/60


Last edited by platinum; 08-09-2008 at 12:58 PM.
08-09-2008, 01:32 PM   #2
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I have looked at the pictures as a whole and it appears that the lens is good, BUT that the combination is backfocusing. The background in the f/2.8 shot is sharper than the car. So solve that first (either by having the combination calibrated at Pentax, or by doing it yourself (via the hidden menu in the 1.10 firmware).
08-09-2008, 02:37 PM   #3
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Original Poster
QuoteOriginally posted by Asahiflex Quote
I have looked at the pictures as a whole and it appears that the lens is good, BUT that the combination is backfocusing. The background in the f/2.8 shot is sharper than the car. So solve that first (either by having the combination calibrated at Pentax, or by doing it yourself (via the hidden menu in the 1.10 firmware).
hello Asahiflex

thanks for you comment.
your words are usefull for me
i was afraid that the lens is not very sharp in f2.8.
i'm afraid that this lens is not a good copy.

about you the problem is only a Front-Back focus problem?

i put another test so you can confirm your opinion about Front-Back focus problem.

TEST NUMBER 2 (railing)


(red point is the point of Autofocus; AF in center)

(each photo 1,5 mb)

RAILING A
f2.8
1/350

RAILING B
f4
1/180

RAILING C
f8
1/60


about you?

and please can you give me an explanation:
i always use Aperture-Priority + exposure multi-segment,
but why "RAILING A" photo is more light then "RAILING C"?

Last edited by platinum; 08-09-2008 at 02:44 PM.
08-09-2008, 03:13 PM   #4
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The center part isn't big or contrasty enough for your two testing photos, this might 'cause focus error. I bet you can take several shots at the same scene might end up with different results.

Better test on something more contrasty, and make sure the center part (for center focusing) is big enough.

08-09-2008, 03:14 PM   #5
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sure you're using the right focus spots? Looks like the image background is sharper in that last image w/ the handrail...
08-09-2008, 03:41 PM   #6
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Original Poster
QuoteOriginally posted by frank Quote
The center part isn't big or contrasty enough for your two testing photos, this might 'cause focus error. I bet you can take several shots at the same scene might end up with different results.

Better test on something more contrasty, and make sure the center part (for center focusing) is big enough.
hello frank

yes, i have other tests.

TEST NUMBER 3 (monumental door)


(red point is the point of Autofocus; AF in center)

(each photo 1,5 mb)

MONUMENTAL DOOR A
iso 400
f2.8
1/1000

MONUMENTAL DOOR B
iso 400
f4
1/500
08-09-2008, 03:42 PM   #7
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Original Poster
QuoteOriginally posted by kenyee Quote
sure you're using the right focus spots? Looks like the image background is sharper in that last image w/ the handrail...
hi kenyee,
i saw again now on my k10d: i use "AF in center".
i'm sure.

so, also for you is a Front-Back focus problem?

ONLY this problem?

and about sharp in 2.8, it is normal for this lens?
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