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08-25-2008, 07:04 AM   #1
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Pentax 28-70mm f2.8 Star - different exposures?

hello :-)

i did a new test with a Pentax 28-70mm f2.8 Star (+ pentax k10d) and now i want your opinion, please!
i did 4 photos: Jpeg, Aperture-Priority; exposure multi-segment; AF in center; 100 iso.

i always used exposure multi-segment but i find different exposures in the photos!
(if you see photos, you can see in f2.8 more light than in f11)
is it normal?

CATHEDRAL A
1/1500
f2.8

CATHEDRAL B
1/750
f4

CATHEDRAL C
1/180
f8

CATHEDRAL D
1/90
f11


(each photo 700 kb)


Last edited by platinum; 08-26-2008 at 04:09 AM.
08-26-2008, 02:51 AM   #2
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please, can you tell me more info about this my question?

:-)
08-26-2008, 02:59 AM   #3
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When I look at the results, the biggest problem I see is that you have the sun in slightly different positions for each shot, making it difficult to evaluate fully since the lighting may be changing on the surface.

If you want a real test, do not use a photo such as this, but take a set of photos using a single uniform surface. Take a shot at each apature setting and then use the selection tool to evaluate the grey scale of the exposure for the central 10% of the area.

If I look at your sequence, the sun is highest sun position is in photo B, which changes the lighting of the overall image. In checking the actual lighting of the front of the cathedral, I looked at the area of the building above and to the left of the rosetta. the grey scale values are as follows for the same atea

Picture A 124
picture B 133
picture C 113
Picture D 103

Overall the shots are within about +/- 1/2 stop of the meadian value and about 3/4 stop from lowest exposure to highest. this is not really too bad.

It is not uncommon to find slight variations in exposure as you stop down. I have found several lenses that seem to have a gradual error in apature, which from wide open to fully stopped down work out to about 1/2 stop. generally over-exposing slightly ( by the 1/2 stop) as you stop down. For the test I suggest test it is important that lighting does not change, therefore you need to shoot with the light behind you.

Last edited by Lowell Goudge; 08-26-2008 at 03:10 AM.
08-26-2008, 04:22 AM   #4
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Original Poster
QuoteOriginally posted by Lowell Goudge Quote

It is not uncommon to find slight variations in exposure as you stop down. I have found several lenses that seem to have a gradual error in apature, which from wide open to fully stopped down work out to about 1/2 stop. generally over-exposing slightly ( by the 1/2 stop) as you stop down. For the test I suggest test it is important that lighting does not change, therefore you need to shoot with the light behind you.
really?

you tell me an info that i don't know.
very thanks

so, about you, this is NOT a real problem if there are different exposures for different apertures?
the lens is good?



please see here:
i did now a new test ("shoot with the light behind you")
(Jpeg, Aperture-Priority; exposure multi-segment; AF in center; 100 iso)


STREET A
1/2000
f2.8

STREET B
1/1000
f4

STREET C
1/250
f2.8

STREET D
1/125
f11

(each photo 700 kb)

08-26-2008, 06:05 PM   #5
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each photo is getting progressively darker even though they have the same exposure, so something isn't right.

have you tried it with other lenses? maybe it's a fault of the camera
08-26-2008, 06:26 PM   #6
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QuoteOriginally posted by platinum Quote
really?

you tell me an info that i don't know.
very thanks

so, about you, this is NOT a real problem if there are different exposures for different apertures?
the lens is good?



please see here:
i did now a new test ("shoot with the light behind you")
(Jpeg, Aperture-Priority; exposure multi-segment; AF in center; 100 iso)
For me, the question is, how does my equipment as a system perform. Normally +/- less than 1/2 stop over the entire exposure range of the lens is not critical. If you find it critical, then you can bracket automatcally. Each photo on it's own (in your first set) would be called suitable, hence there is not really an issue.

For what ever reason, I could not open all the photos in the second set, and you should consider resizing them, if you are posting only for information only on something like exposure, but the real issue is that you need to do exposure tests on a uniformly lit surface that occupies the entire frame.

I looked at the first and last photos, and they are not identical in composition.

With the camera on multi segment, and a variable image, it is impossible to tell exactly what the camera is thinking. and even a slight change in composition can have a big impact on the end result.

Last edited by Lowell Goudge; 08-26-2008 at 06:32 PM.
08-26-2008, 07:27 PM   #7
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QuoteOriginally posted by Lowell Goudge Quote
With the camera on multi segment, and a variable image, it is impossible to tell exactly what the camera is thinking. and even a slight change in composition can have a big impact on the end result.
huh? platinum did the test by setting the exposure and maintaining the same theoretical exposure for each one.
it shifted a bit but it's the same scene
08-27-2008, 01:01 AM   #8
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Out of topic

... excellent flare resistance for your lens

08-27-2008, 01:49 AM   #9
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QuoteOriginally posted by k100d Quote
each photo is getting progressively darker even though they have the same exposure, so something isn't right.

have you tried it with other lenses? maybe it's a fault of the camera
i used other lens and there is not this problem.

i think that my k10d is ok.

MESSAGE FOR ALL:
if another owner of a "Pentax 28-70mm f2.8 Star" is here, please tell us your experience!


QuoteOriginally posted by gkopeliadis Quote
... excellent flare resistance for your lens
yes, it's very good :-)

Last edited by platinum; 08-27-2008 at 01:55 AM.
08-27-2008, 01:58 AM   #10
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Original Poster
QuoteOriginally posted by Lowell Goudge Quote

For what ever reason, I could not open all the photos in the second set, and you should consider resizing them, if you are posting only for information only on something like exposure, but the real issue is that you need to do exposure tests on a uniformly lit surface that occupies the entire frame.

I looked at the first and last photos, and they are not identical in composition.

With the camera on multi segment, and a variable image, it is impossible to tell exactly what the camera is thinking. and even a slight change in composition can have a big impact on the end result.
yes, you are right.
but, trust me, i did others tests and i always see different in exposure
(progressively darker).

when i have more time, i will do anothet test!
08-27-2008, 02:30 AM   #11
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QuoteOriginally posted by platinum Quote
if another owner of a "Pentax 28-70mm f2.8 Star" is here, please tell us your experience!
I’ve never heard of this lens before …
I'm interested in it ...
08-27-2008, 06:15 AM   #12
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eBay Store - Matsuiyastore: Pentax: smc PENTAX FA 28-70mm f 2.8 AL Exc

get them here
08-27-2008, 09:43 AM   #13
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Or buy 3 Tamron 28-75mm f/2.8s and still have some drachmas left over for beers.
08-27-2008, 09:53 AM   #14
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QuoteOriginally posted by Miserere Quote
Or buy 3 Tamron 28-75mm f/2.8s and still have some drachmas left over for beers.
yes, also you are right.
money is always a problem about a lens.

but i think that a "Pentax 28-70mm f2.8 Star" is like a bank cheque:
when you will buy it, you will have more or less the same money.
and so you can see that it is an "economic investment" in long duration
08-27-2008, 09:55 AM   #15
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QuoteQuote:
i always used exposure multi-segment but i find different exposures in the photos!
wouldn't this play a part? I highly doubt its the lens. multi-segment metering is far different from center-weighted. it can easily yield different exposures with the slightest change in lighting conditions, and based on the photos with the sun peering just over the top of the building with only the slightest movement from the camera ie: you, would yield a change in light thus a change in exposure. you might want to do a little reading on multi-segment metering before blaming your equipment. granted multi-segment is best for your photos but you should still understand what you are doing before placing blame. here is a little snippit:

QuoteQuote:
Exposures are constructed around a TTL multi-segment metering system that takes light readings from 16 zones throughout the frame. There are three modes available, selected with a ring surrounding the mode dial. Multi-Segment, Spot, and Center-Weighted metering options are available. Multi-Segment metering functions like other manufacturers’ Matrix and Evaluative systems, taking readings from a number of zones across the composition and determining the best overall exposure for the scene. Spot Metering takes a reading from the center of the frame (roughly 5 percent) or active autofocus point. Center-Weighted metering is a combination of the two; it biases the exposure toward the center but does not neglect the rest of the scene, retaining sensitivity (albeit diminishing) as it moves toward the edges of the frame.

The metering timer is adjustable, and can be set to 10-second (default), 3-second, or 30-second options in the Custom Settings menu.

In terms of design, the K10D’s 16-segment metering system is somewhat dated. Many competitors now feature meters that read from 35 to 256 zones across the frame. In use, however, we found the K10D supplies accurate readings in most lighting conditions. The K10D’s multi-segment setting is generally consistent with competing models in situations with a distinct key light coming from the front or side. It is thrown in some high-contrast, backlit scenes, but no more so than its direct competitors. The camera’s spot meter is right on and quite consistent. The Center-Weighted setting, however, is disappointing. It is easily fooled by backlit subjects – even in the center of the composition – and is generally less effective than its Multi-Segment setting.
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