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04-20-2008, 02:13 PM   #1
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Fade to white - Help! Problem with Tamron 28-75mm ??

I apparently have a problem with my very recently purchased Tamron 28-75mm. On the other hand, perhaps I've missed something in terms of using a third party lens???

I tested it today, and finally figured out what was going on. I ended up testing it against a Pentax FA 1:4 28-70 as a baseline or constant.

Today was a sunny day, no clouds. For the test I picked a point in the distance and used that point for every frame. Just to further make all things equal I set the metering to center-weighted so that the PentaX FA would not use multi-segment metering.

I tested the Tamron as follows:

In Program Mode. Set aperture to 2.8. Fired off a shot. Set Aperture to 3.5. Fired off a shot. I rinsed and repeated until I had reached f/32. As I did this, of course, the shutter speed changed, as would be expected.

As I progressed, I checked the histogram. It moved to the right until at about f/8, when everything was blown out. By f/22 and f/32, the entire thing was in essence white.

I then took my Pentax FA 1:4 28-70 and took it from f/4 to f/22. As you would expect, the histogram remained essentially the same throughout the range. Perfect. I think that this part of the test clearly demonstrates that there is nothing wrong per se with the camera.

BTW, I recorded aperture vs shutter speeds for both lenses and this showed that they both behaved similarly. Basically, the following shows how they performed.

Tarmon

2.8 = 1500
3.5 = 1000
4.0 = 750
4.5 = 500
5.6 = 350
6.7 = 250
8.0 = 180
9.5 = 125
11 = 90
13 = 60
16 = 45
19 = 30
22 = 20
27 = 15
32 = 10

Pentax

4.0 = 1000
4.5 = 750
5.6 = 500
6.7 = 350
8.0 = 250
9.5 = 180
11 = 125
13 = 90
16 = 60
19 = 45
22 = 30

As you can see the values on the Tamron are only 1 f/stop off from the Pentax, all the way down the line. As far as I am concerned this could be the coating. Clearly it is a constant, and there is no progression. Basically I think this says that the camera is probably working this all out fine. It seems to me more like something about the lens.

Then again, I am new to my K10D. Is there something I am missing?

Any help you can lend will be greatly appreciated. I can post the sequence of pictures if it really will help.

Thanks in advance,

woof!

04-20-2008, 03:44 PM   #2
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Have you tried this test in Av mode?
04-20-2008, 03:54 PM   #3
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you may just be learning that different lenses have different personalities.

it really may not be anything but that.

if my simple hypothesis is correct (and I am very often wrong when I am hypothesizing.) the larger the front element, the more light you can get.

QuoteQuote:
Then again, I am new to my K10D. Is there something I am missing?
i dont think its the camera, you are comparing lenses here.

good luck.

mitch
04-20-2008, 04:29 PM   #4
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Pics

Again... this is program mode. The camera sets the exposure based on an adjustment to either the shutter of the aperture. The histograms should be basically the same from one program setting to the next. The only thing that changes is depth of field.

The Pentax behaves correctly. As you can see the Tamron degrades. Basically I tried it in all modes. the one thing that holds true is that the higher the Aperture, the more blown out the Tamron gets. I just used program mode because it illustrates it so well.

I can only include 15 pictures, so I stopped at the seventh exposure for each sequence.

Tamron

2.8 = 1500



3.5 = 1000



4.0 = 750



4.5 = 500



5.6 = 350



6.7 = 250



8.0 = 180



I think you get the picture.


Now the Pentax

4.0 = 1000



4.5 = 750



5.6 = 500



6.7 = 350



8.0 = 250



9.5 = 180



11 = 125



13 = 90




Suffice it to say the Tamron gets worse moving towards f/32 and the Pentax stays the same moving to f/22. Same camera, same program, same conditions. The Pentax behaves as I would expect. The Tamron shows a non-linear behavior. This can't be right, and I don't think this is a matter of "personality."

Shane

04-20-2008, 04:34 PM   #5
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QuoteOriginally posted by Buddha Jones Quote
Have you tried this test in Av mode?
No but I will if that will somehow be more dispositive. I'll have to do it tomorrow as the light is gone for today.

I shot in every mode I could today and was blowing everything out on the high end everywhere. I was in and out of the woods, and it took me the longest time to realize that it was the higher f/stops that were blowing out. Basically, whenever I set it such that the f/stop is above say 5.6, it is noticeably overexposed, especially in bright sunlight.

Shane
04-20-2008, 04:40 PM   #6
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I would guess the iris isn't closing properly, is it a new lens?
04-20-2008, 04:44 PM   #7
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QuoteOriginally posted by borno Quote
I would guess the iris isn't closing properly, is it a new lens?
I have to say I agree with you. That makes the most sense.

Shane
04-21-2008, 07:00 AM   #8
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No more comments? I guess I'll return it to Tamron to see if they will service it.

Thanks.

Shane

04-21-2008, 07:29 AM   #9
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This behavior looks suspiciously like the issue of using M or K lenses on the K10D. The smaller the aperture the more it over exposes.

Couple of quick questions:
First was the lens locked in "A"? If not, do you have use of aperture ring set to "Permitted" in the custom menu?

If the lens was locked in the A position, try cleaning the contacts.
04-21-2008, 07:31 AM   #10
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QuoteOriginally posted by woof Quote
I apparently have a problem with my very recently purchased Tamron 28-75mm. On the other hand, perhaps I've missed something in terms of using a third party lens???

I tested it today, and finally figured out what was going on. I ended up testing it against a Pentax FA 1:4 28-70 as a baseline or constant.

Today was a sunny day, no clouds. For the test I picked a point in the distance and used that point for every frame. Just to further make all things equal I set the metering to center-weighted so that the PentaX FA would not use multi-segment metering.

I tested the Tamron as follows:

In Program Mode. Set aperture to 2.8. Fired off a shot. Set Aperture to 3.5. Fired off a shot. I rinsed and repeated until I had reached f/32. As I did this, of course, the shutter speed changed, as would be expected.

As I progressed, I checked the histogram. It moved to the right until at about f/8, when everything was blown out. By f/22 and f/32, the entire thing was in essence white.

I then took my Pentax FA 1:4 28-70 and took it from f/4 to f/22. As you would expect, the histogram remained essentially the same throughout the range. Perfect. I think that this part of the test clearly demonstrates that there is nothing wrong per se with the camera.

BTW, I recorded aperture vs shutter speeds for both lenses and this showed that they both behaved similarly. Basically, the following shows how they performed.

Tarmon

2.8 = 1500
3.5 = 1000
4.0 = 750
4.5 = 500
5.6 = 350
6.7 = 250
8.0 = 180
9.5 = 125
11 = 90
13 = 60
16 = 45
19 = 30
22 = 20
27 = 15
32 = 10

Pentax

4.0 = 1000
4.5 = 750
5.6 = 500
6.7 = 350
8.0 = 250
9.5 = 180
11 = 125
13 = 90
16 = 60
19 = 45
22 = 30

As you can see the values on the Tamron are only 1 f/stop off from the Pentax, all the way down the line. As far as I am concerned this could be the coating. Clearly it is a constant, and there is no progression. Basically I think this says that the camera is probably working this all out fine. It seems to me more like something about the lens.

Then again, I am new to my K10D. Is there something I am missing?

Any help you can lend will be greatly appreciated. I can post the sequence of pictures if it really will help.

Thanks in advance,

woof!
Before you go off on any tangents, I suggest you modify your test in 2 ways,
- first, go away from using a scene with all sorts of different values to a uniformly lit and uniform color object. I like block walls and paved roads.
- second, don't use the shutter speed but use the histogram value in a photo editor. You should not that slight differences in lenses (even within brand) could cause the camera to sit on one side of a shutter speed selection or another. also use 1/3 stop exposure steps on both.

Take a series of photos and plot f-stop by f-stop the histogram value for each lens. and then compare. After all it is the exposure you are concerned about. Slight changes in lighting are removed by this approach,
04-21-2008, 08:58 AM   #11
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did you lock your ISO to one value?
If you're changing aperture, and lock ISO, it can only change shutter speed to control exposure. You should get an overexposure warning in your viewfinder...

Also, if it's too bright (looks like it's daytime sun) and you don't have an ND filter, it'll hit exposure limits as well...
04-21-2008, 04:34 PM   #12
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>> Before you go off on any tangents, I suggest you modify your test in 2 ways,
>> - first, go away from using a scene with all sorts of different values to a uniformly lit and >> uniform color object. I like block walls and paved roads.

I did this, AND I locked ISO. Same result.

>> - second, don't use the shutter speed but use the histogram value in a photo editor. You
>> should not that slight differences in lenses (even within brand) could cause the camera to
>> sit on one side of a shutter speed selection or another. also use 1/3 stop exposure steps
>> on both.

The shutter speed was to demonstrate that it the shutter speeds were indeed changing in a linear fashion with the changing of the f/stop. Had I not done that, someone would have asked whether the shutter speeds were changing. I only did that to show that they were. And in fact, they were close enough to the Pentax values that it removes any suspicion whatsoever. The difference could easily be accounted for by the coating.

>> Take a series of photos and plot f-stop by f-stop the histogram value for each lens. and
>> then compare. After all it is the exposure you are concerned about. Slight changes in
>> lighting are removed by this approach,

I puzzled over this for a long time. What does the histogram show that my photographs do not show? Then I realized. You replied without seeing my photos. Have a look at the photos above. One from the Tamron and one series from the Pentax.

Nevertheless, I did what you suggested... I took photos of my driveway from high up at an angle late in the day when the driveway was not lighted directly by the sun. Nice uniform gray color. Almost could not auto-focus. I then locked ISO and shot both sequences in Program Mode. Even used a tripod.

The Tamron overexposed by about f/5.6 to f/8.0 and was all white by f/22 (or earlier). The Pentax performed perfectly. The Histogram on the Pentax remained the same... a nice thin mound right about dead center. The histogram on the Tamron looked the same at first, then marched to the right with the change in f/stop, then hit the wall and went straight up.

Need me to post the pictures for that run? The camera did not think anything was overexposing. The calculated vales were in range.

>> did you lock your ISO to one value?
>> If you're changing aperture, and lock ISO, it can only change shutter speed to control
>> exposure. You should get an overexposure warning in your viewfinder...

Great idea. I did this and locked at 800 ISO. The camera does not see it as overexposed. As far as it is concerned, the values it is calculating for each of the stops is in range. Yes, it goes to a very slow speed at the higher f/stops, but I did not hit any limits at all. After everything was said and done, I also tried it in Shutter Priority mode "unofficially" and it behaved the same.

The lens is not stopping down correctly. It looks like a lazy iris. Does anyone concur or are there further tests, things that I need to do? Thanks in advance.

Shane
04-21-2008, 04:38 PM   #13
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I agree- it sounds exactly like the experiences I've had with slow apertures. Try using the DOF preview to make sure.
04-21-2008, 04:55 PM   #14
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Send it in for service...
04-21-2008, 05:08 PM   #15
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QuoteOriginally posted by JMS Quote
I agree- it sounds exactly like the experiences I've had with slow apertures. Try using the DOF preview to make sure.
BINGO!

Thanks very much. Good test. When you hit DOF preview with f/stop set to f/22 for example, it stops down very, very slowly, then fast at the end.

Thanks you very much. Diagnosis confirmed. Lens back to manufacturer. Thanks to all for their input, especially JMS. That is just the kind of thing that will be easy to explain to the mfg. Much better than my photo sequence.

Shane
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