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10-10-2010, 04:35 PM   #1
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Odd overexposure with A mount 28mm 2.8 on K-7

I recently picked up a clean 28mm f.2.8 A lens for my K-7. It appears to function correctly and takes nice sharp images. That said, I have noticed that the K-7's meter tends to want to overexpose through this lens by somewhere between 2/3 and a full stop.

I've tested this by shooting identical scenes with it and another lens (a 16-45 DA) set to 28 mm at the same aperture. Sure enough, the camera generally thinks it needs more light under the same conditions when the 28mm is mounted. Histograms also show the highlights clipped on the right. Just to remove a red herring, I've also played with the Dynamic Range settings, they don't seem to make a difference.

Of course, this is easily correctable by using exposure compensation, but I'm curious if anyone else has experienced something like this, as well as if anyone has any ideas on what might be going on here.

10-10-2010, 04:39 PM   #2
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I've got a Rokinon 28mm 2.8 that is similar. f/2.8 it underexposes by a stop, f/4 is right on, and every other stop is one stop over exposed.

That's when the lens is on the A position. Using stop-down metering, it is right on at every aperture value.
10-10-2010, 05:00 PM   #3
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I just spent September shooting with my A28 2.8 in the "Single In September" comp, using it with my K-x body, and didn't have any overexposures (apart from shooting towards the sun without a lens hood). From memory, shots with my K-7 aren't overesposed either. Maybe the aperture blades on your lens are a little sticky / slow?

Here's a link to my Single in September album -> jackassp's Album: Single In September - PentaxForums.com

Cheers, hope this helps.
10-10-2010, 05:29 PM   #4
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What you should do is to shoot a series of shots of a uniformly lit paved road or block wall and using the camera to control aperture in Av mode to shoot a test shot at each aperture and then measure the grey scale value in a photo editor. This will let you test the exposure and aperture control

10-10-2010, 09:23 PM   #5
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QuoteOriginally posted by Lowell Goudge Quote
What you should do is to shoot a series of shots of a uniformly lit paved road or block wall and using the camera to control aperture in Av mode to shoot a test shot at each aperture and then measure the grey scale value in a photo editor. This will let you test the exposure and aperture control
What Lowell said except that you should not even need to check the values in a photo editor. If the variance is as large as you suggest, the issue should be evident in the LCD on the back of the camera when zoomed out to view the series at a glance.


Steve
10-10-2010, 10:19 PM   #6
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QuoteOriginally posted by stevebrot Quote
What Lowell said except that you should not even need to check the values in a photo editor. If the variance is as large as you suggest, the issue should be evident in the LCD on the back of the camera when zoomed out to view the series at a glance.


Steve
I used the histogram to check the exposure. It's quite easy to see when your exposure is off if you know how to read it.
10-11-2010, 01:02 AM   #7
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I just took some shots using the A28 and my K-7, and the exposure histograms looked close to perfect at F2.8, F5.6 and F8. One shot did show a peak at the right hand side of the histogram, but I'm sure that was from some blown highlights from the bit of sky in the shot though.

Hope this helps.
John
10-11-2010, 03:41 AM   #8
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btw, I don't have a k-7, but I note my 16-45 tends to under-expose with the K100D, sometimes by quite a bit. Generally speaking the k100d meters the A 28 well.

Once you know these sorts of things it is easy enough to always dial in ev comp when you mount the lens.

10-11-2010, 09:47 AM   #9
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QuoteOriginally posted by alohadave Quote
I used the histogram to check the exposure. It's quite easy to see when your exposure is off if you know how to read it.
The reason I use a block wall is to remove the question of what the matrix metering does with a complex scene. Measuring with an editor when you know that with neutral settings a 45 change in vreyscale is 1 stop
10-15-2010, 08:59 AM   #10
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Thanks all - It's definitely overexposing in some situations, AlohaDave is right, it's clearly visible in the histogram and to the naked eye.

The thing that's odd about it is that the k7's meter is choosing to over expose (it's not a stuck diaphram or other mechanical issue in the lens). For instance, shooting the same scene at the same focal length and same f-stop, it will consistently choose a slower shutter speed with the 28 than with the 16-45.

I have been just setting the EV compensation and enjoying the lens, but I still find this a bit curious.
10-15-2010, 12:09 PM   #11
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Does this happen at all aperture settings? Or does it happen only at some settings (in particular, the largest and/or the smallest apertures)?

The overexposure may be caused by a bent (or loose) aperture lever.

I once had a Tokina with overexposure only at F/2.8, the largest aperture. It took me a while to find out the cause was some play in the aperture lever. I used a piece of plastic to jam it tight. That fixed the problem.
10-15-2010, 12:47 PM   #12
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QuoteOriginally posted by petergottlieb Quote
Thanks all - It's definitely overexposing in some situations, AlohaDave is right, it's clearly visible in the histogram and to the naked eye.

The thing that's odd about it is that the k7's meter is choosing to over expose (it's not a stuck diaphram or other mechanical issue in the lens). For instance, shooting the same scene at the same focal length and same f-stop, it will consistently choose a slower shutter speed with the 28 than with the 16-45.

I have been just setting the EV compensation and enjoying the lens, but I still find this a bit curious.
Again

I cannot stress this enough, you need to test the lens with the camera against a uniformly lit surface, i.e.block wall or paved road are very good for this.

Many of the cameras suffer from exposure non linearity with legacy lenses and the K7 is no different.

I have attached a chart of the cameras I own with my K50F1.4 to show this.



I can also tell you that even new lenses have exposure errors,

My Tamron 28-75 F2.8 is not perfect. wide open it meters and exposes with the image at about 105 greyscale (contrast and brightness neutral) it rises linearly from that point to about 145 by F32 when controlled in the A position, by the camera body. This represents a drift of a full stop over the entire aperture range.

My Sigma 70-200F2.8 meters perfectly on my K10D by itself, but add the sigma 1.4x TC and it needs -.7 EV compensation, and with the 2x TC it needs -1.3 EV compensation.

You need to test your cameras and lenses as a system against a uniformly lit surface, so that you eliminate any interpretation done by the matrix metering, and are able to correctly understand the results.
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