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05-26-2014, 08:12 AM   #16
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Sigma. 18-250 Hood

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05-26-2014, 09:04 AM   #17
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Focus a bit farther away and they will look identical.
05-26-2014, 09:10 AM   #18
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I too think its the hood. The HSM is the focus system. Why should it show big dark blobs in the corners?


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05-26-2014, 09:16 AM   #19
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It can't be the sensor either as the picture would also be tilted to the same degree.

05-26-2014, 11:09 AM   #20
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QuoteOriginally posted by i_trax Quote
i think from the sun and very high humidity
Sorry to keep pressing on this, but looking at the photo you use as an example of the problem there was cloud cover, so no sun.

Humidity will make the lens wet, possibly, but not overheat.

What made you come to the conclusion the lens had overheated?
05-26-2014, 11:44 AM   #21
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It's something very close to the lens (look at OOF edges), circular, and black in color. In other words, lens hood.
05-26-2014, 03:49 PM   #22
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Assuming the hood is not the issue, what could go wrong if a lens overheats?

The Sigma 18-250 has a plastic lens body. If a uniform level of heat was applied to it, it is conceivable that some deformation of the lens barrel may occur, for example, which could throw out the alignment of something like the front lens group. Or at a certain temperature, some internal components of the lens could have expanded or deformed substantially and be impacting optical performance.

Hence the warnings about leaving cameras in cars or on car dashboards during hot days. So heat damage is not an impossibility. Hence white or silver lenses.

Having said all that, walking about with a lens, even on a hot, humid day in direct sun, should at least provide some ventilation, which ought to stop the lens cooking to the point where it gets damaged.
05-26-2014, 04:40 PM   #23
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Heat damage to plastic is permanent. Unless it is memory plastic

05-26-2014, 04:55 PM   #24
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QuoteOriginally posted by rawr Quote
Assuming the hood is not the issue, what could go wrong if a lens overheats?

The Sigma 18-250 has a plastic lens body. If a uniform level of heat was applied to it, it is conceivable that some deformation of the lens barrel may occur, for example, which could throw out the alignment of something like the front lens group. Or at a certain temperature, some internal components of the lens could have expanded or deformed substantially and be impacting optical performance.

Hence the warnings about leaving cameras in cars or on car dashboards during hot days. So heat damage is not an impossibility. Hence white or silver lenses.

Having said all that, walking about with a lens, even on a hot, humid day in direct sun, should at least provide some ventilation, which ought to stop the lens cooking to the point where it gets damaged.
Still, an element getting dislodged can't tilt rotate the image of the hood but at the same time keep the subject straight.
05-26-2014, 07:01 PM   #25
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Not sure why so embarrassing? I think this has happened to everyone once...
05-26-2014, 07:59 PM   #26
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QuoteOriginally posted by calsan Quote
Not sure why so embarrassing? I think this has happened to everyone once...
That's why everyone is recognizing it too!
05-27-2014, 12:39 AM   #27
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QuoteOriginally posted by JohnX Quote
Sorry to keep pressing on this, but looking at the photo you use as an example of the problem there was cloud cover, so no sun.

Humidity will make the lens wet, possibly, but not overheat.

What made you come to the conclusion the lens had overheated?
felt very tight, zoom was sticky
it was well over 30deg C, no wind like in a greenhouse

---------- Post added 05-27-14 at 15:46 ----------

well I did some photos and you are most likely right, can get very similar results with tilted hood,
but that was the first thing I did check when black blobs appeared in the viewfinder, the hood was locked (? maybe not)
thank you for your time
kind regards
jack
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