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02-13-2010, 10:48 PM   #1
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What and why

Not sure if this is the right place - so someone please direct me...

Here's the problem - kinda a whiteish bright spot in the center of an image. Camera K10, (had same effects on a K100) Lens is a Chinon 50mm 1.7 but I am using 2 hollowed out teleconverters as extension tubes. This problem showed up using f22 and 1/2 second exposure. I have not noticed it wide open, but the depth is so narrow...any ideas.

I am going to try and experiment with more f stops and time values and see where it starts.

This was a raw image simply converted to jpeg and resized nothing else done.

Thanks, Chris

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02-14-2010, 02:33 AM   #2
Ash
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Is this problem reproducible - or is it an intermittent problem? Does t happen with any other lens?

What it looks like to me is that a bright foreign body lodging itself on one of the lens's elements in the centre, which you should be able to see when you look through the lens wide open.
If it's clean, check your camera's sensor for any obvious sign of contamination.
02-14-2010, 08:30 AM   #3
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That is a reflection of the sensor off the rear element of the lens.
02-14-2010, 04:09 PM   #4
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QuoteOriginally posted by Ash Quote
Is this problem reproducible - or is it an intermittent problem? Does t happen with any other lens?

What it looks like to me is that a bright foreign body lodging itself on one of the lens's elements in the centre, which you should be able to see when you look through the lens wide open.
If it's clean, check your camera's sensor for any obvious sign of contamination.
Yes, (reproducible) Haven't tried other lenses with extension tubes. Happens with long exposures.

Nope nothing on the lens.

Thanks.

02-14-2010, 04:10 PM   #5
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QuoteOriginally posted by Wheatfield Quote
That is a reflection of the sensor off the rear element of the lens.
Interesting - any way to avoid such?
02-14-2010, 05:37 PM   #6
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QuoteOriginally posted by DblArrow Quote
Interesting - any way to avoid such?
Avoid taking pictures like the one you have there.
Seriously.
It's the large white area that does it, I think.

Here's one I managed:



That was shot with a Pentax A100/2.8 macro lens, which is a very good lens.
Anyway, it's something I hardly see in my pictures, but a bright background image can sure bring it out.

Using "designed for digital" lenses may help, if part of the design is rear element anti reflective coatings designed to minimize this type of flare.
02-14-2010, 06:12 PM   #7
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http://thesybersite.com/minolta/sensor-reflection/index.htm
02-14-2010, 11:29 PM   #8
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QuoteOriginally posted by Wheatfield Quote
Avoid taking pictures like the one you have there.
Seriously.
It's the large white area that does it, I think.


Using "designed for digital" lenses may help, if part of the design is rear element anti reflective coatings designed to minimize this type of flare.
Thanks, muchly appreciated.

02-14-2010, 11:30 PM   #9
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Perfect - Thanks.

I've only seen this on one lens and that only when using extension tubes. Will have to play some more.
02-15-2010, 03:58 AM   #10
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Consider a ringlite. Mine was about US$40 on eBay. Even with a contrasty subject on white background, I don't see that sensor effect with my K20D + FA 50/1.4 + Raynox DCR-250 (and I just now peeked at some recent shots to be sure).

EDIT: I just read the Minolta article and realized that the setups aren't comparable, and I haven't used the ringlite with a film-era lens, especially uncoated. But a ringlite might work anyway. Can't hurt, eh?

Last edited by RioRico; 02-15-2010 at 04:15 AM. Reason: addendum
02-15-2010, 09:03 AM   #11
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So, there is actually a way around.
Just find out which f-stop it is less pronounced or non-existent.
Good info 'coz I think I also have a lens that does this.
I'm thinking it was not a good lens when actually it might have just been the sensor reflection all the time!
02-15-2010, 11:44 AM   #12
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I have a similar problem with a rikenon 135 F2.8 screw mount.

lens is OK except in bright light (daylight in my case)

has a white low contrast donut in the middle.

it is the only lens I have out of 10 screw mounts with an issue, it gets worse as yuou stop down

someone commented it may be due to a flat rear element in combination with the reflection from the sensor
02-16-2010, 10:12 AM   #13
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QuoteOriginally posted by Lowell Goudge Quote
I have a similar problem with a rikenon 135 F2.8 screw mount.

lens is OK except in bright light (daylight in my case)

has a white low contrast donut in the middle.

it is the only lens I have out of 10 screw mounts with an issue, it gets worse as yuou stop down

someone commented it may be due to a flat rear element in combination with the reflection from the sensor
Interesting, I always thought it was kinda prone to glare or the like but could actually be a sensor reflection problem.
I have to do further tests!
Thanks for all the info!
02-16-2010, 10:37 AM   #14
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QuoteOriginally posted by GerryL Quote
Interesting, I always thought it was kinda prone to glare or the like but could actually be a sensor reflection problem.
I have to do further tests!
Thanks for all the info!
try the same shot on a film body

seriously I am thinking to do the same but when you look at the shots in the link posted by ma318, they are identical to what I get.

I may try a polarizing filter, and am also toying with the idea of mounting a small filter in the back barrel.

I used ot have some old filters from a speed graphic.

If I can find them and demonstrate a solution I will post it
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