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PostPosted: Sun Nov 09, 2008 10:55 am 
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I was totall shocked this morning! I was test-shooting against the morning sun (but with the sun not in the area of view!) with my new Nikon AF 85/1.8 the same scene at f/1.8, f/4.0 and f/8.0. Reviewing the images I found out that the highest contrast was achieved fully open at f/1.8 :shock: 8) :?: As you can imagine I was quite stunned...
The worst contrast was with f/8.0 where clearly flare raised its ugly head - or was it "ghosting"? I reshot and got the same results so without calling my procedure scientific I assume the results are valid (believe me for now, I'll post images later).

What might be the reason? Well, if you look into the lens from the front side and press the aperture activation button at f/4.0 or f/8.0 something bright and gleaming is reflecting the light: The aperture blades!
Hmmmm, so it might be that the reflections of the sun shining on the aperture blades reflecting back to the rear of the front lens(group) were the reason for the reduced contrast :?

Now someone tell me: Did you ever encounter a similar effect?

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PostPosted: Sun Nov 09, 2008 11:23 am 
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Now here's the proof:

Image

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PostPosted: Sun Nov 09, 2008 12:30 pm 
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Hi Thomas,

That's a bit baffling. :lol:

Bob.

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PostPosted: Sun Nov 09, 2008 12:58 pm 
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I wonder if Gordon has some specifics on this. I think you're right about the blades creating some reflection problems. Pretty interesting info though. Now I know that if I'm shooting against the sun and getting problems I can try different aperture.

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PostPosted: Sun Nov 09, 2008 1:06 pm 
Never thought about the aperture blades that way :?


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PostPosted: Sun Nov 09, 2008 1:54 pm 
WOW, i might expect a tiny difference, but that is like the difference between night and day! You learn something every day!


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PostPosted: Sun Nov 09, 2008 2:56 pm 
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Bob wrote:
That's a bit baffling.

You mean because a baffle is some kind of aperture?!

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PostPosted: Sun Nov 09, 2008 3:19 pm 
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Thomas wrote:
You mean because a baffle is some kind of aperture?!

Just joking, of course, but optical baffles are usually inserted to prevent unwanted light from intruding. Baffles might not be the answer, however, as it may be that the designer forgot to factor in any light reflected out from the iris and how it might subsequently be reflected back in towards the camera's sensor, presumably from an internal lens surface. But heck, what do I know about it? Nothing like a spot of armchair quarterbacking, though. :)

Bob.

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PostPosted: Sun Nov 09, 2008 3:54 pm 
So the aperture is causing a form of ghosting? I thought they were painted black to stop this very thing from happening.


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PostPosted: Sun Nov 09, 2008 6:15 pm 
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I'll post some photos looking into the lens at various apertures. I hope to capture the "gleam" of the aperture blades.

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PostPosted: Tue Nov 11, 2008 8:14 pm 
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Now here you can see the reason for the effect:
Image
from left to right: f/1.8, f/4.0, f/8.0. Shot with sun-shade.

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Last edited by Thomas on Wed Nov 12, 2008 6:15 am, edited 1 time in total.

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PostPosted: Tue Nov 11, 2008 8:43 pm 
I think it is caused by diffraction: http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm


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PostPosted: Tue Nov 11, 2008 9:23 pm 
So pretty much to get the "best contrast" you gotta take multiple shots at the lowest possible F-number and combine these images with different "places" in focus so to speak....does that make any sense?


I m going to check this out as well....but with the 18-55 and the 50mm, which i guess was the lens u used...?


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PostPosted: Tue Nov 11, 2008 9:39 pm 
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@Old-shaker-hand: Diffraction becomes measurable at f/5.6 and perhaps visible at f/8.0 the earliest. This here does clearly not result from diffraction. it is the result of the aperture blades reflecting the light back to the back of the front lens which in turn reflects the stray-light back towards the sensor. Ugly :evil:
@ Alex: This is the Nikon AF 85/1.8!

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PostPosted: Tue Nov 11, 2008 10:05 pm 
That's pretty amazing!

Could this effect be caused or worsened by a so-so internal lens coating (or lack of it)? Also, I have the same doubt as grahamnp, I presumed that the blades were matte black or some other pretty unreflective colour to avoid such problems?

Bye!


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