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Author Topic: Sensor Dust & Small Aperture  (Read 1358 times)

walter.sk

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Sensor Dust & Small Aperture
« on: June 08, 2012, 03:32:12 PM »

I cleaned my sensor successfully, as I have been doing for many years.  I always do my test shots at f/22 or smaller apertures to see the dust spots.  I had thought that we stopped down to increase the depth of field to see the dust, but a few years ago I was told, probably on these forums, that DOF was not a factor, but there was a different reason for stopping down.

I have completely forgotten what the reason was, and would appreciate if somebody could remind me. 

By the way, I have given up on saying I am having a "senior moment."  They come so often now that I only mention the lucid moments  :-[
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Gary Brown

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Re: Sensor Dust & Small Aperture
« Reply #1 on: June 08, 2012, 04:25:50 PM »

Thom Hogan's sensor-cleaning essay mentions the range of angles at which light hits the sensor filter, as one factor:

“At f/1.8, the multiple angles at which light is hitting the sensor filter and the shallow depth of field work in your favor. At f/22 any dust spot will be pretty much be resolved in your image.”
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Ellis Vener

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Re: Sensor Dust & Small Aperture
« Reply #2 on: June 08, 2012, 09:15:53 PM »

here's a clearer analogy: with the lens wide open it's like illuminating a set wit ha relatively large softbox, the light wraps around the object (in this case the dust particle) When you stop the lens all the way down it's like switching to a small spotlight, so the edge of the object's shadow is more clearly defined.

Interestingly enough, when I was first starting to shoot digitally (back in September 2001 with a D1X), I was having a persistent dust spot problem but it was only with one lens, a 20mm f/2.8D AF-Nikkor.  The spot was really obvious when the lens was stopped down. We couldn't find it on the sensor and then my assistant had the bright idea of looking through the lens with a magnifying glass.  There was a really tiny little spec on the surface of an internal  element. I bought a replacement lens the next day as we were in the middle of an assignment that was due at a publishers by the end of the week.

Later I went back and started to look at really big projections of slides I'd been shooting over the previous year with that lens; now that I was aware of what to look for I could see the spec in those images too. I suppose I'd either been unconsciously ignoring it or had  just gotten accustomed to its presence.
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Ellis Vener
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Creating photographs for advertising, corporate and industrial clients since 1984.

walter.sk

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Re: Sensor Dust & Small Aperture
« Reply #3 on: June 09, 2012, 12:39:44 PM »

Thanks, guys: 
the cobwebs have cleared away, and I remember now.  I can sleep again!
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arlon

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Re: Sensor Dust & Small Aperture
« Reply #4 on: June 13, 2012, 05:44:36 PM »

Recently got a D800E, it resolving power is amazing. It's dust resolving ability is equally amazing. Dust may be this cameras "achilles heel"..  )-:}
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