Approve the Cookies
This website uses cookies to improve your user experience. By using this site, you agree to our use of cookies and our Privacy Policy.
OK
Index  •   • New posts  •   • RTAT  •   • 'Best of'  •   • Gallery  •   • Gear  •   • Reviews
Guest
New posts  •   • RTAT  •   • 'Best of'  •   • Gallery  •   • Gear  •   • Reviews
Register to forums    Log in

 
FORUMS Canon Cameras, Lenses & Accessories Canon EOS Digital Cameras 
Thread started 09 May 2007 (Wednesday) 18:42
Search threadPrev/next
sponsored links
(this ad will go away when you log in as a registered member)

Dust spect sharpness and apertures?

 
manipula
Cream of the Crop
Avatar
5,290 posts
Joined Jan 2007
Location: English Wookie in Wellington, NZ.
     
May 09, 2007 18:42 |  #1

Following a discussion at work today with a professional with a 5D and some current dust issues, we sat scratching our heads as to the technical reason why dust specs appear sharper when the aperture's stopped down then when wide open?

I got to thinking about circles of confusion etc and started banging my head against a brick wall when I realised that regardless of aperture the point of light will only ever be a point when the lens is focusing the image on the film plane, and that those points of light, or if out of focus, circles of confusion, will be the same size on the sensor. Surely dust should be rendered the same whether the light hitting the sensor is coming from something sharp or from something soft/bokeh-tastic as these points of light/circles of confusion will be the same size across the sensor regardless of their origin?

At which point I had to get back to work, stop thinking about camera physics and we agreed to ask the fellow nerds of the interweb if they knew then science behind it? I'm sure it's painfully obvious when it's explained but amongst the million other things I've done today I'm going right in circles.

Over to you dweebs. :p


Cheers, Dave.
www.manipula.co.nz (external link) :: Gear list for the nerds (external link) :: flickr (external link) :: ModelMayhem (external link)
:: insert scathing quip here! ::

  
  LOG IN TO REPLY
sponsored links
(this ad will go away when you log in as a registered member)
JC4
Goldmember
Avatar
2,610 posts
Likes: 3
Joined Apr 2007
Location: Columbus, Ohio
     
May 09, 2007 18:53 |  #2

My guess:

When the lens is wide open, more points of light hit each sensor. Each of these points of light are coming in at different angles, converging on this one sensor. More of them make it around the dust spec.

When the lens is closed down, fewer points of light hit each sensor, more chance for the dust to get in the way. The dust blocks a greater percentage of light to that sensor.

Just a guess. In pure layman's terms, because that's what I am :)


John Caputo

  
  LOG IN TO REPLY
AirBrontosaurus
Goldmember
Avatar
3,814 posts
Likes: 1
Joined Sep 2005
Location: Indianapolis, Indiana
     
May 09, 2007 18:54 |  #3

JC4 said it better.


Chris | My Flickr (external link) | AirBrontosaurus.com (external link) | Peleng 8mm Fisheye writeup (external link)
Body
: Canon 5D
Lenses: Canon 24-105mm f/4 L | Canon MP-E 65mm Macro | Canon 85mm f/1.8 |

  
  LOG IN TO REPLY
AirBrontosaurus
Goldmember
Avatar
3,814 posts
Likes: 1
Joined Sep 2005
Location: Indianapolis, Indiana
     
May 09, 2007 18:54 |  #4

JC4 wrote in post #3180280 (external link)
My guess:

When the lens is wide open, more points of light hit each sensor. Each of these points of light are coming in at different angles, converging on this one sensor. More of them make it around the dust spec.

When the lens is closed down, fewer points of light hit each sensor, more chance for the dust to get in the way. The dust blocks a greater percentage of light to that sensor.

Just a guess. In pure layman's terms, because that's what I am :)

That's much better my explanation. Thanks for that!


Chris | My Flickr (external link) | AirBrontosaurus.com (external link) | Peleng 8mm Fisheye writeup (external link)
Body
: Canon 5D
Lenses: Canon 24-105mm f/4 L | Canon MP-E 65mm Macro | Canon 85mm f/1.8 |

  
  LOG IN TO REPLY
manipula
THREAD ­ STARTER
Cream of the Crop
Avatar
5,290 posts
Joined Jan 2007
Location: English Wookie in Wellington, NZ.
     
May 09, 2007 18:59 |  #5

Makes sense. By that you're implying that when stopped down all the millions of points of light are converging onto the sensor at a much narrower angle of view, but how does that relate to the fact the regardless of the aperture, the angle of view is still sufficient to cover the whole sensor? Does that mean when the aperture's wide open there's light being aimed at a much greater square mm area then when close down?


Cheers, Dave.
www.manipula.co.nz (external link) :: Gear list for the nerds (external link) :: flickr (external link) :: ModelMayhem (external link)
:: insert scathing quip here! ::

  
  LOG IN TO REPLY
kpt4321
Member
46 posts
Joined Mar 2007
Location: Masachusetts
     
May 09, 2007 19:57 |  #6

Depth of field. The specs are closer to being in focus when you stop it down, thus appear sharper.




  
  LOG IN TO REPLY
Mark_Cohran
Cream of the Crop
Avatar
15,765 posts
Gallery: 2 photos
Best ofs: 1
Likes: 1818
Joined Jul 2002
Location: Portland, Oregon
     
May 09, 2007 22:08 |  #7

Well, I don't know how good an analogy this is, but in my mind it's much like the difference between a point source of light and a diffused source of light. Remember, the dust is on the hot mirror/filter in front of the sensor. Thus the dust is actually casting a shadow on the sensor and that's what we see on our images. So, with a small aperture, the light entering the lens is focused pretty sharply and hits the dust/filter/sensor much like the light from a undiffused strobe. You cover all of the area, but there are harsh, dark shadows on your background (the sensor). With a wide open aperture, the light wraps around the dust much like what happens when we use a diffuser to make our light sources appear larger, and the shadows are much softer or non-existent.

Mark


Mark
-----
Some primes, some zooms, some Ls, some bodies and they all play nice together.
Forty years of shooting and still learning.
My Tumblr Site (external link) (NSFW)
Follow Me on Instagram (external link)

  
  LOG IN TO REPLY
droiby
Member
Avatar
62 posts
Joined Apr 2007
Location: Sydney
     
May 10, 2007 08:25 |  #8

manipula wrote in post #3180309 (external link)
Makes sense. By that you're implying that when stopped down all the millions of points of light are converging onto the sensor at a much narrower angle of view, but how does that relate to the fact the regardless of the aperture, the angle of view is still sufficient to cover the whole sensor?

Because all pixels on the sensor will still be able to see something through the aperture. Your typical lens projects a circle of light on the sensor. Changing the aperture size changes how well-defined this circle of light is. This is why you notice vignetting in large apertures.

Does that mean when the aperture's wide open there's light being aimed at a much greater square mm area then when close down?

Not sure what you mean, but think of it this way...

Imagine that you're a pixel on the sensor. In front of you is the aperture, and in front of that is the lens. What you can see is basically all the light that is coming through the aperture. Regardless of which pixel you are on the sensor, you'll still be able to "look" through the aperture. Your pixel value is like an "average" of all the light rays that you see (more technically, it's called a "convolution").

When the aperture is wide open, then you're averaging over a large area, and hence a dust spec in front of you doesn't have as well-defined an edge because a lot of rays hitting you can go around the dust spec. When the aperture is stopped way down, then you're averaging over a small area, and hence a dust spec has a well-defined edge because not much light can go around it.


Canon 30D | 70-200mm f/2.8L IS | 100mm f/2.8 macro | 50mm f/1.4 | 16-35mm f/2.8L | 17-55mm f/2.8 IS | 580EX

  
  LOG IN TO REPLY
gjl711
"spouting off stupid things"
Avatar
55,963 posts
Likes: 2696
Joined Aug 2006
Location: Deep in the heart of Texas
     
May 10, 2007 10:22 |  #9

I think one has to look to the simplest of all cameras, the good old pinhole camera for the answer. A pinhole camera in theory has an almost infinite depth of field. The smaller the aperture, the closer to infinity the DOF. By closing the aperture as small as possible you have in essence created a pinhole camera. A lens in focus only focuses on one plane and the DOF is dependant on the aperture. For example, shoot a picture of something distant through a chain link fence with the fence links maybe three feet in front of the lens at f/2.8. In the resulting picture the chain link fence will not be visible or you may see some shadow areas but nothing clearly. Now stop it down to f/22 and the links become visible. Same thing applies I think. As you stop down the lens, the DOF increases.


Not sure why, but call me JJ.
I used to hate math but then I realised decimals have a point.
.
::Flickr:: (external link)
::Gear::

  
  LOG IN TO REPLY
Jon
Cream of the Crop
Avatar
69,628 posts
Likes: 226
Joined Jun 2004
Location: Bethesda, MD USA
     
May 10, 2007 10:30 |  #10

Mark_Cohran wrote in post #3181070 (external link)
Well, I don't know how good an analogy this is, but in my mind it's much like the difference between a point source of light and a diffused source of light. Remember, the dust is on the hot mirror/filter in front of the sensor. Thus the dust is actually casting a shadow on the sensor and that's what we see on our images. So, with a small aperture, the light entering the lens is focused pretty sharply and hits the dust/filter/sensor much like the light from a undiffused strobe. You cover all of the area, but there are harsh, dark shadows on your background (the sensor). With a wide open aperture, the light wraps around the dust much like what happens when we use a diffuser to make our light sources appear larger, and the shadows are much softer or non-existent.

Mark

Spot on. Look at the difference in a shadow from a 40 w. fluorescent tube and a 40 w. incandescent bulb (at the same distance) for a "real world" illustration.


Jon
----------
Cocker Spaniels
Maryland and Virginia activities
Image Posting Rules and Image Posting FAQ
Report SPAM, Don't Answer It! (link)
PERSONAL MESSAGING REGARDING SELLING OR BUYING ITEMS WITH MEMBERS WHO HAVE NO POSTS IN FORUMS AND/OR WHO YOU DO NOT KNOW FROM FORUMS IS HEREBY DECLARED STRICTLY STUPID AND YOU WILL GET BURNED.
PAYPAL GIFT NO LONGER ALLOWED HERE

  
  LOG IN TO REPLY
kitacanon
Goldmember
4,706 posts
Gallery: 1 photo
Likes: 35
Joined Sep 2006
Location: West Palm Beach
     
May 10, 2007 10:42 |  #11

And the reason some don't care so much is because at apertures smaller than F:11-16 diffraction starts to decrease the sharpness of the image, even if the DoF increases....and F:11-16 usually has enuf DoF of most situations.


My Canon kit 450D/s90; Canon lenses 18-55 IS, 70-210/3.5-4.5....Nikon kit: D610; 28-105/3.5-4.5, 75-300/4.5-5.6 AF, 50/1.8D Nikkors, Tamron 80-210; MF Nikkors: 50/2K, 50/1.4 AI-S, 50/1.8 SeriesE, 60/2.8 Micro Nikkor (AF locked), 85mm/1.8K-AI, 105/2.5 AIS/P.C, 135/2.8K/Q.C, 180/2.8 ED, 200/4Q/AIS, 300/4.5H-AI, ++ Tamron 70-210/3.8-4, Vivitar/Kiron 28/2, ser.1 70-210/3.5, ser.1 28-90; Vivitar/Komine and Samyang 28/2.8; 35mm Nikon F/FM/FE2, Rebel 2K...HTC RE UWA camera

  
  LOG IN TO REPLY
StewartR
"your nose is too big"
Avatar
4,269 posts
Joined Jun 2006
Location: Maidenhead, UK
     
May 10, 2007 10:55 |  #12

Mark_Cohran wrote in post #3181070 (external link)
Well, I don't know how good an analogy this is, but in my mind it's much like the difference between a point source of light and a diffused source of light. Remember, the dust is on the hot mirror/filter in front of the sensor. Thus the dust is actually casting a shadow on the sensor and that's what we see on our images. So, with a small aperture, the light entering the lens is focused pretty sharply and hits the dust/filter/sensor much like the light from a undiffused strobe. You cover all of the area, but there are harsh, dark shadows on your background (the sensor). With a wide open aperture, the light wraps around the dust much like what happens when we use a diffuser to make our light sources appear larger, and the shadows are much softer or non-existent.

Mark

Best explanation I've seen. Nice one Mark.


www.LensesForHire.co.u​k (external link) - complete with matching POTN discussion thread
Photos: Cats (external link) | London by day (external link) | London by night (external link) I My POTN photo sharing threads (external link) | Official "Where Am I Now?" archive (external link)
Gear: 350D | Sigma 18-200mm | EF-S 10-22mm | EF 50mm f/1.4

  
  LOG IN TO REPLY
sponsored links
(this ad will go away when you log in as a registered member)

1,467 views & 0 likes for this thread
Dust spect sharpness and apertures?
FORUMS Canon Cameras, Lenses & Accessories Canon EOS Digital Cameras 
AAA
x 1600
y 1600

Jump to forum...   •  Rules   •  Index   •  New posts   •  RTAT   •  'Best of'   •  Gallery   •  Gear   •  Reviews   •  Member list   •  Polls   •  Image rules   •  Search   •  Password reset

Not a member yet?
Register to forums
Registered members may log in to forums and access all the features: full search, image upload, follow forums, own gear list and ratings, likes, more forums, private messaging, thread follow, notifications, own gallery, all settings, view hosted photos, own reviews, see more and do more... and all is free. Don't be a stranger - register now and start posting!


COOKIES DISCLAIMER: This website uses cookies to improve your user experience. By using this site, you agree to our use of cookies and to our privacy policy.
Privacy policy and cookie usage info.


POWERED BY AMASS forum software 2.1forum software
version 2.1 /
code and design
by Pekka Saarinen ©
for photography-on-the.net

Latest registered member is spinraj
756 guests, 233 members online
Simultaneous users record so far is 15144, that happened on Nov 22, 2018

Photography-on-the.net Digital Photography Forums is the website for photographers and all who love great photos, camera and post processing techniques, gear talk, discussion and sharing. Professionals, hobbyists, newbies and those who don't even own a camera -- all are welcome regardless of skill, favourite brand, gear, gender or age. Registering and usage is free.