Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
07-16-2010, 08:16 AM   #16
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
QuoteOriginally posted by newarts Quote
Ben I think the diffraction limit of resolution is a function only of F-Number, ie. (1/(angle subtended by aperture)) times wavelength.
Resolution is not defined by the f-number (relative aperture), but by the real or effective aperture or entrance pupil (which is all the same). In astrophotography, which is the subject here, you usually talk about a fully open aperture (as scopes don't have a diaphragm), but ofcourse the basic formula applies to any lens:

res(angular in arcsecs) = 1.22 (W/D) 206265"

W = Wavelength of the light
D = Diameter of the lens (or entrance pupil)
the absolute numbers are only there to normalize for arcsecs.

Another, even simpler rule would be: res = (115" * 1mm)/D
this is normalized to mm as the measure for the diameter and the wavelength ids placed around 560nm through the fixed factor

Anyway: both lenses describe the "diffraction limited" angular resolution of a lens. The smaller the diameter gets (in a photographioc lens this would be to close the aperture) the bigger the resulting number gets, which is nothing else, but a shrinking resolution.

As you see, we are talking not about the f-number, as the focal length does not come into play. We are only talking about effective diameters of the lens.

If you substitute "aperture" in your formula with "diameter" it is correct.

Ben

07-16-2010, 08:17 AM   #17
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
QuoteOriginally posted by smigol Quote
I use two thin rubber bands, as these are the most flexible. The ones I use come from bundles of onions at the grocery store, maybe 2 mm in diameter.

I do this exercise every time I use the lens, as temperature is different and I can never remember exactly the right position!
That's a neat solution: easy and fast. Thanks for showing that, I will certainly use it in the future.

Ben
07-16-2010, 08:42 AM   #18
Site Supporter
smigol's Avatar

Join Date: Sep 2008
Location: Menlo Park, CA
Photos: Albums
Posts: 391
Thanks!

On my main scope, when I'm going for prime focus, I use a Bahtinov mask. That way I can see which way the spikes need to move to get focus.

Also, when I do that, I go after a star, not a planet. The spikes are much sharper.
07-16-2010, 09:04 AM   #19
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
Focusing masks are fairly common with scopes, but I never really thought about using them on a photographic lens… But the rubber band mask is so easy to use, that I'll change my habits.

Ben

07-16-2010, 09:56 AM   #20
Veteran Member




Join Date: Sep 2007
Location: USA
Posts: 1,812
not to derail any of the worthwhile discussion on this thread -
but there was a very detailed thread on this forum previously
that talked a lot about astro-photography using the k-x -
far more than I know about -

K-x is king for long exposures and astrophotography!

maybe worthwhile checking out?
07-16-2010, 10:13 AM   #21
Inactive Account




Join Date: Dec 2008
Location: Ames, Iowa, USA
Photos: Albums
Posts: 2,965
QuoteOriginally posted by Ben_Edict Quote
Resolution is not defined by the f-number (relative aperture), but by the real or effective aperture or entrance pupil (which is all the same). In astrophotography, which is the subject here, you usually talk about a fully open aperture (as scopes don't have a diaphragm), but ofcourse the basic formula applies to any lens:

res(angular in arcsecs) = 1.22 (W/D) 206265"

W = Wavelength of the light
D = Diameter of the lens (or entrance pupil)
the absolute numbers are only there to normalize for arcsecs.

Another, even simpler rule would be: res = (115" * 1mm)/D
this is normalized to mm as the measure for the diameter and the wavelength ids placed around 560nm through the fixed factor

Anyway: both lenses describe the "diffraction limited" angular resolution of a lens. The smaller the diameter gets (in a photographioc lens this would be to close the aperture) the bigger the resulting number gets, which is nothing else, but a shrinking resolution.

As you see, we are talking not about the f-number, as the focal length does not come into play. We are only talking about effective diameters of the lens.

If you substitute "aperture" in your formula with "diameter" it is correct.

Ben
But I was thinking they are the same thing and in photographic terms we usually talk in terms of f-number not absolute aperture.

QuoteOriginally posted by http://www.licha.de/astro_article_mtf_telescope_resolution.php :
Obviously the resolution limit or resolving power is connected to that Airy Disk, because no detail imaged by the telescope can be smaller than this disk. The radius of the Airy Disk can be estimated with:

q lin = 1.22 * ( f * λ ) / D (for linear resolving in cycles/mm)
q ang = 1.22 * λ / D (for angular resolving in arc sec)
Now I understand that in the camera case, if we refer the resolution back to the subject plane (rather than sensor plane) the focal length cancels out!

q_lin_subject_plane=q_lin_image_plane*subject_distance/focal_length=1.22*subject_distance*λ/D

Thanks for your help,
Dave
07-16-2010, 12:22 PM   #22
Senior Member




Join Date: Sep 2009
Location: Beautiful Bavaria :-)
Posts: 123
QuoteOriginally posted by Ben_Edict Quote
It simply does NOT matter, whether any light will then fall outside the sensor area.

WHAT DOES MATTER is, that the subject's projection gets bigger through the tc and occupies a larger area ON the sensor! This means, the image is distributed over a larger part of the sensor and accordingly the light from the subject is distributed over the larger area and gets dimmer at each single point (the inverse aquare law).
Hi Ben,

it DOES matter: the light that does no longer reach the image area (be it film, be it a sensor) is exactly the amount of light or photoelectric energy that is missing from the image area and thus resulting in a magnified, but darker or fainter image. The TC acts like an aperture system stopping down the lens.

The initial subject's projection by the lens alone covers the image circle of the medium (film or sensor). The 2x TC magnifies the center of that projection to the size of the medium, cutting off the outer area of the original aerial picture of the lens alone, but of course the TC uses or distributes the same amount of light that had entered through the lens. The image is not distributed over a larger area of the sensor, but over a larger area overall as you correctly state. The sensor size is fixed and is always fully used. Because not all photons that had entered throug the front lens reach the film/sensor anymore, the combination lens + TC acts like a slower lens or a stopped down lens as it concerns the lens speed.
07-16-2010, 01:13 PM   #23
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
QuoteOriginally posted by Vranx Quote
Hi Ben,

it DOES matter: the light that does no longer reach the image area (be it film, be it a sensor) is exactly the amount of light or photoelectric energy that is missing from the image area and thus resulting in a magnified, but darker or fainter image. The TC acts like an aperture system stopping down the lens.

The initial subject's projection by the lens alone covers the image circle of the medium (film or sensor). The 2x TC magnifies the center of that projection to the size of the medium, cutting off the outer area of the original aerial picture of the lens alone, but of course the TC uses or distributes the same amount of light that had entered through the lens. The image is not distributed over a larger area of the sensor, but over a larger area overall as you correctly state. The sensor size is fixed and is always fully used. Because not all photons that had entered throug the front lens reach the film/sensor anymore, the combination lens + TC acts like a slower lens or a stopped down lens as it concerns the lens speed.
No - that is a completely wrong misconception.

The light "loss" introduced by a tc is ONLY a product of simple physics, the inverse square law.

It does not matter at all, whether the image circle gets bigger or not and whether the sensor has a larger or smaller area. WHAT DOES MATTER is, the size of the subject ON the sensor. The larger the object size on the sensor, the wider is the finite amaount of light spread out and thus dimmer at any given point.

As photographers we do not usually image point sources, we photograph instead subjects with dimensions. And thus, when magnified, the area brightness gets lower.

Ben

07-16-2010, 07:04 PM   #24
Veteran Member




Join Date: Dec 2007
Posts: 923
QuoteOriginally posted by Ben_Edict Quote
No - that is a completely wrong misconception.

The light "loss" introduced by a tc is ONLY a product of simple physics, the inverse square law.

It does not matter at all, whether the image circle gets bigger or not and whether the sensor has a larger or smaller area. WHAT DOES MATTER is, the size of the subject ON the sensor. The larger the object size on the sensor, the wider is the finite amaount of light spread out and thus dimmer at any given point.

As photographers we do not usually image point sources, we photograph instead subjects with dimensions. And thus, when magnified, the area brightness gets lower.

Ben
Let me try to explain in simple maths.
If the amount of light energy falling on the sensor is originally A.
After you put on a 2x Teleconverter, the amount of light coming through the lens doesn't change, and it doesn't matter how big the image circle is now.
The image falling on the sensor is now a 1/2 x 1/2 = 1/4 of the area inside of the original view. That's how you get the "magnifying effect".
So the light energy falling on the sensor is now 1/4 A, even though the physical lens aperture opening has not changed at all. That's 2 stops less light.
07-16-2010, 08:30 PM   #25
Site Supporter
Ex Finn.'s Avatar

Join Date: Nov 2007
Location: Southern Maryland.
Photos: Gallery | Albums
Posts: 6,123
QuoteOriginally posted by kittykat46 Quote
Let me try to explain in simple maths.
If the amount of light energy falling on the sensor is originally A.
After you put on a 2x Teleconverter, the amount of light coming through the lens doesn't change, and it doesn't matter how big the image circle is now.
The image falling on the sensor is now a 1/2 x 1/2 = 1/4 of the area inside of the original view. That's how you get the "magnifying effect".
So the light energy falling on the sensor is now 1/4 A, even though the physical lens aperture opening has not changed at all. That's 2 stops less light.
Thank`s, I was starting to get a headache.
07-17-2010, 07:29 AM   #26
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
QuoteOriginally posted by kittykat46 Quote
Let me try to explain in simple maths.
If the amount of light energy falling on the sensor is originally A.
After you put on a 2x Teleconverter, the amount of light coming through the lens doesn't change, and it doesn't matter how big the image circle is now.
The image falling on the sensor is now a 1/2 x 1/2 = 1/4 of the area inside of the original view. That's how you get the "magnifying effect".
So the light energy falling on the sensor is now 1/4 A, even though the physical lens aperture opening has not changed at all. That's 2 stops less light.
You are not far off what I have been writing above: The light loss can ONLY be explained by the subject's size increase ON THE SENSOR. It is the old and well-proven inverse square law, that has given us the explanation long, long ago…

Secondly: the "physical aperture opening", as you put it has not changed, BUT THE F-NUMBER has! If you increase focal length (by means of the tc), but the physical aperture size stays fixed (maximum aperture of the lens at best), the f-number goes down. Because the f-number is a ratio! The ratio between focal length and entrance pupil! If you increas fl, the f-number needs to go down, very simple maths, indeed.

You could compensate for the light loss by using a lens with a faster aperture.

The only thing, that needed clarification, was Vranx' notion, that the light loss could be attributed to the "cut-off!" image, because some light would not reach the sensor, when magnified. That is a very misleading explanation, because the role of the image circle is non-existent, as we are simply dealing with magnification increase.

Ben
07-17-2010, 07:15 PM   #27
Forum Member




Join Date: Sep 2008
Location: Sydney
Posts: 91
Original Poster
Excellent!

So, the moral of the story is: If you need 400mm FL, go for a 400mm lens!!
07-17-2010, 07:54 PM   #28
Inactive Account




Join Date: Dec 2008
Location: Ames, Iowa, USA
Photos: Albums
Posts: 2,965
QuoteOriginally posted by me:
in the camera case, if we refer the resolution back to the subject plane (rather than sensor plane) the focal length cancels out!

q_lin_subject_plane=q_lin_image_plane*subject_distance/focal_length=1.22*subject_distance*λ/D
This might lead one to think that a 300mm f:4 is always better than a 100mm f:2...Hmmm..

On further thought, it follows that a particular field of view dictates a particular focal length; therefore if field of view (therefore focal length) is fixed - like to image Orion's Belt, then the smallest f-number for the appropriate focal length (perfect) lens should be used.

Dave
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
barn, camera, dfs, door, dslr, exposures, photography
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Quick question Raptorman Pentax DSLR Discussion 5 12-27-2009 11:03 AM
DFS question WillCarney Pentax DSLR Discussion 8 11-20-2009 06:03 PM
astrophotography equipment question sholtzma Pentax DSLR Discussion 0 05-18-2009 08:56 AM
Quick question ACImages Pentax SLR Lens Discussion 7 03-26-2009 11:56 AM
Quick Question Fl_Gulfer Post Your Photos! 2 04-16-2008 05:19 PM



All times are GMT -7. The time now is 06:36 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top