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100-400ii + 2xiii: usable?
  
 
splathrop
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p.2 #1 · p.2 #1 · 100-400ii + 2xiii: usable?


You need a really crystal clear night, usually cold, to make significant gains with optics. Multi-quote Like

There are probably times when that is true. But astronomers distinguish clarity (often maximized in the cold) from "seelng," which is less temperature dependent, but often only mediocre during cold conditions.

To show astronomical detail (think planetary detail) you need "seeing." To distinguish structure in extended faint objects (distant galaxies), you need clarity. Some of the best amateur astrophotos (using telescopes) are taken from Florida, which presents superior seeing more often than most other spots in the nation.

An excellent resource not only for discussion of these topics, but for local forecasts of imaging conditions is at: http://www.cleardarksky.com/csk/

Note that a forecast of excellent astronomical seeing for your location may also indicate a great time to take long-distance telephoto terrestrial shots with minimal atmospheric disruption. You can learn to recognize good seeing. If a distant vista with which you are familiar seems somehow closer than usual, that is often because you are observing more detail than you are accustomed to, because atmospheric conditions aren't smearing it. Good seeing.



Oct 24, 2016 at 08:10 PM
Ben Horne
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p.2 #2 · p.2 #2 · 100-400ii + 2xiii: usable?


You will want to stop down about one stop beyond your best aperture when using a TC. If you shoot the 100-400mm w/ 2x TC @ f/11, you are going to have a soft image. If you stop down a third, it gets better, 2/3 even better, and 1 stop is about max. You are also going to be battling diffraction so you need to factor that in as well.


Oct 25, 2016 at 01:13 PM
rabbitmountain
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p.2 #3 · p.2 #3 · 100-400ii + 2xiii: usable?


Thanks for that . Isn't diffraction due to the light passing through only the extreme centre area of the lens? In that case 100-400ii + 2x ext @400mm @f/11 is still with lens wide open and f/22 is only 2 stops down. Or am I missing something? With stacked 2x + 2x extenders f/22 would be lens wide open.


Oct 25, 2016 at 05:45 PM
alundeb
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p.2 #4 · p.2 #4 · 100-400ii + 2xiii: usable?


The MTF of the lens at 400mm f/8 is very close to diffraction limited, it is about equal to a perfect diffraction limited lens at f/10. So with a 2x TC attached it will resolve as a perfect lens at f/20.


Oct 25, 2016 at 06:46 PM
jcolwell
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p.2 #5 · p.2 #5 · 100-400ii + 2xiii: usable?


rabbitmountain wrote:
Thanks for that . Isn't diffraction due to the light passing through only the extreme centre area of the lens?.


No. It's more or less the opposite of that.

Diffraction happens when light (and ocean waves) pass by an edge (in which case, the light at and very near to the edge bends) or through a small hole (after which, the light spreads). When a lens aperture is wide open, you get diffraction (bending of waves) all around the outer edge of the optics, but it's a very small proportion of the total amount of light entering the lens (i.e. the diffracted light rays are only those at and very close to the edge, while almost all of the light passes into the lens without getting close enough to an edge to get diffracted. When you stop down the lens, the amount of light diffracted (bent) at the edges of the aperture becomes a much larger proportion of the total light entering the lens, and so a much greater proportion of the image is affected (usually by blurring), by the diffraction.



Edited on Oct 25, 2016 at 08:27 PM · View previous versions



Oct 25, 2016 at 08:09 PM
Sjjindra
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p.2 #6 · p.2 #6 · 100-400ii + 2xiii: usable?


Jcolwell
Thanks for that explanation of diffraction. Simple and clear. I hadn't quite grasped it before.
Steve



Oct 25, 2016 at 08:24 PM
howard
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p.2 #7 · p.2 #7 · 100-400ii + 2xiii: usable?


John_T wrote:
...with the moon, I find adding more lens often just compresses more atmospherics with no net gain in detail of the subject. You need a really crystal clear night, usually cold, to make significant gains with optics.


This baffles me -- regardless of what lens, TC you use, don't they have to go through the same amount of atmosphere to reach the moon?



Oct 26, 2016 at 03:08 AM
 

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alundeb
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p.2 #8 · p.2 #8 · 100-400ii + 2xiii: usable?


howard wrote:
This baffles me -- regardless of what lens, TC you use, don't they have to go through the same amount of atmosphere to reach the moon?


There will only be a difference in local turbulence very close to the lens with different aperture sizes, like if you stand in a warm room and shoot through an open window or door out in the cold.

For the angle of view of the moon seen from earth, and an aperture size of about 70 mm, the local atmosphere seen by the lens, and affected by a change in aperture, extends to roughly 10 m from the lens. The effect diminishes gradually with distance, there is no defined limit. At 100 m distance though, the effect is guaranteed to be negligible with all realistic aperture sizes.




Oct 26, 2016 at 07:37 AM
howard
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p.2 #9 · p.2 #9 · 100-400ii + 2xiii: usable?


alundeb wrote:
There will only be a difference in local turbulence very close to the lens with different aperture sizes, like if you stand in a warm room and shoot through an open window or door out in the cold.

For the angle of view of the moon seen from earth, and an aperture size of about 70 mm, the local atmosphere seen by the lens, and affected by a change in aperture, extends to roughly 10 m from the lens. The effect diminishes gradually with distance, there is no defined limit. At 100 m distance though, the effect is guaranteed to be
...Show more

This does not answer my question. John_T was implying that "more lens" (I take it he meant longer focal length) will compress "atmospherics" more, I say focal length has nothing to do with it as you have to penetrate the same amount of atmosphere to reach the moon.




Oct 26, 2016 at 03:56 PM
jcolwell
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p.2 #10 · p.2 #10 · 100-400ii + 2xiii: usable?


howard wrote:
..., I say focal length has nothing to do with it as you have to penetrate the same amount of atmosphere to reach the moon.


Agreed, as long as you view the images from different focal lengths at the same magnification (viewing size/sensor size). Also, increasing focal length with an extender makes the image more susceptible to blur from camera shake, but this usually isn't a problem if you take proper care.




Oct 26, 2016 at 07:08 PM
howard
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p.2 #11 · p.2 #11 · 100-400ii + 2xiii: usable?


jcolwell wrote:
Agreed, as long as you view the images from different focal lengths at the same magnification (viewing size/sensor size). Also, increasing focal length with an extender makes the image more susceptible to blur from camera shake, but this usually isn't a problem if you take proper care.



This, upon reflecting on this some more, I think what John_T meant was probably the following case: normally, a longer focal length will let you get greater magnification over a longer distance (think of a Zebra on the savanna). To get the same or similar magnification, you will have to get closer. Thus, with the longer lens, you do "compress" more atmosphere.

But in the case of the moon, we can't get any closer , thus it is a special case where you will get different magnification with different focal lengths, but the amount of atmosphere stays the same.

Not to be nitpicking, just an interesting mental exercise of my aging mind



Oct 26, 2016 at 09:23 PM
rabbitmountain
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p.2 #12 · p.2 #12 · 100-400ii + 2xiii: usable?


surely we can get closer by the time space tourism will blossom. Well book a window seat, take off our extenders and mount a 50L to throw the blackground out of focus...
Until then, we'll need to improvise.



Oct 26, 2016 at 09:47 PM
John_T
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p.2 #13 · p.2 #13 · 100-400ii + 2xiii: usable?


Right. You gain no additional detail with with additional FL, while magnifying whatever is floating around in the atmosphere, aberration, dust, pollution, cloud, water, etc. Such is not always directly visible, maybe a hint of cloud, striation of dust, but it's the same veil brought closer. Your moon is bigger, which might give you the illusion you have more detail, but you have only gotten a bigger image, not better. I sometimes get dust off the Sahara, impenetrable. You think the image is OOF, motion blurred, your gear is upfudded, the next day you are maniacally MFAing, sending the body and lens to CPS, kicking the dog, all over dust.

Shot on a clear night, or at altitude on top of a mountain above the lower atmosphere, depending on the resolution and magnification your gear achieves, you get significantly more detail.



Oct 26, 2016 at 10:49 PM
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