Velocity Reviews - Computer Hardware Reviews

Velocity Reviews > Newsgroups > Computing > Digital Photography > Now, the next BIG contest is ON!

Reply
Thread Tools

Now, the next BIG contest is ON!

 
 
Doc Johnson
Guest
Posts: n/a
 
      06-09-2010
On Wed, 9 Jun 2010 13:59:10 +0100, "whisky-dave"
<(E-Mail Removed)> wrote:

>Earth is flat because the Earths surfacwe curves less than the 1/4
>wavelengh. IIRC


A bad mix of apples and oranges but a fun concept if the surface were
perfectly smooth, and then only applicable to one small portion of the
earth at a time.

Though I know one section of earth where it's even flatter, than that
bad-analogy. A section of North Dakota. I surmise it has to do with the
continental plains slowly rising to meet the Rocky Mountains further out
west--this slow geological rise negating the curvature of the earth. When
traveling through N. Dakota in 1979 to go photograph the total eclipse, I
was driving all night just the night before--to catch up with the event
next morning. I had been noticing that when I would first spot a car's
headlights in the distance on the flat and straight roads (at 2-3 am
there's so little traffic too), that it seemed like an astoundingly long
time before our cars would pass each other. The duration being so long is
what first caught my attention. When driving at night and you see another
car's headlights you instinctively expect it to pass in an acceptable
amount of time, even if it was sitting still on the side of the road in
front of you. You can almost judge if a car is standing still or traveling
toward you just by how long it's taking to meet those oncoming lights. This
expected time delay becomes almost instinctual, not worth thinking about.
Something was highly amiss that night. I decided to time just how long it
took to meet up with some oncoming headlights (prompted by the
environmentally induced boredom too).

I was speeding along that night at 70-75 mph in order to be there on time
for the solar spectacle. From the moment I first noticed the tiny glint of
another oncoming car's headlights on the distant horizon (and looked down
at the clock on the dashboard to note the time), until the time we passed
each other, it took (drum roll please) ... 35 MINUTES. There was no break
in seeing those headlights during this time so I know they were coming from
the same vehicle.

If the other car was traveling toward me at a similar rate of speed this
means that I spotted another car about 20.4 to 21.8 miles away. Keeping in
mind those headlights are only a couple feet from the ground and being seen
from the viewing height in the average car-seat.

Knowing that the distance of the horizon on a calm ocean as seen by a 6 ft.
tall person from a standing height is only 3 miles away, it puts that
section of N. Dakota's flatness into perspective. If viewing the ocean's
horizon from a seated position knock off about another half mile or so,
it's about 2 to 2.5 miles away. If wanting to see the ocean's horizon 21.5
miles away you'd have to do so from a viewing height of about 308 ft.

That one stretch of road to Minot, N. Dakota should be in a record-book
somewhere as the flattest spot on earth. (Until another area can be found
to best it.)


p.s. The eclipse photographs turned out spectacular. Using my OM2n I got
some nice images of bright crimson prominences, among many other unique
images. Localized events like crescents of light projected on snow-drifts
by pin-holes formed between shadows of hands and fingers, etc. (I engaged
some nearby kids on that impromptu viewer's-hill-gathering to help, and to
help entertain and educate them.) Though I had no idea that it would get so
dark that changing a roll of film during totality would be as difficult as
it was, my most frantic film-change ever. A light overcast sky that morning
(which at first concerned me but only added to the event) also afforded
some specialized solar-eclipse effects. The light high-altitude haze acting
as a mild rear-projection screen had relayed waves of rainbow colors in the
sky just moments before totality approached. Then during totality the whole
sky seemingly inverted in an instant, as if seeing the universe in
negative. The darkest spot in the sky was the sun. It was astounding, right
to the very core of your DNA.

Afterward I stopped off at a large nearby truck-stop for an early lunch.
About 300-400 others were there. (Minot had become an eclipse destination
that morning.) You could have heard a toothpick drop in that large
dining-hall. Nobody was talking during their whole meal. Only hushed and
solemn whispers when placing orders or paying at the register. As if they
were all put in their place for once in their small lives. Now knowing with
absolute certainty for the first time in their lives that they cannot be in
control of everything. They were all genuinely awe-struck by what had just
occurred, as was I. I can only imagine how fearful someone would have been
if they didn't know what had just taken place. Here it is 31 years later
and I still get goose-bumps reminiscing about it. It's as if all throughout
the evolution of life on earth that DNA has never quite been able to adapt
to the solar eclipse experience. Your DNA doesn't know how to react to it.
I understand the "eclipse chaser". Experiencing totality is a mental,
emotional, and physical rush, unlike any other. That one in Minot that
morning, with the high-altitude rear-projection screen unveiling another
awe-inducing layer of its effects, even more so.





 
Reply With Quote
 
 
 
 
Doug McDonald
Guest
Posts: n/a
 
      06-09-2010
On 6/9/2010 11:17 AM, Doc Johnson wrote:

>
> Though I know one section of earth where it's even flatter, than that
> bad-analogy. A section of North Dakota. I surmise it has to do with the
> continental plains slowly rising to meet the Rocky Mountains further out
> west--this slow geological rise negating the curvature of the earth. When
> traveling through N. Dakota in 1979 to go photograph the total eclipse,


I've only seen one total eclipse of the sun, in Costa Rica in, I believe, 1991.

This eclipse was teh beginning of the eclipse craze; the travel company I went
with was oblivious to it happening before I asked them if they could
do a pre-existing trip "add-on" ... which they did, getting one
other customer. After that, they have done lots of eclipse trips.

This was awesome ... it lasted 7 minutes. I had a 600mm mirror lens
to photograph it. This was on a camera on a small mini-tripod about
a foot high, which turned out to be quite adequate. The thing of note was that
the feet of the tripod were slightly awash in the Pacific Ocean. The beach
was packed with eclipse-gazers (this beach, Manuel Antonio, is the most
popular in Costa Rica.)

Doug McDonald
 
Reply With Quote
 
 
 
 
Wolfgang Weisselberg
Guest
Posts: n/a
 
      06-09-2010
DanP <(E-Mail Removed)> wrote:
> On 8 June, 02:53, Wolfgang Weisselberg <(E-Mail Removed)>
> wrote:
>> DanP <(E-Mail Removed)> wrote:
>> >> I guess you could do a thought experiment.
>> >> If 2 lenses gather the same amount of parallel rays along
>> >> their lens axis, they must have the same diameter. *Go from
>> >> there.
>> > Right, at maximum lens aperture bigger lenses gather more light.
>> > Close the bigger lenses aperture to match the small ones and you get
>> > the same amount of light.

>>
>> Please clarify, what exactly do you mean "to match the small
>> ones"? *f/stop? *Diameter? *Area? *What exactly?
>>
>> -Wolfgang


> f/stop.


> Big lens: 42mm diameter, 50mm focal length, f/1.2 aperture.
> Small lens: 18mm diameter, 50mm focal length, f/2.8 aperture.


> Set them both to f/2.8 or smaller and you get the same light.


So use a 200mm as the big lens and a 50mm as the small lens
and answer again.

-Wolfgang
 
Reply With Quote
 
DanP
Guest
Posts: n/a
 
      06-10-2010
On Jun 9, 11:46*pm, Wolfgang Weisselberg <(E-Mail Removed)>
wrote:
> DanP <(E-Mail Removed)> wrote:
> > On 8 June, 02:53, Wolfgang Weisselberg <(E-Mail Removed)>
> > wrote:
> >> DanP <(E-Mail Removed)> wrote:
> >> >> I guess you could do a thought experiment.
> >> >> If 2 lenses gather the same amount of parallel rays along
> >> >> their lens axis, they must have the same diameter. *Go from
> >> >> there.
> >> > Right, at maximum lens aperture bigger lenses gather more light.
> >> > Close the bigger lenses aperture to match the small ones and you get
> >> > the same amount of light.

>
> >> Please clarify, what exactly do you mean "to match the small
> >> ones"? *f/stop? *Diameter? *Area? *What exactly?

>
> >> -Wolfgang

> > f/stop.
> > Big lens: 42mm diameter, 50mm focal length, f/1.2 aperture.
> > Small lens: 18mm diameter, 50mm focal length, f/2.8 aperture.
> > Set them both to f/2.8 or smaller and you get the same light.

>
> So use a 200mm as the big lens and a 50mm as the small lens
> and answer again.
>
> -Wolfgang


200mm?
I am talking about lens diameter not focal length.


DanP
 
Reply With Quote
 
Martin Brown
Guest
Posts: n/a
 
      06-11-2010
On 09/06/2010 23:46, Wolfgang Weisselberg wrote:
> DanP<(E-Mail Removed)> wrote:
>> On 8 June, 02:53, Wolfgang Weisselberg<(E-Mail Removed)>
>> wrote:
>>> DanP<(E-Mail Removed)> wrote:
>>>>> I guess you could do a thought experiment.
>>>>> If 2 lenses gather the same amount of parallel rays along
>>>>> their lens axis, they must have the same diameter. Go from
>>>>> there.
>>>> Right, at maximum lens aperture bigger lenses gather more light.
>>>> Close the bigger lenses aperture to match the small ones and you get
>>>> the same amount of light.
>>>
>>> Please clarify, what exactly do you mean "to match the small
>>> ones"? f/stop? Diameter? Area? What exactly?
>>>
>>> -Wolfgang

>
>> f/stop.

>
>> Big lens: 42mm diameter, 50mm focal length, f/1.2 aperture.
>> Small lens: 18mm diameter, 50mm focal length, f/2.8 aperture.

>
>> Set them both to f/2.8 or smaller and you get the same light.


Only exactly on axis or very near to the axis.

Actually in practice that may not be entirely true. A small lens with
maximum aperture f2.8 might well vignette at the extreme corners of the
field of view whereas the f1.2 stopped down to f2.8 will be almost
perfect uniform film plane illumination for a well designed lens.

Most lenses designed down to a price need to be stopped down around half
a stop to avoid some corner vignetting. Vignetting at the corners of an
image can be a nuisance when you don't want it.

Regards,
Martin Brown
 
Reply With Quote
 
DanP
Guest
Posts: n/a
 
      06-11-2010
On 11 June, 08:23, Paul Furman <(E-Mail Removed)> wrote:
> On 6/10/2010 12:25 PM, DanP wrote:
>
>
>
>
>
> > On Jun 9, 11:46 pm, Wolfgang Weisselberg<(E-Mail Removed)>
> > wrote:
> >> DanP<(E-Mail Removed)> *wrote:
> >>> On 8 June, 02:53, Wolfgang Weisselberg<(E-Mail Removed)>
> >>> wrote:
> >>>> DanP<(E-Mail Removed)> *wrote:
> >>>>>> I guess you could do a thought experiment.
> >>>>>> If 2 lenses gather the same amount of parallel rays along
> >>>>>> their lens axis, they must have the same diameter. *Go from
> >>>>>> there.
> >>>>> Right, at maximum lens aperture bigger lenses gather more light.
> >>>>> Close the bigger lenses aperture to match the small ones and you get
> >>>>> the same amount of light.

>
> >>>> Please clarify, what exactly do you mean "to match the small
> >>>> ones"? *f/stop? *Diameter? *Area? *What exactly?

>
> >>>> -Wolfgang
> >>> f/stop.
> >>> Big lens: 42mm diameter, 50mm focal length, f/1.2 aperture.
> >>> Small lens: 18mm diameter, 50mm focal length, f/2.8 aperture.
> >>> Set them both to f/2.8 or smaller and you get the same light.

>
> >> So use a 200mm as the big lens and a 50mm as the small lens
> >> and answer again.

>
> > 200mm?
> > I am talking about lens diameter not focal length.

>
> He's talking about both.
>
> 200mm with 42mm aperture f/4.5
> * 50mm with 18mm aperture f/2.8
>
> or
>
> 200mm with 18mm aperture f/11
> * 50mm with 18mm aperture f/2.8- Hide quoted text -
>
> - Show quoted text -


I only have 3 lenses:
EF-S 18-55mm f/3.5-5.6 42mm diameter
EF 50mm f/1.8 18mm diameter
EF-S 55-250mm f/4-5.6 (did not measure lens diameter but close to
42mm)

I wanted to take some pictures last night as it was clear but I live
in Glasgow and at half past midnight I could not see any stars from my
window.

Any tests that I will do will have fixed aperture (f number) on all
lenses and focal lengths, my point being is only the aperture (f
number) that matters in terms of gathering light.


DanP
 
Reply With Quote
 
Wolfgang Weisselberg
Guest
Posts: n/a
 
      06-14-2010
DanP <(E-Mail Removed)> wrote:
> On Jun 9, 11:46*pm, Wolfgang Weisselberg <(E-Mail Removed)>
> wrote:
>> DanP <(E-Mail Removed)> wrote:
>> > Big lens: 42mm diameter, 50mm focal length, f/1.2 aperture.
>> > Small lens: 18mm diameter, 50mm focal length, f/2.8 aperture.
>> > Set them both to f/2.8 or smaller and you get the same light.


>> So use a 200mm as the big lens and a 50mm as the small lens
>> and answer again.


> 200mm?
> I am talking about lens diameter not focal length.


And I am trying to point out that a 200mm f/2.8 lens at f/2.8 will
capture more light from a star than a 50mm f/2.8 lens at
f/2.8.

It's patently obvious that a 200mm f/2.8 lens has a larger
diameter than a 50mm f/2.8 lens.

-Wolfgang
 
Reply With Quote
 
David J Taylor
Guest
Posts: n/a
 
      06-15-2010
> And I am trying to point out that a 200mm f/2.8 lens at f/2.8 will
> capture more light from a star than a 50mm f/2.8 lens at
> f/2.8.
>
> It's patently obvious that a 200mm f/2.8 lens has a larger
> diameter than a 50mm f/2.8 lens.
>
> -Wolfgang


More light (photons) in a given period, when other things are similar.

Cheers,
David
 
Reply With Quote
 
J. Clarke
Guest
Posts: n/a
 
      06-15-2010
On 6/15/2010 2:10 AM, David J Taylor wrote:
>> And I am trying to point out that a 200mm f/2.8 lens at f/2.8 will
>> capture more light from a star than a 50mm f/2.8 lens at
>> f/2.8.
>>
>> It's patently obvious that a 200mm f/2.8 lens has a larger
>> diameter than a 50mm f/2.8 lens.
>>
>> -Wolfgang

>
> More light (photons) in a given period, when other things are similar.


I don't see Wolfgang's post, but it contains the usual misconception
about photographic vs astronomical imaging.

The 200 will "capture more light" in the sense that more light will
enter the lens. However for a given sensor size, a smaller proportion
of that light will strike the sensor. If you make adjustments so that
both cover the same field of view, for example by adding two 2x
teleconverters to the 50, _then_ the 200 will show its "greater light
gathering", and this will be reflected in the photographic formulae, as
at that point the 50/2.8 will be functioning as a 200/11.2.

Astronomers generally work with a magnified field to begin with--in
other words they almost always use some kind of teleconverter--so that
they are working at effective focal lengths longer than that of their
primary lens or mirror. In that situation, larger aperture always
results in a brighter image. Photographers, on the other hand, hardly
ever use a teleconverter these days, so effect of the diameter of their
lens is accurately reflected in the f/ratio.
 
Reply With Quote
 
Wolfgang Weisselberg
Guest
Posts: n/a
 
      06-15-2010
Remmy Martin <(E-Mail Removed)> wrote:

> Yes, I realize it is an exercise in futility in trying to educate you.
> But others reading this who have an IQ above 100 will be able to glean some
> knowledge from the above.


You belong to the IQ-below-70 group.
Plonk.

-Wolfgang
 
Reply With Quote
 
 
 
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
GIDS 2009 .Net:: Save Big, Win Big, Learn Big: Act Before Dec 29 2008 Shaguf ASP .Net 0 12-26-2008 09:29 AM
GIDS 2009 .Net:: Save Big, Win Big, Learn Big: Act Before Dec 29 2008 Shaguf ASP .Net Web Controls 0 12-26-2008 06:11 AM
GIDS 2009 Java:: Save Big, Win Big, Learn Big: Act Before Dec 29 2008 Shaguf Python 0 12-24-2008 07:35 AM
GIDS 2009 Java:: Save Big, Win Big, Learn Big: Act Before Dec 29 2008 Shaguf Ruby 0 12-24-2008 05:07 AM
CurrentElement->next = CurrentElement->next->next (UNDEFINED?) Deniz Bahar C Programming 2 03-09-2005 12:45 AM



Advertisments