On Thu, 14 Dec 2000 22:05:09 GMT, "Alexander Ibrahim"
>"Neuman - Ruether"
>news:3a3add0a.3305058@newsstand.cit.cornell.edu...
[...]
>Sci.physics may be more appropiate for this discussion.
Perhaps....;-)
Another "real-life" example...:
Meter the side of a house close up; with no changes in the
illumination of the house, and without changing the angle
of measurement, meter the side of that same house from
across the valley (or some such similar great distance -
assuming practicality of measurement of such a small
angle...;-); I think those measurements will be the same.
Going back to the moon example, an exposure metering of the
fully-illuminated face of the moon results in an exposure
not unlike what one gets metering a similar tone value in
full sunlight on earth (both targets are roughly the same
distance from the sun, and in our example, roughly the
same tone, and removing most of the effects of atmosphere by
specifying clear air, it may surprise you, but not me, that
the close-up metering of the earth-tone and the distant
metering of the moon tone from the earth results in roughly
the same metered light value...;-).
[...]
>> If the f-stop is the same, and the designs equally
>> roll off edge illumination, there is no difference
>> in light transmission due to front-element diameter
>> of a lens.
>Ah...but they are not the same lens. Even if the TRV900 had a scaled down
>version of the XL-1 lens, or vice versa...they would have different optical
>qualities...hence different roll off illumination. It so happens that
>scaling the lens elements would probably make this comparison worse rather
>then normalize things.
I was trying to simply things. Transmission values of
two same-speed lenses are likely to be slightly different,
but the differences are normally small enough to ignore
in this comparison. It is a common misconception in video
that a larger-fronted lens accepts more light than a smaller
fronted lens of the same speed and focal-length. This is
not necessarily true even at wide stops, and it is probably
not true at all by mid stops. A good example is comparing
a Tessar-type lens, which generally has a front element
barely larger than the aperture, with a 6-element
symmetrical design, which often has a front element
far larger than the aperture. It these lenses are the same
FL and speed, it is very likely their apertures are similar
in size, as are their light-transmissions (though the
6-element lens may have been designed to cover a larger
angle of view...).
>>A tiny 20mm f2 lens makes as bright an
>> image as a big 200mm f2;
>Uh...noooo. First off I am not certain whether or not you are talking about
>the diameter of the front of the glasss (phi) or the focal length.
FL, the only really relevant spec. for a given format...
And..., yeeeessss....;-) The relative aperture defines the
(approximate, but very nearly true) brightness of the passed image.
>>a smallish-front 20mm f2
>> (all else being equal) passes as much light as a
>> largish-front 20mm f2...)
>Maybe you mean to talk about both options...
Yes. I have two 35mm f2 Nikkors. One is the older
MF version, with a front element that is quite
large. The other is a newer AF design, with a front
element that is quite small. Both are f2 wide-open;
both are 35mm in FL; and both pass nearly the same
image in all respects, including brightness.
>I don't really know how to explain this David...at least not without giving
>a lesson in optics. How the zoom collects differing amounts of light from
>different areas of the subject is complicated and counterintuitive.
I would have to agree...! ;-)
>OTOH how a larger lens face (phi) gathers more light follows directly from
>geometry. The greater the surface area of the front of the lens the more
>light it can bend towards the imaging surface...how much more is given
>theoretically by the ratio of surface areas. (For a circular lens this is
>given by PI*r^2.) In a real environment this is not precisely correct
>because some of the light is absorbed, bent, reflected and interfered
>with...Further most lenses are not "perfectly" efficient at bending light
>onto a focal plane. (Our imager surface.) (I am not even getting into
>quantum effects which are relevant in many real world conditions.)
I think you are confusing designing for lens angle of view
and lens speed... (see the 35mm Nikkor examples,
above...;-). Lens speed is not defined by the size of the
lens front, but by the clear aperture, related to FL...
>Here is another common sense version of this...take an opaque disc of any
>sort. Hold it up to a light. look at its shadow. Move it closer...the shadow
>grows because a greater amount of the emmited light is incident on the disc.
>Move it further away the shadow gets smaller. Change the size of the
>disc...if you make the disc bigger the shadow is bigger, smaller...the
>shadow is smaller. If you measure the shadows the disc and the distance from
>the light you will find it follows an inverse square law
Yes, but if you measure the illumination in the shaded
area, it should remain about the same...;-)
>>If the area of the diaphragm
>> of one lens is four times that of the other at the same
>> focal-length, it passes two stops more light, hardly
>> a negligible amount of difference...;-)
>You can't have it both ways now David. Either area is important or it is
>not.
Area of clear aperture (related to the diaphragm maximum
opening size, not of the front element size...).
>Physics and me say it is...hence how your diaphragm, or iris operates.
>Notice how doubling the radius of your iris opening gives you four times the
>amount of light...doesn't that sound suspiciously like what I am saying ?
No - more like what I'm saying...;-)
>If
>you open two stops you should double the distance to the subject to maintain
>correct exposure.
This does not follow at all... If it did, we would not be
able to see very far at all! ;-) Our eyes have similarities
to lenses/cameras (and some differences), and if what
you say is true, landscape features at various distances would
darken rapidly with greater distance. I've never observed
this...! ;-)
>When you double the distance, you'll probably zoom to
>retain your framing...meaning you'll get a good part of your two stops back,
>BUT NOT ALL OF IT!
How? the zoom lens is more likely to lose most of a stop
as it is zoomed long. The imaged brightness of an object
in its view does not change more than this with zooming
OR moving (and not at all if you just move, without zooming).
You are misusing physics, methinks....;-)
>> BTW, in my reviews, at:
>> www.David-Ruether-Photography.com/camcorder-comparison.htm ,
>> I based my conclusions on overall performance, though
>> in specific light levels, the conclusions could very
>> well be different. I liked the picture quality of
>> neither Canon reviewed, though in a specific room
>> interior light-level, both performed at their best
>> (and better than some others - but it is reasonable
>> to assume that not everything will be shot at this one
>> light level, and in most other situations I preferred
>> the picture quality of other camcorders...;-).
>FINALLY something I can say YES to. Reviews are opinion pieces. I have
>comments to make about your testing process etc. Then again so should anyone
>who reads any review. You have conclusions and I think you stated your
>reasons for reaching them clearly. In the end that makes a critique
>valuable.
Thanks for the comments.
>I liked your review, though I disagree based on the cameras in your test I
>have worked with personally. When I get around to doing mine I hope you like
>them too, even though I am sure you'll disagree.
Mebbe... The weightings of what each person considers
relatively important/unimportant may be different, but
the observations should be similar...;-) I prefer the
sharpest images with the least negative picture artifacts
(color-bias errors, over-sharpening effects,
"stairstepping", changes in sharpness with distance, etc.)
under the widest range of lighting conditions...
>> David Ruether
>> d_ruether@hotmail.com
>> http://www.David-Ruether-Photography.com
>> Hey, check out www.visitithaca.com too...!
>You attend Cornell ? If so take one of their University Physics courses. The
>third semester (modern physics at most Universities) covers Optics in some
>depth. Your school may also have a course on optics for technical
>photographers, or better yet a physics course on optics that is not for
>physics majors.
"Likewise, I'm sure...!" ;-)
>Understanding light and optics properly is key to masterful cinematography.
>
>Alexander
>http://www.zenera.com
Ummmm...., I'm not too sure of that....! ;-), ;-), ;-)