Why no long telephoto zoom lenses specifically for cropped frame DSLR's

"Compared to FF, 4/3 format receives half the total number of photons, but is also half the physical size."<P>

That should be corrected to:<P>

Compared to FF, 4/3 format receives one quarter the total number of photons, but is also one quarter the area. The same number of photos per sq mm are striking the same number of photo cells per sq. mm. <P>

<center><img src="ttp://jdainis.com/film_photons2.gif"></center><P>

"It is equivalent to a 600mm/2.8."

When people use the word "equivalent" in that sense they mean "equivalent angle of view". That should read:"<P>

It has an equivalent angle of view as a 600mm f/2.8.<P>

Playing fast and loose with math, arguing semantics or throwing in a bunch of red herrings doesn't change the fact that the cone of light from the f/2.8 lens is the same whether it is falling on a larger or smaller sensor.<P>

<center><img src="http://jdainis.com/tree3.jpg"></center>

<blockquote>

<p>Playing fast and loose with math, arguing semantics or throwing in a bunch of red herrings doesn't change the fact...</p>

</blockquote>

<p>I quite agree...</p>

<blockquote>

<p>"It is equivalent to a 600mm/2.8." When people use the word "equivalent" in that sense they mean "equivalent angle of view". That should read:"It has an equivalent angle of view as a 600mm f/2.8.</p>

</blockquote>

<p>No, it should read "It has the equivalent angle of view as a 600mm"</p>

<p>Once you threw in the f2.8, and you started making statement about "when people use the word 'equivalent'", you became the one guilty of the offenses you describe: "arguing semantics or throwing in a bunch of red herrings"</p>

<p>If you want to learn what people <strong>actually</strong> mean when they talk about what is "equivalent", instead of painting your herrings red, stop by the Oly forums. Most of the four thirds "advocates" scream bloody murder if you try to make the equivalence argument only about angle of view. They honestly use it to describe all aspects of photography, including justifying the most expensive 300mm f2.8 from any major manufacturer.</p>

"No, it should read "It has the equivalent angle of view as a 600mm""

I know you're done with me.

But I honestly don't understand this.

I do understand that the exposures from lens to lens with the same aperture setting may be a little off.

And I also understand that metering through different focal length lenses can produce different exposures. Intuitively I

know that a wider angle of view may have more dark or light objects or space in them. So if I meter through two different lenses the reading will be different.

I can also see where taking a lens and adding say a 2x teleconverter will subtract 2 stops of light to the film or sensor.

I can't see how a light metered reading of 2.8 is going to produce a similar exposures on two different focal length lenses

with one lens being 2 stops smaller than the other(5.6). Or the same focal length lens on different sized sensors.

A 50mm at 2.8 and a 100mm at 2.8 should produce similar exposures on the same camera. This should also be true to 25 to 50, 100 to 200, 300 to 600.

A 50mm at 2.8 on an FF or DX camera should produce similar exposures(obviously with differing angles of view). The amount of light coming through the lens does not change respective to camera or sensor size.

If I put in an extension tube, say, and increase the length the light travels, that will change my exposure; that is to say that increased distance from objective to sensor plane results in less light.