Notorious T.O.D. Loc: Harrisburg, North Carolina

imagemeister Loc: mid east Florida

The 24-70 is a "constant aperture zoom" meaning it can be f2.8 at ALL zoom settings. Most CAZ's are non-extending (internal) zooms - but the Canon 24-70 is not one of them as it extends when zoomed out - the 24-105 is another one and the Nikon 200-500 is another one.

Charles 46277 Loc: Fulton County, KY

Ah--so the fact that it does not extend when zoomed means that the aperture hole can remain constant?

imagemeister Loc: mid east Florida

This is generally true - but there are the exceptions that I mentioned - and there could be a few more ?

When it does not extend, it is EASIER for the lens designers to make it constant aperture.

John_F Loc: Minneapolis, MN

Yes, it is contradictory. In the optics equations both focal length and lens aperture figure into DoF calculations. So the two must vary in a consistent manner in order for DoF not to vary.

Apaflo Loc: Anchorage, Alaska

The phyical size of the aperture is constant, but what actually determines the f/stop is the "effective aperture". The effective aperture changes depending on how the focal length is zoomed. The focal length be changed using only the front element (a "positive" element), or a rear element (i.e. retro focus), or a mix of changing focal length by adjusting both front and rear elements.

Changing only the positive front element magnifies the effective aperture by exactly the same amount it magnifies the focal length. The f/stop therefore stays exactly constant as the lens is zoomed (even though the physical size of the opening of the aperture does not change),

If only a rear element is used to change the focal length it will have the opposite effect, with a significant change in the effective aperture. If both front and rear elements are used to zoom the focal length there will be some change in f/stop, but not as much as if only a rear element is changed.

A "constant-aperture" zoom is nicer, but more expensive to design and manufacture. But it absolutely does not give up a wider f/stop at shorter focal lengths in order to be the same f/stop as at longer focal lengths.

Note that this does not relate to whether the front of a zoom lens extends when zoomed. Also there is a difference between "Internal Focus" design that does not extend the front of a lens when focused, and whether zooming extends the front of the lens.

Apaflo Loc: Anchorage, Alaska

But DOF does vary!

Even with a constant aperture zoom lens the changing focal length causes DOF to vary.

SteveR Loc: Michigan

The other advantage, ofc, is that the lens will always focus at f2.8 before closing down to the selected aperture, allowing for fast focus.

Screamin Scott Loc: Marshfield Wi, Baltimore Md, now Dallas Ga

Plus a constant aperture will allow the same amount of light into the viewfinder regardless of the focal length selected...

John_F Loc: Minneapolis, MN

Every collection of spherical refracting surfaces my mathematically transformed to the equivalent simple lens. The calculation will produce four cardinal planes from which all object and image distanes are measured. These are the principal and focal planes. (quality lenses will have their principal planes nearly tangent to the two external surfaces). The focal planes are 'focal length' distant from the principal planes. If there are no aperture leaves the aperture will be the edge of the lens tube. Aperture is a physical property of the lens.

Apaflo Loc: Anchorage, Alaska

Yes but that includes the magnification from either the front or rear elements or a combination of both. The f/stop is not a function of just the physical size of the opening.

Architect1776 Loc: In my mind

Absolutely not better engineered. It is a choice to maintain a size and weight goal take a 100-400mm constant aperture lens using the larger aperture as the size would create an unwieldy and heavy lens virtually unusable except on a tripod.
Yet the engineering of the lens can be superb or even better than the fixed aperture lens as is shown in the 100-400mm example.

John_F Loc: Minneapolis, MN

The front and rear lens elements may be mathematically combined such that there will be a single focal length and a single set of cardinal planes and cardinal points. The behavior of the light beam passing through the lens will be in accordance with the lens formulas. For magnification, the ratio of object size to image size, O/I, will equal the ratio of the object distance from front principal plane, s, to image distance from rear principal plane, s', s/s'. The lens equation will be 1/s + 1/s' = 1/f where f is the focal length. All matters assiciated with the formation of images by means of light refracting surfaces are governed by the jathenatical physics of optics, for which there are many text books. Mine is Valasek's Theoretical and Experimental Optics (1949).

The f-stop, N, is the ratio of the focal length/aperture diameter; it is a defined quantity.

BebuLamar

So in another word if you set the lens at a certain f stop the diaphragm will not move as you zoom.

Apaflo Loc: Anchorage, Alaska

True.