That's one I missed, but I just zapped it. It's been dealt with now, so there shouldn't be any more. I'm really glad about that as well - it takes a long time to delete all that nonsense. (Although some of it is potentially dangerous nonsense)

That sounds pretty much what I expected - it's certainly never been a lens on my Wants List....

The wider the zoom ratio of a lens, the more difficult it is to make and the chances are that the quality will be lower as well. This particular design is not at all bad, but the caveat is "for its zoom range" and it will not be up to the standard of more modest zooms or primes.

Having said that, it would be undeniably convenient and would probably produce excellent 7" x 5" prints.

Sorry to disagree, but a vari-focal lens is one which has a variable focal length. This is subtely different from a zoom. The zoom is supposed to maintain it's point of focus as you change the focal length (essential in a movie, useful for still photography) but a vari-focal has to be refocused after each alteration of focal length.

One of the early Vivitar lenses (was it a 38-80mm?) was widely acknowledged even by Vivitar as a Vari-focal design.

Perhaps we are being too complex in our deliberations!

But I can address your specific question - most definitely most modern zooms are demonstably not a constant aperture as you zoom. The movement of various elements and/or groups within the lens may help to keep the effect under control to a greater or lesser degree, but the effect can be demonstrated by simple metering tests, as suggested by KimC.

The actual difference this makes to exposure can be very small, given the latitude of modern films abd/or CCDs.

The reason is optics. My example was grossly simplified to make the point, and your extension of the concept to 200mm is logical, but only in the same basic way.

Lenses are more complex thatn this, especially zooms, so there are many more design factors involved.

Even something like focal length becomes a blurred concept - what point do we measure it from for example in a zoom that is maybe 10cm long or more - where is the "nodal point" and does it move as you zoom...

Also, focal length often alters as we focus as well as zoom, usually because of Internal Focusing.

So although my simplification is true in that it explains the baisc principle, it becomes less true as we delve deeper, which I mentioned in the previous posts. I just love lenses!

Hi George

Tricky, but here goes:

1. In a variable-aperture zoom design, there is no compensation as you zoom. This is cheaper to manufacture and may not matter with auto exposure cameras, but it will matter with manual exposure. The aperture changes continuously, but only the extremes are quoted. Some zooms have small lines etched to mark out the degree of discepancy from the marked value.

2. We have to have something marked on the barrel otherwise there would be no clue whatsoever as to what the aperture was. The widest aperture is usually inscibed. If we were being really pedantic, a very complex zoom will have some light loss so a physical aperture of say f8 will have a transmission aperture slightly less, usually referred to as a T stop. T stops are not used in still cameras these days, but were once upon a time.

3. It's close enough, but you're right that there is a discrepancy and the usual advice is to meter after zooming. If you zoom again, the metering will change.

We get into some interesting territory in these discussions. Of course part of the problem is always that the more deeply one goes into something, the less true many practical truths become.....

Hope that is more helpful.

There are two types of aperture correction for zooms - mechanical and optical. The pentax zooms that I have with constant f numbers are all of the mechanically compensating type - that is, as you zoom out to wide angle the aperture closes down slightly. Optical correction is also possible, but I don't know of any examples of this.

If you do not change the configuration as you zoom, you end up with a changing aperture value, and there are lots of zoom lenses around that have variable f numbers.

Think about it this way - if a 50mm lens has a front element diameter of 25mm then it will be f2 (50 divided by 25). If the lens could become a zoom and was zoomed to 100mm it would be f4 (100 divided by 25).

George, the other side of the coin is the pure delight of the classic Spotmatic design. It's a beautiful bit of engineering, and a joy to use. The lenses are also superb, although to be fair they could indeed be used on more recent bodies.

There are still some good ones around, but it's a bit like fishing, you need to be patient!

I can understand your disappointment - I've had great success and great failure when buying on eBay as well.

There are some nice Spotmatic F bodies around, but there seem to be rather more KMs, which are of course the bayonet version of the same camera.

Sorry I can't offer you a Spotmatic F, but maybe someone else will come up with the goods.