A550 Zoom Does Not Open

[simpsongb]

So, maybe its me, but I have two different lenses on my SOny, and both behave the same way. I have a Tamron 18-250 f3.5 and a Sigma 28-105 f2.8 . For both lenses, if I zoom out, the available aperture settings start to creep up, and so when I get them all the way out, the widest aperture is no longer the 3.5 for the Tamron or the 2.8 for the SIgma. Put another way, I cannot zoom the Sigma all the way to 105 and also choose the 2.8 aperture setting.

So am I doing something wrong with the Alpha, or is this to be expected???

My daughter has a Canon, and a Tamron f2.8 lens, and she is able to open it wide even at the longest zoom......

Help!!

[TCav]

There are some lenses that have a constant maximum aperture throughout their zoom range, but most do not, including the two you own.

For instance, the Minolta 70-210mm f/4 "Beercan" has a maximum aperture of f/4.0, whatever focal length the lens is set you. Your Tamron 18-250mm superzoom lens has a maximum aperture of f/3.5 at 18mm, but the maximum aperture gets steadily smaller (numerically larger) as you zoom, until at 250mm the maximum aperture is only f/6.3. That's why, engraved on the barrel, it says 18-250mm 1:3.5-6.3.

It's very likely that your daughter's Tamron lens does have a constant maximum aperture of f/2.8.

[simpsongb]

Het TCav

Actually I have watched her use it on her Canon, and she is able to stop it down to 2.8 even at the longest zoom....but, she also paid like $300+ for the lens...

[TCav]

Yes, from what you say, I'm confident that what she has is a zoom lens with a constant maximum aperture of f/2.8.

F-Number is a function of the focal length of a lens and the inside diameter of the diaphragm. A lens with a 50mm focal length and a 25mm diameter diaphragm has a maximum aperture of f/2.0 (50mm/25mm), while a lens with a 100mm focal length but the same 25mm diameter diaphragm has a maximum aperture of f/4.0 (100mm/25mm.)

A zoom lens has a variable focal length but uses the same diaphragm whatever the focal length might be at any given time. It takes a special lens design to have the F-number remain constant as the focal length changes, and that often means that the lens costs more than a conventional zoom lens might.

Take for instance, the Tamron 55-200mm F/4-5.6 ($199) and the Tamron 70-200mm f/2.8 ($769). They are both very good lenses, but the 70-200/2.8 costs almost 4 times as much as the 55-200/4-5.6 even though they have very similar zoom ranges.