Go Back   Steve's Digicams Forums > Digital SLR and Interchangeable Lens Cameras > Sony Alpha dSLR / Konica Minolta dSLR, Sony SLT

Reply
 
Thread Tools Search this Thread
Old Aug 20, 2013, 9:01 AM   #1
Junior Member
 
Join Date: Aug 2013
Posts: 2
Default A550 Zoom Does Not Open

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!!
simpsongb is offline   Reply With Quote
Sponsored Links
Old Aug 20, 2013, 11:06 AM   #2
Senior Member
 
TCav's Avatar
 
Join Date: Sep 2005
Location: Washington, DC, Metro Area, Maryland
Posts: 13,543
Default

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.
__________________
  • The lens is the thing.
  • 'Full Frame' is the new 'Medium Format'.
  • "One good test is worth a thousand expert opinions." - Tex Johnston, Boeing 707 test pilot.
TCav is offline   Reply With Quote
Old Aug 20, 2013, 11:30 AM   #3
Junior Member
 
Join Date: Aug 2013
Posts: 2
Default

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...
simpsongb is offline   Reply With Quote
Old Aug 20, 2013, 1:35 PM   #4
Senior Member
 
TCav's Avatar
 
Join Date: Sep 2005
Location: Washington, DC, Metro Area, Maryland
Posts: 13,543
Default

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.
__________________
  • The lens is the thing.
  • 'Full Frame' is the new 'Medium Format'.
  • "One good test is worth a thousand expert opinions." - Tex Johnston, Boeing 707 test pilot.

Last edited by TCav; Aug 20, 2013 at 6:50 PM. Reason: sp
TCav is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 7:23 AM.