Buying a sharp copy lens from camerashop


Apr 18, 2009
796
0
0
Singapore
#1
Hi,

I am going to buy a Tamron 17-50 f2.8 for Pentax K-x. This is my 1st time buying lens only. i am thinking to buy it at SLR Revo. I am quite worried about getting a bad copy. how do you guys test the lens at the store? do you bring charts? Could anyone one give me advice? Thnks!
 

madmartian

Senior Member
May 2, 2009
20,218
11
0
Outer Space
#2
Hi,

I am going to buy a Tamron 17-50 f2.8 for Pentax K-x. This is my 1st time buying lens only. i am thinking to buy it at SLR Revo. I am quite worried about getting a bad copy. how do you guys test the lens at the store? do you bring charts? Could anyone one give me advice? Thnks!
Bring your camera there & test the lens with it. Take a few shots & zoom in from the preview screen to check its sharpness. Also make sure moving parts move smoothly.
You don't need the whole lab team to be there just to purchase 1 lens ;)
 

Apr 18, 2009
796
0
0
Singapore
#3
So i just
1) use a simple test chart like this ---> lllllllllllllllllll
2)with arms resting on a stable platform, focus the target at 45 degree with lens wide open at F2.8 and from 17mm, 35mm, 50mm
3)using LCD, check if the target is the sharpest line

is it a need to test with lens at F8 F22? just wide open will do?

if the target is in focus but is NOT at the center of the focus range, is it suffering front/back focusing?
 

Octarine

Moderator
Staff member
Jan 3, 2008
12,491
26
48
Pasir Ris
#4
is it a need to test with lens at F8 F22? just wide open will do?
The very basics about Depth of Field should tell you why the test is done at the widest aperture. How does the DOF change when you close aperture? How likely is it to find a focusing error then?
if the target is in focus but is NOT at the center of the focus range, is it suffering front/back focusing?
This is provided there is no other error or problem in your test setup. Chances of human error are usually higher than chances of technical faults. Keep in mind that the focus sensors in the camera are bigger than viewfinder markings. Be careful when focusing, especially hand held. Also, before you setup all your test charts and block half the shop you can simply verify the 7 days 1:1 exchange policy that many other shops have. Get the lens and test at home. Don't forget a tripod.
 

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