Is my lens faulty?

Raymond Lin

Raymond Lin

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Raymond
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I got a sigma 24-70 on Thursday, it had a front focusing problem, so went back to jessops and they swapped it with another one. Got home and the same thing is occurring.

I focus on the line that starts off with "Payments you make....", using only centre focus point on the 30D but clearly that line is out of focus and the lines 2 lines down is in focus instead. This can't be normal surely.

 
That paper's not exactly flat :) Flat sheet of newsprint, camera on tripod, remote/timer release...

Will be getting my 24-70mm next week, will check mine for you then...
 
Fair point.

Try the newspaper thing, tripod, remote release, timer / mirror lockup and at 45 degrees to the print.

What focal length / aperture?
 
Here is something flat :)

The center focus point is on Bristol.

The focus is just slightly in front of it.


 
try do it again with the highest f/stop?
 
You can get a test chart Here, print it on photo paper if you can.


C a n 't t y p e f a s t e n o u g h. . . . .
 
I don't have a cable release, but a cable release won't cause front focusing, if it's camera shake then nothing will be sharp.

 
use mirror lockup?
 
The problem with doing focus tests at an angle is that the AF sensor is larger than you think and it will favour the part that is sharpest and closest to the camera which will lead to false front focus errors. Repeat the test but without the angle and see what the results are like. Also check over several distances not just min. focus.
 
mirror lockup = less vibration
= doesnt need cable release

i think... lol
 
Try the battery test? Rather than newsprint, for the reasons mentioned above.
 
Raymond Lin said:
Its fricking going back

3 shots, dead straight

Click to expand...

That doesn't look good does it? :(

Assuming you've used the delayed timer to release the shutter that doesn't bode well at all. A test I sometimes use is to focus on one letter of the keyboard from a low acute angle at full aperture.
 
CT said:
That doesn't look good does it? :(
Click to expand...

It's at 1/30th though and there's definitely signs of shake. Still doesn't look that promising though
 
Kerioak said:
Have you tried hand held in case your tripod is not holding it securely enough?
Click to expand...

lol?

Fact is the lens is not doing what it is supposed to. I have complete faith that raymond knows how to use a camera. It's just another example of Sigma's legendary quality control.
 
LOL, its a manfrotto 055 pro with 141RC head, not a cheap one and 1/30 on a tripod should be pin sharp, I took 3 shots and its all the same.
 
Just done the test as per instructions, by the way thats a good one to have in a folder when you buy a len's... the 24-105 L was spot on the 18-125 sigma was sopt on as well but not as spot on if you kow what i mean , going to try the others later.

Regards mark.
 
line a few batteries up in a line going away from you and equally spaced. focus on one of the with the lens wide open and see if it's in focus
 
Just printed out the test above that Dogfish Magnet gave. Looks really useful I'm going to have a go tomorrow with all my lenses - Then I can only blame myself for bad focusing :)
 
Don't use focus test charts, notorious for finding problems that don't exist in the real world.

Chuck Westfell himself says do not use focus test charts.

Try the battery test, reliable and won't give false positives.

Two "bad" lens in, and I start to suspect the body.
 
puddleduck said:
Don't use focus test charts, notorious for finding problems that don't exist in the real world.

Chuck Westfell himself says do not use focus test charts.

Try the battery test, reliable and won't give false positives.

Two "bad" lens in, and I start to suspect the body.
Click to expand...

Yes I can imagine re test charts.

You say two bad lenses and so you suspect the body but what about other lenses how are they working with the camera?
 
cowasaki said:
Yes I can imagine re test charts.

You say two bad lenses and so you suspect the body but what about other lenses how are they working with the camera?
Click to expand...

Well in Nikon land its very hard for a non AF-S / HSM lenses to backfocus as this is controlled solely by the body.

Spherical aberations can confuse the AF sensors (especially if they are caliberated to f/5.6 accuracy for a typical consumer lens), with fast (f/2.8 or better) lenses shot wide open at close to minimal focus distance.
 
I had the same lens and it was soft from 2.8-6.3 . Returned it and bought a 24-105.
 
When you took the lens back, did they swap it in front of you? Or did they take it "out the back to get a new one" and appear with said new one?

Don't suppose you noted the serial number before you went in the 1st time?

I thought it was 99% the body that caused back/front focus?
 
jonnypb said:
line a few batteries up in a line going away from you and equally spaced. focus on one of the with the lens wide open and see if it's in focus
Click to expand...

I have an issue with that though - batteries are round, and therefore the same problem can occur as with the focus test chart- you don't actually know exactly where your camera's focus will be taken from within the focus indicator. Unless you use square batteries, but then they are so thick that you wouldn't necessarily see the part that is actually in focus....

I'm surprised someone has not designed and built some kind of stepped piece of kit to check focus.....
 
OutLore said:
I have an issue with that though - batteries are round, and therefore the same problem can occur as with the focus test chart- you don't actually know exactly where your camera's focus will be taken from within the focus indicator. Unless you use square batteries, but then they are so thick that you wouldn't necessarily see the part that is actually in focus....

I'm surprised someone has not designed and built some kind of stepped piece of kit to check focus.....
Click to expand...

What about a 9V pp3 sideways or what about some other small rectangular object!
 
cowasaki said:
Just printed out the test above that Dogfish Magnet gave. Looks really useful I'm going to have a go tomorrow with all my lenses - Then I can only blame myself for bad focusing :)
Click to expand...

Have also tried this test, results are here:-

http://forum.warehouseexpress.com/message.asp?FTopicID=3335

Blimey Ray, that lens is well out of focus.

Really daft I know and I am really trying to help and know you're a regular on here, but is the lens set to AF?

Sorry if you find this an insult; it is not intended to be.

Cheers, Mark
 
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