Why focus varies from one lens to another

[SylBer]

Hello fellow Pentaxians,

Although the title is pretty explicit, let me explain my question. I own a few Pentax lens. A few days ago, I decided to test focus using Yvon Bourque's chart. To my surprise, all my lens are spot on, except one. My very much appreciated DA35 has a 3mm FF problem. So my question to all the experts on this forum: Could anyone explain to me how come all my lenses are spot on and one suffers from FF? Should I return my DA35 to Pentax?

Thanks for your advices.

Sylvain

[Torphoto]

It's called sample variation, when they test the lens what ever rig they use can be a few mm off from the next one or the one they were using before. 3mm FF is minor as long as the lens does what it should and isn't FF'ing shots out of focus, try adjusting it in the firmware if you can.

[nostatic]

If you don't shoot pictures of focus charts, odds are your problem will be solved

[farfisa]

Originally Posted by SylBer

Hello fellow Pentaxians,

Although the title is pretty explicit, let me explain my question. I own a few Pentax lens. A few days ago, I decided to test focus using Yvon Bourque's chart. To my surprise, all my lens are spot on, except one. My very much appreciated DA35 has a 3mm FF problem. So my question to all the experts on this forum: Could anyone explain to me how come all my lenses are spot on and one suffers from FF? Should I return my DA35 to Pentax?

Thanks for your advices.

Sylvain

Did you use Yvon's separate Macro chart for the DA35?

[Miserere]

Originally Posted by nostatic

If you don't shoot pictures of focus charts, odds are your problem will be solved

While this is normally the case, this is a macro lens, and front-focusing can be a huge problem at macro distances.

[Buddha Jones]

Originally Posted by nostatic

If you don't shoot pictures of focus charts, odds are your problem will be solved

Originally Posted by Miserere

While this is normally the case, this is a macro lens, and front-focusing can be a huge problem at macro distances.

An attempt at humor foiled by logic.

[SylBer]

Yes, I did use the appropriate chart. As far as macro is concerned, I can always focus manually. My point was, if all my other lens are correct, this probably means my K10D is not affected by the BF/FF problems reported on different forums. Is it possible for Pentax to fix this lens problem? If you were me, would you send it back?

Thanks guys.

[nostatic]

Originally Posted by Buddha Jones

An attempt at humor foiled by logic.

bah humbug!

[Parallax]

Originally Posted by Buddha Jones

An attempt at humor foiled by logic.

Made me laugh.

[Class A]

Originally Posted by Torphoto

It's called sample variation, when they test the lens what ever rig they use can be a few mm off from the next one or the one they were using before.

The body does the AF, not the lens. If the body works well with a number of lenses, how can one lens be off?
Even if the lens is odd in some respect, the feedback loop in the AF mechanisms should keep adjusting until the subject is in focus.

Apparently, individual lenses can throw off the AF system, but as to why exactly is one of the great lens mysteries to me.

[gkopeliadis]

Originally Posted by nostatic

If you don't shoot pictures of focus charts, odds are your problem will be solved

concealed ... not solved at all

[Igilligan]

Originally Posted by nostatic

If you don't shoot pictures of focus charts, odds are your problem will be solved

Well that may be true, unless you shoot faces... then the beautiful eye you think is in focus become a blur and the tip of the nose is so sharp it bleeds...

That is what I am getting with a couple of my lenses on the K20... and you know I love shootin' those close in face shots..

Humor and logic gets tossed when I am standing on the ledge wanting to jump... haha hehe

[Canada_Rockies]

Originally Posted by Class A

The body does the AF, not the lens. If the body works well with a number of lenses, how can one lens be off?
Even if the lens is odd in some respect, the feedback loop in the AF mechanisms should keep adjusting until the subject is in focus.

Apparently, individual lenses can throw off the AF system, but as to why exactly is one of the great lens mysteries to me.

Lighting can throw it off as well. It seems, from some of my reading, that tungsten lighting does not focus the same as daylight. I suspect that colour temperature will affect focus somewhat. Lenses [used to] have focusing marks for infrared for just this problem, but at a greater difference in frequency, of course. I suspect that tungsten would require a technical correction to the focus, and the detector might be thrown off.

[alohadave]

Originally Posted by Igilligan

Well that may be true, unless you shoot faces... then the beautiful eye you think is in focus become a blur and the tip of the nose is so sharp it bleeds...

That is what I am getting with a couple of my lenses on the K20... and you know I love shootin' those close in face shots..

Humor and logic gets tossed when I am standing on the ledge wanting to jump... haha hehe

Last time I checked, the tip of my nose is a lot further from my eye than 3mm.

[Igilligan]

Not every face shot has to be straight on...

and my nose is a LOT more than 3mm, but I am turning and its getting closer...........