Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
06-23-2010, 08:06 PM   #1
Veteran Member




Join Date: Feb 2010
Location: Scarborough, Ontario
Photos: Albums
Posts: 488
Not true F4?

Hey guys, my Sigma 17-70mm is rated f2.8-f4. I just noticed though... that when I get to 70 it turns into f4.5. Is it a defect? or is it normal?

EDIT: it also shows up as 50mm on the EXIF data when I shoot things above 50mm.


Last edited by NecroticSoldier; 06-24-2010 at 06:25 AM.
06-23-2010, 08:26 PM   #2
Veteran Member
dgaies's Avatar

Join Date: Oct 2009
Location: Maryland / Washington DC
Posts: 3,917
QuoteOriginally posted by NecroticSoldier Quote
Hey guys, my Sigma 17-70mm is rated f2.8-f4. I just noticed though... that when I get to 70 it turns into f4.5. Is it a defect? or is it normal?
Do you have the newer Sigma 17-70mm? The older one is f/2.8-4.5.
06-23-2010, 08:42 PM   #3
Veteran Member




Join Date: Feb 2010
Location: Scarborough, Ontario
Photos: Albums
Posts: 488
Original Poster
QuoteOriginally posted by dgaies Quote
Do you have the newer Sigma 17-70mm? The older one is f/2.8-4.5.
The newer one.
06-23-2010, 08:49 PM   #4
Veteran Member
dgaies's Avatar

Join Date: Oct 2009
Location: Maryland / Washington DC
Posts: 3,917
QuoteOriginally posted by NecroticSoldier Quote
The newer one.
Yeah, I didn't see the lens in your signature after I posted

Does this happen when you were focusing on something close? If you focus on something far away, can you get the aperture down to f/4 @ 70mm?

06-23-2010, 08:55 PM   #5
Veteran Member




Join Date: Feb 2010
Location: Scarborough, Ontario
Photos: Albums
Posts: 488
Original Poster
QuoteOriginally posted by dgaies Quote
Yeah, I didn't see the lens in your signature after I posted

Does this happen when you were focusing on something close? If you focus on something far away, can you get the aperture down to f/4 @ 70mm?
Sorry ;( about to sleep soon... but umm... let me check.. this happens both ways ;(. Not sure if it was like this before... just noticed it now... first time bringing it out to shoot like 300 pictures.
06-23-2010, 08:59 PM   #6
Veteran Member
dgaies's Avatar

Join Date: Oct 2009
Location: Maryland / Washington DC
Posts: 3,917
QuoteOriginally posted by NecroticSoldier Quote
Sorry ;( about to sleep soon... but umm... let me check.. this happens both ways ;(. Not sure if it was like this before... just noticed it now... first time bringing it out to shoot like 300 pictures.
Yeah, not sure what's going on. I've never used this particular lens so hopefully someone who owns one can chime in. Good luck.
06-24-2010, 11:25 AM   #7
Veteran Member




Join Date: Feb 2010
Location: Scarborough, Ontario
Photos: Albums
Posts: 488
Original Poster
QuoteOriginally posted by dgaies Quote
Yeah, not sure what's going on. I've never used this particular lens so hopefully someone who owns one can chime in. Good luck.
;( hope someone can help... maybe it's the K-X's fault?
06-24-2010, 12:54 PM   #8
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
QuoteOriginally posted by NecroticSoldier Quote
I just noticed though... that when I get to 70 it turns into f4.5
What do you mean it "turns into" 4.5? If you are at 70mm and in Av mode, and turn the dial as far as it goes, are you saying it stops at f/4.5?

If so, are you *sure* it's the new version?

06-24-2010, 02:03 PM   #9
Veteran Member
Laurentiu Cristofor's Avatar

Join Date: Nov 2009
Location: WA
Photos: Albums
Posts: 3,044
Could be some issue with the camera sensing badly the aperture. On one of my lenses I've noticed that the camera was displaying different aperture values when the lens was wide open. Even more strange is the fact that yesterday I was playing with a Tamron adaptall lens connected via a plain K mount adapter and I noticed the camera was recording some random aperture values even though the display was showing f--.

Does the camera overexpose slightly when it displays f/4.5? If you notice this, then your lens is f/4 but the body reads that badly - it should affect exposure, but doesn't make your lens slower.

Another idea is to try and clean the mount and lens contacts - might help.
06-24-2010, 02:46 PM   #10
Veteran Member




Join Date: Feb 2010
Location: Scarborough, Ontario
Photos: Albums
Posts: 488
Original Poster
QuoteOriginally posted by Marc Sabatella Quote
What do you mean it "turns into" 4.5? If you are at 70mm and in Av mode, and turn the dial as far as it goes, are you saying it stops at f/4.5?

If so, are you *sure* it's the new version?
Yes, in Av mode ;( I turned it as far as possible.

Yes it is the new version... because the old version doesn't have HSM and OS.

It clearly says 2.8.-4 though...



QuoteOriginally posted by Laurentiu Cristofor Quote
Could be some issue with the camera sensing badly the aperture. On one of my lenses I've noticed that the camera was displaying different aperture values when the lens was wide open. Even more strange is the fact that yesterday I was playing with a Tamron adaptall lens connected via a plain K mount adapter and I noticed the camera was recording some random aperture values even though the display was showing f--.

Does the camera overexpose slightly when it displays f/4.5? If you notice this, then your lens is f/4 but the body reads that badly - it should affect exposure, but doesn't make your lens slower.

Another idea is to try and clean the mount and lens contacts - might help.
Eh... the exposure seems fine to me... but is there a way to check?

I'll try to clean the lens contacts.

On second thought... I don't know what I'm doing... so I researched on how to clean the lens contacts... rubbing alcohol on a cloth? is that correct?

I actually do see a black line across, looks like it was from putting it on a camera (or a similar motion..?)

Last edited by NecroticSoldier; 06-24-2010 at 03:15 PM.
06-24-2010, 04:58 PM   #11
Veteran Member
Laurentiu Cristofor's Avatar

Join Date: Nov 2009
Location: WA
Photos: Albums
Posts: 3,044
QuoteOriginally posted by NecroticSoldier Quote
On second thought... I don't know what I'm doing... so I researched on how to clean the lens contacts... rubbing alcohol on a cloth? is that correct?
The way I do it is with a qtip dipped in alcohol (I squeeze the cotton, so it's not dripping). I just gently wipe the grease/dust that gets on the contacts over the time.

For checking the exposure, you could try this: stop down the lens to a position where the ring and the camera are agreeing on the aperture. Take a shot and note the shutter speed. Then calculate what that should be if you take the shot at f/4 (if you take the shot at f/5.6 then at f/4 you'd need a shutter speed twice as fast - 1/500 instead of 1/250, for example). Then move the ring to f/4 (where the camera shows f/4.5) and take another shot. The camera should have exposed a little longer than you calculated for f/4, so the shot should be somewhat lighter than the first shot - it won't be a huge difference, but if you notice it, then it confirms that the lens is f/4 but the camera uses it as if it was f/4.5, so it will slightly miss the exposure and you'll need to compensate when using the lens wide open.

Of course, you should do this experiment under steady lighting conditions.

Last edited by Laurentiu Cristofor; 06-24-2010 at 05:00 PM. Reason: added a clarification in the description of the steps
06-24-2010, 05:29 PM   #12
Veteran Member




Join Date: Feb 2010
Location: Scarborough, Ontario
Photos: Albums
Posts: 488
Original Poster
QuoteOriginally posted by Laurentiu Cristofor Quote
The way I do it is with a qtip dipped in alcohol (I squeeze the cotton, so it's not dripping). I just gently wipe the grease/dust that gets on the contacts over the time.

For checking the exposure, you could try this: stop down the lens to a position where the ring and the camera are agreeing on the aperture. Take a shot and note the shutter speed. Then calculate what that should be if you take the shot at f/4 (if you take the shot at f/5.6 then at f/4 you'd need a shutter speed twice as fast - 1/500 instead of 1/250, for example). Then move the ring to f/4 (where the camera shows f/4.5) and take another shot. The camera should have exposed a little longer than you calculated for f/4, so the shot should be somewhat lighter than the first shot - it won't be a huge difference, but if you notice it, then it confirms that the lens is f/4 but the camera uses it as if it was f/4.5, so it will slightly miss the exposure and you'll need to compensate when using the lens wide open.

Of course, you should do this experiment under steady lighting conditions.

I think... 4.5 IS 4.5.... because both pictures are the same... I took a picture at... F5.6 1/60 and then a F4.5 1/125. The 1/125 one seems darker.

But when I set it to Av again (I did M for the second picture) it changed it to 1/100 on 4.5, I compared the first and this one and it is the same exposure level.

I cleaned the contacts and it still seems to be saying the same thing. Also... on the image data viewed on the Pentax utility, is it normal to have a third party lens such as my Sigma not to show it's name?
06-24-2010, 06:53 PM   #13
Loyal Site Supporter




Join Date: Mar 2009
Location: Southern Indiana
Photos: Gallery
Posts: 14,963
Yes, Sigma doesn't pay licensing fees and so they use their own number to mark the lens, but Pentax's software won't recognize it. If you use Adobe, it may recognize it, depending on how new the lens is and how new your version of the software is.
06-24-2010, 07:07 PM   #14
Veteran Member




Join Date: Feb 2010
Location: Scarborough, Ontario
Photos: Albums
Posts: 488
Original Poster
QuoteOriginally posted by Rondec Quote
Yes, Sigma doesn't pay licensing fees and so they use their own number to mark the lens, but Pentax's software won't recognize it. If you use Adobe, it may recognize it, depending on how new the lens is and how new your version of the software is.
>;( is that just for the name of the lens? or also the Aperture readings?

Also... it's F4.0 RIGHT before 70... but when you get to 70... BAM they add that .5
06-24-2010, 07:30 PM   #15
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,314
QuoteOriginally posted by NecroticSoldier Quote
>;( is that just for the name of the lens? or also the Aperture readings?

Also... it's F4.0 RIGHT before 70... but when you get to 70... BAM they add that .5
someone commented that this may be also a function of focusing distance, does the aperture change if you focus to infinity and read F4? I did not see a response to that. Also how close to 70 does it get to and remain at F4?

I would be interested. it may be they are playing on permissable tolorances, and if so, that would be too bad, i.e. at 65 its F4 but at 70 it is 4.5 . Also can you tell us the filter diameter, and the diameter of the front element.

simply put an F4 lens at 70mm should be a diameter of about 17.5mm only.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
50mm, k-mount, pentax lens, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
True ISO of K-X Eagle_Friends Pentax DSLR Discussion 6 04-04-2010 06:04 AM
Is it true? graphicgr8s General Talk 14 09-12-2009 08:12 AM
True to form graphicgr8s General Talk 25 07-09-2009 01:58 PM
Can this be true? rburgoss Photographic Technique 4 09-09-2008 10:25 AM
Is it true?! madisonphotogrl Pentax DSLR Discussion 76 03-19-2008 05:09 PM



All times are GMT -7. The time now is 08:00 PM. | See also: NikonForums.com, part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top