Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
12-04-2014, 11:39 AM   #1
Loyal Site Supporter
UncleVanya's Avatar

Join Date: Jul 2014
Photos: Gallery | Albums
Posts: 9,513
When 135mm isn't...

Today as part of some ongoing play time with my K50 I took some shots using my DA 18-135, my F100 Macro, and my Takumar Bayonet 135 f2.5

Without moving the tripod or changing settings I took a shot of a lamp with the Takumar and the DA set to 135mm. Imagine my surprise when I saw that the composition was entirely different. I realize the centering of the main subject is different - the head was free to rotate on the tripod when I did this so I wasn't surprised by that. The field of view differences however were startling. I have heard about focus breathing and I know people have complained that some of the larger zooms (60-250 I think is one) exhibit this - and this was at around 5' not a long range shot at all. I assume what I am seeing is focus breathing but I plan to take a couple of infinity shots later today to verify that.

In any case, the 18-135 produced a wider Field of View than the Takumar for this test.

Attached Images
View Picture EXIF
PENTAX K-50  Photo 
View Picture EXIF
PENTAX K-50  Photo 
12-04-2014, 12:02 PM   #2
Pentaxian
LensBeginner's Avatar

Join Date: Sep 2014
Photos: Albums
Posts: 2,490
Remember than all focal length number are approximations.
The Limited by Pentax are unique in that they really are that precise focal length (and hence the fact that those numbers are sometimes "strange" e.g. 77mm... )
I believe one could well be a 131mm and the other a 138mmm...
Add some other factors like focus breathing and you've got a plausible explanation.
12-04-2014, 12:04 PM   #3
Ace
Junior Member




Join Date: Jul 2013
Posts: 38
Wow, that is a shocking difference. I have read about focus breathing but assumed it would create a very minor change in field of view. I look forward to seeing your infinity shoots.
12-04-2014, 12:05 PM - 1 Like   #4
Site Supporter
boriscleto's Avatar

Join Date: Dec 2009
Location: Liverpool, NY
Photos: Gallery
Posts: 12,297
The 18-135 is an internally focusing lens. It will only be at its maximum focal length at infinity focus.

12-04-2014, 12:39 PM   #5
Pentaxian




Join Date: Sep 2013
Location: Pasadena, CA
Photos: Albums
Posts: 1,103
Could be worse. The new Nikon 70-200mm is actually 130mm at the long end at short distances.
12-04-2014, 01:25 PM   #6
Loyal Site Supporter
narual's Avatar

Join Date: Jun 2013
Location: South Bend (Notre Dame), Indiana
Posts: 1,935
You have to be farther away before they'll match. Screwy, but that's how they can make them so small.
12-04-2014, 01:53 PM   #7
Pentaxian
LensBeginner's Avatar

Join Date: Sep 2014
Photos: Albums
Posts: 2,490
QuoteOriginally posted by rrstuff Quote
Could be worse. The new Nikon 70-200mm is actually 130mm at the long end at short distances.
???
I'd call that false advertising!!!
12-04-2014, 02:01 PM   #8
Loyal Site Supporter
UncleVanya's Avatar

Join Date: Jul 2014
Photos: Gallery | Albums
Posts: 9,513
Original Poster
QuoteOriginally posted by LensBeginner Quote
Remember than all focal length number are approximations.
The Limited by Pentax are unique in that they really are that precise focal length (and hence the fact that those numbers are sometimes "strange" e.g. 77mm... )
I believe one could well be a 131mm and the other a 138mmm...
Add some other factors like focus breathing and you've got a plausible explanation.

I wonder where we could get this info and if we could add this to the list of data collected here about lenses in the user database.

12-04-2014, 02:31 PM   #9
Loyal Site Supporter
UncleVanya's Avatar

Join Date: Jul 2014
Photos: Gallery | Albums
Posts: 9,513
Original Poster
Looks like Focus Breathing is the culprit. Here's a couple of shots made at about 30-35' - nothing fancy and forgive the focus on the Takumar I was in a hurry between work calls and live view isn't all that easy sometimes depending on subject matter. Another oddity - the fact is that I used manual mode and used the green button to do stop down metering. The light did not change between the shots - there is that much difference between how they meter on the same body in the same conditions.
Attached Images
View Picture EXIF
PENTAX K-50  Photo 
View Picture EXIF
PENTAX K-50  Photo 
12-04-2014, 02:39 PM   #10
Pentaxian
LensBeginner's Avatar

Join Date: Sep 2014
Photos: Albums
Posts: 2,490
QuoteOriginally posted by bradshea Quote
I wonder where we could get this info and if we could add this to the list of data collected here about lenses in the user database.
I don't recall where I read this, but I believe it was on a Dpreview thread. Naturally I can't find it now that I (we) need it...
12-04-2014, 02:58 PM   #11
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 26,202
QuoteOriginally posted by bradshea Quote
In any case, the 18-135 produced a wider Field of View than the Takumar for this test.
QuoteOriginally posted by boriscleto Quote
The 18-135 is an internally focusing lens. It will only be at its maximum focal length at infinity focus.
What he said...

Focal length is measured at infinity. Both of your lenses are 135mm when focused at the horizon. That being said, there is a difference between how focus is done for internal vs. external focus lenses. With external, focus is attained by extension (effective focal length is increased). With internal focus lenses focus is attained by changing the relative position of internal elements with NO change to effective focal length. As a result, internal focus lenses typically shoot WIDER at moderate to close focus than an external focus lens of the same nominal focal length.

The difference can be fairly substantial. I was shocked when I found that my Sigma 17-70/2.8-4 (C) at 70mm and 1.5m distance had the same FOV as my DA 50/1.8.


Steve
12-04-2014, 03:58 PM   #12
Loyal Site Supporter
UncleVanya's Avatar

Join Date: Jul 2014
Photos: Gallery | Albums
Posts: 9,513
Original Poster
It's one thing to know this is the case and another to see it in action. I'm glad I had this accidental discovery today it was fun to share it. I think I will try this with my M50 f2 vs. the 18-135 set to 50mm's. I really wish I had an older M or A series that was internally focused to check against to see if that largely removed the differences as Steve indicated it might.
12-04-2014, 04:15 PM   #13
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,314
This issue was first observed with the K28/2 which had a fixed rear element design(the forerunner of internal focusing).

As others have stated the international standards for validating focal length are based upon infinity focus only.
12-04-2014, 06:01 PM   #14
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 26,202
QuoteOriginally posted by Lowell Goudge Quote
As others have stated the international standards for validating focal length are based upon infinity focus only.
Yes, by definition, though it is not validation as much as convention for measurement.

What I find funny is that people don't complain that the FOV of their external focus primes becomes narrower with close focus.


Steve

Last edited by stevebrot; 12-04-2014 at 06:21 PM.
12-04-2014, 06:33 PM   #15
Loyal Site Supporter
UncleVanya's Avatar

Join Date: Jul 2014
Photos: Gallery | Albums
Posts: 9,513
Original Poster
No complaints here - just observations. I repeated the test with 100mm setting and 50mm setting then compared these to my F100 Macro and M50 both of which had larger images (smaller FOV) than the zoom. The difference was nearly non-existent on the 50 at the 4-5' range I was using. The 100mm showed a large difference like 135 had done. All in all a great deal of fun without any real practical implication to the way I shoot.
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!
135mm, camera, da, field, focus, pentax help, photography, shot, shots, takumar, tripod, troubleshooting, view
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Abstract Life isn't fair! robjmitchell Post Your Photos! 8 11-03-2014 06:57 PM
Why GoProís Success Isnít Really Isn't about the Cameras interested_observer General Photography 16 07-01-2014 05:05 PM
Nature Isn't she gorgeous Panteg Post Your Photos! 13 07-22-2013 08:16 AM
What to do when 1:1 Macro isn't quite close enough pentaxman Pentax SLR Lens Discussion 14 11-29-2008 05:14 PM
Camera cleaning - When the dust isn't on the sensor? cputeq Pentax DSLR Discussion 9 05-17-2008 11:08 PM



All times are GMT -7. The time now is 04:22 AM. | 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