Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
05-09-2011, 07:39 PM   #1
New Member




Join Date: May 2011
Posts: 5
When 200mm isnt, really...

So... I have a 50-200mm dal kit lens, and I got my hands on a 18-200mm promaster, which is a rebadge of the tamron lens of the same specs.
Thing is, the view through the 200mm end of the kit lens is a good deal closer than the 200mm end of the 18-200mm... see below:



as opposed to:




and, just for kicks, here's both of them at 50mm (or 48mm on the promaster - couldnt quite get it to 50mm...)




vs




Is this a result of some immutable law of optics im unaware of?

I look forward to your wise and learned responses.

Thanks!

05-09-2011, 08:39 PM   #2
Site Supporter
SpecialK's Avatar

Join Date: Dec 2006
Location: So California
Photos: Gallery
Posts: 14,622
I believe focal length is measured when focused to infinity. As you focus closer, the FL changes, at least on some lenses, and that change might be different from lens to lens.
05-09-2011, 08:50 PM   #3
Pentaxian
twitch's Avatar

Join Date: Nov 2009
Posts: 4,571
I saw a post about the Nikon 18-200 fully extended being only 135mm at close focus distances, so it's a common thing. Supposedly internally focusing lenses are more prone to this.
05-10-2011, 01:00 AM   #4
Site Supporter
Stone G.'s Avatar

Join Date: Jun 2010
Location: North Zealand, Denmark
Photos: Gallery | Albums
Posts: 1,514
It is far from unusal that FL decreases at close focusing distances. This depends upon the specific lens designs (e.g.: Internal Focus and/or Fixed Rear Elemements) and thus, may even happen with certain primes. See for example Dojidar Dimitrov's Pentax K-Mount Page:

Focal Length Experiments

As another example I have measured the actual FL of my Tamron SP 35-80 mm CF Macro lens (Model 01A) at closest focusing distance and at the 80 mm macro setting. I find the actual FL to be 55 rather than 80 mm - see "Example 1" at the bottom of this page:

http://www.gyes.eu/photo/x-macro.htm


Last edited by Stone G.; 05-10-2011 at 01:07 AM.
05-10-2011, 02:59 AM   #5
Pentaxian
aurele's Avatar

Join Date: Jan 2011
Location: Paris, France
Photos: Albums
Posts: 2,917
i guess, you try both of your lense, with your Dslr on a tripod.

it's a bad idea

to measure well that kind of thing, it is not the Dslr that need to not move, it's the part of the lense that is the closest to the subject.

i guess the 2nd lense come out much more than the first one.
05-10-2011, 06:11 AM   #6
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,345
The previous posts regarding focal length are correct. focal length is only measured at infinity focus.

The posts are also correct that depending upon design, specifically internally focused lenses and fixed rear element lens designs, the focal length can change considerably when focused close.

This is especially true of super range zooms, i.e. lenses with more than 5-10 x focal length ranges.

It is also true of many zooms which can do macro, as you will find that the maximum magnification ratio is only at minimum focal length (somewhat counter intuitively)

I started a post for people to report the true focal lengths of Internally focused lenses at minimum focus, but it sort of died.
05-10-2011, 04:12 PM   #7
New Member




Join Date: May 2011
Posts: 5
Original Poster
thanks, all. i guess i just wanted to make sure that there wasnt something wrong with the lens.
as regards the note about internal focus making a difference, that explains why a different lens i tried, a 28-210mm, also seems much closer at 210mm than the kit lens is at 200mm.

thanks!
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!
200mm, flickr, k-mount, kit, lens, pentax lens, promaster, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
HELP my 540 isnt working HELP!!!! Death2man Pentax Camera and Field Accessories 3 01-09-2010 02:30 PM
Isnt PP = Post Processing or NOT? BBear Photographic Technique 15 07-01-2008 10:00 AM



All times are GMT -7. The time now is 03:43 AM. | See also: NikonForums.com, CanonForums.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