Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
03-04-2008, 12:18 PM   #1
New Member
mito's Avatar

Join Date: Dec 2007
Posts: 14
Pentax 18-250 is a 18-180!

Hi guys,
While trying my Pentax 18-250 lens, I found out that the advertised focal length does not correspond with the actual focal length. The lens is something like a 18-180 (which is still quite amazing, but not what I paid for...).
Here are some quick and dirty shots that prove my claim. All pictures are taken with a tripod, f/8, about 5m far from the subject. The height of the crackers box is about 30cm.
The first one is taken with a Quantaray 100-300mm (which is a rebadged Sigma lens) with a focal length of 250mm.
The second shot is taken with the Pentax 18-250 at the same focal length, while the third is again from the Quantaray but at 200mm. Note that the size of the subject in the third picture is still bigger than the size of the subject taken with the Pentax lens
At 100mm the differences between the two lenses are not so evident.

Of course the advertised focal length of the Quantaray lens may be wrong, that's why I'd like to hear from other 18-250 owners if they can reproduce my measurements.

I did not alter the Exif data in the pictures and I can provide more examples if anybody asks.

Attached Images
View Picture EXIF
PENTAX K10D  Photo 
View Picture EXIF
PENTAX K10D  Photo 
View Picture EXIF
PENTAX K10D  Photo 
03-04-2008, 01:19 PM   #2
Pentaxian
jsherman999's Avatar

Join Date: Dec 2007
Photos: Gallery
Posts: 8,228
.

I have noticed the same thing. Somewhere, I forget where, I read something that gave a bit of an explanation - basically, at say 200 meters, you get the full 250mm, but closer in, you get less. I forget why, or where I read it right now....



.
03-04-2008, 01:19 PM   #3
Senior Member




Join Date: Sep 2007
Location: Vancouver, BC, Canada
Posts: 131
I believe the focal lengths are advertised as when focus is at infinity.

At least on photozone, I think, they mentioned that the focal length gets shorter when focusing closer distances.
03-04-2008, 01:25 PM   #4
New Member
mito's Avatar

Join Date: Dec 2007
Posts: 14
Original Poster
Both pictures were taken from the same distance, so it should not make much difference. I will post the picture with focus at infinity as soon as I come back home

03-04-2008, 01:43 PM   #5
Senior Member




Join Date: Nov 2007
Location: Moscow, Russia
Posts: 239
The optical scheme of THIS lens (18-250) acts like this at close focus distances. On macro shots it's focal length is like 150mm only. The further the object gets from the lens, the closer to 250mm it's real focal length gets.
Not all tele lenses act this way, but this one does.
You can check it by shooting a really distant object with these two lenses.
02-23-2009, 07:41 PM   #6
Loyal Site Supporter
charliezap's Avatar

Join Date: Feb 2009
Location: saugus ma
Photos: Gallery | Albums
Posts: 8,153
i just got the pentax 18-250. it falls short as advertised. divide 250 by 18 and you get a factor of 13.888. i took a picture at 18mm and at 250mm and printed them out and measured an object in the shots with a machinist scale and eye loupe.the factor in the shots came out as a 10.185 differential which with my math puts it at 183.25mm--not 250mm.to be fair i will do the same comparison at a greater distance but i still think that this lens will come up short.my quantaray lens set at around 178mm gives me the same size image as the pentax set at 250mm. i think mito is spot on.
02-23-2009, 07:58 PM   #7
Site Supporter
jamesk8752's Avatar

Join Date: Dec 2006
Location: Suburban Detroit, MI, USA
Posts: 226
It's well established that zoom lenses which have internal focusing may show significant reduction of their focal length when focused on targets shorter than infinity. Many if not all IF zooms show this behavior; it may be more pronounced in the 18-250 because of the extreme zoom factor of this lens. I've noticed it in many of my Pentax zooms.

In other words, this is expected behavior for this lens, and you got what you paid for - you just didn't know it!
02-23-2009, 08:16 PM   #8
Veteran Member
arbutusq's Avatar

Join Date: Sep 2006
Location: Vancouver BC canada
Photos: Gallery
Posts: 380
I believe most zoom lenses behave like this. My old Nikon 18-70 was only around 55 mm at close distances and 70 mm at long distances and infinity. I haven't tested any of my Pentax zooms though, can't be bothered.

02-23-2009, 08:28 PM   #9
Loyal Site Supporter
charliezap's Avatar

Join Date: Feb 2009
Location: saugus ma
Photos: Gallery | Albums
Posts: 8,153
i live just a 1/4 mile from rte 1.i usually check out new cameras and lens by shooting from the overpass which gives me over a one mile look in either direction. the highway has many signs and i like to use the bridge to check resolution on my equipment. i will take some shots at infinity with the pentax 18-250 and do the math to see what the figures tell me.i know that the lens isn't big bucks but the quality isn't too bad. i liked the quantary 70-300 which i have since sold when i sold my pentax k10d. i may repurchase the quantaray.it got me picture of the week in parade magazine 2 years ago when i took a picture of an airliner flying by the moon.you can actually see the craters on the moon with the airliner with contrails to its left.it was a luck shot taken during the day with a deep blue sky.
02-23-2009, 10:34 PM   #10
Veteran Member
frank's Avatar

Join Date: Nov 2006
Location: Singapore
Photos: Gallery
Posts: 3,202
James is right on this one. If you shoot something far away, like a building several hundred meters away, then you compare both, you'll see the 'actual' focal length of this 18250 lens :0 When you focus at close target, the focal length becomes shorter.
02-24-2009, 12:57 AM   #11
Veteran Member
PentaxPoke's Avatar

Join Date: Jan 2008
Location: Oklahoma
Posts: 1,411
mmmmm crackers...
02-24-2009, 01:45 AM   #12
Site Supporter
Vylen's Avatar

Join Date: Dec 2008
Location: Sydney, Australia
Photos: Gallery
Posts: 1,248
out of curiosity, and if anyone knows, is it the same with the 55-300mm?

since i dont have a 300mm lens other than that one, i cant try it out for myself
02-24-2009, 02:25 AM   #13
Veteran Member
frank's Avatar

Join Date: Nov 2006
Location: Singapore
Photos: Gallery
Posts: 3,202
QuoteOriginally posted by Vylen Quote
out of curiosity, and if anyone knows, is it the same with the 55-300mm?

since i dont have a 300mm lens other than that one, i cant try it out for myself
55-300 isn't an IF lens, and it's not a mag zoom lens either. Normally this is more obvious to IF and mag zoom lenses

I've done a quick comparison between the DA18250 and DA50200, the subjects taken by DA50200 in close range look quite a bit bigger than those by the DA18250 at the same focal length.

Here is one set of the comparison results, DA18-250 @ 200mm F8 VS DA50-200 @ 200mm F8 (center crop, camera on tripod):

02-24-2009, 03:41 AM   #14
Site Supporter
Vylen's Avatar

Join Date: Dec 2008
Location: Sydney, Australia
Photos: Gallery
Posts: 1,248
QuoteOriginally posted by frank Quote
55-300 isn't an IF lens, and it's not a mag zoom lens either. Normally this is more obvious to IF and mag zoom lenses

I've done a quick comparison between the DA18250 and DA50200, the subjects taken by DA50200 in close range look quite a bit bigger than those by the DA18250 at the same focal length.

Here is one set of the comparison results, DA18-250 @ 200mm F8 VS DA50-200 @ 200mm F8 (center crop, camera on tripod):

hm yea, i figured but i just wanted to make sure
02-24-2009, 06:33 AM   #15
Veteran Member




Join Date: Mar 2007
Location: Toronto (for now)
Posts: 1,749
All the more reason why those muppets saying the 18-250 covers the 55-300 aree just plain wrong. It barely covers the 50-200.
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!
k-mount, length, lens, pentax, pentax lens, pictures, quantaray, size, slr lens, subject, third
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Sigma 18-250 or Pentax/Tamron 18-250? mjbens01 Pentax SLR Lens Discussion 3 07-08-2010 09:10 PM
Pentax 18-250 always same as Tamron 18-250? kitkat Pentax SLR Lens Discussion 12 07-08-2010 08:30 AM
1/250 HSS vs 1/180 X-sync Prognathous Pentax Camera and Field Accessories 7 06-01-2009 12:25 PM
Leitz Elmarit-R 1:2.8/180 and Pentax K20D Piotr Pentax SLR Lens Discussion 8 02-18-2009 07:09 AM



All times are GMT -7. The time now is 07:31 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