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08-15-2011, 05:28 AM   #1
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Tamron AF 18-250mm F/3.5-6.3 Di II LD is it really 250 in the long end

hi guys

today i got a tamron 18-250mm but i also have a pentax 55-300mm. my question is why does taking pics from the same position at 250mm the pentax lens seems to produce more zoomed images.is the tamron really 250mm

thankz

08-15-2011, 05:46 AM   #2
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I've asked a similar question before. My 70 ltd is not as zoomed in as my Tamron 70-300 at 70. The replies of most people are that there are some tolerances in marking the focal lenght.
08-15-2011, 05:59 AM   #3
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The 18-250mm is 250mm, but only when it's focussed to infinity. Internal focus lenses move internal elements which widens the effective field of view as you decrease the subject distance. I measured vs my 55-300 once, and found:
- at approximately one mile, fov on the 18-250 was about 220mm
- at 30 feet, fov was 200mm
- at 9 feet, fov was 160mm
08-15-2011, 06:06 AM   #4
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QuoteOriginally posted by audiobomber Quote
of view as you decrease the subject distance. I measured vs my 55-300 once, and found: - at approximately one mile, fov on the 18-250 was about 220mm - at 30 feet, fov was 200mm - at 9 feet, fov was 160mm
where these at 250mm zoom position and what was the result for the 55-300mm

thankz

08-15-2011, 08:08 AM   #5
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QuoteOriginally posted by nirVaan Quote
where these at 250mm zoom position and what was the result for the 55-300mm

thankz
The comparison was between the 55-300 and the 18-250mm. The 55-300 is not an internal focus lens, so its focal length does not change significantly with subject distance.

I shot the 18-250 only at 250mm, selecting successfully closer targets. I then shot the same targets with the 55-300mm, selecting a focal length to match the field of view that I was getting from the 18-250. So for example at 30 feet subject distance, a focal length setting of 250mm on the 18-250, corresponded to a focal length of 200mm on the 55-300mm*. If this isn't clear, you can check it with your lenses.

To your original question, is the tamron really 250mm?
1. Yes, but only at infinite subject distances
2. No, it has a wider fov than 250mm at any focal distance below infinity.

*The 55-300 won't actually register 200mm, but the size of the 18-250 image was in between the 210 and 190mm focal lengths on the 55-300mm.
08-15-2011, 08:57 AM   #6
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There was/is a similar recent thread about "focal length ripoff" -- but it's not that lensmakers are trying to take advantage of user ignorance, just that each different optical design has their own character (flaws). Maybe some website somewhere has a database of curves of different zooms, showing their nominal vs actual FL's at various distances. Myself, I don't worry about it, not for casual shooting. If a specific focal length is mandatory, I use a prime. If I want to fill the frame with a composition, a zoom (no matter its optical quirks) will suffice.
08-15-2011, 09:23 AM   #7
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..maybe rather outta question.. How 'noticable' is the "50mm" difference between 18-200 & 18-250 lens? I'm planning to buy one of these..
08-15-2011, 09:45 AM   #8
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QuoteOriginally posted by RioRico Quote
Myself, I don't worry about it, not for casual shooting. If a specific focal length is mandatory, I use a prime. If I want to fill the frame with a composition, a zoom (no matter its optical quirks) will suffice.
It is misleading though. Most people look at the 18-250mm and expect they're getting a lens that's longer than the 50-200 and almost as long as the 55-300. In fact within normal bird length distances with this type of lens, say 10-30 feet, the 18-250 is barely as, or not even as long as, the 50-200.

People constantly compare the 18-250 with the 55-300 as a telephoto, but there's a large difference in fov at close and mid distances.

08-15-2011, 10:17 AM   #9
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The 18-250 or any zoom is (hopefully) just as specified: it is that focal length at infinity focus. When I set a 100-300mm or 70-210mm zoom to 200mm, or use a 200mm prime, and focus to infinity, I see the same FOV (within tolerance) as with the 18-250 @200mm. 'Twould be nice if a lensmaker or tester posts graphs of nominal vs actual FL at various distances. I'd like to see such.

But any variance revealed by such a graph won't bother me when I shoot. I make mental and physical adjustments -- zoom or move differently, frame the shot differently, whatever. Maybe I'm forgiving because 1) I'm lackadaisical|slothful, 2) I just care about results (which can be cropped), and/or 3) I've trained myself with the Schneider Betavaron 50-125mm fixed-focus enlarger zoom -- changing focus means changing focal length AND position. Using that in the field leaves little opportunity for measurement concerns.

I have bigger fish to fry, as it were.
08-15-2011, 12:57 PM   #10
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Thanks for the insight. this is off topic but how do you focus at infinity while on auto focus
08-15-2011, 01:17 PM   #11
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QuoteOriginally posted by nirVaan Quote
Thanks for the insight. this is off topic but how do you focus at infinity while on auto focus
Put the camera on manual and move the focus ring to the end.
08-15-2011, 07:29 PM   #12
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QuoteOriginally posted by audiobomber Quote
The 18-250mm is 250mm, but only when it's focussed to infinity. Internal focus lenses move internal elements which widens the effective field of view as you decrease the subject distance. I measured vs my 55-300 once, and found:
- at approximately one mile, fov on the 18-250 was about 220mm
- at 30 feet, fov was 200mm
- at 9 feet, fov was 160mm
Dan, can you repost the test you did of the two? The images were very educational.
08-15-2011, 07:53 PM   #13
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QuoteOriginally posted by rfortson Quote
Dan, can you repost the test you did of the two? The images were very educational.
I'm sorry that I deleted it. I may have it somewhere in an external hard drive, but I won't be able to look before the weekend.
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