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09-23-2008, 08:12 PM   #1
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250mm Gives Different Angle of View???

I was playing around with my 2 Tamron Lenses 18-250mm, 70-300mm.

I noticed something very strange!!!

Both pics taken by each lens at 250mm from the same distance with K20D on a tripod.

The 70-300mm gives a much much narrower angle of view! In other words, it seems lot longer.

And yes,I also tried 70mm,100mm 135mm... THE SAME Difference!

Could anyone enlighten me on this?

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09-23-2008, 09:10 PM   #2
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Depth of field is a lot narrower in the right hand shot, leading me to believe its closer to its subject (assuming both lenses are set to 250).

If you're certain the camera hasn't moved, then perhaps the markings on the 300 are off, leading you to believe its set to 250 when it looks to be wider.

To be sure, set it all up again, take a shot with each wide, middle and tele in the same order and check again.


Thats all the Sherlock I got.
09-23-2008, 09:20 PM   #3
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.

The 18-250 is notorious for having a true 250mm only at infinity - at anything closer, you actually only get ~ 175 - 220mm. (There are numerous threads about this.)

So - you're not crazy!


.
09-23-2008, 09:27 PM   #4
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The camera was on a 200 worth Tripod, so it couldn't have moved unless there had been a earthquake in the space of that 30 seconds.

This is a not a difference of DOF,its the angle of view. Both lenses at 250mm, the 70-300mm seems to reach a lot longer. I know the 70-300mm is a FF, but isn't 250mm always the SAME angle of view??

Anyone here has the 2 two lenses so you can try like I did? A tripod is not really neccessary,coz the difference is so remarkable that I only needed to see through the viewfinder to see it.

09-23-2008, 09:34 PM   #5
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EXIF is removed in the images ... what does the focal length saved in EXIF have to say about the focal length used in each image?
09-23-2008, 10:01 PM   #6
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QuoteOriginally posted by Snapshot12 Quote
The camera was on a 200 worth Tripod, so it couldn't have moved unless there had been a earthquake in the space of that 30 seconds.

This is a not a difference of DOF,its the angle of view. Both lenses at 250mm, the 70-300mm seems to reach a lot longer. I know the 70-300mm is a FF, but isn't 250mm always the SAME angle of view??

Anyone here has the 2 two lenses so you can try like I did? A tripod is not really neccessary,coz the difference is so remarkable that I only needed to see through the viewfinder to see it.
See JSherman999's message above...

Some lenses only have their true zoom range when focused to infinity. To put it another way...250 on the lens barrel is only 250 if the focus ring is set to infinity. At closer focus some lenses automagically move into their wide-angle range. May I guess that the subject was less than ten feet away from the tripod?

Repeat your test using a distant object and see if you still have the discrepancy.

Steve

Last edited by stevebrot; 09-23-2008 at 10:11 PM.
09-24-2008, 11:41 AM   #7
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Here's what I found:

At 9 feet, the 18-250 has the same FOV as the 55-300 at 160 mm
At 30 feet, the 18-250 has the same FOV as the 55-300 at 200 mm
At about a mile, the 18-250 has the same FOV as the 55-300 at 230 mm

Picasa Web Albums - Dan - FL compare
09-24-2008, 06:37 PM   #8
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QuoteOriginally posted by audiobomber Quote
Here's what I found:

At 9 feet, the 18-250 has the same FOV as the 55-300 at 160 mm
At 30 feet, the 18-250 has the same FOV as the 55-300 at 200 mm
At about a mile, the 18-250 has the same FOV as the 55-300 at 230 mm

Picasa Web Albums - Dan - FL compare
Good post! That's an interesting comparison. I'd heard of the FOV differences but no one has demonstrated it like you did. Thanks!

09-24-2008, 07:27 PM   #9
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It's essentially why a 18-250 is so silly. Why not just do a solid 16-105 and be done with it.
09-24-2008, 07:47 PM   #10
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Cheers for some of your input ,just to put all of us out of misery, I got a reply from Tamron today--


That is because they are internal focus lenses in which the inside elements rotate a move to achieve focus and per industry standards, if compared with each other, you have to set the focus to infinity. This allows for the variable of where the elements may be to be eliminated.

Tamron USA


BTW, Tamron only has a distributer in the UK,and of course no reply from them yet

Just wonder if this applies to all the other 'IF' Zoom Lenses...or just Tamrons.
09-24-2008, 07:56 PM   #11
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QuoteQuote:
I got a reply from Tamron today--


That is because they are internal focus lenses in which the inside elements rotate a move to achieve focus and per industry standards, if compared with each other, you have to set the focus to infinity. This allows for the variable of where the elements may be to be eliminated.

Tamron USA

"rotate a move to achieve". "may be to be eliminated".

What language is this intended to be?
09-24-2008, 08:13 PM   #12
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QuoteOriginally posted by SpecialK Quote
"rotate a move to achieve". "may be to be eliminated".

What language is this intended to be?
I think its intended just to make a fool of all of us...

But I think I got the picture.
09-24-2008, 08:33 PM   #13
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Zoom lens of the internal focus type Document Type and Number:United States Patent 5136431

Abstract: A zoom lens is disclosed comprising a front lens unit, an intermediate lens unit having two lens sub-units with their axial separation varying for zooming purposes, and a rear lens unit, wherein at the wide-angle end, focusing is performed by moving at least the rear lens unit, and, at the telephoto end, focusing is performed by moving the intermediate lens unit and the rear lens unit, whereby the amount of focusing movement of the intermediate lens unit from an infinitely distant object to an object at a certain short distance is greater at the telephoto end than at the wide-angled end.


Zoom lens of the internal focus type - Patent 5136431
09-25-2008, 07:34 AM   #14
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Snapshot... so for everyone's edification. When you do focus on a near-field object of about 10 feet, zoomed @ 250mm, what does EXIF actually say the focal length was? 250mm or around 150mm?
09-25-2008, 07:45 AM   #15
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QuoteOriginally posted by m8o Quote
Snapshot... so for everyone's edification. When you do focus on a near-field object of about 10 feet, zoomed @ 250mm, what does EXIF actually say the focal length was? 250mm or around 150mm?
250mm .
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