Pentax 18-250mm Vs. Tamron 70-300mm, focal length markings are not the same

aweir · 04-04-2009, 08:51 PM

I was doing some test shots of both lenses at the same focal length and I noticed that 70mm on the Pentax was alot wider than 70mm on the Tamron. and likewise 250mm on the Pentax was ALOT wider than the 250mm on the Tamron.

For having the same focal length, why is the Pentax so much wider? I thought bother were built for the 1.5X crop factor, so shouldn't focal lengths be generally consistent?

poco · 04-04-2009, 09:19 PM

Originally posted by aweir

I was doing some test shots of both lenses at the same focal length and I noticed that 70mm on the Pentax was alot wider than 70mm on the Tamron. and likewise 250mm on the Pentax was ALOT wider than the 250mm on the Tamron.

For having the same focal length, why is the Pentax so much wider? I thought bother were built for the 1.5X crop factor, so shouldn't focal lengths be generally consistent?

The markings are made at infinity for IF lenses. With an IF lens, a shorter subject distance will move the focus mechanism enough to shorten the effective focal length. I have seen posts that state the 18-250mm, when used at its closest focus distance and at 250mm, in the "macro' mode, is effectively a 90mm lens.

04-04-2009, 08:51 PM	#1
aweir Forum Member Join Date: Jun 2008 Posts: 58	Pentax 18-250mm Vs. Tamron 70-300mm, focal length markings are not the same I was doing some test shots of both lenses at the same focal length and I noticed that 70mm on the Pentax was alot wider than 70mm on the Tamron. and likewise 250mm on the Pentax was ALOT wider than the 250mm on the Tamron. For having the same focal length, why is the Pentax so much wider? I thought bother were built for the 1.5X crop factor, so shouldn't focal lengths be generally consistent?

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04-04-2009, 09:19 PM	#2
poco Senior Member Join Date: Sep 2008 Posts: 153	Originally posted by aweir I was doing some test shots of both lenses at the same focal length and I noticed that 70mm on the Pentax was alot wider than 70mm on the Tamron. and likewise 250mm on the Pentax was ALOT wider than the 250mm on the Tamron. For having the same focal length, why is the Pentax so much wider? I thought bother were built for the 1.5X crop factor, so shouldn't focal lengths be generally consistent? The markings are made at infinity for IF lenses. With an IF lens, a shorter subject distance will move the focus mechanism enough to shorten the effective focal length. I have seen posts that state the 18-250mm, when used at its closest focus distance and at 250mm, in the "macro' mode, is effectively a 90mm lens.