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03-25-2010, 02:13 PM   #1
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Newbie question regarding lenses

Hello, i've been using my Pentax K-M for a few months now and decided to purchase a Tamron superzoom (18-250) to replace the very good, but limited range, Pentax Kit lens (18-55) and my old Takumar (70-200) zoom.

Before selling the takumar, i decided to compare both lenses. I noticed with the new Tammon, when set to 250mm, covers a wider area than the old Takumar, at 200mm, in other words, the Takumar zooms "further" than the Tamron eventhough the Takumar it's only 200mm. I would say that if the Tamron covers 6 degrees, the old Takumar covers approx 4 degree, at least 1.5x more zoom power, as if the takumar was actually 300 mm and not 200.

I know that regarding camera designs, a lens mounted on a digital camera is about 1.5x more thant on a film camera. But... Different zooms on the same body should be the same?

A bit confused... So is my Takumar a 300mm or the Tamron not really a 250mm?


Thanks
Phil

03-25-2010, 02:18 PM   #2
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The focal length is measured at infinity. Are you comparing the two lenses at infinity focusing?
03-25-2010, 02:47 PM   #3
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QuoteOriginally posted by audiophil Quote

I know that regarding camera designs, a lens mounted on a digital camera is about 1.5x more thant on a film camera. But... Different zooms on the same body should be the same?

A bit confused... So is my Takumar a 300mm or the Tamron not really a 250mm?

I've run into something like this before when comparing a prime lens with the 75-300 Tamron (at a less than infinity distance). I would guess that what's going on is the Tamron is achieving focus at 250mm by shortening the length of the lens. Perhaps that's a necessary compromise in a lens with as great a range as 18-250.
03-25-2010, 03:02 PM   #4
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QuoteOriginally posted by audiophil Quote
I know that regarding camera designs, a lens mounted on a digital camera is about 1.5x more thant on a film camera. But... Different zooms on the same body should be the same?
No no no no no. A 100mm lens is still a 100mm lens, no matter whether it's on a full-frame or half-frame (APS-C dSLR) body. It does not become a 150mm lens when put on your Km. Forget you ever heard of Crop Factor. [/me makes mystic gesture to ward off evil.] Erase "1.5x" from your mind. [/me eats drug.] With a 100mm lens, your Km SEES less of a picture than does my film ZX-M. But the lens remains the same. [/me seeks to hunt down and slaughter whomever coined the term Crop Factor.]

Try this: cut two pieces of paper, each 1 inch x 1 1/2 inches. Now cut one of those in half, so it's now 3/4 inch x 1 inch. Lay that smaller half on top of the bigger piece of paper. That bigger piece is the size of a 35mm film frame. The smaller piece is the size of you Km's APS-C sensor. Now take a magazine or other picture, maybe of someone's head. Lay the bigger paper on it. Now lay the smaller paper on the picture. You haven't changed the focal length; the relationships in the picture remain the same. But with the smaller sensor, you just see less of the picture. That's all.


Last edited by RioRico; 03-25-2010 at 03:09 PM.
03-25-2010, 03:14 PM   #5
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QuoteOriginally posted by RioRico Quote
N

Try this: cut two pieces of paper, each 1 inch x 1 1/2 inches. Now cut one of those in half, so it's now 3/4 inch x 1 inch. all.
Not if I cut it in the direction you don't want me to.

Look! You're so quick and witty that I have to grab these opportunities when they present themselves, okay!?

Your syntax and intention were actually excellent, but since you saved yourself a few keystrokes not explaining which dimension to cut, I had to jump on it.

Yeah--I have no life.
03-25-2010, 03:34 PM   #6
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QuoteOriginally posted by Ira Quote
Not if I cut it in the direction you don't want me to.
You're right. Not if you had cut it the way you wanted. That's why I... oh, forget it. Cut it diagonally. Cut it in spirals. I've seen people making nice little portraits by snipping away with scissors. Of, course, they go blind soon, and not just from self-abuse.

QuoteQuote:
Yeah--I have no life.
Somehow I'm reminded of a line in GHOSTBUSTERS but I'd better not repeat it now.

Meanwhile, I'd like to petition the Word Police to place Crop Factor on the banned list, on Wired's TIRED list, in the file of Phrases Not To Use Any More. Like Moral Majority or Compassionate Conservative or Quantum Meditation or High-Quality Compact Camera. Although the last may be valid, in time. If I live so long.
03-25-2010, 04:48 PM   #7
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Newer lenses often use internal focus instead of moving the whole lens on a helicoid to achieve proper sharpness at a given distance. Those lenses stated focal length is accurate only at infinity. When you move internal elements to achieve proper focus, you alter the focal length, thus explaining the discrepancy you found.
03-25-2010, 05:02 PM   #8
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thank you for the simplest explanation of Crop factor. I will happily show this to my wife and plead my case to buy a new lens I will need a 17 to add to my collection .

03-26-2010, 08:03 AM   #9
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QuoteOriginally posted by SOldBear Quote
The focal length is measured at infinity. Are you comparing the two lenses at infinity focusing?
I did various tests, some with close focus and some at infinity focus, same results.


QuoteOriginally posted by RioRico Quote
No no no no no. A 100mm lens is still a 100mm lens, no matter whether it's on a full-frame or half-frame (APS-C dSLR) body. It does not become a 150mm lens when put on your Km.

Sorry for my bad english, what i meant is the 35mm equivalent focal length. Say a 100mm lens mounted on a DSLR has a 35mm equivalent of a 150mm. At least that is what i read in all reviews.

But in my case, two different lenses with the same focal gives me different results on the same body...
03-26-2010, 08:07 AM   #10
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QuoteOriginally posted by SOldBear Quote
The focal length is measured at infinity. Are you comparing the two lenses at infinity focusing?

Yes i did tests at infinity and close focus with same results
03-26-2010, 08:12 AM   #11
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QuoteOriginally posted by RioRico Quote
No no no no no. A 100mm lens is still a 100mm lens, no matter whether it's on a full-frame or half-frame (APS-C dSLR) body. It does not become a 150mm lens when put on your Km. .

Sorry for my bad english. What i meant to say is the 35mm equivalent of a lens mounted on a DSLR camera, i.e a 200 mm is equivalent to 300 mm. At least from what i read in the reviews. I understand that is is due to a crop factor and that the lens remains a 200mm where ever it's mounted on!!!

But in my case, two different lenses set at 200mm, on the same body, gives me different results.

Last edited by audiophil; 03-26-2010 at 08:20 AM.
03-26-2010, 10:11 AM   #12
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QuoteOriginally posted by audiophil Quote
But in my case, two different lenses set at 200mm, on the same body, gives me different results.
That is strange. I have the Takumar A 70-200 and the Pentax FA 18-250, which is nearly identical to the Tamron 18-250, but I haven't compared their coverage. I will do that right now. [I go outside and test the lenses, also a Pentax FA 100-300 and a Super-Takumar 200/4.]

I could not test at infinity focus because I can not see infinity in this forest, but looking at 25m and 50m, all these lenses at 200mm seem to have the same field of view (FOV). If your Tamron can not get a smaller FOV than your old Takumar, it is possible that the Tamron isn't extending all the way out to 250mm.

Lens specifications say the FOV at 200mm should be 8 degrees, and at 250mm should be 6.5 degrees. If you are seeing a 1.5x difference, or about 12 degrees, that means the Tamron is only extending out to 135mm. A 300mm lens has a FOV of 5 degrees. I don't know if there was ever a Takumar zoom that went out to 300mm; I don't see any in the lens database. Perhaps your Tamron is faulty or damaged. It should certainly reach a smaller FOV than your Takumar. Bon chance!
03-26-2010, 10:25 AM   #13
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Can you post samples shot at infinity focus? The phenomenon you describe is very well known at close distances, but shouldn't be an issue at infinity. But I've never used the 18-250 and can't say how it compares in practice. Some small discrepancy is to be correct - like maybe the actual focal length is 243mm instead of 250mm - but no way should it be coming out less than 200 at infinity.
03-26-2010, 10:27 AM   #14
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Last year I've noticed the same thing as Audiophil when comparing the Tamron 28-300 and Pentax 80-320 on same camera (K20D) . The Tamron when at 300mm gave the same FOV as the Pentax zoomed at 250mm .
03-26-2010, 12:07 PM   #15
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I'll post back with pictures soon
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