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03-24-2009, 01:27 PM   #1
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When is a 200mm lens not a 200mm lens?

So today, I got a package at work containing a nice little MZ-5N plus battery pack and Tamron 28-200mm lens. I was pretty pleased, as I thought I'd be able to sell the 50-200mm lens I got with my GX20, but as I was doing a quick comparison between the two, I came up against something rather unexpected: The 200mm end of the Tamron doesn't bring things as close as the 200mm end of the Schneider.

Here's the Schneider @ 200mm:


and here's the Tamron at the same focal length:


I thought I'd have a look using a fixed focal length to compare the Tamron with, so here are a couple of photos comparing it with my 50mm 1.7. The Tamron had to be at 65mm to match the Pentax's focal length.

Pentax 50mm 1.7:


Tamron 28-200 @ 65mm



This seems very odd to me. It also made me wonder about the image stabilization. If the camera thinks the lens is set to 200mm, but the optics in fact behave as a 180mm or whatever, won't the body make the wrong compensations for movements?

Any thoughts would be appreciated.

Many thanks,

Philip

03-24-2009, 01:35 PM   #2
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I don't know how the shake reduction deals with this, but it's a pretty well-known phenomenon in these mega-zooms.

They only reach their stated max focal length when focused to infinity, and are considerably wider when focused closer.

The same is true with the Pentax/Tamron 18-250mm.
03-24-2009, 01:47 PM   #3
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Thank you Quicksand. I'd never heard of this before - I'm only just starting with zooms really, and I (naively?) just expected to be able to trust the scale.
03-24-2009, 01:59 PM   #4
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I've noticed the same thing back than I was going through a bunch of Ni-Canons... At one instance I was playing with the Canon's 28-105 (pretty known lens) and had an el-cheapo Canon 80-300... well, that last one at 80mm, had the same image frame as the 28-105 at 105mm... go figure!!!

03-24-2009, 03:09 PM   #5
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QuoteOriginally posted by pa79 Quote
Thank you Quicksand. I'd never heard of this before - I'm only just starting with zooms really, and I (naively?) just expected to be able to trust the scale.
I believe if you do a search on internal focus you will find some discussions about it. Such as https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/22081-pentax-18-250-18-180-a.html and others with titles similar.
03-24-2009, 04:14 PM   #6
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Thanks for the link m8o. If I twist my mind enough it sort of makes sense if I think about the mechanics of the lens, and I can actually see the size of objects in the viewfinder change as I sweep from close focus to infinite.

Going back to my other point, does anybody know about the impact of varying effective focal length on image stabilization? Should you only use it with lenses which aren't internal focussing? I noticed that the pictures using the Tamron were significantly more blurred than the ones with the Schneider, even though it was effectively at a shorter focal distance. Could this be because the body was making the wrong corrections? (Unfortunately, the pictures above are much smaller than I was expecting, so you can't tell. I thought I'd linked to larger images from picasa, but that's another thing I'm only really starting with...)
03-24-2009, 04:16 PM   #7
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Sorry to say so, but it's old news...
one of the reasons why I sold my copy of Tamron 28-200 (the second one were it's very slow max appertures)

BR

PS: superzooms are as long as they say only when focused on infinity...
03-24-2009, 05:16 PM   #8
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QuoteOriginally posted by pa79 Quote
Going back to my other point, does anybody know about the impact of varying effective focal length on image stabilization? ...
I'm sorry I don't know that answer. I do know some of the threads on the subject were fairly long, sometimes heated, and a good many sub-topics were touched upon; hopefully this too. Hope you find the answer.

03-25-2009, 07:08 AM   #9
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QuoteOriginally posted by pa79 Quote

Going back to my other point, does anybody know about the impact of varying effective focal length on image stabilization? Should you only use it with lenses which aren't internal focussing?
Zoom lenses that have the A setting in the aperture ring (which means they DO have the Pentax compatible firmware built in) are supposed to transmit focal length to body at all times. This is why with such zooms, when using a flash unit such as the 540, 500 or 360, the flash also zooms when you change the focal length. I presume this information is also considered for the shake reduction feature.

About the "real focal length" with zoom lenses, there was an article by Herbert Keppler (Popular photography) many years ago in this matter. His article was based specifically on the Tamron 28/200 (Pentax rebadged) and his studies and findings (lab tests and such) revealed that ONLY at infinity the lens was a true 28-200. At closer focusing distances the long end shortened a lot, showing an actual focal length of only 135 mm (set at 200) when focusing at closest focusing distance.

This phenomena (or technical condition) is also true with all zoom lenses, where with longer range zooms this issue aggravates.
03-25-2009, 10:43 AM   #10
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If you look at the EXIF metadata for your pictures with a program like PhotoME (for Windows), you'll see a couple of different values.

There's an entry for "Focal Length," and if you scroll down further you'll also see "SR Focal Length."

For the lenses I have, those numbers appear to be the same (e.g., on my Sigma 24-60mm, when the "Focal Length" is 60mm, the "SR Focal Length" is also 60mm). I wonder whether the numbers are different for lenses like yours?

I'm curious.
03-25-2009, 03:44 PM   #11
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Hi Quicksand,

I had a look further down the EXIF table as you suggested, and I thought you had it. But then it got more confusing...

For the first two pictures, the SR for the Schneider is 204 while the Tamron gives 188. So far so good, but when I looked at the second two, I got 52 for the Pentax 50mm and 84 for the Tamron @ 65mm. Very odd.
03-25-2009, 04:51 PM   #12
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Very odd indeed.

I don't have the slightest idea what universe of factors SR considers, but focus distance may be one of them.

Somewhere around here there's a thread that discusses what SR value to input for old manual macro lenses. Shaking is much more pronounced with the high magnifications of macro work, so the conclusion was, for best SR performance, enter a LARGER number as the subject gets closer and magnification gets bigger. Sorry, I don't remember the exact formula, but perhaps a search of this forum will turn it up.

So I wonder whether that "SR Focal Length" -- particularly the strange 84mm value for the Tamron -- takes into account your closeness to the subject, and if it does, whether that increase masks the decrease you get from the internal-focusing superzoom at less-than-infinity distances.

This is all pure unfounded speculation, I'm afraid! And my experience (where lens focal length = SR focal length) isn't really consistent with this explanation.

So I dunno.
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