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06-13-2012, 12:11 AM   #1
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tamron 28-200mm really a 28-128mm?

So I made a home made canon fd to pk adapter (out of a doi 1.5x teleconverter). I wanted to see how much magnification it was giving (this thing was a nightmare and the Frankenstein I wound up with has the entire assembly of glass reversed and shifted back some but strangely it works). So I tested it against some lenses and discovered something strange. Fyi all these lenses are native unmodified pentax mount. I was using them to compare magnification of a canon fd mount vivitar on the adapter I made.

I recently got a tamron 28-200mm xr off ebay along with a pentax 100-300mm and a film body. I used the tamron to try and get the equivalent frame and it was a lot more magnification that I was expecting. So I tried the 100-300mm and got drastically different results. I did a little experimenting. At 28mm on the tamron, it was about the same magnification as 28mm on my kit lens. On the long end though where the tamron should be about 200mm, it has about the same magnification as 128mm on a pentax 100-300mm, a pentax 100-200mm, a sigma 70-300mm, a sigma 100-300mm, and an old manual focus vivitar 28-200mm.

Now for the weird part. The above lens is the forth generation if I am not mistaken (the xr model). I also have a tamron 571d 28-200mm (the second generation if I am not mistaken obtained from a different source). Its the same. At 200mm, its about the same magnification as all the others at about 128mm.

Am I missing something or are tamron super zooms not quite so super? Fyi I wouldn't think crop factor would have any effect but except for the kit lens, these are all old full frame film lenses so it wouldn't matter anyway.


On a side note, I had a really hard time focusing (I need to cut one clearance slot then I'll have aperture control in case its a depth of field issue or something) but here are pics from the Frankenstein lens if any one cares to look. I think there were most certainly focusing issues so I'll have to play with it a little more.

canon_adapter Photo Gallery by Richard Homeyer at pbase.com

06-13-2012, 12:22 AM   #2
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The optical design of this and many other zooms means that the stated range applies only when focused at infinity. Focusing on closer objects brings a progressive decrease in focal length. There are a number of threads about this effect on these forums, I recall a recent one on the 18-250 Tamron/Pentax, which is about 160mm focal length at MFD. I couldn't find a quick link to the optice behind this though a bit more skilfull Googling will probably dig one up!

Last edited by StephenHampshire; 06-13-2012 at 01:03 AM. Reason: minor spelling errors
06-13-2012, 12:48 AM   #3
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Thanks a bunch for the info. I guess its not as much of an issue if it gets its claimed focal length at longer distances where the longer focal lengths are more likely to be used. I was testing at about 10 feet (as I was unaware of this issue). Is this something specific to tamrons? I also have a sigma 28-200mm AF that is on par with the 2 AF pentaxes, the 2 other AF sigmas, and the MF vivitar 28-200mm. All the others seem to keep a lot more of their magnification at closer distances and they all seem to be about the same at 200mm where the tamrons fall much shorter in magnification. I had forgotten about the sigma 28-200mm initially but just checked it (it had minor fungus and while cleaned image quality is really bad below f11 so I generally never use it).

Edit: To clarify, I meant is is specific to tamrons to loose so much magnification as I did catch the part about many zooms being designed this way.
06-13-2012, 01:04 AM   #4
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Others can probably give you a better answer, but I believe this only applies to IF (internal focusing)lenses

06-13-2012, 05:11 AM   #5
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QuoteOriginally posted by StephenHampshire Quote
Others can probably give you a better answer, but I believe this only applies to IF (internal focusing)lenses
Yep. True dat. IF lenses are only their maximum focal length at infinity. As you focus closer, the magnification drops due to the IF design of the lens. Sucks, but since most hyperzooms are IF, (Actually, I don't think I've ever seen a hyperzoom that wasn't IF) you're kinda stuck. My Tamron 28-300 is the same way.
06-13-2012, 06:10 AM   #6
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QuoteOriginally posted by StephenHampshire Quote
Others can probably give you a better answer, but I believe this only applies to IF (internal focusing)lenses
Actually thats a very good answer. While I probably could't explain the technical reasoning myself it now makes perfect sense and I under stand why its happening..
06-13-2012, 06:28 AM   #7
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QuoteOriginally posted by GibbyTheMole Quote
Yep. True dat. IF lenses are only their maximum focal length at infinity. As you focus closer, the magnification drops due to the IF design of the lens. Sucks, but since most hyperzooms are IF, (Actually, I don't think I've ever seen a hyperzoom that wasn't IF) you're kinda stuck. My Tamron 28-300 is the same way.
Thanks for the conformation. It kind of makes me wonder if there are any super zooms (or hyper zooms if that's the right term) that are not internal focusing (and are any good). The sigma 28-200mm I have is not internal focusing and doesn't have this issue. I can not say for sure as my sigma had fungus but the tamron is supposed to be a lot better optically. It's not worth the money to get a different 28-200mm anyway but at some point down the road I wouldn't mind an 18-? super zoom and or a longer 28-?.
06-13-2012, 07:39 AM   #8
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QuoteOriginally posted by ripit Quote
It kind of makes me wonder if there are any super zooms (or hyper zooms if that's the right term) that are not internal focusing (and are any good). The sigma 28-200mm I have is not internal focusing and doesn't have this issue.
Well, the 28-200 qualifies as a superzoom, and I guess if yours isn't IF, there ya go. Now that I think about it, I used to have a non-IF superzoom, but it was manual focus. It was a Sakar 28-200, and it was surprisingly decent. At a distance of 5 feet my Tamron 28-300 at 300mm gave me roughly the same FOV as a non-IF zoom at 220mm. The farther your subject is from the lens, the less pronounced the effect.

06-13-2012, 08:43 AM   #9
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I wonder if internal focusing has become more common with newer lenses that are pretty much all computer designed (I would think anyway) and complex designs are easier due to plastic parts. I have 3 old manual vivitars that are not internal focusing and they are all pretty good lenses (28-200mm, 28-210mm and 35-200mm) and the sigma is an older film lens too. When I was researching them the sigma was said to be of significantly lesser optical quality (and sells for a lot less) so I would rather stick with the tamron and loose the close magnification than get another sigma and loose image quality (then again, even keh wanted less that 50$ for the sigma when they had them and even cheaper on ebay). From what I can tell most if not all of the newer sigma super zooms are internal focusing.
06-13-2012, 09:20 AM   #10
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Like Gibby and ripit, I have old MF non-IF superzooms: Promaster-Tamron 60-300 and Tokina SD 35-200, both PK-A mount. I haven't really thought to compare their output with the DA18-250. Let's see... I focus on my bookshelf about 2.5m away, and the DA @200 is definitely a wider view than the two MFs, which can't quite see the Ansel Adams books that fall in the FA's VF.

Now I whip out my non-IF FA100-300 and zoom for the same view, and the Info screen says I'm at 150mm. So there's the margin: at 2.5m, the IF lens loses 100mm FL! I wuz robbed!! [NOTE: We've had more than a couple I WUZ ROBBED! threads on this subject.] Damn those thieving optical designers!

Last edited by RioRico; 06-13-2012 at 08:27 PM.
06-13-2012, 10:09 AM   #11
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QuoteOriginally posted by RioRico Quote
Now I whip out my non-IF FA100-300 and zoom for the same view, and the Info screen says I'm at 150mm. So there's the margin: at 2.5mm, the IF lens loses 100mm FL! I wuz robbed!! [NOTE: We've had more than a couple I WUZ ROBBED! threads on this subject.] Damn those thieving optical designers!
Yes, and they'd have gotten away with it too if it wasn't for those meddling kids......
I'm glad my 16-45 is not IF, 45mm is a bit short at the best of times. The 16-50 DA* IS IF though. Short changed all round!
06-13-2012, 10:25 AM   #12
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QuoteOriginally posted by RioRico Quote
Like Gibby and ripit, I have old MF non-IF superzooms: Promaster-Tamron 60-300 and Tokina SD 35-200, both PK-A mount. I haven't really thought to compare their output with the DA18-250. Let's see... I focus on my bookshelf about 2.5m away, and the DA @200 is definitely a wider view than the two MFs, which can't quite see the Ansel Adams books that fall in the FA's VF.

Now I whip out my non-IF FA100-300 and zoom for the same view, and the Info screen says I'm at 150mm. So there's the margin: at 2.5mm, the IF lens loses 100mm FL! I wuz robbed!! [NOTE: We've had more than a couple I WUZ ROBBED! threads on this subject.] Damn those thieving optical designers!
I feel a little less ripped off as you get what is promised at longer distances which is where I would think you would use the longer focal lengths the most (unless perhaps you do a lot of macro which I don't, and I have other lenses for that). I also understand the physics of it and can see why an internal focusing lens would have to do that.

Still, it might be nice if they made it a little more obvious that they do it instead of just putting 200mm* and in the fine print put *at infinity focus with no mention of the massive amount of magnification you loose closer (I assume that is how your average publicist would state it to not make the lens look bad in any way). I'm kind of curious how much you loose at medium distances like 10-100ft though. You obviously loose a lot at 10ft. I might have to compare a little when I have more time just to satisfy my curiosity. They are still very nice lenses with great image quality for what they are and what they cost used. Oddly the older tamron seemed to have better image quality (if nothing else colors look better). I'm not really playing with or using the older one much though as it is technically not mine. The seller refunded and then seemed to abandon it even though he claimed to want it back. Over 2 months and he has not provided the return shipping label he promised (he did just about everything possible wrong against me and himself on this transaction).
I'm not really sure at what point I disregard anymore need to hold it for him (its sitting in the box ready to be closed and shipped right now. It has been over a month since he has sent any message and I got tired of trying to send messages to him after he refunded but took not action about return and took a week to respond each time (most times). Its broken anyway (won't focus to infinity lower than 50mm unless you press the front of the lens towards the camera as the front and rear haves are starting to separate). It arrived with a piece broken off from inside though from where I have no idea.
06-13-2012, 11:08 AM   #13
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QuoteOriginally posted by ripit Quote
Still, it might be nice if they made it a little more obvious that they do it instead of just putting 200mm* and in the fine print put *at infinity focus with no mention of the massive amount of magnification you loose closer
Solution: Disclaimers.

On rear-view mirrors: CAUTION: Objects may be close than they seem.
On IF zooms: CAUTION: Focal lengths may be shorter than you expect.
On politicians / pundits: CAUTION: Statements may not map to reality.
06-13-2012, 12:09 PM   #14
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QuoteOriginally posted by RioRico Quote
Solution: Disclaimers.

On rear-view mirrors: CAUTION: Objects may be close than they seem.
On IF zooms: CAUTION: Focal lengths may be shorter than you expect.
On politicians / pundits: CAUTION: Statements may not map to reality.
Yea you have to read the fine print on everything these days including politicians....
06-13-2012, 12:39 PM   #15
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QuoteOriginally posted by ripit Quote

------------
Edit: To clarify, I meant is is specific to tamrons to loose so much magnification as I did catch the part about many zooms being designed this way.
Actually, ANY lens - not only IF lenses - may show this type of behaviour if they have gropups of lenses that move realtive to each other depending upon focusing distance. Even prime focal lenses may show this. But in all other aspects the reasoning is correct: You can only count on FL specifications at infinity focus.

I suggest that you take a look here: Focal Length Experiments
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