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02-02-2015, 08:25 AM   #1
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Help with lens

I have several lenses and just noticed something odd. I have/had a Sigma 28-200mm, Tamron 28-200mm (sold them) a Promaster 28-210... When comparing these lenses the Promaster seemed more like a 200mm than 210mm when compared with the Sigma and Tamron because when zoomed to 210 the images size was very close to the other two... Ok now comes the interesting part - I have a Promaster 28-300mm which is considerably longer in focal length than the Promaster 210mm so I can assume that it is 300mm... Now here is where I need help and just noticed the odd thing... I have a Pentax kit lens 50mm to 200mm and at the 200mm mark it equals the Promaster at the 300mm mark - Is it possible that there was a mixup at the factory and I actually have a 50-300mm that was mislabeled... I am using these on my K-50 with 1.5 crop factor... Anyone have any ideas what is up with these lenses... Seems like sense the Promaster 210 equaled the Signma and Tamron at the 200 mark, one has to assume that this is pretty accurate and the Promaster 300 is considerably longer. Why would the Pentax 50-200 equal the 300mm Promaster at full focal when it is only a 200mm lens? Crazy...

02-02-2015, 08:32 AM   #2
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Your Promasters are actually made by another company (Sigma, Tokina, Tamron, whoever). Try prowling the lens database on the forum for that focal length and see if you have any "twins" that look the same as yours. You may find your answers there.
02-02-2015, 08:56 AM   #3
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QuoteOriginally posted by Buckaroo50 Quote
I have several lenses and just noticed something odd. I have/had a Sigma 28-200mm, Tamron 28-200mm (sold them) a Promaster 28-210... When comparing these lenses the Promaster seemed more like a 200mm than 210mm when compared with the Sigma and Tamron because when zoomed to 210 the images size was very close to the other two... Ok now comes the interesting part - I have a Promaster 28-300mm which is considerably longer in focal length than the Promaster 210mm so I can assume that it is 300mm... Now here is where I need help and just noticed the odd thing... I have a Pentax kit lens 50mm to 200mm and at the 200mm mark it equals the Promaster at the 300mm mark - Is it possible that there was a mixup at the factory and I actually have a 50-300mm that was mislabeled... I am using these on my K-50 with 1.5 crop factor... Anyone have any ideas what is up with these lenses... Seems like sense the Promaster 210 equaled the Signma and Tamron at the 200 mark, one has to assume that this is pretty accurate and the Promaster 300 is considerably longer. Why would the Pentax 50-200 equal the 300mm Promaster at full focal when it is only a 200mm lens? Crazy...
Try comparing them at infinity focus.
02-02-2015, 10:31 AM   #4
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As boriscleto says, it is very common with zoom lenses for the maximum focal length only to be true at infinity focus. The lenses focus at closer distances by reducing the focal length.

02-02-2015, 11:03 AM   #5
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It is called focus breathing. Try focusing on something very far in the distance with all three to see what they come out as. Also, even in ideal situations they may be 10-15mm short or long.
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