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12-12-2012, 06:44 PM   #1
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Is 300mm REALLY 300mm?

Okay, I was playing around and testing a lens I bought since it got here today. For fun I wanted to compare it to a couple more lenses I have in the same focal length on the long end. I was surprised at my results. These have had nothing done other than resizing. No cropping, nothing. They were shot at ISO 1600, 1/80 second. I found something strange. Here they are!

Sigma 70-300 DL Macro Super

Name:  sig70-300DLmacrosuper.jpg
Views: 573
Size:  31.8 KB

Sigma DG Macro 28-300

Name:  sig28-300DGmacro.jpg
Views: 602
Size:  31.7 KB

Tamron AD Aspherical XR Di LD [if] Macro 28-300

Name:  tam28-300xrdildifmacro.jpg
Views: 574
Size:  31.6 KB

So, if 300mm is 300mm then how in the world did I get different sizes? That braid is a part of my roomates cosplay, and is the same size all the way up.

12-12-2012, 06:46 PM   #2
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Short version - 300mm on a zoom isn't necessarily 300mm, especially if we're talking about close focus.

Some close focus lenses will allow you to focus even closer than others - i.e. magnification.
12-12-2012, 06:50 PM   #3
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These were all taken from my computer desk and the braid is about 10-12 feet from me. Ohh, they were all resized with Prish Resizer, so you get the weird banding in the background.
12-12-2012, 06:50 PM   #4
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You're not shooting at something distant, you're shooting something that's rather close.

You'd probably get much more of an expected result if you shot something at maximum range rather than up close and personal.

Maybe. Just my 2 cents.

12-12-2012, 06:54 PM   #5
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The more consumer-oriented your zoom lens is, the more they approximate the focal lengths. For instance, the Pentax 18-135mm is more like a 105mm at the long end.

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12-12-2012, 06:54 PM   #6
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I may give it a shot (pardon the pun) from my balcony tomorrow after work. There is a telephone pole in the field behind the apartments across from me.
12-12-2012, 07:04 PM   #7
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I have heard a whispering that internal focusing lenses change the focal length as you bring the subject closer?
12-12-2012, 08:47 PM   #8
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QuoteOriginally posted by Louicio Quote
I have heard a whispering that internal focusing lenses change the focal length as you bring the subject closer?
Yes, this is what I have read also, but can't seem to find the link right now. But it does make a difference if the lens is internal focus (IF)

I know that with my DA* 60-250 when focused at close range it doesn't go much past 200mm.

12-12-2012, 08:50 PM   #9
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Makes me wonder what my Sigma 150-500 HSM will do.
12-12-2012, 09:02 PM   #10
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Zoom focal lengths are at infinity. At short distances they can be quite a bit less. But this can also be a blessing as the design allows for closer focusing of the lens at longer focal lengths. For instance, the DA 18-250 tops out at around a true focal length of 180mm on anything but distanct subjects. On the other hand, it can focus on stuff a foot away at any zoom setting...

Last edited by vonBaloney; 12-12-2012 at 10:19 PM.
12-12-2012, 09:11 PM   #11
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QuoteOriginally posted by Adam Quote
The more consumer-oriented your zoom lens is, the more they approximate the focal lengths. For instance, the Pentax 18-135mm is more like a 105mm at the long end.
Nothing to do with that, it's strictly whether it's an internal focus lens, and subject distances below infinity. The 18-135 is only 135mm at infinity. The consumer-oriented 55-300 is 300mm at the long end, regardless of subject distance because it is not an IF lens. The pro quality DA 60-250 loses range as subject distance decreases.

Last edited by audiobomber; 12-12-2012 at 10:18 PM.
12-12-2012, 09:15 PM   #12
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QuoteOriginally posted by VoiceOfReason Quote
Makes me wonder what my Sigma 150-500 HSM will do.
Here's the graph:

Focal Length versus Subject Distance - Sigma 150-500mm
12-12-2012, 09:15 PM   #13
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QuoteOriginally posted by vonBaloney Quote
Zoom focal lengths are at infinity. At short distances they can be quite a bit less. But this can also be a blessing as the design allows for closer focusing of the lens at longer focal lengths. For instance, the DA 18-250 tops out at around a true focal length of 180mm on anything but distance subjects. On the other hand, it can focus on stuff a foot away at any zoom setting...
What he said...

Focal length is always measured at infinity and many zooms grow "shorter" in terms of magnification as you focus closer. Traditionally this has been less true with the more expensive lenses.


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12-12-2012, 09:24 PM   #14
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QuoteOriginally posted by stevebrot Quote
What he said...

Focal length is always measured at infinity and many zooms grow "shorter" in terms of magnification as you focus closer. Traditionally this has been less true with the more expensive lenses.


Steve
Nope, strictly a property of internal focus. Internal Focus prime lenses lose range at short subject distances too, just not as much as zooms. The A*300mm for example is not IF and does not do this, the DA*300 does because it's IF.

Last edited by audiobomber; 12-12-2012 at 10:19 PM.
12-12-2012, 09:35 PM   #15
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I learned new stuff tonight. That makes it a good night! I'm still going to sit on the balcony and do some tests tomorrow to see what happens.
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