On my 70D, why is 250mm by Canon a larger zoom than 250mm by Sigma?

Started 5 months ago | Questions
Sir Atfus
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On my 70D, why is 250mm by Canon a larger zoom than 250mm by Sigma?
5 months ago

I posted a thread in the Sigma section, but I'm think maybe I should ask 70D owners. Just bought the Sigma 18-250mm to replace my Canon 18-55 and 55-250. But I'm realizing that at 250mm, the Canon lens is zooming in closer. 250 on Sigma actually feels more like 200mm.

Can someone explain why? I've been digging through Amazon reviews, and nobody has mentioned this. When searching the internet, I did find someone asking the same question through Yahoo, but there were no good answers.

This is the Sigma lens i bought. http://www.amazon.com/Sigma-18-250mm-f3-5-6-3-Digital-Cameras/dp/B008B48AAE/ref=sr_1_1?s=electronics&ie=UTF8&qid=1406343454&sr=1-1&keywords=sigma+18-250

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Canon EOS 70D
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Howard
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Re: On my 70D, why is 250mm by Canon a larger zoom than 250mm by Sigma?
In reply to Sir Atfus, 5 months ago

Some zoom lens only attain their claimed focal length when focused at infinity, their focal lengths become shorter when focused on something closer. I would think that lenses with higher zoom ration tend to "lose" focal length more because of the design compromise.

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Howard
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Sir Atfus
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Re: On my 70D, why is 250mm by Canon a larger zoom than 250mm by Sigma?
In reply to Howard, 5 months ago

Thanks Howard.. I think Focus Breathing is the lingo. I've never heard of this.

http://www.bobatkins.com/photography/technical/focus_breathing_focal_length_changes.html

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Sir Atfus
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Re: On my 70D, why is 250mm by Canon a larger zoom than 250mm by Sigma?
In reply to Sir Atfus, 5 months ago

I loaded some pictures in this thread.

http://www.dpreview.com/forums/post/54091986

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Abu Mahendra
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Re: On my 70D, why is 250mm by Canon a larger zoom than 250mm by Sigma?
In reply to Sir Atfus, 5 months ago

It could even be simpler than focus breathing. It could well be a case of false or "optimist" labeling.

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R2D2
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Re: On my 70D, why is 250mm by Canon a larger zoom than 250mm by Sigma?
In reply to Sir Atfus, 5 months ago

Sir Atfus wrote:

Thanks Howard.. I think Focus Breathing is the lingo. I've never heard of this.

It's pretty common actually.

Re-shoot your test at Infinity and see how things stack up.

R2

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drbird
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Re: On my 70D, why is 250mm by Canon a larger zoom than 250mm by Sigma?
In reply to R2D2, 5 months ago

Sadly it is the even the new big white ones are not free from it... but it kicks in really late, jus about when one focus closer than the pre-version allowed. The L 70-300 is one which suffers quite a bit from it as well as the 100-400 see

All the super zooms suffer from it, and often very substantial, just look at the magnification at the maximum focal length for instance for the 70-300L it is 0.21 at 1.2m which exactly the same for the 70-200L = means the 300mm are gone at 1.2m and the lens has the same focal length as the 70-200L at this focusing distance.

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