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After I bought Sigma 150-600mm f5-6.3 Contemporary DG OS HSM Lens I ran some tests. This test was to see the difference in focal length compared to a crop sensor lens vs this full frame lens on a same Crop Sensor body.

 

Since it's a full frame lens - it s/b equal to 240-960 MM on a Canon Crop Sensor. However when I did the side by side comparison of Canon EF-S55-250mm vs Sigma 150-250mm C, I found that they have identically same field of view and the composition fills up the same too.

I was expecting Sigma to be closer to subject than Canon 55-250mm at the same focal length (150 at sigma, 146 at Canon) . Applying the crop factor of 1.6, Sigma should have given me a field of view of 240mm however it does not.

Please take a moment and look at the video here, I've done side by side comparison.

Focal length test, Crop vs Full frame lens

marked as duplicate by mattdm, Michael C canon Jan 17 '18 at 20:57

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You are misunderstanding the application of crop factor in regards to your 55-250 lens.

Yes, there is a 1.6 crop factor when using a full frame lens on a crop body, but it also applies to using a crop lens on a crop body.

So your 55-250 has the same field of view as a 88-400. Focal length markings on a lens do not reflect which body they were design for. Take a look at a compact sized Point and Shoot camera and you will see something like 4-20mm which might be equivalent to 24-120mm.

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Crop factor does NOT affect the lens in any way. If it is a 150-600 mm lens, it is always a 150-600 mm lens on any camera body. The sensor size cannot affect what the lens actually does, but a smaller sensor can only see a smaller crop of it.

What does change is that the body with a smaller sensor sees a smaller field of view, because the smaller sensor crops the view to be smaller.

It has been said that "your 55-250 mm has the same field of view as a 88-400", but that is very incomplete which causes us confusion if said that way. That actual meaning of this 1.6 crop factor is only that your 55-250 mm lens on the cropped sensor body has the same field of view as a camera with full sensor would see with a 88-400 mm lens (because its sensor is larger, seeing a wider view, so needing a 1.6x longer lens to reduce the full frame field of view back to what the smaller sensor would see). The field of view is also determined by the sensor size... the only issue is that a cropped sensor size sees a smaller field of view..

The "Equivalent Focal Length" only apples to the full size sensor. It does not apply to the cropped sensor. The lens does what it always does, and the sensor size sees what it always sees. A smaller sensor simply crops a smaller view.

Crop factor is about the sensor size, and it is about the field of view, but crop factor is NOT about the lens. Equivalent focal length is a comparison which corresponds to the lens, due to sensor size, but it only applies to the full frame sensor body.

Users with years of experience with full frame 35 mm film cameras know the field of view expected from various focal lengths. The purpose of this Equivalent Focal Length merely tells them what field of view to expect from the new digital camera with the smaller sensor, in full frame terms that they understand.

If you don't have experience with full frame cameras, then this Equivalent Focal Length has no meaning to you. It does NOT apply to your cropped sensor body. The Equivalent Focal Length ONLY applies to the full frame camera. If the lens is 150-600 mm, it is ALWAYS 150-600 mm. However, smaller sensors do see a smaller cropped view from it than a larger sensor would see.

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