Is Zoom on 55-250mm @ 135mm the same as 70-200mm @ 135mm on a cropped sensor?
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Yes, it's exactly the same.
The focal lengths given in the specs are always the actual focal lengths, so 135mm on any lens will always give the same field of view when used on the same sensor format. The difference between the two is that the 55-250 EF-s projects a smaller image circle, so would give black corners if mounted on full frame (which you can't do because of the rear elements protruding into the body).
Try using the calculator at the bottom of this page to make a comparison between 200mm and 250mm, if you need to know exactly how different the lengths are.
The focal lengths given in the specs are always the actual focal lengths, so 135mm on any lens will always give the same field of view when used on the same sensor format. The difference between the two is that the 55-250 EF-s projects a smaller image circle, so would give black corners if mounted on full frame (which you can't do because of the rear elements protruding into the body).
Try using the calculator at the bottom of this page to make a comparison between 200mm and 250mm, if you need to know exactly how different the lengths are.
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if you are on a Canon APS-C just times your number by 1.6 for eg 55mm x 1.6 = 88mm
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Thanks folks. ^
I was positive that when I set my 55-250mm @ 135mm and my 18-135mm @ 135mm one of them was more zoomed in than the the other hence the question.
So I would lose 80mm with a 70-200mm but could always then use a TC.
Sort of.
The relative difference in FOV between 200mm and 250mm and 320mm and 400mm is the same.
The difference between focal lengths isn't different because of the cropped sensor.
The difference between your 55-250 and 18-135 could be inaccurate focal length markings on one of the lenses, or that the 18-135 is actually slightly longer/shorter than 135mm at the long end.
If both lenses are set perfectly at 135mm, the FOV will be exactly the same.
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Sort of as above
You'll loose an effective 80mm (50mm focal distance x 1.6 crop factor), BUT you will gain in the IQ of the lens and be able to crop in to the same factor (hope this makes sense) easily.
Yes you can use a 1.4 TC on the F4 lens, but you'll loose 1 stop of light - so will be a 280 @ F5.6 or the 1.4 / 2.0 TC on the 2.8
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As others have said, 135mm on one lens should be the same as 135mm on another lens. The measurements all refer to the physical focal length, and do not depend on whether it's a full-frame / EF / FX lens or a crop sensor / EF-S / DX lens.
But there are a couple of reasons why you might observe a difference in practice.
This is one:
But there are a couple of reasons why you might observe a difference in practice.
This is one:
Another is a phenomenon called focus breathing: the focal length can vary depending on the focus distance. Focal length is quoted with the lens focussed at infinity. When focussed closer, it can be quite a lot less, especially at the long end of the zoom. So it's quite possible that "135mm" on an 18-135 zoom is actually less than "135mm" on a 55-250 zoom. (And I'd bet it's that way round.) This effect is more common with consumer-grade lenses but it can even affect pro-spec lenses: the Nikon 70-200mm f/2.8 VR II is a notorious breather.