Tamron Lens APS-C figures

<p>Hi!<br>

Alright, silly question coming up!<br>

The Tamron 17-50 really is 17-50 on an APS-C sensor due to its EF-S design - yes? What about the 70-300 VC, is that basically an EF lens and so actually 112-480, or thereabouts? Or is that an EF-S style lens as well?<br>

Please let me know either way!</p>

<p> </p>

<p>You are confused. Focal Length is focal length. Manufacturers don't "adjust" it for APS-C format.</p>

<p>If you adjust the 70-300 to be "112-480" to take into account sensor size then you also have to adjust the 17-50 to be "27-80". It doesn't matter that one lens is full frame and the other is for crop sensors. Both quote the <strong><em>actual</em></strong> focal length, not "effective full frame equivalent focal length on a crop sensor"</p>

<p>Only P&S digicams routinely quote the "effective focal length" taking sensor size into account</p>

<p>Reading <a href="http://www.bobatkins.com/photography/tutorials/crop_sensor_cameras_and_lenses.html">http://www.bobatkins.com/photography/tutorials/crop_sensor_cameras_and_lenses.html</a> might help resolve the confusion for you.</p>

<p>No. All lenses are identified by their actual focal length rahter than their "effective" focal length (as determined by the sensor size). </p>

<p>The focal length of a lens is an optical property which does not change. So for example a 50mm lens will always produce an image about 50mm away from it. </p>

<p>If you now compare the image produced from that 50mm lens mounted on a camera with a full frame sensor to the image produced with the same lens on a APS-C size sensor you will see they are different. The bigger full-frame sensor gives a wider angle view. The difference is called the 'crop factor'.</p>

<p>Below is an example of a shot taken with a full frame sensor camera showing the size of image given by the APS-C sized sensor camera. For a 50mm lens on an APS-C camera the 'effective focal length' will commonly be about 80mm (actual focal length x crop factor). But the lens will still have a focal length of 50mm.</p>

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<p>One other piece of the puzzle that may help: when manufacturers designate a lens as being designed for a crop sensor camera (like the Canon EF-S designation or the corresponding one, which I have forgotten, from Tamron), that only means that the lens has been designed to take advantage of that form factor and may not work on other cameras. E.g., some of them have a narrower FOV and will vignette on a 'full frame' sensor camera, and others won't mount because the larger mirror on those cameras will hit the rear lens element. However, in focal length, as others have said, they are the same. If you have a crop-sensor camera, you can use any of them. If you use an EF lens, you are carrying around more weight than you need because the camera won't see the outer parts of the lens, but that is the only cost. I use a crop-sensor camera, and I use a mix of EF, EF-S, and Tamron full frame lenses.</p>