Lens question

labpaws

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Im mostly new to all this so sorry if my question is an easy one but want to make sure im thinking the correct way. Im interested in a Sigma 17-70mm lens. Now there is an f2.8 and a f2.8 to 4.5 version. The f2.8 will be a fast lens at all stops but a small DOF. The 2.8 to 4.5 lens will not be as fast on the long end but have a larger DOF. Is this the correct way of thinking or am i all wrong?
Oh and a word on HSM. If my body has a motor is HSM needed and/or just an added bonus to have HSM
 
the 2.8 is a constant aperture wile the 2.8-4.5 varies with the focal leingth. And you have the basics correct but variable apreture is not exactly basic :lol: The 2.8 will be 2.8 at both 17mm and 70mm wile the 2.8-4.5 means that the maximum aperture will be 2.8 at 17mm and 4.5 at 70mm.
 
Also when you see or hear the phrase 'fast lens' it means that it's max. aperture is wide enough to allow you to use a fast shutter speed, not that it will be fast at all stops.
 
Im mostly new to all this so sorry if my question is an easy one but want to make sure im thinking the correct way. Im interested in a Sigma 17-70mm lens. Now there is an f2.8 and a f2.8 to 4.5 version. The f2.8 will be a fast lens at all stops but a small DOF. The 2.8 to 4.5 lens will not be as fast on the long end but have a larger DOF. Is this the correct way of thinking or am i all wrong?
Oh and a word on HSM. If my body has a motor is HSM needed and/or just an added bonus to have HSM

Battou answered the first question quite concisely.

As for your other question about the DOF. Both of those will be able to have a large DOF. The 2.8 is just the max aperture, they will both have a minimum of about 22, creating a large DOF.
 
It all indicates the starting aperture. You can go up from there.
 

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