Lens Sizes / Uses / Meanings

[Caroline_7]

I currently have the EF-S 18-55mm and EF 75-300mm lens for my Canon Rebel T3i. I was graciously gifted a 40mm lens for Christmas with the option to exchange it for another lens. One would want to think that the 40mm is covered within the 18-55mm lens but I am completely unsure how this works so any assistance with suggestions, size meanings and/or what these are greatly used for would be greatly appreciated! Thank you in advance!

[ebiggs1]

And now she has both sides.   She can either take Tim's excellent explanation or she can take mine.  We strive to give people choices, don't we?  

[Skirball]

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@ScottyP wrote:
People always talk about the DOF being shallower on FF.

Is that only if you adjust the distance to the subject to create the equivalenf framing for both lenses in the comparison? (You increase the distance to subject on the crop to get equivalent framing, and longer distance = thicker DOF?)

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Not sure if you're referring to my comment or not.  I wasn't referring to the increased bokeh (presumably, thinner DoF) with respect to FF; I was comparing a f/1.8 50mm versus the f/4.5 or whatever the kit lens would be at 50mm.

[ScottyP]

Hi Skirball. No, I was following up on Tim's comment explaining crop v. FF.

[Caroline_7]

Thank you to everyone who contributed to this thread!