Voyager, here's a hypothetical situation.
Let's say I own two lenses - A canon 50mm f/1.4 and an 18-55 f/3.5-5.6. I also own a 450D (I do actually) which is good enough at ISO 800 but only good at ISO 1600 when the light is acceptable to correctly expose.
I've left my 50mm f/1.4 at home by accident and am stuck in a dark area (I'm into Urbex

) with only my camera and the slow 18-55 lens. I want to take a picture of the room I am in, but it's too dark to handhold the camera, get a correctly exposed image WITHOUT camera shake. If I use the flash, it reflects off of dials and stuff that ruins the image, so that's out of question - as is a tripod as I don't have one.
We have a situation here where I have reached the limit of my equipment - had I remembered to bring the
more expensive prime lens along, I'd have got the shots.
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