Focus and Zoom

redhed17

redhed17

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I was out taking pictures last night of fireworks and there was a chap next to me who asked what settings I was using, so I told him. At the end I said if you zoom in or out you may need to re-focus, which I have to do with my lenses, but he said that he doesn't need to because his lens had a constant f2.8 aperture. :eek:

Seeing as I don't have a lens with that wide or constant aperture, is that the case that if you focus on something with that type of lens on a subject, would the focus not change of you zoomed in closer, or indeed further away? :shrug:
 
The aperture is the amount of light being passed through the lens, not the focus.
 
A true zoom (parfocal) lens should not require re-focusing at different focal lengths. Lenses that allow you to change the focal length, but require you to refocus are called varifocal. You might notice a focusing error more if you zoom in, but if it's a parfocal it should change merely because you've zoomed.

In either case though, as noted above, it's nothing to do with the aperture, fixed or variable. I think the guy was getting confused between aperture and focus.
 
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Thanks for the replies. :)

I think my lenses must be varifocal lenses, because I have to re-focus should I zoom in or out. His 70-200 VR must be a parfocal lens. I thought one had to always refocus and hadn't heard of parfocal and varifocal so thanks for the links Musicman. :)

Paul Male said:
The aperture is the amount of light being passed through the lens, not the focus.
Click to expand...

I know the difference between focus and aperture, I just didn't know whether a lens with a fixed aperture throughout it's zoom range kept the point of focus when zooming, maybe because the internal lenses re-positioned in a different way to variable aperture zooms. My (not expensive) lenses certainly don't their focus. ;)
 
redhed17 said:
I was out taking pictures last night of fireworks and there was a chap next to me who asked what settings I was using, so I told him. At the end I said if you zoom in or out you may need to re-focus, which I have to do with my lenses, but he said that he doesn't need to because his lens had a constant f2.8 aperture. :eek:

Seeing as I don't have a lens with that wide or constant aperture, is that the case that if you focus on something with that type of lens on a subject, would the focus not change of you zoomed in closer, or indeed further away? :shrug:
Click to expand...

ROFL I almost wet myself at that...

He either doesn't know what hes talking about or was drunk :lol: (Or having you on!)

I have a Nikon 24-70 2.8
As with any lens (other than primes... Obviously) You have to focus again when you zoom.
 
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