attack_donut
enthusiast
Reged: 16/12/2006
Posts: 284
Loc: East Coast
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Is the view from a 50mm prime the same as 50mm view from an 18-55mm?
I ask because I am always amazed at the sheer number and variety of lens configurations and wondered if there was some advantage to buying a (say) 35-70 to a 18-70 if shooting at the same focal length.
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Overread
Not a veteran
Reged: 17/01/2008
Posts: 1578
Loc: UK; Suffolk
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It should be the same view yes - however as I understand it there is often a little play in the manufacture of zoom lenses so 50mm as marked on the barrel is a general guide only.
I would say that this is really only a problem if you are dead set on shooting at 50mm only - and at that point a prime would be the better option. If you are using a zoom there is no reason not to zoom back and forth as needed between shoots and let the focal length be what it will (within reason since there is a noticable difference between 18mm and 55mm of course)
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El Sid
Going potty
Reged: 14/04/2003
Posts: 10755
Loc: Sussex-by-the-Sea
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Nominally yes but I suspect in reality probably not exactly. Aside from the fact that marked scales on the zoom barrel are for guidance only the lenses do have a different optical configuration - which probably means that at any given focus the absolute field of view is not quite identical. Mind you the difference is probably so marginal that only a measurebator would notice... 
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Nigel
Completely BSRIPN 
ElSid Gallery
Terrorist?............or potential photographer?....... 
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zx9
Pooh-Bah
Reged: 22/06/2007
Posts: 1875
Loc: London
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The main differences between a 50mm prime lens and a zoom at 50mm will be the wider maximum aperture for low light shooting, better sharpness and lower distortion. These differences can also be seen between a zoom lens with a small ratio, say 35mm to 70mm and a super zooms of say 18mm to 200mm.
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ZX9 (Keith Hudson)
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TimF
Taking it strictly
Reged: 30/07/2001
Posts: 18967
Loc: Herts/Beds border
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Like the others have said. However, just to throw an extra dimension in, not all 50mm are the exact marked focal length either!
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Tim BSRIPN
If I had all the money I've spent on drink, I'd spend it on drink
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daft_biker
Action Man!
Reged: 11/10/2006
Posts: 10579
Loc: Oot n aboot
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I compared the 50mm mark on a zoom with my 50mm once. It was miles out.
Even if they were the same focal length at infinity they could be pretty far apart by the time you get to minimum focus. There's often exposure differences between infinity focus and minimum too - macro lenses usually come with a table to show you how much to compensate for at various magnifications when using an external meter.
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Andrew (BSRIPN) ... My pics on Flickr.
We do not inherit the earth from our ancestors, we borrow it from our children.
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MickLL
Too Grand, Man
Reged: 01/08/2004
Posts: 3938
Loc: SE England
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Quote:
There's often exposure differences between infinity focus and minimum too - macro lenses usually come with a table to show you how much to compensate for at various magnifications when using an external meter.
That's because, as you focus closer, you are effectively adding extension. A bit like adding continuously variable extension tubes. Therefore, as with tubes, you lose light and more exposure is required.
MickLL
PS (Edit) Of course with modern auto-exposure you don't have to worry about it. The camera does all the hard work for you and as a close-up/macro nerd I am very glad that it does. If it didn't I may well have stayed away from the macro genre.
Edited by MickLL (09/02/2010 11:18)
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TimF
Taking it strictly
Reged: 30/07/2001
Posts: 18967
Loc: Herts/Beds border
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A lot of macro lenses are actually a shorter focal length at close range than their stated value.
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Tim BSRIPN
If I had all the money I've spent on drink, I'd spend it on drink
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daft_biker
Action Man!
Reged: 11/10/2006
Posts: 10579
Loc: Oot n aboot
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Quote:
....
PS (Edit) Of course with modern auto-exposure you don't have to worry about it. The camera does all the hard work for you and as a close-up/macro nerd I am very glad that it does. If it didn't I may well have stayed away from the macro genre.
Add in tilt/shift movements that AE doesn't work with and things get even more interesting! I use the trial and error approach of tweaking the shutter speed until the histogram looks right. (wonder if I can get a live histogram to set the exposure with in liveview? wonder if it's any quicker!)
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Andrew (BSRIPN) ... My pics on Flickr.
We do not inherit the earth from our ancestors, we borrow it from our children.
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daft_biker
Action Man!
Reged: 11/10/2006
Posts: 10579
Loc: Oot n aboot
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Quote:
A lot of macro lenses are actually a shorter focal length at close range than their stated value.
The internal focus ones?
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Andrew (BSRIPN) ... My pics on Flickr.
We do not inherit the earth from our ancestors, we borrow it from our children.
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