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Author Topic: Lenses and comparitive field of view  (Read 2468 times)
walter.sk
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« on: July 18, 2012, 08:09:22 PM »
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A few years ago, I added a Tamron 28-300 to my collection.  I was quite surprised at the time to see that the field of view at 28mm was quite different compared to my 16-35 at 28mm, and the same was true when comparing the Tamron at 300mm to my Canon 100-400 at 300mm.  Somebody educated me at that time as to why this was so, but I have long forgotten the reason.  This weekend a friend reported the same phenomenon and I couldn't explain it.

I would be grateful for information on what parameters are used in determining the focal length of lenses that could explain this issue.
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bernhardAS
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« Reply #1 on: July 18, 2012, 09:30:11 PM »
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My first guess would be that one or both lenses that you compare have "focus breathing". That means that the lenses only have their nominal Focal Length at indefinite focusing. The Focal length changes (becomes usually smaller) when focusing nearer.

Without knowing the details of the Tamron lens, I suspect that most super/yooms have focus breathing, to enable a more compact (and cheaper) design.
(compare for example size and weight with the early Nikon or Canon 50-300.)

This effect is of course different from lens design to lens design, so even comparing lenses at the same nearer distance might yield different results.
The best shot is to compare both at indefinite setting. 

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Colorado David
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« Reply #2 on: July 18, 2012, 09:40:36 PM »
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Most of these lenses are variable focal length lenses and not zooms.  A true zoom lens will remain in focus throughout its focal length range while a variable focal length lens will require refocusing as the focal length is changed. for example, the lenses on high end video cameras are zooms.  The proper technique is to zoom in tight to your subject and set focus.  Then when you zoom out to any focal length, your subject will remain focused.  Some of the newer low-cost autofocus video cameras will depend on adjustments from the autofocus.  This is probably a long-winded way of saying that I agree with the theory that the difference you're seeing may be from focus breathing.
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Wayne Fox
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« Reply #3 on: July 19, 2012, 03:58:38 AM »
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Along with what has been added (with such lenses I believe focal length can only be measured when focused on infinity), on such lenses focal lengths are approximate, and setting the lens based on the markings on the lens doesn't mean you get that exact focal length.  So setting the 100-400 on "300" is only approximately 300, and the 16-35 at "28" is only approximately 28, and those would be skewed if the lens is not focused on infinity.
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theguywitha645d
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« Reply #4 on: July 19, 2012, 10:17:07 AM »
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A couple of reasons. First and most likely is the intermediate focal length markings are not accurate, but approximate. Also, IF (Internal Focus) lenses will not have the same focal length when focused away from infinity as a lens that changes the lens to sensor distance--IF lenses focus by changing focal length (that is how you can focus a lens without changing lens to sensor distance). IF focal lengths are only true at infinity.
« Last Edit: July 19, 2012, 10:20:09 AM by theguywitha645d » Logged
theguywitha645d
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« Reply #5 on: July 19, 2012, 10:18:44 AM »
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Most of these lenses are variable focal length lenses and not zooms.

The definition of a zoom lens is a variable focal-length lens.
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walter.sk
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« Reply #6 on: July 19, 2012, 06:33:11 PM »
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Thanks for the replies.  I think I now have enough to understand the differences between the various types of lens, focusing at infinity, and focus "breathing."  But rather than try to explain this to my friend I'll send him the link to this thread.
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Colorado David
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« Reply #7 on: July 19, 2012, 07:40:35 PM »
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The definition of a zoom lens is a variable focal-length lens.

It's true that a zoom is a variable focal length lens, but not all variable focal length lenses are zooms. A true zoom will retain focus through its zoom range.
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bernhardAS
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« Reply #8 on: July 19, 2012, 10:45:09 PM »
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In the days on Manual Focus the difference was apparent.
One lens example I have is the Vivitar Series 1  35/85 2.8 (for FD) which explicitly needed to be focused after zooming.
I think in the days of Auto Focus the manufacturers have become a bit cavalier about it, as they assume that the user will not notice anyway. 

Should you have set your Camera to auto focus only with the extra button and not with the shutter release button you might see the difference in modern lenses. 
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theguywitha645d
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« Reply #9 on: July 20, 2012, 11:26:45 AM »
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It's true that a zoom is a variable focal length lens, but not all variable focal length lenses are zooms. A true zoom will retain focus through its zoom range.

Never seen that definition in any of my sources.
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leuallen
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« Reply #10 on: July 20, 2012, 06:53:47 PM »
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"Zoom lenses (sometimes referred to as "true" zoom) are ideally parfocal in that focus is maintained as the lens is zoomed (focal length and magnification changed), which not only is convenient, but also has the advantage of allowing more accurate focusing at maximum focal length and then zooming back to a shorter focal length to compose the image.[1]

Many zoom lenses, particularly in the case of fixed lens cameras, are actually varifocal lenses, which gives lens designers more flexibility in optical design trade-offs (focal length range, maximum aperture, size, weight, cost) than parfocal zoom, and which is practical because of auto-focus, and because the camera processor can automatically adjust the lens to keep it in focus while changing focal length ("zooming") making operation practically indistinguishable from a parfocal zoom."

Wikpedia
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theguywitha645d
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« Reply #11 on: July 20, 2012, 11:40:33 PM »
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"Zoom lenses (sometimes referred to as "true" zoom) are ideally parfocal in that focus is maintained as the lens is zoomed (focal length and magnification changed), which not only is convenient, but also has the advantage of allowing more accurate focusing at maximum focal length and then zooming back to a shorter focal length to compose the image.[1]

Many zoom lenses, particularly in the case of fixed lens cameras, are actually varifocal lenses, which gives lens designers more flexibility in optical design trade-offs (focal length range, maximum aperture, size, weight, cost) than parfocal zoom, and which is practical because of auto-focus, and because the camera processor can automatically adjust the lens to keep it in focus while changing focal length ("zooming") making operation practically indistinguishable from a parfocal zoom."

Wikpedia

Thanks, but that is a very suspect reference from Wikipedia. There is only one source to an internet article published in 2003 by one author. I think it might be a case of someone making "facts."
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leuallen
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« Reply #12 on: July 21, 2012, 01:10:44 AM »
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Google it. I have read similar information elsewhere before. There was a debate about this in the 70's or 80's when certain manufactures came out with 'vari focal' lenses which had to be refocused for each focal length.
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Wayne Fox
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« Reply #13 on: July 21, 2012, 02:28:52 AM »
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hmmm, I've never had anyone call my store and ask for a "variable focal length" or varifocal lens ... they all ask if I have a xx-xxx zoom.  I've never seen it used as a the "technical" term for a parfocal lens, and if it is, it's pretty meaningless at this point since no one uses it with that distinction any longer.  With still photography and autofocus there is little need for it, although with so many cameras being used for video I could see where a parfocal zoom design could be advantageous.
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theguywitha645d
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« Reply #14 on: July 21, 2012, 10:41:41 AM »
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Google it. I have read similar information elsewhere before. There was a debate about this in the 70's or 80's when certain manufactures came out with 'vari focal' lenses which had to be refocused for each focal length.

I did search for it which is why I only found one author for the term. There are Varifocal lens for security cameras but they are simply zoom lenses. It is rather an obscure term if real.
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walter.sk
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« Reply #15 on: July 21, 2012, 12:21:48 PM »
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I do seem to remember the term "parfocal," or something similar, referring to a lens with constant focal point throughout the focal length range, and the procedure was to focus at the longest focal length and them zoom out to the desired setting.
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RazorTM
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« Reply #16 on: July 24, 2012, 06:53:22 PM »
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"Zoom lenses (sometimes referred to as "true" zoom) are ideally parfocal in that focus is maintained as the lens is zoomed (focal length and magnification changed), which not only is convenient, but also has the advantage of allowing more accurate focusing at maximum focal length and then zooming back to a shorter focal length to compose the image.[1]

Many zoom lenses, particularly in the case of fixed lens cameras, are actually varifocal lenses, which gives lens designers more flexibility in optical design trade-offs (focal length range, maximum aperture, size, weight, cost) than parfocal zoom, and which is practical because of auto-focus, and because the camera processor can automatically adjust the lens to keep it in focus while changing focal length ("zooming") making operation practically indistinguishable from a parfocal zoom."

Wikpedia

That says zoom lenses are ideally parfocal, not normally parfocal.  Ideally means in the best possible examples of a zoom lens.
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