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06-12-2010, 08:45 AM   #1
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Relationship between focal lenths and reachability in terms of zoom

Hi

I searched on net but did not find the answers. I am stuck with a doubt regarding focal length and zoom capabilities. To say exactly, if I have two lenses 18-250mm, and 70-300mm which lens can bring a given subject more closer from a given point at the highest focal lengths?

How to calculate this reachability with any given lens and also in fixed focal length lenses? Does getting the zoom by diving the focal range numbers (250/18 = 14x and 300/70 = 4X) has any explanation in finding out which brings the subject more closer?

Please clarify.

Cheers
Gunasekhar

06-12-2010, 09:20 AM   #2
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QuoteOriginally posted by gut1kor Quote
Hi

I searched on net but did not find the answers. I am stuck with a doubt regarding focal length and zoom capabilities. To say exactly, if I have two lenses 18-250mm, and 70-300mm which lens can bring a given subject more closer from a given point at the highest focal lengths?

How to calculate this reachability with any given lens and also in fixed focal length lenses? Does getting the zoom by diving the focal range numbers (250/18 = 14x and 300/70 = 4X) has any explanation in finding out which brings the subject more closer?

Please clarify.

Cheers
Gunasekhar
Basically, no, there's no need to divide focal lengths from the zoom range. That has nothing to do with magnification: ( they just sell point and shoot cameras that way to give an idea how much range the zoom can cover, compared to other such cameras, but it's not a true measure of what magnification you get or related to like telescope magnifications )


It's simpler than that. The focal length, thus magnification/'reach' of a zoom lens is variable: thus a 70-300 can go longer than an 18-250: 300mm is always longer than 250mm, regardless of what else the lens can do.

That is to say, consider each focal length a lens can be set to, separately from any other it can be set to, when considering magnification/field of view. Focal length is an absolute. 300 is longer than 250mm.


The 18-250 can go much *wider* than the 70-300, though, so it may be much more versatile. And nearly as long.

Last edited by Ratmagiclady; 06-12-2010 at 09:32 AM.
06-12-2010, 10:03 AM   #3
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Unless you need to focus on small things. Then everything goes out the window.

But if you are shooting objects at large (10+ feet) then the larger the focal length the better (i.e. 70-300mm is better than 50-200mm, and 50-500mm is better still).

If you are shooting small things (i.e bugs, flowers etc.) ask, but I won't try to explain it now. It will just confuse the issue.
06-12-2010, 10:09 AM   #4
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QuoteOriginally posted by Ratmagiclady Quote
Basically, no, there's no need to divide focal lengths from the zoom range. That has nothing to do with magnification: ( they just sell point and shoot cameras that way to give an idea how much range the zoom can cover, compared to other such cameras, but it's not a true measure of what magnification you get or related to like telescope magnifications )
It's simpler than that. The focal length, thus magnification/'reach' of a zoom lens is variable: thus a 70-300 can go longer than an 18-250: 300mm is always longer than 250mm, regardless of what else the lens can do.
That is to say, consider each focal length a lens can be set to, separately from any other it can be set to, when considering magnification/field of view. Focal length is an absolute. 300 is longer than 250mm.
The 18-250 can go much *wider* than the 70-300, though, so it may be much more versatile. And nearly as long.
So its basically the focal length that matters when it comes to magnification/bringing the subject closer. Enlightening :-)
Thank you.

06-12-2010, 10:19 AM   #5
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QuoteOriginally posted by omega leader Quote
Unless you need to focus on small things. Then everything goes out the window.

But if you are shooting objects at large (10+ feet) then the larger the focal length the better (i.e. 70-300mm is better than 50-200mm, and 50-500mm is better still).

If you are shooting small things (i.e bugs, flowers etc.) ask, but I won't try to explain it now. It will just confuse the issue.

Actually I just ordered the Pentax k-7 with 18-55mm kit lens. I was looking towards 18-250mm Tamron lens for wildlife photography and then I was stumbled upon the Sigma 70-300mm with macro capabilities. As it is clear now that the more the focal length, the far I can reach, I feel Sigma 70-300mm is a better deal as it also has macro capabilities (and it also grabbed good reviews).

The 18X, 20X etc...point-and-shoot terminology is all confusing and misleading many people like me. If I am right, because of the FOV factor in point and shoot cameras, the smaller focal lengths seem to reach more. Isn't it true?

Gunasekhar
06-12-2010, 01:57 PM   #6
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QuoteOriginally posted by gut1kor Quote
and then I was stumbled upon the Sigma 70-300mm with macro capabilities. As it is clear now that the more the focal length, the far I can reach, I feel Sigma 70-300mm is a better deal as it also has macro capabilities (and it also grabbed good reviews).
One thing to keep in mind and the other members can correct me if I am wrong, but just because a lens is labeled "Macro" you can not necessarily expect to capture extreme closeup photos.

Most true macro lenses are close focusing and and are 1:1, meaning the image on the sensor is the same size as it is in real life. Most zooms labeled macro are 1:2 to 1:4.

This is not to say that the lens will not work for what you need it to but if you want true macro photography you have to go with a dedicated macro lens.
06-12-2010, 02:44 PM   #7
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QuoteOriginally posted by bigdog104 Quote
Most true macro lenses are close focusing and and are 1:1, meaning the image on the sensor is the same size as it is in real life. Most zooms labeled macro are 1:2 to 1:4.
Yes, I did read about this. Thanks for bringing this up.
06-12-2010, 02:49 PM   #8
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The ratio you calculated is the zoom ratio. What you really need to consider is the image size. Image size is a simple calculation

image size = subject size x focal length / distance

the longer the focal length the bigger the image

06-13-2010, 09:33 AM   #9
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QuoteOriginally posted by gut1kor Quote
Hi

I searched on net but did not find the answers. I am stuck with a doubt regarding focal length and zoom capabilities. To say exactly, if I have two lenses 18-250mm, and 70-300mm which lens can bring a given subject more closer from a given point at the highest focal lengths?

How to calculate this reachability with any given lens and also in fixed focal length lenses? Does getting the zoom by diving the focal range numbers (250/18 = 14x and 300/70 = 4X) has any explanation in finding out which brings the subject more closer?

Please clarify.

Cheers
Gunasekhar
Gunasekhar, the higher the focal length number for a lens the greater the magnification of the subject you see though the viewfinder,just like looking through binoculars or a telescope.
As the magnification gets bigger the total area you see gets smaller, so when you look through your viewfinder with the lens set at 18 you will see from side to side [and up and down] more than if you set it to 55, however at 55 the subject in the middle of the frame will be bigger,
so to answer your question the 300 will give you the largest view of your subject.but you will see less of the total area you see with your eye.
try this .what you see with your eye is approximately the same size as what you see through the viewfinder with 55 mm lens on a Pentax DSLR but what you see with your eye from side to side is approximately what you see with a 18mm
lens.
so now you run into the problem of trying to balance what you see width wise against magnification!! which is why we stitch images together sometimes to give the viewer what they would have seen if they were there.
get complicated doesn't it?,
welcome to the wonderful world of photography!
regrad
Alistair
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