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-   -   zooming question (https://forums.steves-digicams.com/add-lenses-49/zooming-question-171036/)

rayzor May 18, 2010 12:29 PM

zooming question
 
I am a bit confused about zooming. How can I find the effective zooming distance between two lenses? I just found out the multiplier tells me nothing.

Example:
Let's say I want to fill the screen with an object with lens A. I can zoom in.. The object moves further and further away.. I zoom in some more. At some point (without losing clarity) I can't zoom no more. Lens B with a "bigger zoom" allows me to do the same thing but further. How much further can I go?

TCav May 18, 2010 12:51 PM

Different cameras have different size image sensors. Smaller image sensors provide narrower angles of view than larger image sensors. Different cameras have lenses with different focal lengths. Shorter focal lengths provide wider angles of view than longer focal lengths. Put them together, ans you really can't compare the angles of view of two different cameras without some common frame of reference.

The specification you should be looking for is the "35mm Equivalent Focal Length". That tells you the angle of view is equivalent to the angle of view a specific focal length lens on a 35mm film camera. All camera manufacturers use it, and it's the only way to compare the angle of view that a particular camera is capable of. In fact, some cameras don't give the actual focal length. And some manufacturers provice the "35mm Equivalent Focal Length" in the EXIF data in each image.

rayzor May 18, 2010 1:05 PM

I was actually talking about lenses on a SLR. This should provide a better frame of reference. For example a 18-55mm lens vs a 18-200mm lens. The latter zooms further.. but how much further? about 4x further than the former? If so, what is that in terms of distance?

TCav May 18, 2010 1:39 PM

Actually, what I said applies to dSLRs as well.

And you need to understand that zooming doesn't affect the angle of view linearly. A 10mm lens is a lot wider than a 20mm lens, but a 210mm lens is only a little longer than a 200mm lens. And while a focal length of 200mm is a lot longer than a focal length of 50mm (4 times longer), and the angle of view at 200mm is 7.8 while the angle of view at 50mm is 30.2 (almost 4 times narrower), the angle of view at 12.5mm is 94.4 (just a little over 3 times wider that a 50mm lens.) And since the angle of view doesn't change linearly with the focal length, neither does the distance.

Perhaps Tamron's Find the Right Lens webpage and Canon's Focal Length Comparison webpage will help you.

rayzor May 18, 2010 2:46 PM

I'm still confused.. Basically my question is this... Let's say I'm standing at the upper deck of a football stadium.. Let's say that distance is 300 feet. What lens do I need to magnify the player so that it looks like I am standing 10 feet from him. Is there a way to calculate the distance that a particular lens can "zoom" to based on the specs of the lens.

Addition:
I just checked out those sites.. That's what I'm looking for! Thanks! But is there a mathematical formula to calculate this? or does it vary by manufacturer?

TCav May 18, 2010 2:53 PM

Yeah. It's trigonometry.

The tangent of half the horizontal angle of view times the distance to the subject equals half the width of the view at that distance.

Does that help?

rayzor May 18, 2010 4:45 PM

It would be more easier if lens makers just say, this lens will zoom X feet. I just want to see how far away a lens will actually magnify to. Is there a reason why this isn't done?

TCav May 18, 2010 5:17 PM

If I understand you correctly, you'd like a ratio of the width of a scene to the distance to the scene, which is the angle of view but not expressed in degrees. And while that ratio would be smaller as the focal length got longer, but it would get quite large for wide angle lenses. For lenses with a 180 angle of view, the ratio would be 1:∞. How would you express that ratio for lenses with angles of view greater that 180?

dnas May 18, 2010 11:35 PM

Quote:

Originally Posted by rayzor (Post 1095912)
It would be more easier if lens makers just say, this lens will zoom X feet. I just want to see how far away a lens will actually magnify to. Is there a reason why this isn't done?

It can't really be expressed that way.

An 18-55mm lens will zoom to 100 feet or infinity, just the same as a 55-200mm will. The difference will be the magnification.

Or are you actually saying that in order to fill the whole width of the frame with an object of a certain size, the zoom will do so at x feet away from the camera?

So for example, if a 50mm lens will fill the frame width with a 6 foot wide object at 5 feet, then a 200mm lens will fill the frame width with a 6 foot wide object at 20 feet. So the zoom = 5 feet for the 50mm, and 20 feet for the 200mm. Is this what you mean??

Streets May 21, 2010 9:36 AM

If you are 300 feet from an object and are using a 50mm focal length lens. then you wish to change to a lens that will make it appear that you are now just 10 feet from the object; simply multiply the original focal length by 30. You will need a 1500mm telephoto lens and a very sturdy tripod to accomplish this particular task.


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