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nyoman
08-05-2006, 02:12 AM
I have seen or heard tele lens for digital SLR such as Sigma 50-500mm or Tamron 200-500 mm

I am completely new to this 'mm' measurement terms. I think that means the focal length right? (but you know I'm not a native english speaker and I don't quite understand it).

Does 50-500 mm lens mean that it will enlarge an object 10X optically ? because with my Panasonic DMC FZ7, the 36 mm - 432 mm lens mean that it has 12X optical zoom ability.

And, does 200-500 mm lens mean that it will enlarge an object 2.5 X ?

If I want to photograph birds that is very far, about 100 m far or so, what kind of tele lens that is good for that?

Thank you for your help.

coldrain
08-05-2006, 02:24 AM
The focal length is in mm's. The X x optical zoom is just confusing marketing speak with no real value or meaning about what the lens can do.
An 18-200mm lens would have a "11x optical zoom range", a 50-500mm a "10x", but both lenses have NOTHING in common.

To make things more confusing, it depends on the size of the sensors of a DSLR of how big it will get the subject on photo with a certain lens.
That is why as reference they take a 35mm film SLR, and with the crop factor of the sensor determine the field of view compared to the 35mm SLR.

So, a 50-500mm lens on a 1.5x crop factor camera like a Nikon D50 or Sony A100 gives the field of view of a 75-750mm lens.

That does give you some zoom power to catch birds with
To get an idea about what those mm's mean in terms on what you get on the photo, play around here:
www.tamroneurope.com/flc.htm

AlexMonro
08-05-2006, 06:27 AM
To get an idea of the "magnification factor" of a telephoto lens you probably need to compare the focal length with that of a "normal" lens - one which gives a field of view of roughly that that our eyes percieve as a normal scene (excluding the "corner of the eye" peripheral vision). This normal focal length is about equivalent to the diagonal of the image formed on the film / sensor - about 43mm for a standard 35mm film. For historical reasons, 50mm has become accepted as normal for 35mm film.

To achieve some standardisation to compare fields of view of digital cameras, which have many different sizes of sensors, lenses for compact cameras are usually quoted in terms of "35mm equivalent" - i.e. give the same field of view as a lens of that focal length on a 35mm film camera. This is complicated by the fact that a standard 35mm film frame has an aspect ratio of 3:2 (36 x 24mm), whereas most digital cameras have aspect ratios of 4:3. Your FZ7 actually has a focal length of 6 - 72mm, which, because its 1/2.5"sensor is 1/6 the linear dimensions of the 35mm film frame, gives the 36-432mm quivalent focal length. However, because it starts from the slightly wide angle 36mm equivalent focal length, the magnification at full zoom is roughly equivalent to an 8x - 10x telescope.

Digital SLRs tend to quote the actual focal lengths, together with a "crop factor", as coldrain says. This crop factor describes how much smaller the DSLR's sensor is than the 35mm film frame. The Nikon D50 has a sensor 23.7 x 15.5 mm, which gives a diagonal of about 28.3 mm, or 1/1.5 times the 43.3 mm diagonal of the 36 x 24 mm standard 35mm film frame. So at the 50mm end of the Sigma zoom, you get a field of view the same as a 75mm lens on a standard 35mm film camera.

Because the standard focal length for 35mm film is about 50mm, you can get an idea of the "magnification factor" by dividing the focal length by 50. So, on a 35mm film camera, full zoom on the lenses mentioned (500mm) will give a magnification roughly equivalent to looking through a 10x telescope (note that this is only by coincidence the same as the Sigma's 10x zoom range - the Tamron still has 10x magnification, though only 2.5x zoom range).

When used with a cropped sensor DSLR, you need to multiply by the crop factor for the sensor size - e.g. 1.5 for the Nikon, or 2 for a four thirds format camera, such as the Olympus E500, which would have a magnification similar to a 20x telescope when used with a 500mm (actual) focal length lens.

Hope this explains things a little. If you need more details, this page might be helpful:

http://www.cambridgeincolour.com/tutorials/camera-lenses.htm

tate30
08-05-2006, 04:20 PM
This is somewhat off topic and please forgive my ignorance! I have a camera with a X10 zoom. I would think an object should be 10x larger at max zoom ( 1" would be 10"). My test showed only about a 20% increase in size. Is this right or did I miss something?

Shosta
08-05-2006, 04:28 PM
This is somewhat off topic and please forgive my ignorance! I have a camera with a X10 zoom. I would think an object should be 10x larger at max zoom ( 1" would be 10"). My test showed only about a 20% increase in size. Is this right or did I miss something?
Assuming that it is optical zoom, x10 just means that at longest focal length you have ten times of the shortest focal length.
So a 17-70 mm zoom have 70/17 = x4 zoom.
For a 50-500 mmm you have x10 zoom.

My S2 have an equivalent 36-432 mm = x12 zoom but at 432 mm things are smaller than at 500mm

Regards
Shosta

cwphoto
08-10-2006, 09:51 PM
This is somewhat off topic and please forgive my ignorance! I have a camera with a X10 zoom. I would think an object should be 10x larger at max zoom ( 1" would be 10"). My test showed only about a 20% increase in size. Is this right or did I miss something?

There should be a 10x difference in the object height - so maybe something's wrong somewhere.:confused:

cdifoto
08-10-2006, 09:53 PM
This is somewhat off topic and please forgive my ignorance! I have a camera with a X10 zoom. I would think an object should be 10x larger at max zoom ( 1" would be 10"). My test showed only about a 20% increase in size. Is this right or did I miss something?

Truth in advertising, or lack thereof?

David Metsky
08-11-2006, 07:49 AM
This is somewhat off topic and please forgive my ignorance! I have a camera with a X10 zoom. I would think an object should be 10x larger at max zoom ( 1" would be 10"). My test showed only about a 20% increase in size. Is this right or did I miss something?
You missed something. How did you perform your test?

-dave-