What’s Your Preference?

NikCan · #1 (**permalink**) 11-21-2010, 08:15 AM

First I'll say I am new to this forum.
Sorry if this question is in the wrong place.

I picked my username as a Joke as
A Cross between Nikon and Canon

Anyway; My question is I think I have the focal lengths down pat.
But I would Like to be kindly corrected if I am wrong.

I came up with these by dividing the focal lenghs by 18mm
such as in a 18-55mm kit lens

55mm = to 3X zoom

100mm = to 5X zoom

200mm = to 10X zoom

300mm = to 15X zoom

400mm = to 20x zoom

500mm = to 25x zoom

600mm = to 30x zoom

800mm = to 40x zoom

Am I Correct???

inkista · #2 (**permalink**) 11-21-2010, 08:35 AM

Nope. Sorry.

If you're calculating zoom the way it's described for the marketing of P&S cameras, then the "zoom" of a lens is a relative thing. You divide the long focal length of a zoom by its short focal length.

So, the 18-55 is a 55/18 => 3x zoom
And a 100-300 lens is a 300/100 => 3x zoom

this is actually kind of useless for figuring out the "reach" of a lens, which is why lenses are more accurately described by focal lengths. Focal lengths are absolute values, not relative ones. This is a good demo of what focal lengths mean in terms of reach.

If you're talking magnification as with binoculars, I believe (although I could be wrong on this, because this is more of an apples'n'oranges thing), you just divide the focal length by 50mm, as 50mm lens gives the same magnification/power as the unaided human eye. So, a 400mm lens would be like using 8x binoculars, a 300mm lens like 6x.

NikCan · #3 (**permalink**) 11-21-2010, 09:57 AM

Quote:

Originally Posted by inkista

If you're calculating zoom the way it's described for the marketing of P&S cameras, then the "zoom" of a lens is a relative thing.

Could someone elaborate more on this for me???
Why is it a relative thing???
I mainly just wanted all of my calculations corrected
if that's even possible?
Thanks In Advance!!!

RecurrentNerve · #4 (**permalink**) 11-21-2010, 10:04 AM

It's relative as shown by inkista's calculations:

Quote:

So, the 18-55 is a 55/18 => 3x zoom
And a 100-300 lens is a 300/100 => 3x zoom

Both give '3x' zoom, but the 100-300 lens will obviously 'zoom' more than the 18-55.

daft_biker · #5 (**permalink**) 11-21-2010, 01:11 PM

Quote:

Originally Posted by NikCan

Could someone elaborate more on this for me???
Why is it a relative thing???
I mainly just wanted all of my calculations corrected
if that's even possible?
Thanks In Advance!!!

The figures look right enough. They're pretty meaningless other than as an illustration of how long a lens you might need if upgrading to a SLR from a compact.

It's relative because the wide end of zooms is often different. You don't always start from the same focal length.

With SLRs you might be able to get a range of lenses to cover from 8mm up to 1200mm which you would need a 150x zoom lens to cover but if you start at 18mm you only need a 67x lens.

Elmo · #6 (**permalink**) 11-21-2010, 02:00 PM

There are two different mag factors. The first is zoom specific where as noted the longest FL is divided by the shortest. The second has more meaning overall it is the magnification factor. Here it FL used (Can vary with zoom) is divided by the Normal FL for the camera sensor or film. Rough approximations for the normal FL is 43 mm for Full Frame sensor and 28 mm for 1.6 sensors. so a 300mm zoom can reach up to 6.9x magnification for a 70-300 zoom and 10.7x for reduced sensor.

The theoretical FL is the diagonal distance across the sensor. However the accepted value for FF is 50 mm making the 1.6 reduced sensor 32mm.

sk66 · #7 (**permalink**) 11-21-2010, 02:23 PM

Quote:

Originally Posted by daft_biker

The figures look right enough. They're pretty meaningless other than as an illustration of how long a lens you might need if upgrading to a SLR from a compact.

It's relative because the wide end of zooms is often different. You don't always start from the same focal length.

With SLRs you might be able to get a range of lenses to cover from 8mm up to 1200mm which you would need a 150x zoom lens to cover but if you start at 18mm you only need a 67x lens.

+1
They would be right if there was such a thing as an 18-400 or 18-800, but there isn't.
For this kind of thing most would consider 50mm as a generic starting point (FF) as it is the "normal fov".

So a 200mm would be 4x normal (zoom) and a 25mm would be a .5x (wide angle).
It's still pretty "generic" information....

sk66 · #8 (**permalink**) 11-21-2010, 02:34 PM

Quote:

Originally Posted by inkista

This is a good demo of what focal lengths mean in terms of reach.

Cool. First time I've seen something that makes getting an idea of FOV "simple"...

Using the info provided and swagging 50mm= 50* fov then 200mm = 4x zoom and 1/4 fov =10*, and 25mm would be .5x/1.5 fov or 75*

Hah! Not exact by any means but kinda nifty....