Zoom and Magnification???

[woty87]

Hi all,

I apologize for the open ended nature of this question, but can anyone make the concept of "zoom" and "magnification" easier for me to understand.

On CIC and other places, I see people refer to their lenses as 2x or 4x or such. I'm never really quite sure what that means. I have to believe its related to the focal length but what is the correlation exactly? What does it really mean for something to have 4x zoom etc..?

Also, how does this relate to magnification? I know that if a lens has a magnification of .25x, that means the image on the sensor will be 25% of the actual object size. But doesn't that change with distance from the object? So, is magnification a function of how close you are and lens? Or is it just a function of only the lens itself?

I'm sure you can tell I'm pretty lost on the subject. I've been through the CIC tutorials, but I think I just got further confused. If anyone can make it simpler for me, it would be very much appreciated.

Thanks!

[revi]

Let's start with the zoom factor: that's simply the ratio between the longest and shortest focal length of the lens.
So a 75-300 mm would be a 4x zoom, same as a 15-60mm (note that these don't even have overlapping focal ranges...). The wider zoom ranges (like 18-200mm, 11x) can mean you need less lenses, and have less to carry around for a given range of focal lengths. No idea about the optical quality of such zooms though (compared to the more traditional 4x zooms)

The magnification factor is exactly what you think it is. And, for a given lens, it indeed depends on the distance to the object, so the maximum magnification given for a lens is the magnification at the closest possible focusing distance, without any close-up lenses or extension tubes. (same as when you look at something: the closer you get, the larger the object seems to be, but get too close, and you can't focus any more).

The magnification factor for a given distance to the object depends on the lens: longer focal lengths give a higher magnification. That's the reason to use longer focal lengths for macro lenses: you can (and must) stay farther away, and still get 1:1 images. The longer working distance means less problems with lighting the subject, and less chance to disturb e.g. insects (flowers and such are less likely to run away, in my experience)

So, magnification depends on the actual focal length of your lens and the distance to the subject. And for the record: the zoom factor has NO relation to the magnification.

Remco

[Dave Humphries]

Very good answer Remco, nothing to add.

[enaiman]

I won't go too deep into this because Remco already gave a fine explanation

If you are into calculus, this page: http://www.mystd.de/album/calculator/magnification.html gives plenty of explanations

[FrankMi]

One thing I would add, because it confused me initially, is that to compare the zoom of one lens to another, you really need to consider the focal length as each relates to the 35mm Full Frame Equivalent standard. For example, my D3100's 55-300mm telephoto lens has a 5.4X Zoom. The Nikon S9100 has an 18X zoom (4.5-81mm) so does the S9100 have more (or less) magnification than my 55-300mm at max zoom?

To find out, I have to convert both to 35mm Full Frame Equivalent (FFE) sizes.

The D3100's 55-300mm converts to a FFE of 82.5-450mm.
The S9100's 4.5-81mm converts to a FFE of 25-450mm.

They both have the same telephoto magnification of 450mm in FFE terms, but the wide-angle capability of the S9100 is better.

If I include my Tokina's 11-16mm wide-angle lens and the Nikon 18-55mm kit lens in the mix, the total zoom for my D3100 is 11-300mm or a FFE of 16.5-450mm, a whopping 27X.

Hope this helps!

[rpcrowe]

Remco answered this concisely and correctly. I would like to add one bit of information. Given today's zoom lens technology; you cannot get a zoom lens with a constant f/2.8 aperture that has a zoom range of much over 3x.

As an example the Canon 70-200mm f/2.8 IS ii lens has a zoom ratio of about 2.86X calculated by dividing the shortest focal length of the lens into the longest focal length: or 70/200 = 2.86. My 17-55mm f/2.8 IS lens has a zoom ratio of just a tad over 3.0x: or 17/55 = 3.24...

However the new Canon 70-300mm f/4-5.6L IS lens has a zoom ratio of 4.29x or 70/300. This lens is a variable aperture which starts out at f/4 at 70mm, stops down to f/5 at 200mm and ends up as f/5.6 at 300mm. This lens is one of Canon's newest zooms and is a very expensive lens. However, the technology is just not there to provide a commercially viable lens with that long of a zoom range and a constant f/2.8 aperture.

Variable aperture lenses are also less expensive to manufacture. The very inexpensive Canon 18-55mm f/3.5-5.6 kit lens has a zoom ratio of only 3.0x but was designed as a variable aperture lens because it could be produced relatively inexpensively. The 18-55mm kit lens runs $170 (USD) at Adorama in NYC while my 17-55mm f/2.8 runs $1,059 (USD) from the same supplier. Of course there are other advantages to my lens besides the constant f/2.8 aperture, but the constant aperture is a major reason that the lens is so expensive...

[woty87]

Thanks all. You are a wealth of information as always. All cleared up!

[Glenn NK]

I would like to add one bit of information. Given today's zoom lens technology; you cannot get a zoom lens with a constant f/2.8 aperture that has a zoom range of much over 3x.

Very interesting Richard; I did not know this.

Further to those comments, the Canon TSE 17-55, constant f/2.8 has a zoom ratio = 3.24 (slightly higher than three). What's perhaps more significant (for Canonites shooting crop bodies), is that this is a seriously sharp lens. Quite an achievement. The competition (Tamron's 17-50 f/2.8 has a ratio of 2.94, and Sigma's 17-70 with a ratio of 4.12, drops to f/4.5 at the long end).

There are often comparisons of these lenses on various forums, but the obvious excellence of the Canon 17-55 in this respect never seems to be mentioned.

Thanks for the interesting information,

Glenn