How to calculate X Zoom ?

[yb98]

I wonder if my method to calculate X zoom from the lentgh in mm is right or no ?

Suppose I have a 300mm lens with a DRebel. I multiply 300*1.6=480 which gives me the equivalent with a 35mm camera. And then I divide 480/35=13,7.

So the 300mm lens on a Drebel means 13,7 X Zoom.

Am I right ?

[Guillermo Freige]

Wrong :)

The X number has nothing to do with the absolute focal lenght or compared to the 35mm one. It measures the zoom ratio. To calculate it you must divide the largest focal length by the smallest one. So the 18-55mm lens is a 3x one, and the 100-300mm is also a 3x one, despite the range of focal lenght covered is very different. So a 300mm lens (a prime one) actually has no X number, because it isn't a zoom. Or actually it's a 1X lens :)
X numbers are meaningful in fixed lens digicams, because most of them have a smallest focal lenght of 28 or 35mm, so the relation is always against a wideangle. In SLR cameras, the X numbers are not used, but instead the absolute range covered (as 28-135) because the smallest focal lenght can be as small as 12 or as big as 100 in zoom lenses, and to give only the X number is meaningless.

[Zeke]

I am confused.

I have a 'point & shoot' digi stills camera, which has a 10X optical zoom. I would like to do some sports (cricket) photography and as such would require at least equivalent to 10X optical zoom. What sort of lens would I require for something between 10X - 20X optical zoom?

[Guillermo Freige]

Very simple.
As I-ve said, most digicams start at 28 or 35mm equivalence, so a 10x can be a 280 or 350 at the largest focal lenght. The drebel at 18mm has a 29mm equivalence (similar to the wideangle end of digicams), so the 55-200 will give you a 11x zoom (counting the focal lenghts covered by the kit lens), or 16x with the 75-300 or 22x with the 100-400, with some gap between 55 and 75 or 100 in those cases,

[Case]

Zeke,

you need to find out what the "35mm" equivelant focal lenght of your p&s is. It's probably something like 28mm-280mm. With a dSLR you'll need to buy lenses to cover the specific range you want. I don't think there is many slr lenses that have a ratio of 10x, a few maybe (sigma's 50-500 for example). There are plenty of lenses which will cover the range you're looking for (say 250-500mm). It's just a matter of funds :roll:
The problem with dSLR's is there always something else.....that new L lense, new flash etc etc

Chris

PS don't forget when looking at lenses for a 300d/10d to multiply the lenses focal length by 1.6 to account for the sensors smaller size...ie a 100mm lense becomes a 160mm lense.

[yb98]

I thought that the X zoom was relative to the human eye. So I thought that if I shot with 1xzoom, I'll get the same view as my eyes see it.

If we consider this definition of zoom (relative to human eye), how do you calculate it given a focal length with a DRebel ?

[Zeke]

Zeke, you need to find out what the "35mm" equivelant focal lenght of your p&s is.

Any ideas how I do this? Is it something that is likely to be in the documentation, or more something that I need to work out manually?

[Guillermo Freige]

I thought that the X zoom was relative to the human eye. So I thought that if I shot with 1xzoom, I'll get the same view as my eyes see it.

If we consider this definition of zoom (relative to human eye), how do you calculate it given a focal length with a DRebel ?

This is true for binocular magnifications. In cameras or camcorders is the smaller/larger focal lenght coefficient.
In 35mm equivalences, the normal lens is the 50mm, so you can use that length to calculate the X factor compared to the human eye. In the Drebel, the equation is: lens focal length x 1.6 (sensor crop factor) / 50. so a 300mm lens has a 300x1.6/50, so it's almost a 10x magnification.

[Zeke]

Ok, I have found the info regarding my camera:

10X optical zoom, 6.3-63.2 mm (35 mm equivalent: 38-380 mm)

[Guillermo Freige]

Zeke, you need to find out what the "35mm" equivelant focal lenght of your p&s is.

Any ideas how I do this? Is it something that is likely to be in the documentation, or more something that I need to work out manually?

It's in the docs. Check for "35mm equivalence" in the zoom definition, or just check any online review from your camera. That info is always specified

[Zeke]

Zeke, you need to find out what the "35mm" equivelant focal lenght of your p&s is.

Any ideas how I do this? Is it something that is likely to be in the documentation, or more something that I need to work out manually?

It's in the docs. Check for "35mm equivalence" in the zoom definition, or just check any online review from your camera. That info is always specified

Yep - see above post.

[yb98]

I thought that the X zoom was relative to the human eye. So I thought that if I shot with 1xzoom, I'll get the same view as my eyes see it.

If we consider this definition of zoom (relative to human eye), how do you calculate it given a focal length with a DRebel ?

This is true for binocular magnifications. In cameras or camcorders is the smaller/larger focal lenght coefficient.
In 35mm equivalences, the normal lens is the 50mm, so you can use that length to calculate the X factor compared to the human eye. In the Drebel, the equation is: lens focal length x 1.6 (sensor crop factor) / 50. so a 300mm lens has a 300x1.6/50, so it's almost a 10x magnification.

Okay I understand now. So the 18-55 mm lens kit provided with the DRebel has only a (55*1.6)/50=1,76 magnification factor.

My powershot S30 is (in 35mm equivalent) 35-105mm , so the highest magnification factor should be 105/50=2x magnification factor.
So the 3x optical zoom indicated on my S30 is not the magnification factor, but it corresponds to the ratio between the longest and shorest focal lenght (105/35=3). Am I right ?

[Case]

So the 3x optical zoom indicated on my S30 is not the magnification factor, but it corresponds to the ratio between the longest and shorest focal lenght (105/35=3). Am I right ?

Yep

:)

[Jim_T]

Zoom and magnification have little to do with each other.. They're two different things.

Zoom only describes how much you can change the focal length of a lens. For instance, if you have a 28-135 lens that's set to 135mm, and you're too close to get your whole subject in the viewfinder, you can ZOOM OUT to a smaller focal length to make things fit.. It's common to think zoom is designed to bring things closer.. but you have to consider you can zoom 'away' or 'out' as well.

When you twist a zoom lens in and out, your subject appears to rush towars you, or away from you... Or in other words, zoom close or zoom far.. Or zoom in and zoom out.. Zoom is a verb that's used to describe this apparent motion caused by varying the focal length.

The amount a lens can zoom in (or out) is determined by dividing the maximum focal length by the minimum focal length.. 135/28 = 4.82 X (135mm is 4.82 times greater than 28mm)

A fixed lens (ie EF 1200 f/5.6L USM) has NO zoom. That's right.. 1200mm and you have NO zoom. You have a fixed focal length, you can't zoom in or out.. (You have to use your feet :) ) You'll never find a fixed focal length lens described as a zoom lens.

If you go to the Canon lens site.. You'll see they do not call *any* of their prime lenses 'zoom' lenses.. http://www.usa.canon.com/eflenses/lineup/index.html

The magnification of a lens gets complicated. It depends on what format they're designed for.. A 100mm lens for a medium format camera will have less magnification than a 100mm lens designed for a 35mm camera.

Generally, it's accepted that for the 35mm film format, a 50mm lens approximates the human eye.. (Not exactly, but close). You'll notice that the 50mm prime lens is described as a 'Normal' lens or a 'Standard' lens. As focal lengths decrease, you move towards wide angle and as focal lengths increase, you move towards telephoto.

Since 50mm is roughly what we see.. You can get an approximate of lens magnification by dividing by 50. So a 100mm lens would magnify like a 2X telescope. A 300mm lens would be like a 6X telescope.

The 10D/300D have a 1.6 crop factor, so you have to take that into account. 300mm nets the same field of view as a 480mm lens.. So a fixed 300mm lens on a 10D/300D will give you no zoom and 9.6X magnification..

If you have a 15-30mm lens, you have 24-48mm.. Despite the fact the lens has a 2X zoom rating, at the maximum of 48mm, it has no magnification at all..

Zoom and magnification are two different things.

[yb98]

Thanks to you all for the explanations.
It's very clear now.

Regards,

[amollambe]

Very simple.
As I-ve said, most digicams start at 28 or 35mm equivalence, so a 10x can be a 280 or 350 at the largest focal lenght. The drebel at 18mm has a 29mm equivalence (similar to the wideangle end of digicams), so the 55-200 will give you a 11x zoom (counting the focal lenghts covered by the kit lens), or 16x with the 75-300 or 22x with the 100-400, with some gap between 55 and 75 or 100 in those cases,


hi,
sir kindly elobrate the calculation of x zoom. As i have Canon EF 100-300mm f/4.5-5.6 USM Lens (http://www.the-digital-picture.com/Reviews/Canon-EF-100-300mm-f-4.5-5.6-USM-Lens-Review.aspx)(how much x zoom dose it have & how to calculate it)
Thanking You,
Amol

[Jon]

That's a 3x zoom (300/100=3). But so's a 70-210 mm zoom. Or (roughly) an 18-55 mm. So you can see why the "x" of a zoom is pretty much useless information. This is, BTW, a 3 year old thread.

[Andy_T]

Actually, the second post in that thread gave a very simple and easy-to-understand method to calculate that:
The X number has nothing to do with the absolute focal lenght or compared to the 35mm one. It measures the zoom ratio. To calculate it you must divide the largest focal length by the smallest one. So the 18-55mm lens is a 3x one, and the 100-300mm is also a 3x one, despite the range of focal lenght covered is very different.

However, I am totally amazed by the amount of cr@p and misinformation that was posted afterwards.
Obviously it was sufficient to totally confuse everybody who continued to read the thread...

Best regards,
Andy

[rowdyred94]

LOL @ Andy. "I'm surprised how stupid and repetetive you all are. Regards."

Sometimes it needs to be said in other ways. Seems to me, most of the following posts were also correct.

[Milez5858]

Wrong :)

The X number has nothing to do with the absolute focal lenght or compared to the 35mm one. It measures the zoom ratio. To calculate it you must divide the largest focal length by the smallest one. So the 18-55mm lens is a 3x one, and the 100-300mm is also a 3x one, despite the range of focal lenght covered is very different. So a 300mm lens (a prime one) actually has no X number, because it isn't a zoom. Or actually it's a 1X lens :)
X numbers are meaningful in fixed lens digicams, because most of them have a smallest focal lenght of 28 or 35mm, so the relation is always against a wideangle. In SLR cameras, the X numbers are not used, but instead the absolute range covered (as 28-135) because the smallest focal lenght can be as small as 12 or as big as 100 in zoom lenses, and to give only the X number is meaningless.

The answer is technically correct, but creative listening to what the real question is would yeild an answer of Yes. He's looking for a way to compare a point and shoot, so he's done it exactly correct. 1.6 for Canons built in factors x mm length / 35 will yeild the closest human comparison between and apple and an orange.

[SkipD]

For all intents of a simple way to create a comparison of Zoom x on a point and click and know if you have close to the same magnification on your SLR, the answer to the original question should have simply been "YES".

Sure there are other factors that complicate the issue, but the guy wasn't asking for the theory of relativity. Most people who ask this question are simply trying to make sure the will "Get closer" to their subject than they were previously able to with their point and click.The original poster (OP) of this ANCIENT thread (it's 7 years old) was totally unaware of the truths involved. It sounds to me like you don't know how to make the comparison between focal lengths on point-n-shoot cameras and SLR lenses either.

Here's an example of how it's done: Mary Sue has been using a Canon SX120 IS point-n-shoot camera and is wanting to use a Canon 50D DSLR. She is, of course, interested in what focal lengths she would need to keep the versatility of the SX120 camera's 10X super-zoom lens. The SX120 lens is actually a 6.0 mm to 60.0 mm lens, but the advertising also shows the "35 mm equivalent" focal length range as 36 mm to 360 mm. To know the focal lengths needed for the 50D, merely divide the "35 mm equivalent" values by 1.6. In other words, Mary Sue would need 22.5 mm on the short end and 225 mm on the long end for the 50D to have the same field (angle) of view coverage as her SX120 IS camera.