View Full Version : optical zoom conversion question

TeddTucker

07-03-2007, 09:37 AM

Very newbie question; I’m looking to get a digital camera SLR and I’m having a hard time understanding the optical zoom length. I know you take the “70-300mm” and times it by the focal length multiplier (say 1.6) and this gives me 122-480mm, but what does that mean? How many times zoom is that? I want to know if it is like 7x 10x…. so I can compare it to my point and shoot.

David Metsky

07-03-2007, 09:50 AM

X times zoom means nothing without context. A 100-300 mm lens is a 3x zoom no matter what camera you put it on. It's simply a ratio of the minimum to the maximum zoom length. Its just shortcut for saying the real numbers.

In most P&S cameras the low end zoom is between 32 and 36mm (equiv) so you can use the numbers almost interchangebly. But once you go to a dSLR with separate lenses, it becomes meaningless.

In this case, your lens is approximately a 4x zoom. But since it starts and 122mm, and no P&S starts anywhere near 122mm there's no way to compare. This lens covers the long end of most ultrazooms, but not the wide end.

erichlund

07-03-2007, 12:34 PM

Very newbie question; I’m looking to get a digital camera SLR and I’m having a hard time understanding the optical zoom length. I know you take the “70-300mm” and times it by the focal length multiplier (say 1.6) and this gives me 112-480mm, but what does that mean? How many times zoom is that? I want to know if it is like 7x 10x…. so I can compare it to my point and shoot.

Well, if you know your point and shoots 35mm equivalents, you can compare directly. To get "x" value of a lens, divide the large number by the small number. 300 / 70 = 4.3, so it's a 4x lens (Except for the marketing dept, which always rounds up). You will notice that 480 / 112 gives the same ratio, so also rounds to 4x.

If you look at my sig below, my widest angle is 17mm and my longest telephoto is 300, so my lenses combined give a range of 17.6 or 18x. If I get the Tokina 12-24 then my range will jump to 25x. But I don't have any one lens that covers that full range.

SpecialK

07-03-2007, 01:26 PM

Also, an "ultra zoom" for a DSLR usually does not have the best image quality. That is one reason people have more than one lens.

TeddTucker

07-03-2007, 02:47 PM

Thanks for the reponses, but I’m still a little confused. Currently, I own a Canon powershot a710 which has 6x zoom. If I buy a rebel XTi and a 70-300mm lens I’m going to have less zoom? The main reason I’m upgrading is because I can’t get close enough to wildlife without scaring them away and I thought a SLR with this type of lens would give me that reange/distance

No you will have more zoom. The camera you have has a range of 35-210 which is smaller then the 70-300. You will need another lens for the wide end though

David Metsky

07-03-2007, 09:13 PM

Your Canon A710 is 35-210.

That lens on the XTi is 122-480.

Multiplier numbers mean nothing, those numbers above tell what your cameras can do. If you want more zoom, you want bigger numbers, and this camera/lens combination will give you that.

Tom_N

07-03-2007, 09:24 PM

The zoom factor of a 70-300mm lens is approximately 4.3x (300/70). On a Digital Rebel XTi, the field of view will be equivalent to what a 112-480mm lens would provide on a 35mm SLR.

Assuming that your current camera has a field of view that is equivalent to what a 35-210 lens would provide on a 35mm SLR, the 70-300mm lens will provide about 2.3x more magnification at the far end (480/210 ~= 2.3). It will bring you a bit closer to birds and other wildlife.

Honest Gaza

07-03-2007, 11:44 PM

Ok, you wanted a rough guide...here it is :p

A Point & Shoot with x3 Optical Zoom is roughly equivalent to 100mm in SLR Lens terms.

A Point & Shoot with x10 Optical Zoom is roughly equivalent to 300mm in SLR Lens terms.

A Point & Shoot with x12 Optical Zoom is roughly equivalent to 430mm in SLR Lens terms.

So far so good ?

When you shove a 300mm SLR Lens onto an SLR camera, it gives you 300mm (like the x10 Optical Zoom of the Point & Shoot).

Unfortunately, when you purchase a Digital SLR, it may have a focal length multiplier (Crop Factor determined by Sensor size), which also "alters" the final "perceived" focal length.

Generally, lower end Canon DSLR cameras use multiplier of 1.6

Generally, lower end Nikon DSLR cameras use multiplier of 1.5

Therefore, my Canon 400D (XTi) using a 300mm lens will be the equivalent of 480mm (slightly better than the rough equivalent of 430mm that a Point & Shoot with x 12 Optical Zoom would provide).

Thanks for the reponses, but I’m still a little confused. Currently, I own a Canon powershot a710 which has 6x zoom. If I buy a rebel XTi and a 70-300mm lens I’m going to have less zoom? The main reason I’m upgrading is because I can’t get close enough to wildlife without scaring them away and I thought a SLR with this type of lens would give me that reange/distance

Binoculars make this terminology difficult because they are actually talking magnification. A 7x35 binocular means an object will be 7 times closer to you with a 35mm field of view. Not so with a camera lens, a camera lens is only talking about the difference between the smallest focal length and the largest focal length.

The 10x or 4x or 3x figure has absolutely nothing at all to do with the magnification of a camera lens, it's simply a number to explain how much zoom(zoom doesn't mean magnification)a particular lens has. In simple terms a 10mm-100mm lens is a 10x lens because 10mm(the widest focal length)multiplied by 10(10x)is 100mm. A 4x lens with the same starting focal length would be a 10mm-40mm, a 3x lens would be a 10-30, etc. etc.

On a camera lens you kind of have to "know" what type of lens a given focal length is: 28mm=wide angle, 50mm=normal lens, 100mm=telephoto, 400mm=super-telephoto....The Field of view changes with each lens and the lens focal length is stated in mm basically the number of mm the front element is from the focal plane. The farther the front element is from the focal plane the more magnification a lens has and the smaller the field of view.

Your A710 has a 35mm-210mm zoom lens. The difference between the smallest focal length(35mm a moderate wide angle)and the largest focal length(210 short telephoto)is 6x(35 multiplied by 6 is 210).

The 70-300 lens you talk about putting on a rebel is just a little more than a 4x lens(70x4.28=300). But the 300mm max focal length is more of a telephoto lens than the 210mm focal length of the A710.

To top all of this off, with digital cameras you have to apply a crop factor the accepted focal lengths when comparing them to a 35mm camera's accepted focal lengths. 35mm cameras were the most popular cameras in film and the idea that a 28mm was a wide angle and a 300mm was a telephoto was widely known to most photographers using 35mm equipment. Digital cameras use a sensor that is much smaller than a frame of 35mm film rendering the old standard used for 35mm film useless.

GarethP

07-04-2007, 03:23 AM

This sort of links across to my post so I hope you don't mind me butting in??

When I get my DSLR it's main purpose will be to shoot Field Hockey images from the sideline (approx 60m from the goal). Currently on full zoom with my S3 the goal (approx 4m x 2m) fills the frame. What sort of focal length would I be needing on a DSLR to achieve the same result?

Currently on full zoom with my S3 the goal (approx 4m x 2m) fills the frame. What sort of focal length would I be needing on a DSLR to achieve the same result?

the S3 has a 35mm equivalent focal length of 36-432mm. so as Big Gaz pointed out in his post, for a dslr you need a 300mm focal length; 300 x 1.5 crop factor from dslr = 450mm 35mm equiv. the 70-300mm range lens' are pretty common telephoto lens'.

then if you add a 1.4 teleconverter for minimal IQ loss thats 300 x 1.5 x 1.4 = 630mm. thats a fair bit of reach. lol

GarethP

07-04-2007, 04:31 AM

OK, cool - I think I've got the hang of this now!! :rolleyes:

Honest Gaza

07-04-2007, 05:37 AM

the S3 has a 35mm equivalent focal length of 36-432mm. so as Big Gaz pointed out in his post, ....

Rooz....You promised that would be our little secret :o :D

Rooz....You promised that would be our little secret :o :D

lmao just talking you up in front of all these foreigners mate. :D

Here's examples of focal lengths in photos:

http://www.tamroneurope.com/flc.htm

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