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PostPosted: Mon Sep 14, 2009 9:16 pm 
If I was to buy a 70mm-300mm lens on a camera with 1.5x reduction like a a200 for example, to give me an idea, what would this roughly convert to in optical zoom (10x?) I have read all about this but im still a little confused :oops: ??


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PostPosted: Mon Sep 14, 2009 11:01 pm 
if i undersrand correct you?? The zoom of that lens will be 300/70 = 4.1 or something, that is the zoom, whether you are using aps-c or full frame. On the aps-c, like your a200 the actual focal length will be 1.5 * 70-300, thus it will be 105- 450mm, this means you are able to get closer with a crop sensor than a full frame, with the same lens. Hope this helps.


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PostPosted: Mon Sep 14, 2009 11:10 pm 
to get 10x zoom, you will have to get an 18-200 or 250 lens, but watch out, the longer zoom range lenses are not always very good optically, though the sony ones are quite good.


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PostPosted: Tue Sep 15, 2009 2:36 am 
The 1.5x crop factor doesn't change the zoom ratio. 70*1.5 = 105 and 300*1.5=450. 450 / 105 is still approximately a 4.3x zoom**

If you are getting in SLRs, I would advise you not to look at the zoom ratio and look at the focal length instead. A 10-100mm lens and a 50-500 are both 10x zooms but they are completely different in practice. :wink:

**Edited for embarassing maths.


Last edited by grahamnp on Wed Sep 16, 2009 1:26 pm, edited 1 time in total.

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PostPosted: Tue Sep 15, 2009 5:26 am 
Hey there, no question is too stupid to ask!

as others have said:
"optical zoom" Is something that is used to market point and shoot digital cameras. It refers to a ratio of the shortest focal length and the longest focal length, for example if a digicam that goes from 20-200mm would have a 10x optical zoom. To convert your lens' focal length into optical zoom you would need to compare it to the focal length of a specific camera.

So if your friend has a digicam with 10x optical zoom that goes out to 225mm, your 450mm equivalent lens has 20x optical zoom.

In other words, forget about optical zoom, just think in focal length.


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PostPosted: Tue Sep 15, 2009 9:11 pm 
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Yep, as the others have said, just divide the big mm number by the small mm number to get the 'times' magnification.

So a 70-300mm has 300/70= 4.2x magnification.

The equivalent focal length is different - on a cropped body, it would be 70-300mm x 1.5 = 105-450mm. But the times ratio is still the same: 4.2x.

This guide could be useful to you:

http://www.cameralabs.com/lens_guide/Le ... uide.shtml


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PostPosted: Tue Sep 15, 2009 9:58 pm 
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Assuming you have a kitlens such as the 18-70mm, the optical zoom figure you're looking for is of course significantly higher. You'll be able too zoom from 18mm (27mm) to 300mm (450mm). You'll get 16.67x optical zoom that way.
It'd be good to read a bit about focal lengths though, get to know the basics of lens technology.

- Bjorn -

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PostPosted: Wed Sep 16, 2009 12:49 pm 
yeah 10x zoom for example is not a universal specification. It varies from point and shoot to point and shoot and is more of a selling point to attract customers even though it's not that useful. You have to look at the real focal lengths of the lenses to compare apples to apples.


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PostPosted: Wed Sep 16, 2009 2:05 pm 
ClayC wrote:
Hey there, no question is too stupid to ask!

as others have said:
"optical zoom" Is something that is used to market point and shoot digital cameras. It refers to a ratio of the shortest focal length and the longest focal length, for example if a digicam that goes from 20-200mm would have a 10x optical zoom. To convert your lens' focal length into optical zoom you would need to compare it to the focal length of a specific camera.

So if your friend has a digicam with 10x optical zoom that goes out to 225mm, your 450mm equivalent lens has 20x optical zoom.

In other words, forget about optical zoom, just think in focal length.


Thanks, I'm used to motoring forums where you get flamed for asking basic questions! :) Am I correct in thinking 300mm on a 70-300 lens will look the same as 300mm on a 18-300 lens? The problem is I am after a budget dslr and an extra lens mainly for motorsport, where I will be stood at the edge of the track and need to get close up to the cars, my current 5x optical zoom compact doesn't get close enough so i want to buy a lens that gets me closer than that!


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PostPosted: Wed Sep 16, 2009 4:55 pm 
willgti6 wrote:
ClayC wrote:
Hey there, no question is too stupid to ask!

as others have said:
"optical zoom" Is something that is used to market point and shoot digital cameras. It refers to a ratio of the shortest focal length and the longest focal length, for example if a digicam that goes from 20-200mm would have a 10x optical zoom. To convert your lens' focal length into optical zoom you would need to compare it to the focal length of a specific camera.

So if your friend has a digicam with 10x optical zoom that goes out to 225mm, your 450mm equivalent lens has 20x optical zoom.

In other words, forget about optical zoom, just think in focal length.


Thanks, I'm used to motoring forums where you get flamed for asking basic questions! :) Am I correct in thinking 300mm on a 70-300 lens will look the same as 300mm on a 18-300 lens? The problem is I am after a budget dslr and an extra lens mainly for motorsport, where I will be stood at the edge of the track and need to get close up to the cars, my current 5x optical zoom compact doesn't get close enough so i want to buy a lens that gets me closer than that!


Yes, 300mm is 300mm and thus the two focal lengths will give you the same feld of view on the same camera.


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PostPosted: Thu Sep 17, 2009 2:17 pm 
What do you think of this, seems cheep for 500mm? :?click here

Edit: I have just read up on mirror lens'


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PostPosted: Thu Sep 17, 2009 8:11 pm 
The problem with generic cheap optics in my experience is that image quality can be pretty bad. For example I found that when I used a cheap 2x teleconverter on one of my lenses the image quality was so bad that it looked better to just do a 2x digital crop later without using that glass at all. You might be better off with a sigma or tamron 70-300 telephoto.

That being said Ive never used that lens or brand.


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