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Old Jan 6, 2007, 7:26 AM   #1
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Quick Question: :?

I have a 30D which has a 1.6 crop factor...How does this effect the focal length of a lens..?? Also i here folk's saying that 35mm on a SLR has more focal length..If i've got it right...

How do you work out the focal distance on a lens..Say...200-500 or is it the same...

Just trying to work this out...Very Confused..:?:?
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Old Jan 6, 2007, 7:38 AM   #2
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The crop factor does not change the focal length of the lens.

A 200-500mm lens remains a 200-500mm lens crop factor or not.

The crop factor however gives you a field of view equivalent to a lens with a focal length of 320-800mm.




**edited for poor maths skills ! **
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Old Jan 6, 2007, 7:41 AM   #3
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Thanks...Oh well learn new things every day...
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Old Jan 6, 2007, 7:57 AM   #4
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Sintares is absoultly correct, but although the focal lenght does not change the narrow field of view does make the image appear larger on your sensor, although really no closer than the same lens on a full frame camera.
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Old Jan 6, 2007, 10:14 AM   #5
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So when someone says 35mm on a SLR is the equivelent of say 48mm just for arguement sake..I know this is wrong but i'm just using it as an example:

Or 50mm becomes 80mm what are they on about...???
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Old Jan 6, 2007, 10:30 AM   #6
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Hmm, I'm not sure what you're asking in your last question.

Say you take a 50mm lens. (Like it says 50mm when you buy it). If you attach that lens to a 35mm SLR, then you will have 50mm of reach.

Now if you take the same lens and attach it to a DSLR, the lens is still 50mm (it doesn't grow bigger when you attach it to a DSLR). Except what you actually see is the lens focal length multiplied by the crop factor (in your case, 50mm x 1.6 = 80). That means you will be working with a reach of 80mm, not 50mm.

This is because the sensor is smaller in the DSLR than a piece of film. If you were shooting with film, you would capture the entire picture, but shooting with a DSLR sensor is like cropping out the middle of the picture. The crop factor is telling you how much is cropped out. Is that making any sense or am I just rambling?:blah:
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Old Jan 6, 2007, 10:56 AM   #7
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When they talk of 35mm equivalent focal length they are referring to applying the crop factor in your case. To get the wide angle/normal/telephoto so it makes sense with 50mm roughly being one power you have to apply the correction.

The reason you should understand that the focal length doesn't actually change is that some things like depth of field are calculated from the focal length without the crop factor being applied.

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Old Jan 6, 2007, 11:08 AM   #8
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Morag2 wrote:
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This answers my Question...I just wanted to understand what Folk's mean when they say 50mm and then say Equivelent to like you said 80mm
Quote:
Thanks hope you understand what i was trying to find out....
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Old Jan 6, 2007, 3:49 PM   #9
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Sintares wrote:
Quote:
The crop factor does not change the focal length of the lens.

A 200-500mm lens remains a 200-500mm lens crop factor or not.

The crop factor however gives you a field of view equivalent to a lens with a focal length of 320-600mm.
Why is it 320-600mm and not 320-800mm?
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Old Jan 6, 2007, 4:11 PM   #10
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It is 800mm i just worked it out on the Calculator..Lol
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