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Focal Length Help Please

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Old Monday 13th March 2006, 18:52   #1
dicklynch
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Focal Length Help Please

Hello

Can someone help this dummy understand focal length as it applies to digital cameras?

First, am I correct when I think that when I used a 135mm lens (non zoom) on a 35mm body (non-digital) the focal length of all pictures will be 135mm?

If the above is correct, then again I think (trouble area here) that if I use the same 135mm lens on a digital body; the focal length is not 135mm.

If my last thought is correct, how do I convert the numbers listed under “Properties” for a digital picture to the focal length I am familiar with, e.g. the non digital 35mm numbers?

Here is where I am having problems.

If I take a picture using the Pentax ist DS, and the Sigma 170-500mm zoom, the focal length listed under Properties can read from 170 to 500 depending on the zoom selection. Can I convert this to the “correct” focal length … or is this correct?

I become even more confused when I hang the Nikon CP-4500 on my Pentax PF-80 spotting scope using a Williams 28mm eyepiece (focal length 24mm). The focal lengths for those photos taken and listed under Properties have a range from 13mm to 32mm. Again I’m guessing this depends on how I zoom, when using the camera’s 1-3 optic zoom. Can this be converted?

Hope this is a bit clearer than Tapioca.

Thanks in advance

Ciao

Dick
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Old Monday 13th March 2006, 19:47   #2
IanF
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Yes in the first instance 135mm with film camera is 135mm focal length.

I'm not 100% au fait with all the technicalities but my take on this is -

With digital you have to bear in mind that there are several different sized CCD sensors. Pro DSLRS have 'full sized' sensors that equate to 35mm in size and so a 135mm lens on them is 135mm focal length!

Most DSLRs though have smaller sensors so a 35mm designed lens on a smaller sensor causes some cropping and apparent increase in focal length. With the likes of the Canon 350D and 20D the crop factor is 1.6x the focal length of the lens. With the Nikon D70 the crop factor is 1.5x. Most other models fit into one of these as many cameras use sensors from the same sources.

A further twist is the new breed of DSLR designed lenses some of which are designed for smaller sensors so one of theses 135mm lenses will give a true 135mm focal length!

I suspect a full sized sensor with a frame filling shot will always beat a smaller sensor with a crop factor lens - but for amateur bird photography the crop factor makes a big difference in more reach for no extra cost!

Compact cameras use even smaller sensors together with smaller lenses. manufactuers usually give a 35mm equivalent for the camera zoom range so you can get an approximation of focal length. I think one of our members did come up with a digiscoping converter to calculate actual focal length, but I can't recall the details as it was never something that greatly interested me. Hopefully another member will recall it.
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Old Wednesday 15th March 2006, 20:25   #3
dicklynch
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Quote:
Originally Posted by IanF
Yes in the first instance 135mm with film camera is 135mm focal length.

I'm not 100% au fait with all the technicalities but my take on this is -

With digital you have to bear in mind that there are several different sized CCD sensors. Pro DSLRS have 'full sized' sensors that equate to 35mm in size and so a 135mm lens on them is 135mm focal length!

Most DSLRs though have smaller sensors so a 35mm designed lens on a smaller sensor causes some cropping and apparent increase in focal length. With the likes of the Canon 350D and 20D the crop factor is 1.6x the focal length of the lens. With the Nikon D70 the crop factor is 1.5x. Most other models fit into one of these as many cameras use sensors from the same sources.

A further twist is the new breed of DSLR designed lenses some of which are designed for smaller sensors so one of theses 135mm lenses will give a true 135mm focal length!

I suspect a full sized sensor with a frame filling shot will always beat a smaller sensor with a crop factor lens - but for amateur bird photography the crop factor makes a big difference in more reach for no extra cost!

Compact cameras use even smaller sensors together with smaller lenses. manufactuers usually give a 35mm equivalent for the camera zoom range so you can get an approximation of focal length. I think one of our members did come up with a digiscoping converter to calculate actual focal length, but I can't recall the details as it was never something that greatly interested me. Hopefully another member will recall it.

Thanks for the info Ian.

Like you, to me, it really doesn’t matter all that much as I’m stuck w/the results … kinda like seeing the temperature in Celsius and trying to convert to Fahrenheit, certainly does not change any thing, but it might make you feel warmer/colder.

Ciao

Dick
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Old Wednesday 15th March 2006, 22:00   #4
rezMole
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The focal length of the lens REMAINS the same whatever body is used. The only difference with most DSLRs is that they have smaller sensors, so they only "use" the centre part of the image seen by the lens. This has the "effect" of seemingly bringing the image closer.

What most birders would probably love is for someone to bring out a sensor that has a crop factor of something like 3x. Then a 500mm lens would "appear" to be 1500mm!!!
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Old Thursday 16th March 2006, 09:31   #5
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Quote:
Originally Posted by rezMole
The focal length of the lens REMAINS the same whatever body is used. The only difference with most DSLRs is that they have smaller sensors, so they only "use" the centre part of the image seen by the lens. This has the "effect" of seemingly bringing the image closer.

What most birders would probably love is for someone to bring out a sensor that has a crop factor of something like 3x. Then a 500mm lens would "appear" to be 1500mm!!!
It is also important to remember that the resolution of the lens is not changed. However, as the centre of the lens is being used, the centre resolution is often better than the peripheral resolution.
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