Advice - cropping factors and F Nos

TwentySixDays

TwentySixDays

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Stuart
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Hi I have a 500 x F5.6 lens M42 lens.
If I buy an adaptor and mount it on my D300 what is the equivalent F No.?

Taking F No = Focal lenght (f) divided by aperture (a).
in this case f=500, Fn=5.6 therefore a = 500/5.6 = 90ish.

Cropping factor on D300 = 1.5
therefore the effective focal length = 500 * 1.5 = 750 and aperture is still 90
which gives an F No of 750/90 = 8.4.

My confusion comes in the amount of light hitting the sensor in terms of lums per sq mm is the same. So will I be shooting at 5.6 or 8.4? :thinking:

Thanks in anticipation
 
TwentySixDays said:
Hi I have a 500 x F5.6 lens M42 lens.
If I buy an adaptor and mount it on my D300 what is the equivalent F No.?

Taking F No = Focal lenght (f) divided by aperture (a).
in this case f=500, Fn=5.6 therefore a = 500/5.6 = 90ish.

Cropping factor on D300 = 1.5
therefore the effective focal length = 500 * 1.5 = 750 and aperture is still 90
which gives an F No of 750/90 = 8.4.

My confusion comes in the amount of light hitting the sensor in terms of lums per sq mm is the same. So will I be shooting at 5.6 or 8.4? :thinking:

Thanks in anticipation
Click to expand...


still be f5.6, the focal lenght is still 500mm crop or full frame.

Hugh
 
boyfalldown said:
still be f5.6, the focal lenght is still 500mm crop or full frame.

Hugh
Click to expand...

Thanks for the clarification Hugh - I take then it's the effective image size in the picture that makes it look like a 750mm lens
 
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