effect of DSLR cropping factor on F-number ?

[ekoswara]

Hello all,

I have a Nikon D-SLR which have a cropping factor of 1.5x. If Iam using a 50mm f/1.8 lens at wide open, does this mean my lense will turn into a 75mm f/1.8 or will it become 75mm f/2.7 ?

Please enlighten me. Thank you

 

[blurblock]

Hello all,

I have a Nikon D-SLR which have a cropping factor of 1.5x. If Iam using a 50mm f/1.8 lens at wide open, does this mean my lense will turn into a 75mm f/1.8 or will it become 75mm f/2.7 ?

Please enlighten me. Thank you

It will still be 50mm f/1.8, just with the cropping factor, you get a 75mm effect only. Your lens will not become 75mm f/1.8 or 75mm f/2.7 (whatever that is ;P ).

Just picture this, instead of seeing the whole pictures full frame, you only see portions of it.

 

[ST1100]

It is an effective 75/1.8. The 'f1.8' brightness applies to every single spot in the image, so no matter how small your sensor is, it is still f1.8.

 

[forbytes]

To clearify further the issue, a 75/1.8 and a 50/1.8 has very different depth of view. Both blurblok and ST1100 gave a good explanation of the 1.8 issue.

Assuming you have a subject 2m away from the focal point in the lens, using a fix aperture of 1.8, the results with 50 and 78mm lens are as follows:

focal length,depth of view
50mm,0.169m
78mm,0.068m

As you can see, if the lens does become a 78/1.8, the subject will have sharp nose and super blur ears at f1.8. That is not the case when you shoot such an object. So you will only see 78mm equivalent of image and lens performance is still a 50/1.8. It maybe a bit confusing to state that a 50/1.8 lens become a 78/1.8 lens when put on a DSLR.

Regards.