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Crop factor + Teleconverter??
My question is:How does the math work for this&DSLR crop factor 1.6xteleconverter 1.4x100mm lensIs it(1.6 x 100) x 1.4 =224mmOr(1.6+1.4) x 100= 300mm.

Or do I have everything wrong? I am willing to accept that I have no idea what Im talking about..

Thanks for your time..

Comments (5)

The crop factor is a function relating to the physical size of the *image* that is projected onto the sensor in the camera..

The teleconverter factor is a function relating to the magnification of that image, like zoom. That is to say, the size and detail of the *scene* being projected onto the sensor..

They are unrelated...

Comment #1

Acoomer wrote:.

My question is:How does the math work for this&DSLR crop factor 1.6xteleconverter 1.4x100mm lensIs it(1.6 x 100) x 1.4 =224mmOr(1.6+1.4) x 100= 300mmOr do I have everything wrong? I am willing to accept that I have noidea what Im talking about..

Thanks for your time.

Start with the 100mm lens. Add the teleconverter 1.4x. You now have a 140mm ACTUAL focal length. Next, apply the crop factor of 1.6x and you have 224mm EQUIVALENT focal length.So the answer is just 100 x 1.4 x 1.6 = 224..

But I broke it down into two steps to distinguish between the fact that the converter gives a real change in focal length while the crop factor is just that, a "crop", resulting in an "equivalent" focal length..

I hope this is clear,Regards,Peter..

Comment #2

Graystar wrote:.

They are unrelated..

Wait, sorry, scratch that. I was thinking in a different direction. ..

Comment #3

Thank you Peter, that was very clear..

What about stacked TCs1.4x TC2x TCOn a 100mmWould that be:(1.4x 100)x2=280Or(1.4+2)x100=340Im just trying to learn how to do the math for lenses..

Thanks again,Tony..

Comment #4

Acoomer wrote:.

Thank you Peter, that was very clear..

What about stacked TCs1.4x TC2x TCOn a 100mmWould that be:(1.4x 100)x2=280Or(1.4+2)x100=340Im just trying to learn how to do the math for lenses..

Thanks again,Tony.

In simple terms your first answer is correct:(1.4 x 100) x 2 = 280or equally (1.4 x 2) x 100 = 280..

But I might add this is pretty theoretical, you are very unlikely to use this combination in practice. First, because of the light loss, and second the general loss of sharpness and image quality that is likely to occur..

Regarding light loss, the 1.4x loses one stop; the 2x loses two stops, together you would lose three stops.Say you have a 100mm f/4 lens to begin with.100mm with 1.4x is now 140mm f/5.6100mm with 2x is now 200mm f/8Using both together (as I said, unlikely),100mm with 1.4x and 2x is now 280mm f/11..

So the combination is a 280mm f/11 - perhaps not very useful, especially if the lens is already less than ideal at f/4 and may need to be stopped down even more to attempt an increase in sharpness.Regards,Peter..

Comment #5

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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