Crop Factor and Focal Length Modifier

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dck22

Senior Member
I understand that compact DSLRs have a smaller sensor that full frame DSLRs and that a 50mm lens used on a sensor with a 1.5 crop factor would produce the same field of view as a 75mm lens (1.5x50) on a 35mm full frame sensor.

What I cannot get my head around is lenses that are "optimized" for the smaller sensor like Nikon DX. Does a 50mm Nikon DX lens (optimized for small sensor) on a D5500 (small sensor) provide a 50mm field of view or a 75mm? What would the field of view be for a 50mm FX lens used on a D5500?

I apologize in advance if everyone already understands this concept clearly, but I haven't found the answer in a format that makes sense to me.

TIA
 
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Don Kuykendall_RIP

RIP :(
They can make the lenses smaller since they do not have to cover as much sensor. That is the short simplified answer. That is also the reason that many of us like to use FX lenses on our DX cameras. The DX only uses the center part of the FX lens which is the prime part of the lens.

DX lenses are great though for your DX camera. I see that you have the 18-140, that is a great walk around lens. I much preferred it over the 18-55. In my opinion it has better IQ along with the better reach. Don't know to much about the 55-300 though. I have the 70-300 for use on my 2 bodies.
 
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nickt

Senior Member
Just to say it another way, 50mm focal length is 50mm whether its dx or fx. The distance to the sensor plane is the same too whether dx or fx. The dx sensor is just smaller so any 'extra' image, falls outside the sensor and is not captured. That's why either lens will work fine on a dx camera.

It might be less confusing to say the dx lens is cost reduced rather than optimized. If you were to put a dx lens on a fx camera, you would likely see vignetting. It was designed knowing the outer edges of the image would miss the dx sensor anyway.
 
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WayneF

Senior Member
dck22 said:
I understand that compact DSLRs have a smaller sensor that full frame DSLRs and that a 50mm lens used on a sensor with a 1.5 crop factor would produce the same field of view as a 75mm lens (1.5x50) on a 35mm full frame sensor.

What I cannot get my head around is lenses that are "optimized" for the smaller sensor like Nikon DX. Does a 50mm Nikon DX lens (optimized for small sensor) on a D5500 (small sensor) provide a 50mm field of view or a 75mm? What would the field of view be for a 50mm FX lens used on a D5500?
Click to expand...


Your first sentence answers the question in the second. That is all there is to it.

If it is the SAME 50mm lens on both DX or FX, then it is of course the same lens, and it performs exactly the same on either camera - in every respect if all other conditions like subject distance remain the same. A 50mm lens made for DX might be "optimized" to have smaller diameter glass, which won't project as large an image, but it will be smaller and lighter and cheaper... than a 50mm lens for FX. But the crop factor effect is still the same for exactly the same lens on either camera.

However, the smaller sensor "crops" the field of view from the lens (called cropping), so only the field of view changes ... The DX sensor sees a smaller view captured. The smaller sensor simply sees less of the lens image - and therefore the smaller image has to be enlarged more to view it at the same size as the larger FX image. This extra enlargement is the telephoto effect that makes us compare it to a 75mm view on FX. But the same 50mm lens is exactly the same lens on either body.

Camera Sensor Crop Factor and Equivalent Lens Focal Length
 
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Sandpatch

Senior Member
Thanks dck22 and everyone. This is a concept that I grasp and then quickly lose understanding of. I finally understand it now -- the explanations in these posts are perfect.
 
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dck22

Senior Member
Thank you all for your responses. I now more clearly understand the differences.

When I started to try to understand photography back in the late 60's with my Nikkormat, there was no such thing as digital. Then I walked away from it for 40 years and it is now an entirely different ball game.

I'm not sure why I am trying again, other than it is fun, but I truly appreciate having a group like this to which to pose my questions. Maybe sometime soon, I will get up nerve enough to post my photos.
 
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Blacktop

Senior Member
dck22 said:
I understand that compact DSLRs have a smaller sensor that full frame DSLRs and that a 50mm lens used on a sensor with a 1.5 crop factor would produce the same field of view as a 75mm lens (1.5x50) on a 35mm full frame sensor.

What I cannot get my head around is lenses that are "optimized" for the smaller sensor like Nikon DX. Does a 50mm Nikon DX lens (optimized for small sensor) on a D5500 (small sensor) provide a 50mm field of view or a 75mm? What would the field of view be for a 50mm FX lens used on a D5500?

I apologize in advance if everyone already understands this concept clearly, but I haven't found the answer in a format that makes sense to me.

TIA
Click to expand...

Didn't realize Nikon made a 50mm DX lens. Learn something new everyday.
 
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WayneF

Senior Member
dck22 said:
Thank you all for your responses. I now more clearly understand the differences.

When I started to try to understand photography back in the late 60's with my Nikkormat, there was no such thing as digital. Then I walked away from it for 40 years and it is now an entirely different ball game.

I'm not sure why I am trying again, other than it is fun, but I truly appreciate having a group like this to which to pose my questions. Maybe sometime soon, I will get up nerve enough to post my photos.
Click to expand...


Digital is much easier than film ever was. We can see what we are doing now (while we can still retry it), and we have easy ways to fix it later too. But it does require a little computer skills now, and that can be tough for some of us. That's the huge difference.

Exposure (shutter speed, aperture and ISO) is still the same, and the other hundred menus are pretty much frills. :) Except to me, other than the overall ease of digital, the big difference is White Balance... We didn't worry much about that with film. B&W was "don't care", and the photo lab generally fixed color negatives for us. But digital, we have to do it.
 
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