Crop Sensors Do Not Increase Lens Focal length

[Bailey]

Most people have seen references to crop factors for sensors smaller than 35mm in size (1.6x, 1.5x, 2x etc)

You can multiply the lens' focal length by the camera sensor's crop factor to get the effective focal length.  The effective focal length is the focal length the same lens would need to be set at on a full frame (35mm sensor size) camera to get the same field of view as the smaller sensor would give.

So the field of view seen through the lens changes when it is moved from a full frame camera to a crop sensor camera, not the actual focal length of the lens.

This article, with images/diagrams, hopefully explains clearly (well, to me it does) that field of view changes, not the actual focal length.

www.eos-magazine.com/articles/eospedia/crop-factor.html

[Peterj]

Feb 21, 2019 12:07:11 GMT Bailey said:

Most people have seen references to crop factors for sensors smaller than 35mm in size (1.6x, 1.5x, 2x etc)

You can multiply the lens' focal length by the camera sensor's crop factor to get the effective focal length.  The effective focal length is the focal length the same lens would need to be set at on a full frame (35mm sensor size) camera to get the same field of view as the smaller sensor would give.

So the field of view seen through the lens changes when it is moved from a full frame camera to a crop sensor camera, not the actual focal length of the lens.

This article, with images/diagrams, hopefully explains clearly (well, to me it does) that field of view changes, not the actual focal length.

www.eos-magazine.com/articles/eospedia/crop-factor.html

This is 100% on target ... however this myth is used by camera manufacturers all the time and has become a convenient method to discuss lens' applied use.

On Nikon's site the P1000 is described with these words "Mind-blowing 3000mm zoom"

Then reading the full specs "4.3-539mm (angle of view equivalent to that of 24-3,000mm lens in 35mm [135] format)

Nikon actually uses the correct statement in full specs.

[Bailey]

and occasionally when I stumble over one of those annoying tv shopping channels advertising the "latest and greatest" compact camera they also sometimes imply it has a 1000mm zoom when in fact it has the same field of view as a 1000mm lens on a 35mm camera.

 

[pontiac1940]

Thanks for your interesting post Bailey. An interesting, albeit confusing subject. Inspired by your post, I took photos with the same lens on an APS-C camera and a full-frame camera as I thought a "down home" example might be of interest. (Just some stuff on the stove: a printer test sheet, a ceramic pot with pens and pencils and a can of air.   

You said, "So the field of view seen through the lens changes.." These two photos clearly show the differences between the two sensor sizes with the same lens set at 400 mm. 

These are links to the two images below but full size. APC-S sensor camera. Full-frame camera. (Note: imgbb has a file size limit so the two large versions are saved at IQ=10 vs 12, so there is some compression.) Load both images and enlarge to 100%, then switch back and forth. For most uses, APS-C sensors are great and you'd never see the differences in most prints in the sizes most of use usually make. As noted previously, the IQ from the full frame is quite impressive at high ISOs. These were shot at ISO 250, f7.1 and 1/250 second. Used a tripod and bounce flash.     

Clive

[pontiac1940]

Another aspect ...

APS-C at 100% and the FF image enlarged so the pencil tips are about the same size. Same lens, so you can see differences in the image quality. However, as noted, these would never show up on a print. 

[Bailey]

Hi Clive,

thank you for confirming the information and example in the link I posted which show only the field of view changes when using the same lens at a constant focal length on full frame and APS-C sensor cameras.