I wanted to post these yesterday, but didn't have the time. So here's the difference, mathematically speaking, in a 10mm focal length difference.
For a subject, whose Field of View is located on the far left, it compares the relative working distances (i.e. the distance from the sensor to the subject) between the 40mm and the 50mm
[TABLE="width: 302"]
[TR]
[TD]FOV Ft[/TD]
[TD]WD Ft (40mm)[/TD]
[TD]WD Ft (50mm)[/TD]
[TD]WD Ft (35mm)[/TD]
[/TR]
[TR]
[TD="align: right"]1[/TD]
[TD="align: right"]1.3[/TD]
[TD="align: right"]1.6[/TD]
[TD="align: right"]1.1[/TD]
[/TR]
[TR]
[TD="align: right"]2[/TD]
[TD="align: right"]2.4[/TD]
[TD="align: right"]3.0[/TD]
[TD="align: right"]2.1[/TD]
[/TR]
[TR]
[TD="align: right"]3[/TD]
[TD="align: right"]3.6[/TD]
[TD="align: right"]4.4[/TD]
[TD="align: right"]3.1[/TD]
[/TR]
[TR]
[TD="align: right"]4[/TD]
[TD="align: right"]4.7[/TD]
[TD="align: right"]5.9[/TD]
[TD="align: right"]4.1[/TD]
[/TR]
[TR]
[TD="align: right"]5[/TD]
[TD="align: right"]5.8[/TD]
[TD="align: right"]7.3[/TD]
[TD="align: right"]5.1[/TD]
[/TR]
[TR]
[TD="align: right"]6[/TD]
[TD="align: right"]7.0[/TD]
[TD="align: right"]8.7[/TD]
[TD="align: right"]6.1[/TD]
[/TR]
[TR]
[TD="align: right"]10[/TD]
[TD="align: right"]11.6[/TD]
[TD="align: right"]14.4[/TD]
[TD="align: right"]10.1[/TD]
[/TR]
[TR]
[TD="align: right"]15[/TD]
[TD="align: right"]17.3[/TD]
[TD="align: right"]21.6[/TD]
[TD="align: right"]15.1[/TD]
[/TR]
[TR]
[TD="align: right"]20[/TD]
[TD="align: right"]23.0[/TD]
[TD="align: right"]28.7[/TD]
[TD="align: right"]20.1[/TD]
[/TR]
[/TABLE]
Based on these numbers (and yes I have tried them out to verify their accuracy (I did it with a 50 and an 85)), there is not a significant difference in working distance until about 10 feet. When you want a field of view of 5 ft and you're shooting with a 50mm, and you want the FOV for the 40mm, you only have to step back about 18 inches. At 10 feet, there is a significant difference, in that if you want the same FOV on a 50mm that you would want on a 40mm, you have to step back 3,4, or 5 feet.
Just for funsies, I threw in the 35mm working distance too. Also, remember that wide angles increase distortion and change perspective.