Question about cropping and field of view?

Columbo · 19 Jul 2016 at 15:46

So I've found an old APS-C only prime lens I forgot I had and have tried it on my full frame body. It works perfectly with the obvious heavy vignetting being visible.

I'm actually quite surprised by the results but have a question regarding how cropping changes the effective field of view / focal length and hopefully some here can help!

The below is the full image at 24mp,edge of lens element visible.

The below is cropped to 18.03mp to remove most but not all of the vignetting

Question is with a 6mp crop taken off what would the focal length of the 2nd shot be?

Is it as below:

Original shot is 30mm at 24mp
I've cropped in by 6mp, reducing the mp count by 25%
Do I then take that 25% and add it to the original focal length therefore giving a 37.5mm field of view in the cropped shot?

niko · 19 Jul 2016 at 17:59

can i ask why on earth you want to know?

Columbo · 19 Jul 2016 at 18:31

Because I'm interested.

niko · 19 Jul 2016 at 18:43

Columbo said:
Because I'm interested.

cool, i just wandered if there was an application for it. a way to get a ball park would be recreate the shot using a zoom and see what setting approximated it. would give you an idea if your maths works

Amp34 · 19 Jul 2016 at 20:38

Have you not reduced the image by 20%? There are 5 6's in 30, not 4.

Columbo · 20 Jul 2016 at 07:18

Amp34 said:
Have you not reduced the image by 20%? There are 5 6's in 30, not 4.

No, 24mp to 18mp is a 6mp reduction.
24 / 6 = 4 (or 25%)

greedy123 · 20 Jul 2016 at 10:01

Horizontal viewing angle of 30mm would be 61.9°. Croping this by 25% would give a viewing angle of ~55°. This would make the focal length approx 35mm I believe.

neodude · 22 Jul 2016 at 19:31

I was under the impression that crop sensor only lenses protruded further into the camera body and as such could cause damage to the mirror when used with Full Frame bodies.

Steve M · 22 Jul 2016 at 21:37

That isnt true at all.

Nexus · 23 Jul 2016 at 00:15

It's true of Canon EF-S lenses, they are designed to fit APS-C size bodies. If you try and fit one on an EF body, the protrusion of the lens can either obstruct or result in your mirror slapping the rear of the lens.

So Canon recommend you don't try and do this, and I believe that's why they also put a white square on the lenses, rather than the red circle of EF mounts.

Columbo · 23 Jul 2016 at 11:46

This is on a Sony A99 & Sigma 30mm f1.4 so isn't the Canon system. I've used the combo quite a bit over the last few days and honestly I'm quite impressed with the results once the vignette is cropped out.