FOV crop & FF

[Robin Usagani]

First, I just want to make a public apology to several people on this forum. I probably missed a few names... infidel, solarflare, greybeard. I was misguided a long time ago and just believed what I was told. I did a test a long time ago and something was messed up on the test.

Now I do a new test. I only use 1 camera, which is a full frame canon 5D II with 2 lenses: Canon 35 f/1.4L and Sigma 20 f/1.8. A crop sensor is pretty much a full frame and you crop the out the outer edge. So pretend I have a 1.75X crop factor (35/20). I set the camera on a sturdy tripod. I first shot it with the 35, then change it with all the same setting with the 20. I think this is a much better test because I can make sure the camera doesn't move at all instead of replacing the camera with a crop sensor.

Here is what I have on the 35

Here is what I have on the 20

I put a red tint on the 35 image so I can see the difference. I pasted it to the 20mm image on another layer and set the opacity to 50%. I then shrink the image and try to line it up. In theory, I pretty much used a 1.75X crop factor camera. And the result? Perfectly lined up.

In summary.. yes.. using a wider angle on a crop sensor will generate the same Field of View as a full frame with longer lens (shorter lens X crop factor). The only difference is the Depth of Field. Since the two lenses have the same distance to the focus point, the wider angle will always have deeper depth if both are shot with the same aperture. Enjoy! I did this at 530AM before going to work! I couldn't sleep thinking about it!

 

[DiskoJoe]

Duh, lol. We still love you schwetty!

 

[Robin Usagani]

Duh, lol. We still love you schwetty!

Is it that obvious? It seems confusing hah. I really think the 20mm will really produce a different perspective. I guess it is only controlled by the distance.

 

[Helen B]

Good stuff. Just remember that a lens 'sees' the world from its entrance pupil, so if you want to make a truly careful comparison between two lenses the camera body shouldn't always be in the same position, but the two entance pupils should be.

 

[Samerr9]

I like how you were really thinking about it, because I was too but I slept

So this is a great conclusion for people with crop sensor bodies. You can play with the lenses instead!

 

[Robin Usagani]

Good stuff. Just remember that a lens 'sees' the world from its entrance pupil, so if you want to make a truly careful comparison between two lenses the camera body shouldn't always be in the same position, but the two entance pupils should be.

When you said entrance pupil, is that the aperture (the blades)?

 

[Helen B]

Good stuff. Just remember that a lens 'sees' the world from its entrance pupil, so if you want to make a truly careful comparison between two lenses the camera body shouldn't always be in the same position, but the two entance pupils should be.

When you said entrance pupil, is that the aperture (the blades)?

It's the image of the hole made by the blades, as seen from the front of the lens - ie the image of the hole in the blades formed by the lens elements that are in front of the blades. If there are no lens elements in front of the blades, then it is the actual hole in the blades.

You can estimate its location by looking into the front of the lens, placing your finger on the outside of the lens barrel where you think the image is then closing one eye and moving your head from side to side until there is no parallax between the image and your finger.

It isn't a single point in space, it is an area, and it can move as you change the viewing location: it can tilt and move forwards as you view further towards the side on a very wide angle lens.

Because it is an area, wide apertures (large entrance pupils) behave almost as if they can see round things - because the periphery of the entrance pupil can indeed see things that the centre cannot. This can give wide aperture, close-up images a subtle visual quality that narrow aperture images do not have.

Because it is the location from which a lens sees the world, the entrance pupil, rather than one of the lens' nodal points, is also the correct point of rotation for a stitched panorama.

 

[Robin Usagani]

Good stuff. Just remember that a lens 'sees' the world from its entrance pupil, so if you want to make a truly careful comparison between two lenses the camera body shouldn't always be in the same position, but the two entance pupils should be.

When you said entrance pupil, is that the aperture (the blades)?

It's the image of the hole made by the blades, as seen from the front of the lens - ie the image of the hole in the blades formed by the lens elements that are in front of the blades. If there are no lens elements in front of the blades, then it is the actual hole in the blades.

You can estimate its location by looking into the front of the lens, placing your finger on the outside of the lens barrel where you think the image is then closing one eye and moving your head from side to side until there is no parallax between the image and your finger.

It isn't a single point in space, it is an area, and it can move as you change the viewing location: it can tilt and move forwards as you view further towards the side on a very wide angle lens.

Because it is an area, wide apertures (large entrance pupils) behave almost as if they can see round things - because the periphery of the entrance pupil can indeed see things that the centre cannot. This can give wide aperture, close-up images a subtle visual quality that narrow aperture images do not have.

Because it is the location from which a lens sees the world, the entrance pupil, rather than one of the lens' nodal points, is also the correct point of rotation for a stitched panorama.

OK, ill try to do more reading. Very hard to understand. So you are saying theoretically my test is flawed a little bit since the location of the pupil is not the same? Yikes.. getting more and more complicated hahah. Very small margin of error though.

 

[Helen B]

Yes, it's likely to be a very small difference in entrance pupil location with the two lenses you mentioned. With other lenses, it could be quite significant.