difference in field of view

Question: How could I take a picture with one focal length (say an 85) and "crop" it so I could see what the FOV would be at another focal length (say 135 or 100). I know how to crop the picture, I'm just wondering how much to crop.

Boring background info...

I'm trying to decide if I should rent a 135 F2 for an upcoming performance. I have the 85 1.8, but I think it may not give me the reach I need from my seat. I have the opportunity on Wednesday to go to rehearsal and shoot from the seat I will occupy on Sunday. I want to see what the FOV difference would be if I had a 135 or 100. While I want to get a little closer to the performers, I don't just want head & shoulder shots.

Thanks!!

bobtex said:

Question: How could I take a picture with one focal length (say an
85) and "crop" it so I could see what the FOV would be at another
focal length (say 135 or 100). I know how to crop the picture, I'm
just wondering how much to crop.

Click to expand...

Use the ratio of focal lengths: to simulate the FOV of a 135mm lens, start with an 85mm photo, and take a center crop with 85/135 of the original width and height. On the 350D/Rebel XT that would be 2176x1451.

--
Alan Martin

Alan Martin said:

Use the ratio of focal lengths: to simulate the FOV of a 135mm
lens, start with an 85mm photo, and take a center crop with 85/135
of the original width and height. On the 350D/Rebel XT that would
be 2176x1451.

Click to expand...

Correct, although I would add that this only applies when the distance to the subject is considerably greater than the focal lengths involved; it doesn't apply at macro distances.

Oh, and on the DReb/300D the crop would be 1934x1289 pixels, and on the XTi/400D it would be 2448x1632.

Doug Pardee said:

Alan Martin wrote:
Correct, although I would add that this only applies when the
distance to the subject is considerably greater than the focal
lengths involved; it doesn't apply at macro distances.

Click to expand...

To add to Doug's tip, here is the formula to find the ratio of FOV for all subject distances:

R-FOV(f1,f2,D) = (f2 * (D-f1)) / (f1 * (D-f2))

Where:

R-FOV(f1,f2,D) implies the ratio of FOV at focal length f1 vs f2 when subject distance from sensor is D (all f1, f2 and D in mm)

As can be claerly seen from above, for D many orders of magnitude larger then f1 and f2, this will simply tend to f2/f1

--
PicPocket

Problem solved!

--
Conglomoreum is dead! Long live Conglomoreum!
http://www.artistlies.com

Only if I can get a 70 - 200 F1.8....

To Alan, Doug and Pic - great answers. Much appreciated.

Doug, I am very new to all this and was wondering why the ratio is different for these two cameras. I thought that they both used a form factor of 1.6.

PippysDI said:

Doug, I am very new to all this and was wondering why the ratio is
different for these two cameras. I thought that they both used a
form factor of 1.6.

Click to expand...

The ratios are the same: 85/135. The number of pixels is different for the different cameras. The DReb/300D is a 6 megapixel camera, the XT/350D is 8, and the XTi/400D is 10 megapixels. So the same percentage ends up with a different number of pixels.

(Of course, it's possible that I miscalculated something, but the intent was 85/135 of the sensor dimensions in each model.)

Come on, my answer was awesome. That was some of the best answering I've ever done. Man, I got robbed.



(just kidding)

--
Conglomoreum is dead! Long live Conglomoreum!
http://www.artistlies.com

never thought about that.
Thanks