# Thread: need help with a calculation

1. ## need help with a calculation

I do not know how to calculate the following.

A two image stitch with 10mm shift left and 10mm shift right.

Vertical position and then crop to exact same aspect ratio as a (one) normal landscape image.

Using a Schneider 35 XL.

How wide a lens would that correspond to a (1) 36x48 sensor and (2) 33x44 sensor?

Thanks

2. ## Re: need help with a calculation

You can use the Alpa lens calculator for these calculations:

http://www.alpa.ch/en/products/tools...alculator.html

In the xls file, it's the "Free calculation" sheet which allows for the calculation you want to do.

CI has provided something similar but I don't have a link at hand.

Chris

3. ## Re: need help with a calculation

Alfa calc looks good, and calculates results for shift and stitch, but Barnak has a focal length slider which you could use to find the focal length.

...or you can look up the formulae and do some trigonometry. (I presume that these on-line and down-loadable calculators are more accurate than the simple "thin lens" formula.)

Originally Posted by danlindberg
I do not know how to calculate the following.

A two image stitch with 10mm shift left and 10mm shift right.

Vertical position and then crop to exact same aspect ratio as a (one) normal landscape image.

Using a Schneider 35 XL.

How wide a lens would that correspond to a (1) 36x48 sensor and (2) 33x44 sensor?

Thanks

4. ## Re: need help with a calculation

Actually, it is easy. Calculate the diagonal of the image area of both formats/image areas. Take the focal length used on one format and divide it by the diagonal of that format. Take the diagonal of the second format and multiply it by the product of the first calculation; that will be the focal length you need.

This does assume infinity focus, for closeups it is trickier, but this will be a good ballpark.

5. ## Re: need help with a calculation

Dan,
Here's what I get
36x48
To keep the same ratio, stitched image would be 56x42
70mm diagonal, so focal length is 30mm

33x44
Stitched image would be 53x39.75 (again, keeping the 1.33 ratio...)
66.25mm diagonal, so focal length is 29mm

Dave

6. ## Re: need help with a calculation

Assuming that a lens acts like a glorified pin hole at the focal length (plus extension if the subject is not at infinity) from the sensor, the focal length is half sensor dimension divided by the tangent of half the angle of view?

The sensor dimension can be the width, height or diagonal, depending which angle of view you want.

7. ## Re: need help with a calculation

To get the 56x42: a vertically shifted 36x48 would be 36+10+10 = 56x48, but that's the wrong ratio, so you would crop off 6mm to get to 56x42 (1.33:1).

The base sensor diagonal is 60mm, and the shifted / cropped diagonal is 70mm. 35mm*60/70 = 30mm

Dave

8. ## Re: need help with a calculation

Thank you all, so much! What a great forum this is