Crop Photo to Certain Focal Length - Math Question

I have a 20mm f/1.4 lens on a full-frame 5D3 (5,760 x 3,840 pixels) and would like to crop a number of photos to 24mm to cut out star coma in the corners. E.g. make the photos look like they were originally shot on a 24mm lens.

Is this the correct math to determine the final resolution of the image cropped to 24mm?

20mm + 20% = 24mm
5,760 - 20% = 4,608
3,840 - 20% = 3,072

So, if I cropped the original resolution photo (5,760 x 3,840) down to (4,608 x 3,072), would this be the equivalent of a 24mm image? I need the final resolution to make an action in photoshop. Thanks!

I think you must have done your math wrong, I make 5760/1.2 to equal 4800 (24/20), if you do it as 20/24 and multiply it's 0.8333×5760 = 4800 too. Then just divide that by 1.5 to get short side of 3200.

Alan

Thanks! I figured I was not thinking about this correctly!

Interesting question. I am wondering, however, why it is important that the image appear to have been taken at a specific focal length? If you keep the ratio of width to height the same as the original image (as the conversion you are talking about does), any crop tool would allow you to crop just enough to remove the coma. In the final image, the focal length is seldom (ever?) evident.

BigAl007 wrote in post #17980185
Then just divide that by 1.5 to get short side of 3200.

I think you mean divide that by 1.2

Perfectly Frank wrote in post #17980800
I think you mean divide that by 1.2

He means 1.5 - Aspect ratio is 1.5 (3:2, or 36mmx24mm sensor, assuming OP is shooting with the Canon 5DIII listed in the OP's sig line).

kirk

kirkt wrote in post #17980812
He means 1.5 - Aspect ratio is 1.5 (3:2, or 36mmx24mm sensor, assuming OP is shooting with the Canon 5DIII listed in the OP's sig line).

kirk

Ok, got it. Thanks.

I was thinking 3840/1.2 = 3200.
But it's all good.

The way I read Al's comment was that he was taking the computed scaled width and computing the scaled height from it by using the aspect ratio.

I got your drift. I did not mean to seem contrarian.

Kirk

Just crop it as needed, don't worry about focal lenght simulation.

I think that the OP is attempting to crop to a specific FOV associated with a specific focal length.

kirk

If your goal is to remove the coma, just crop out the coma.

If you want to compare the 20 to a 24, shoot the same scene from the same tripod, and crop the 20 to match the 24.

If you are a math geek, you'll need to use the angle functions on your calculator, arctan, etc.

skid00skid00 wrote in post #17981017
If your goal is to remove the coma, just crop out the coma.

If you want to compare the 20 to a 24, shoot the same scene from the same tripod, and crop the 20 to match the 24.

If you are a math geek, you'll need to use the angle functions on your calculator, arctan, etc.

You can do hyperbolic cosines if you want, but most would just divide by 24/20, which is 1.2.

I simply used the frame aspect ratio, based on the fact that the OP was talking about using Canon DSLR lenses, because when I did the math I already had the computed size of the long edge in the calculator. Initially I did 20/24 for the amount to crop by and hit equals, then multiplied the result by the number of pixels in the long edge, so only had to input those digits. Then divided the result by 1.5 as then I only needed to enter two more digits, and a decimal point. Since I have M.E, which affects my concentration I find it easier if I don't have to enter four or five digit numbers too often.

Alan

Archibald wrote in post #17981023
You can do hyperbolic cosines if you want, but most would just divide by 24/20, which is 1.2.

Except that doesn't cover the angles covered by the lenses, which is why I made my post in the first place.