Question for the Techies
Mar 7, 2015 21:48:03 #
If I take a photo with a 420mm equivalent lens and crop away everything but 10% of the image is the effective focal length 4200mm?
Mar 7, 2015 21:54:21 #
you have essentially digitally zoomed but I'm not sure of the math, because it's angle of view not % of the crop that determines effective focal length. And you didn't specify the crop aspect ratio.
If the crop aspect ratio is the same as the sensor aspect ratio then the ratio of diagonals is 1/10. But the angle of view..? Gets into some trig and I'm one Marquita in tonight so im going to defer that answer to the smart people.
If the crop aspect ratio is the same as the sensor aspect ratio then the ratio of diagonals is 1/10. But the angle of view..? Gets into some trig and I'm one Marquita in tonight so im going to defer that answer to the smart people.
Mar 7, 2015 22:19:13 #
If the area of the crop is about 10% of the full image then the equivalent focal length of about 1350mm.
Mar 7, 2015 22:37:25 #
Mar 7, 2015 22:39:23 #
Rongnongno wrote:
Why would you do this in the first place?
:shock: :hunf:
:shock: :hunf:
He's just asking a math question.
Mar 7, 2015 22:40:56 #
Is this a puzzle? If not, why would you do that? How big a print could you get from the 10%, and would it be worth the trouble?
Mar 7, 2015 22:42:37 #
mcveed wrote:
Is this a puzzle? If not, why would you do that? How big a print could you get from the 10%, and would it be worth the trouble?
It's a hypothetical question.
Mar 7, 2015 22:54:17 #
This is a tough one because you are mixing area and optical angles !
I'll have to wait for the Mensa guy to show up! 8-)
I'll have to wait for the Mensa guy to show up! 8-)
Mar 7, 2015 22:59:18 #
mcveed wrote:
Is this a puzzle? If not, why would you do that? How big a print could you get from the 10%, and would it be worth the trouble?
He's probably shooting a 5" bird thirty feet away and he's not printing. Loved your comment about the puzzle.
Mar 7, 2015 23:01:35 #
BebuLamar wrote:
If the area of the crop is about 10% of the full image then the equivalent focal length of about 1350mm.
Assuming 10% say horizontally, if you look at doubling, then 2,4,8,16 etc but we only have 10 so 8 to 10 is 1.25x.
So 420 doubled is 840, then 1680, then 3360, then 4200 (using 1.25).
The fov calculators seem to say the same thing.
The 1350 seems to be close to 420x3.25. Not sure about that but not opposed to understanding ...
Mar 7, 2015 23:10:14 #
Rongnongno wrote:
Why would you do this in the first place?
:shock: :hunf:
:shock: :hunf:
Because you can
:?: :-D
Mar 7, 2015 23:11:27 #
joer wrote:
If I take a photo with a 420mm equivalent lens and crop away everything but 10% of the image is the effective focal length 4200mm?
I don't know the correct answer but we know there is a relationship between angle of view and focal length.
I also know that it is not a linear relationship. So I posed a question (just numbers) in hopes that someone would know the formula.
There are tables of AOV and corresponding FL, but I could not find one that goes beyond 2.5 degrees which corresponds to 1200mm.
I routinely do huge crops which look very good on a 24 inch monitor and in prints (8.5x11) and am curious as the effective focal lengths.
Mar 7, 2015 23:16:26 #
joer wrote:
If I take a photo with a 420mm equivalent lens and crop away everything but 10% of the image is the effective focal length 4200mm?
No. You would just have a cropped image you've done nothing to increase the magnification of the image.
Mar 7, 2015 23:30:26 #
joer wrote:
on't know the correct answer but we know there is ... (show quote)
Look at the one at tawbaware.com. Google fov calculator to find it.
Mar 7, 2015 23:32:13 #
LoneRangeFinder wrote:
No. You would just have a cropped image
you've done nothing to increase the magnification of the image.
Best answer. :thumbup:
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