Question for the Techies
Mar 7, 2015 23:39:40 #
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?
If you took a photograph, then cut away all but 10% of it with scissors, what have you got?
Confetti.
Mar 8, 2015 00:06:45 #
UtahBob wrote:
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 ...
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 ...
It's 1328 it's 3.16 * 420mm. 3.16=square root of 10.
Mar 8, 2015 04:56:42 #
10% of the area is not the same as 10% of the length of a side (or the diagonal). 10% of the area would give a crop factor of 3.16, whereas 10% of the diagonal would give a crop factor of 10.
(Reducing the side dimensions by 1/10 gives an area of 1/100th of the original).
(Reducing the side dimensions by 1/10 gives an area of 1/100th of the original).
Mar 8, 2015 05:21:04 #
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
Mar 8, 2015 05:43:26 #
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?
It looks like the focal length is increased because the field of view is considerably closer. But your image, in pixels, will be tiny.
Mar 8, 2015 05:56:52 #
wolfd
Loc: Vancouver, Canada
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.
Mar 8, 2015 06:14:38 #
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?
How are you figuring the 420mm equivalent? The crop factor on a crop sensor camera does not effect the focal length. I think LoneRangeFinder has the right answer.
Mar 8, 2015 06:35:31 #
Mar 8, 2015 06:39:01 #
Leitz
Loc: Solms
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?
Shoot a scene with a 420mm lens, then with a 4,200mm lens, and compare the difference. (Yes, I am no mathematician!)
Mar 8, 2015 06:43:10 #
waegwan wrote:
How are you figuring the 420mm equivalent? The crop factor on a crop sensor camera does not effect the focal length...
His use of the term "equivalent" indicates that he is referring to the 35mm equivalent focal length (and therefore the equivalent field of view).
If his starting point was, say, a DX camera lens with an indicated focal length of 280mm, that would give the same field of view as a full frame lens with a focal length of 420mm.
The actual focal length is always as indicated on the lens, but when focal length is quoted for the purpose of indicating field of view, it is common practice to quote the 35mm equivalent focal length. In the case of crop sensor lenses, doing so gives a common reference point for comparing them to other lenses, whatever the crop ratio of the camera's sensor.
Mar 8, 2015 06:53:36 #
OldEarl
Loc: Northeast Kansas
What you have is an untenable image. It will come apart enlarged to anything bigger than a postage stamp.
I admit it worked for James Bond and the Mission Implausible team but that was fiction. One of my students had to get an image out of a postage stamp area of a 4x5 negative so we used a 50mm enlarging lens designed for 35mm film--different medium, same principle--and got a wallet size pic for the sports page. Not good.
I admit it worked for James Bond and the Mission Implausible team but that was fiction. One of my students had to get an image out of a postage stamp area of a 4x5 negative so we used a 50mm enlarging lens designed for 35mm film--different medium, same principle--and got a wallet size pic for the sports page. Not good.
Mar 8, 2015 07:00:10 #
UtahBob wrote:
Look at the one at tawbaware.com. Google fov calculator to find it.
Good suggestion. I used the lens equivalence calculator and worked it backward reducing the sensor size to 10% of the original (full frame linear dimensions), plugged in the FL and aperture and the calculator gave the equivalent of 4200mm.
I'll accept that.
Thank you all for participating.
Mar 8, 2015 08:37:00 #
geezer7
Loc: Michigan
BebuLamar wrote:
It's 1328 it's 3.16 * 420mm. 3.16=square root of 10.
This is correct. To get 10% area we need to multiply the linear dimensions by square root of .1 which is .316. Thus the ratio of linear dimensions between cropped image and non cropped image is 1/.316=3.16. Equivalent focal length (in terms of field of view - we have been reminded countless times that the actual focal length doesn't change!!) must be 3.16x420=1328.
Interesting question over morning coffee.
Mar 8, 2015 08:37:16 #
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. Nothing changes the focal length of the lens (unless it is a zoom, of course). What you have done is what many P&S and bridge cameras will do in-camera. It's called digital zoom and is in addition to whatever optical zoom the camera has. It enlarges and crops the photo while degrading the resolution.
Mar 8, 2015 08:39:08 #
TucsonCoyote wrote:
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-)
I just woke up. Give my IQ a chance to get up to its full 198. :D
If you want to reply, then register here. Registration is free and your account is created instantly, so you can post right away.