| Author | Thread |
|
|
01/30/2006 10:46:23 AM · #1 |
| How does one calculate the math of effective focal length, etc. on, say a Nikon D70? I have seen people reference effective numbers, but was looking for the somewhat-official version of how do do the math? |
|
|
|
01/30/2006 10:48:15 AM · #2 |
| Are you talking about the crop-factor due to the sensor being smaller than a 35mm film size? If so, it's approximately 1.5 to 1.6 x the focal length. Nikon specs ought to have this figure so you can know for sure. |
|
|
|
01/30/2006 10:53:50 AM · #3 |
|
|
|
01/30/2006 10:57:22 AM · #4 |
| Sorry, I should have been more clear. Here is an example: if I were to purchase a sigma 15mm f/2.8 fisheye to use on my Nikon, the basics of it is calcuation of 1.5x the focal length, making it a ~22.5mm lens. (1) does it affect 1.5x the f-stop as well, making f/2.8 into f/4.2? and (2) how does mathematics apply when trying to determine mathematical conversion to correct for 'fisheye' when using a tool like Adobe Photoshop CS2? |
|
|
|
01/30/2006 11:08:23 AM · #5 |
the 1.5 conversion does not effect the f-stop. The way it works in reality is you have a smaller field of view, smaller angle of view. So the optics don't change, so the f-stop doens't change.
Why woul dyou use a fisheye lens and then correct for it?
|
|
|
|
01/30/2006 11:08:29 AM · #6 |
I can answer #1: It doesn't have any effect on aperture. It also doesn't have any effect on depth of field. Technically, it also doesn't have any effect on focal length, that is a property of the lens. What is does do is affect the field of view of the image. So when "effective focal length" is mentioned, what is really meant (whether the person saying it knows it or not) is that a certain lens has the same field of view with a APS sensor as a longer lens with a 24mm x 36mm sensor/film.
As far as #2 goes, I have no idea, I'm just a poor boy and can't afford CS2.
|
|
|
|
01/30/2006 11:11:05 AM · #7 |
Originally posted by Prof_Fate:
Why would you use a fisheye lens and then correct for it? |
To get the wide field of view without the distortion.
|
|
|
|
01/30/2006 11:17:37 AM · #8 |
This is an excellent converter if you can't to the math yourself.....
lense conversion |
|
|
|
01/30/2006 11:25:27 AM · #9 |
Originally posted by Spazmo99:
To get the wide field of view without the distortion. |
It'd be better to use the right lens. I mean with 'everyone' so worried about lens quality and photographic integrity, this seems to run counter to both.
|
|
|
|
01/30/2006 11:35:40 AM · #10 |
Originally posted by Prof_Fate:
Originally posted by Spazmo99:
To get the wide field of view without the distortion. |
It'd be better to use the right lens. I mean with 'everyone' so worried about lens quality and photographic integrity, this seems to run counter to both. |
Huh? A fisheye is just a different "mapping" of the spherical coordinates of the scene onto the flat sensor. The fisheye and rectilinear mappings are equally valid, just different. A 15mm fisheye does have a significantly wider FoV than a 15mm rectilinear lens. You get different TYPES of distortion, but you get distortion with each. Ever look at people photographed near the edges of the frame with an ultra-wide rectilinear lens? They're about twice as wide as they should be. Not real flattering to the subject.
Bottom line, a fish is great way to achieve a really wide FoV, but not appropriate for all uses. A good ultra-wide rectilinear is also a great thing, but also not appropriate for all uses.
|
|
|
|
01/30/2006 11:39:25 AM · #11 |
Originally posted by kirbic:
Originally posted by Prof_Fate:
Originally posted by Spazmo99:
To get the wide field of view without the distortion. |
It'd be better to use the right lens. I mean with 'everyone' so worried about lens quality and photographic integrity, this seems to run counter to both. |
Huh? A fisheye is just a different "mapping" of the spherical coordinates of the scene onto the flat sensor. The fisheye and rectilinear mappings are equally valid, just different. A 15mm fisheye does have a significantly wider FoV than a 15mm rectilinear lens. You get different TYPES of distortion, but you get distortion with each. Ever look at people photographed near the edges of the frame with an ultra-wide rectilinear lens? They're about twice as wide as they should be. Not real flattering to the subject.
Bottom line, a fish is great way to achieve a really wide FoV, but not appropriate for all uses. A good ultra-wide rectilinear is also a great thing, but also not appropriate for all uses. |
What he said. |
|
|
|
01/30/2006 11:45:53 AM · #12 |
But a wide angle lens is trying to show the world as it is, while a fisheye is purposely distorting it.
If you want a 'regular' view then use a regular WA lens. You might have to correct it, but it would require a lot less corredtion than a fisheye.
It's easier to add a fisheye effect in PS than remove it I would think (i have added one, but not tried to remove it)
|
|
|
|
01/30/2006 11:54:06 AM · #13 |
Originally posted by Prof_Fate:
But a wide angle lens is trying to show the world as it is, while a fisheye is purposely distorting it.
If you want a 'regular' view then use a regular WA lens. You might have to correct it, but it would require a lot less corredtion than a fisheye. |
Agreed, the rectilinear mapping does try to replicate what we see, but falls distinctly short of that for very wide angle lenses. My point was, both the fisheye and rectilinear mappings have their purposes, and fisheye does not purposely distort any more than rectilinear, just distorts differently. It's the same problem as how to put a map of a spherical planet onto a flat piece of paper. It's impossible to do it without distortion, but you may want to choose different mappings based on the objective of the particular map.
Originally posted by Prof_Fate:
It's easier to add a fisheye effect in PS than remove it I would think (i have added one, but not tried to remove it) |
Actually, it's just as easy to do either one. The process is reversible. It's even valid to partially convert between the two, just depends on your end goals.
|
|
Home -
Challenges -
Community -
League -
Photos -
Cameras -
Lenses -
Learn -
Help -
Terms of Use -
Privacy -
Top ^
DPChallenge, and website content and design, Copyright © 2001-2025 Challenging Technologies, LLC.
All digital photo copyrights belong to the photographers and may not be used without permission.
Current Server Time: 03/12/2025 10:04:40 AM EDT.
