Crop factor - misleading

Gavjenks

TPF Noob!
Joined
May 9, 2013
Messages
2,976
Reaction score
588
Location
Iowa City, IA
Can others edit my Photos
Photos OK to edit
The angle that various sizes of sensors can record changes with their size
Right, which is what I and everybody else in the world is talking about when they discuss field of view or the angle of a lens without specifically saying otherwise. THIS one is what it means when you say "angle of view" or "field of view" or anything like that just by itself.
Angle of view - Wikipedia, the free encyclopedia <-- "...[angle] that is imaged by the camera"
Understanding Your Camera: Focal Length, Field of View and Angle of View Defined : Martin Pot - Photography Blog <-- "angle that is projected onto the camera's sensor"
Understanding Camera Lenses <-- notice the calculator requires a crop format to be specified and changes depending on it
$cropped_sensor_view.jpg

but the angle that the lens projects does not change.
Nobody ever cares or talks about this except for a few people shooting large format view cameras who want to know if a certain lens will cover film on an unintended film size. And even then they don't use the term angle, because they know better than to confuse with the common term that everybody would interpret it to mean (the above one). They talk about film sizes covered instead.

A 300mm lens had the same FOV no matter what camera you put it on.
Also no. Nobody ever ever defines or talks about FOV in terms of the total image circle possible.
FOV refers only to the angle as seen by the sensor. i.e. it relates to the first thing you said i quoted above, not the entire image circle.
Field of view - Wikipedia, the free encyclopedia <--see first paragraph
AbelCine - Field of View Calculator <--notice that it changes when you change sensor size
Digital Camera Sensor Sizes: How it Influences Your Photography <--do a page search for field of view
 

Gavjenks

TPF Noob!
Joined
May 9, 2013
Messages
2,976
Reaction score
588
Location
Iowa City, IA
Can others edit my Photos
Photos OK to edit
BUT, focus breathing works in the opposite direction. The lens is shorter/wider than one would think. The issue here is that it's longer/narrower than expected. I have no idea how to account for that except maybe it's a "comparison issue" and the "other lens" has the issue.
No it goes both ways, because it entirely just depends on marketing, not physics.
So for instance, let's say at a given indicated focus length on the barrel, the actual focus length goes from 35-45mm from minimum focus to infinity.
Depending on whether the manufacturer wishes to emphasize how long their lens reaches, they may choose to label this as "35mm" on the barrel, or as "45mm" on the barrel.
Depending on which they choose, you would see either unexpected lengthening or unexpected shortening in normal usage, see?

And of course, when somebody is talking about something in the neighborhood of 18mm, it's more marketable to choose the wider of the two numbers, and when somebody is talking about something in the neighborhood of 300mm, it's more marketable to choose the more tele of the two numbers.

So in general, you should expect to always get the opposite of what you want, in terms of focus breathing versus marked number.
 

table1349

Been spending a lot of time on here!
Joined
Sep 9, 2006
Messages
0
Reaction score
2,772
BS. I assume that means that you are involved in Evolutionary Psychology since using the correct terms doesn't seem to matter to you. To the following statement about the angle of a LENS is absolutely correct. If you are not talking about the LENS then say so, but the PHYSICS associated with the LENS DOES NOT CHANGE.
quote_icon.png
Originally Posted by Gavjenks

The angle of a lens does not change merely by placing it on a different camera.
By the way, yes it absolutely does. In fact, not only does it change based on sensor length, but angle of view isn't even defined without reference to a specific film or sensor size. If I just tell you "300mm" without reference to any specific sensor or film, it is completely impossible to give me any corresponding angle, because you don't have all the parameters you need.

A 300mm lens on a crop sensor is about 5 degrees of visual angle, and a 300mm lens on a large format camera is 29+ degrees of visual angle...

FYI. The rest of the world likes to use the correct terms when discussing concepts such as this. They don't make a habit of SWAGGING terms there "Global psychologist."
 

Gavjenks

TPF Noob!
Joined
May 9, 2013
Messages
2,976
Reaction score
588
Location
Iowa City, IA
Can others edit my Photos
Photos OK to edit
Gryphon, I can't help but notice that while I found half a dozen links to prominent websites that ALL use my definitions, you failed to provide a single example of anybody but you using yours.

Yes, the world likes to use correct terms. And in this case, the correct definitions for angle of view and field of view are both in terms of the imaged portion of the scene, NOT the projected portion of the scene. That's why the world uses them that way, not your way.

Here's yet another one for you from another major photography learning source (digital photography school.com)

http://digital-photography-school.com/photography-101-lenses-light-and-magnification/
Field of View: "The final variable in this initially confusing balancing act of optics is the field of view. This refers to how much of the world the camera can see. A lens&#8217; field of view depends on the focal length of the lens and the size of whatever the image is projected onto."
 

Gavjenks

TPF Noob!
Joined
May 9, 2013
Messages
2,976
Reaction score
588
Location
Iowa City, IA
Can others edit my Photos
Photos OK to edit
And just for giggles / to throw onto the dogpile (not ever my first choice for primary resource...):
Angle of View
(He talks about it in the opposite direction - the furthest points you can see on the final image, but this of course is still the imaged angle, not the projected angle, as it changes with sensor size)
 

Helen B

TPF Noob!
Joined
Sep 16, 2007
Messages
3,296
Reaction score
467
Location
Hell's Kitchen, New York
Can others edit my Photos
Photos NOT OK to edit
BUT, focus breathing works in the opposite direction. The lens is shorter/wider than one would think. The issue here is that it's longer/narrower than expected. I have no idea how to account for that except maybe it's a "comparison issue" and the "other lens" has the issue.
No it goes both ways, because it entirely just depends on marketing, not physics.
So for instance, let's say at a given indicated focus length on the barrel, the actual focus length goes from 35-45mm from minimum focus to infinity.
Depending on whether the manufacturer wishes to emphasize how long their lens reaches, they may choose to label this as "35mm" on the barrel, or as "45mm" on the barrel.
Depending on which they choose, you would see either unexpected lengthening or unexpected shortening in normal usage, see?

And of course, when somebody is talking about something in the neighborhood of 18mm, it's more marketable to choose the wider of the two numbers, and when somebody is talking about something in the neighborhood of 300mm, it's more marketable to choose the more tele of the two numbers.

So in general, you should expect to always get the opposite of what you want, in terms of focus breathing versus marked number.

The focal length of a photographic lens is defined at infinity focus. It's not the manufacturer's choice.
 

Gavjenks

TPF Noob!
Joined
May 9, 2013
Messages
2,976
Reaction score
588
Location
Iowa City, IA
Can others edit my Photos
Photos OK to edit
The focal length of a photographic lens is defined at infinity focus. It's not the manufacturer's choice.
Fair enough, but then they're simply wrong all the time, or flat out lying. Because it very often does not match up at infinity.
 

480sparky

Chief Free Electron Relocator
Supporting Member
Joined
Mar 8, 2011
Messages
24,902
Reaction score
8,880
Location
Iowa
Website
pixels.com
Can others edit my Photos
Photos NOT OK to edit
The focal length of a photographic lens is defined at infinity focus. It's not the manufacturer's choice.
Fair enough, but then they're simply wrong all the time, or flat out lying. Because it very often does not match up at infinity.

Does what not match up?
 

unpopular

Been spending a lot of time on here!
Joined
May 17, 2011
Messages
9,504
Reaction score
2,002
Location
Montana
the focal length is the distance from the optical center of the lens (not the rear element) to the image plane when a point at infinity is at maximum sharpness. In practice, it's defined in it's design and has nothing to do with the measured performance of the lens, or for that matter, the PR department.
 

Helen B

TPF Noob!
Joined
Sep 16, 2007
Messages
3,296
Reaction score
467
Location
Hell's Kitchen, New York
Can others edit my Photos
Photos NOT OK to edit
To the OP, Reflex: have you any way of testing the angle of view on a full frame camera?

(The focal length is measured from the rear nodal point to the image plane. The lens (and only the lens) may be rotated about the rear nodal point without the image of objects at infinity moving.)
 

Gavjenks

TPF Noob!
Joined
May 9, 2013
Messages
2,976
Reaction score
588
Location
Iowa City, IA
Can others edit my Photos
Photos OK to edit
The focal length of a photographic lens is defined at infinity focus. It's not the manufacturer's choice.
Fair enough, but then they're simply wrong all the time, or flat out lying. Because it very often does not match up at infinity.

Does what not match up?
Trigonometrically calculated angle of view and the theoretical one for the FL written on the barrel. At least not at extremes (I have tested close and infinity on some of my lenses when bored and on most of them, NEITHER is actually the stated number. But only at their limits, wasn't that bored. E.g. 300mm on my 70-300)
 

unpopular

Been spending a lot of time on here!
Joined
May 17, 2011
Messages
9,504
Reaction score
2,002
Location
Montana
For one thing, the image circle is generally larger than the sensor... This might not seem like much, but when you're talking about a ratio from, say 24mm to several hundred yards' view at infinity, a few millimeters would be significant.
 

unpopular

Been spending a lot of time on here!
Joined
May 17, 2011
Messages
9,504
Reaction score
2,002
Location
Montana
404 file not found ... not sure if that was what you were linking to, but nonetheless appropriate.
 

Gavjenks

TPF Noob!
Joined
May 9, 2013
Messages
2,976
Reaction score
588
Location
Iowa City, IA
Can others edit my Photos
Photos OK to edit
For one thing, the image circle is generally larger than the sensor... This might not seem like much, but when you're talking about a ratio from, say 24mm to several hundred yards' view at infinity, a few millimeters would be significant.
Not sure why image circle has to do with anything. A 300mm lens could have almost any size image circle. For example, there are 300mm lenses for crop cameras that won't cover a full frame, 300mm lenses for large format cameras that cover 8x10 sized pieces of film. They're all 300mm lenses. (Also how do you measure image circle? It doesn't even have a defined edge since it usually softly feathers out to black)

What I did for the infinity end was this:
1) Went outside on a clear night
2) Focused the lens on the moon at "300mm" (so the barrel says) for the 70-300 lens
3) Snapped a photo.
4) Calculated my distance to the moon's center for the current date using this (well, not for Austin, of course, although I don't think it really matters, I did use cedar rapids -->) Moon distance for Austin, U.S.A. - Texas in year 2014
5) Calculated the distance represented across the frame based on pixels taken up by the moon's diameter. (taking into account altitude above center of the earth)
6) Trigonometried up an angle horizontal.
7) Compared to published theoretical angles for different focal lengths.

And it came out pretty far off. I will try to find the papers from back when I did this. If I can't I'll go out tonight and try it again if you want, it just so happens to be the lunar perigee tonight!

(For nearest focus I just walked to the other end of my room until the wall was in focus and used that, but as per Helen's comment, infinity is apparently all that matters)




Edit: I guess you could just use constellations instead, but that's not the way I thought of doing it *shrug*
 
Last edited:

Most reactions

New Topics

Top