Focal length vs magnification

[Gavjenks]

There are factors of magnification... focal length is certainly one, and subject distance.

Yep. Which is why:

long lenses magnify the subject

And a 100mm lens obviously magnifies 2x compared to a 50 mm.

The magnification of the longer lens (2x focal length)

Are all wrong statements, since they all ignore of the two necessary variables. You cannot say anything meaningful about magnification while only referencing focal length and leaving distance ambiguous. Magnification is only defined at each specific distance at given focal lengths.

It is exactly as wrong, and wrong for the same reason, as the following statements would be:

Heavy rocks are more dense.

A 100 kilogram rock is obviously twice as dense as a 50 kilogram rock.

The density of the heavier rock (2x mass)

Sure, you can imagine a possible situation where a 100kg rock happens to be twice as dense as a 50kg rock. But so what? You can also describe a situation where they are the same, or where the 50kg rock is denser, or anything else. So making any specific claim about their relationship based on mass alone is nonsensical.

 

[WayneF]

Are all wrong statements, since they all ignore of the two necessary variables. You cannot say anything meaningful about magnification while only referencing focal length and leaving distance ambiguous. Magnification is only defined at each specific distance at given focal lengths.

If I stand in the same place, and switch my lens (to get a magnified view of the subject), distance is obviously NOT a variable. A factor yes, but not a variable in this case (more terms). Not much I can do about distance except to move.

But in use, I switched the lens specifically because it changes the magnification of the scene I see, which was the only purpose (often the purpose is view width, which is also magnification).
The formula clearly computes 2x magnification from a 2x longer focal length at the same distance. Focal length is an extreme factor of magnification. It is the reason why we have different focal lengths.

I suspect you know this obvious stuff, surely still just trying to defend the first wrong crap. I'm past bored, and am abandoning this. I don't care what you think, but it seems a sin to publicly spread wrong stuff to newbies.

 

[Gavjenks]

If I stand in the same place, and switch my lens (to get a magnified view of the subject), distance is obviously NOT a variable.

You assigned a value to a variable and then said it's not a variable in the same sentence. If it truly wasn't a variable, then you wouldn't have to say anything about it at all. But you do. Because it is.

The formula clearly computes 2x magnification from a 2x longer focal length at the same distance.

Yes.

"A 100mm lens gives 2x magnification at the same distance" is true.
However, repeatedly in this thread, you have said things like
"A 100mm lens obviously gives 2x magnification over a 50mm lens" without specifying anything about distance.

The first claim is true. The second claim is false. You don't seem to acknowledge the distinction, is the problem.




It's like saying a broken watch "works" just because it happens to be right twice a day... Why would you only focus on that one special case?

 

[480sparky38 37]

 

[JerryLove]

Perhaps answering a question will help?

Does a telescope magnify the image of, say, the moon? Because it is less than moon-sized on my eye (or camera when I attach one), though far bigger than I see it without the telescope?

 

[Gavjenks]

Perhaps answering a question will help?

Does a telescope magnify the image of, say, the moon? Because it is less than moon-sized on my eye (or camera when I attach one), though far bigger than I see it without the telescope?

"Magnify" as a verb implies a comparison, which means you need to specify a baseline, which needs both a FL and a distance. If you are comparing a telescope on Earth to your eyeball on Earth, as you seem to be asking, then yes, the telescope will have a higher magnification.

However, if you were Neil Armstrong in July 1969, for instance, then your eyeball would achieve a much higher magnification of the moon than your telescope does from Earth. Many orders of magnitude higher magnification.

That's why it's generally confusing and unnecessary and not very useful to talk about magnifying as a verb.




Notice that the only cameras that advertise "__x magnification!" are the crappy cheapest of the consumer point and shoots. As soon as they market to any segment of the population that might know what they're talking about, they stop talking about __x magnification, and if anything, advertise maximum system magnification as a proper ratio, not in comparison to anything (like for macro), because there's no room for ambiguity and it is better standardized and scientific.

 

[TreeofLifeStairs]

Even though this has been a heated conversation, the back and forth has actually really helped with my understanding of the subject. Thank you to both of you.

 

[petrochemist115 140]

Perhaps answering a question will help?

Does a telescope magnify the image of, say, the moon? Because it is less than moon-sized on my eye (or camera when I attach one), though far bigger than I see it without the telescope?

Your question actually contains a key part often missed out, which may be the cause of much of this confusion.
This is 'magnify THE IMAGE'
The magnification of telescopes is magnification of the image not the item itself, whilst in macro magnification refers to the items dimensions.

Gavjenks' original reply is probably the clearest explanation I've seen of magnification but still it took the best part of 2 pages of discussion to follow it up!