Lenses focal length

[Almass]

There are now two threads where there is mention that a lens of any focal length "magically morphs" into a sensor size factorial of 1.5 or whatever, thus giving additional reach.

Let's once and for all put this issue to re3st.

Focal length does not change on a full frame or cropped frame whatsoever.

What changes is the CrOP.

Let me explain for the visually impaired:
You have a 250gm steak on a plate surrounded with French Fries. You shoot the plate with a cropped sensor and you get only the steak, you shoot the plate with a full frame, you get the steak and the fries. The steak visual size does not change. (camera to subject distance being constant)

So if you have a 300P on a cropped sensor of 1.5 it does NOT make your lens a 300x1.5= a 450mm
and if you add a doubler of say factor 2, you DO NOT get a 900mm but a 600mm with fstop handicap factored in.

Just take you full frame camera and shoot any subject. Change your image size in the menu or change camera to cropped and shoot the same subject again keeping the same camera to subject distance. Now you have two pics. Crop the full frame to the same size of the cropped frame and.........magic both pics are the same size.....Dohhh.

If some would like to be anal about the issue. The only difference between a same lens used on FF or Dx, besides the crop, is in the DOF......which is very minute difference between the two but is still there for the discerning eye.

[Bjørn Rørslett]

We all ought to know these basic facts and people still, to this day, muddle the issue. Perhaps they have a lens with ability to understand what camera it is attached to and thus can automagically change its optical inherent properties? Perhaps a magnifying element manifests itself from thin air and inserts itself into the optics? Must be magic or snake oil or both.

For old-timers having used anything from "small format" 24x36 mm up to 8x10", the belief in "crop factors" and change of focal length from one format to another is - well - unbelievable. The Internet is capable of making myths become an accepted "fact" if the errors are stated a sufficient number of times, apparently.

Oh well.  Between the alternatives of bashing one's head against the wall in desperation or ignore the expressed nonsense, I prefer the latter one. Produces less headaches in the longer run.

Repeat 1000 times: focal length is focal length is focal length .... It is an inherent property of the optical design. It cannot change just by moving the lens onto another format. Period.

[MFloyd]

Thanks Bjørn. You made me definitely belonging to the "old farts" category i.e. I entirely agree with your analysis ...

[Frank Fremerey]

Oh well.  Between the alternatives of bashing one's head against the wall in desperation or ignore the expressed nonsense, I prefer the latter one. Produces less headaches in the longer run.

If I shoot birds and have to crop to one tenth using a 24x36-sensor of 21Megapixels with a 300mm lens I get a 2 Megapixel bird shot.

If I shot the same bird from the same position (photographers call it "perspective") using a 16x24-sensor of 21 Megapixels with a 300mm lens I get a 4.5 Megapixel bird shot.

If the 300mm lens is not up to the task, the details I get from the 16x24-sensor-4.5-Megapixels-crop are exactly the same as from the 24x36-sensor-2-Megapixel-crop.

If the 300mm-lens IS up to the task I get more details through more magnification like I get more details from a D810-File than I get from a D3-File.

Terminology can be confusing, but with some practice everybody shall be able to learn in the long run.

[Bjørn Rørslett]

"If the 300mm-lens IS up to the task I get more details through more magnification like I get more details from a D810-File than I get from a D3-File."

Magnification and resolution are two entirely different concepts. Obviously people confuse them.

[MFloyd]

Also the myth of increased DOF (Depth Of Field) with DF sensors: if you want to frame the same scenery with a DF sensor (compared to a FX sensor) you have to maintain a bigger distance from the subject: increased distance = increased DOF. Or the other way around: with a bigger sensor and the same focal length, you get to be closer to the subject: shorter distance = shorter DOF. QED (Quod Erat Demostrandum).

[Bjørn Rørslett]

Keeping the concept of an unaltered focal length firmly in mind suffices. The black voids of such bizarre ideas as 'equivalence' and 'equivalent' f-stops (sic) should be avoided as they only can throw mud in one's eyes. Some never recover.

[Les Olson]

Repeat 1000 times: focal length is focal length is focal length ...

Except when it isn't.  Which in the case of any internal focusing lens, which is most modern lenses, is nearly all the time.   

"Crop factor" is a metaphor.  And none the worse for that - so are "light ray" and "depth of field" and "focal length".  Problems arise in all those cases when people forget that those terms are metaphors and use them as the names of real things. 

[Merco_61]

Except when it isn't.  Which in the case of any internal focusing lens, which is most modern lenses, is nearly all the time.   

"Crop factor" is a metaphor.  And none the worse for that - so are "light ray" and "depth of field" and "focal length".  Problems arise in all those cases when people forget that those terms are metaphors and use them as the names of real things.

Focal length is still focal length. The shortening when not at infinity is lens behaviour, not focal length .

[Bjørn Rørslett]

Focal length is, like aperture (f-number), defined for infinity focus.

Many lenses these days are akin to zoom lenses in that their focal length alters when they focus from infinity to the near limit. Still whatever focal length the optics exhibit at a given focused distance will remain unchanged from one format to another. which after all is the main theme here.

[Les Olson]

Focal length is still focal length. The shortening when not at infinity is lens behaviour, not focal length .

Well, "lens behaviour" is a metaphor as well, and a notably vague one.  So is "infinity", though not so vague.   

The focal length of a lens is the distance from the rear principal plane to the plane in which the image of an object at infinity is in sharp focus.  So focal length is a theoretical quantity because nothing is infinitely distant.   Objects closer to the lens than infinity are in sharp focus at greater distances from the rear principal plane than the focal length, and an alternative way of putting it is that the focal length is the shortest distance from the real principal plane to the sensor/film at which anything can be in focus.  That is a handy thing to know if you are using a view camera, but it has no particular importance for cameras with fixed lens to sensor/film distances.  Focal length is used because everyone is used to it, but angle of view would at least as good and in some ways better as a descriptor (because then you would not have people thinking that an 18-140 lens has a bigger zoom range than a 16-80). 

An internal focusing lens does not have a single focal length: it focuses closer by changing the focal length.  Of course, for descriptive purposes we use the infinity focal length, but that is perfectly arbitrary (and handy for advertising, because that is always the longest focal length).  But the focal length at infinity is no more the focal length than any other. 

[Frank Fremerey]

Will some people join who do not know the facts or will we knowers scratch each others back to infinity?

[Eddie Draaisma]

The German "brennweite" (burn length) and Dutch "brandpuntsafstand" (burn point distance) say it all. Much more descriptive than focal length... 

[Bjørn Rørslett]

"But the focal length at infinity is no more the focal length than any other.  "

Except for being the definition of focal length. Which in turn enters the definition of aperture (f-number).

One should probably give up at this point.

[Bjørn Rørslett]

The German "brennweite" (burn length) and Dutch "brandpuntsafstand" (burn point distance) say it all. Much more descriptive than focal length... 

It's exactly the same concept and in fact the same nomenclature. Focal point = focused point = burn point, length = distance.

Thus 'focal length' == distance to burn point.