PHRubin Loc: Nashville TN USA

You also left out the fact that the 2 cameras have different pixel counts, 20% different.

wdross Loc: Castle Rock, Colorado

I find the easiest way for me to talk "equivalent" is to reference the angle of view. The angle of view (4.1°) for my 300mm 4/3rds lens is the same angle of view (4.1°) as the Sony, Canon, Nikon, and any other full frame lens of 600mm. And f4 is f4 for any lens of any format. But depth of field for the f4 will change between formats if keeping the angle of view and aperture the same. For me shooting wildlife, more depth of field is desirable, especially if shooting moving wildlife. Plus I can handhold the 300, 1.4X or 2X teleconverter, and body with no need of a tripod. This is where I feel my 4/3rds provides me an advantage. This is where the word "equivalent" has the most meaning for me.

MJPerini

If we start with prime lenses, a lens' focal length never changes, it's Angle of view changes with sensor size.
In theory it is the same for Zoom lenses, except some zooms change focal lengths with focus distance (so called Focus breathing)
The mess was created in the early days of Crop sensors relative to FF 35mm with the idea of "Equivalent Focal length",(there is nothing wrong with the concept mathematically) because people mistakenly began to think that the focal length somehow changed. It doesn't. The angular view changes.(Because we cropped)
As stated above the math is pretty simple as long as it is based on format diagonal
It is interesting, I grew up with View cameras 8x10 , 5x7 4x5 and lenses were changed from one format to another and as long as it covered the format we used it. A 90mm Schneider Super Angulon @f 45 could almost cover 8x10
No one ever talked about "Equivalent Focal length" We understood that a Normal 300mm lens on would be a long lens on a smaller format. And a "Normal" 6" Dagor would be a wide angle on 8x10 at f/45-64.

The Idea that introducing pixels / mm would somehow simplify a dead simple concept seems like a non starter to me. I have never met a person who did not get the concept after being shown that a smaller sensor is just a crop of a larger sensor. And having been shown that, they usually lose all interest in crop factors and equivalent focal lengths. It just makes sense to them.
I the same way Phone manufacturers NEVER talk abbot focal lengths or equivalencies, they say wide normal tele and no one cares......nor should they

Longshadow Loc: Audubon, PA, United States

No matter how simple a problem seems at first,
it becomes increasingly more complex upon further investigation.

imagemeister Loc: mid east Florida

Internal focus primes focal length does change ......just not as much as zooms .....

BebuLamar

Yes if you focus with the bellow the focal length doesn't change. Internal focusing lenses prime or zoom do change their focal length as you focus closer than infinity. Even macro lenses do this.

bhapke

I read somewhere many years ago that in the early days of photography (all film of course) someone took images at a variety of focal lengths, printed them as 8x10 and put several different people at the location where the photo was taken. They were asked to say if the photo was the same as they saw the scene, more wide angle, or more telephoto. It was found that when the focal length was approximately the same as the film diagonal, the view was rendered normally. This would equate to a 6" lens (approximately 150mm) on 4x5 sheet film, about 43mm for 35 mm film. The camera makers settled on 50mm to 58mm as the "normal" lens.

When digital cameras were initially made the sensors were very small due to the difficulty and high cost of making larger sensors. This resulted in much "shorter" focal lengths being "normal". As a way to make it easier to understand to the average photographer the concept of crop factor was introduced. Since everyone either used or knew about 35mm film, they said "multiply the actual focal length by the crop factor and note the answer". You will get the same field of view as that focal length on a 35mm film camera".

Today, we can just look through the viewfinder or the screen on the back of the camera and compose the shot till it looks good, and ignore all that other stuff.

Linda From Maine Loc: Yakima, Washington

There are times when comparing is instructive.

One example: OMG, you took that grizzly bear photo at 300 mm??? Well, no. My camera has a 2x crop factor, so my equivalent angle of view with that lens is 600 mm.

And then I cropped more

Also, if you're comparing photos with someone who took shots of the same subject (you have a tiny sensor and they have large) then depth of field differences might be questioned if one wasn't familiar with how sensor size affects DOF and aperture choice.

Longshadow Loc: Audubon, PA, United States

All I know is I can adjust the zoom so that the subject looks about the same distance in the viewfinder as it does with my eye. For the Canon APSC body that turns out to be just under 35mm on the lens. And what do you know, 50/1.6 (APSC factor) is 31mm. That's all I need to know, the setting that makes it look like what I see. But I adjust the zoom for what composition I desire, regardless of crop factor or focal length anyway.
I don't need to know the physics behind it.
Or make a molehill into a mountain.

george19

Bingo! Then all you need is a reference, something like a standard field of view on a 24 x 36 mm piece of film is 43 degrees (I made that up) using a 50 mm lens.

SuperflyTNT Loc: Manassas VA

This is a completely invalid comparison. Physical dimensions of a digital image make no sense for comparison.

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Standard fiddle played by faerie pixie in a tux. FoV ratio equals 1122.34%.

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Problem ? WHAT problem ? No problem here !

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The trubble with you is that youre always ignoring UHH Sacred Tradition.

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A mans comparison of lengths ??!!?!!?

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