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So my friends and family are curious about how far my lens can reach. I usually explain with focal length and show them a few pictures.
Is there any easier way to explain what 300mm can get to someone that's not into photography? Is there a formula that states that I can fill the frame of a subject at X meter height and Y meter distance away with effective focal length Zmm?
I have often been asked this question and I use a "rule of thumb" explanation.
On a full-frame (or 35mm) body, a 50mm lens is about the same field of view as a human eye. So you could say that 300 mm is about 6x your normal vision - or similar to a 6 power rifle scope.
If you have a crop sensor, you would multiply that by, say, 1.6... so 6x1.6=9.6 or about 10x your normal vision. The quick (and not entirely accurate) answer is, "It's about 10x your normal vision, it'd be close to a 10 power scope or binoculars."
The other question I get a LOT is "how many times zoom is that lens"... I usually answer it the same way, but you could add that a fixed 300mm is about 6x your normal vision, while a 70-200 is about a 3x zoom (200/70 ~ 3). Does that make sense?
I generally find that with the "common man" in conversation, formula and numbers-driven explanations tend to sometimes confuse more than help. I usually just pull up one of the graphical demonstrations of focal length on the web, such as the Nikkor lens simulator (which also helps explain FoV differences between crop and full frame). An interactive graphic with a slider tends to explain it a lot more easily than any verbiage I can come up with.
A non-technical explanation to a non-photographer... Use simple terms. I would use a very simple measurement method. A person.
Start building from there. At one person's distance, you can see one person's width with a 40mm lens, on a full-frame camera. (I am rounding numbers)
If we have now the same person at double the distance, (again, rounding numbers) we can have the same framing with an 85mm lens.
In photography, we like to round the numbers. So for 4x times the distance, we could think on a 170mm lens.
And for your 300mm lens, we can think about 7-8 person distances.
1 person's distance is too close for a telephoto lens, so let's rise 2 person's distance as starting point of another comparison.
43 mm = You could fit 2 person width
85 mm = 1 person, finger to finger
170 mm elbow to elbow
340 mm = shoulder to shoulder
So my friends and family are curious about how far my lens can reach.
It can "reach" all the way to the stars. What they're really interested in is how much it magnifies the scene. Can you read that street sign three blocks away? Can you get a close-up shot of a kid playing soccer from the other end of the field?
Is there any easier way to explain what 300mm can get to someone that's not into photography?
Use a more intuitive metric than focal length. Angle of view is one that's easy to understand, so instead of saying that your lens has a focal length of 50mm, explain that it "sees" about 40° horizontally, so anything within a 40° angle will fill the frame horizontally. Your other (200mm) lens, on the other hand, has a 10° angle of view, so it provides 4x more magnification than the first lens, but fits only 1/4 as much of the scene into the photo. (You should of course adjust the numbers if you're using a camera with a crop sensor.)
Even better, just let them look through the viewfinder with each lens attached. Or switch to live view so that you can both see.
Is there a formula that states that I can fill the frame of a subject at X meter height and Y meter distance away with effective focal length Zmm
Yes, there is a formula: horizontal field of view = 2 atan(0.5 width / focal length), but it's probably easier to just use the calculator I linked above.
I don't think having a formula is the easier way. Let's compare both aproaches:
Is there a formula that states that I can fill the frame of a subject at X meter height and Y meter distance away with effective focal length Zmm?
Sure, like this one for example:
1 1 1
---- + ---- = ---
S1 S2 f
Where f is the focal length and S1 and S2 are the distances as illustrated in the image below:
The ratio between S1 and S2 is equal to the ratio between Object and Real image.
But how big is the sensor so that the Real Image can fill the frame as you asked for? Also S1 and S2 change when focusing, etc.
The thing is that a formula is just a formula. You can crunch the numbers with it and get some definite results, but it does not provide an intuitive understanding of what's going on. And you have so many dependencies like sensor size, object distance, object size, etc.
show [...] a few pictures [...] to someone that's not into photography
I think that makes a lot more sense, especially if they are not into photography or the maths behind it.
Bryan over at http://www.the-digital-picture.com often shows different zoom settings of zooms lenses in a review.
In this review of a 10-22mm Canon lens you can see the same subject at 10mm, 14mm, 17mm and 22mm focal length.
You don't have a zoom lens of course, but you could integrate the image from your 300mm lens into a series of a zoom lens or any other lens that you have.
The comparison between two images each taken with a different focal length (and no other setting changed) explains a lot to somebody who has no idea about the technical details.
You can still get into more detail if questions like "What if you had a different camera?" comes up.
Another way of doing it is to draw multiple frame lines onto the image taken with the widest lens. Take a look at the manual of Linhof's Technorama 617 III page 12 that illustrates 72mm, 90mm, 180mm and 250mm lenses in one single image.
I chose this intentionally as an example for a camera with different aspect ratio and different sensor size.
tl, dr;
or
This way you can say: "If you see it this way, you'd see it that way with the lens".
The bottom line is that the formula could be necessary for you to do the math, but you should give your audience some visual comparison and not the recipe to do that comparison themselves.
Without getting into formulas, I think the easiest way to visually explain what focal length is is to use an empty 35mm film slide as a framing guide. (Note, that as time goes on, fewer and fewer people know what a 35mm film slide looks like, so the visual guide is less apt...)
First, you have to explain that focal length is a property of the lens. Just like a milk jug might hold 1 or 1/2 gallon or 1 liter, or a certain water bottle might hold 1/2 liter, any particular lens has a particular focal length. (In this analogy, zoom lenses are like collapsible water bottles, that have a certain minimum volume when collapsed, and a maximum volume when expanded). Just as the volume is a property of that particular bottle, so is focal length a property of that particular lens.
(Note: I didn't have to use bottle volume for the analogy. I could have just as easily used the height of the bottle as the property. It doesn't matter — this is just an analogy)
Further extending the analogy, it doesn't matter if the bottle is full, or half full, or empty — the capacity of the bottle is fixed. Just so with a lens: it doesn't matter if it's focused far away, or up close — the focal length of the lens is unchanged.
Related: What is focal length and how does it affect my photos?
Now back to cameras. Lenses of different focal lengths change the field of view when mounted on a certain camera. Conversely, when mounting different cameras (with different film or sensor sizes) on a particular lens, the field of view is also affected.
Here's where the 35mm slide comes in when explaining to people: for a lens of a focal length ƒ (say, 50mm), if it were mounted on a 35mm film camera (the ones most people who used film cameras are familiar with), then you would get the same field of view as is you held a 35mm film slide at a distance of ƒ (50mm, or about 2 inches, in this case) in front of your eye.
Another example: early in the evening of a full moon night, when the moon is low on the horizon and it looks impressive, if you wanted to capture it at full glory, imagine holding an empty 35mm slide at arm's length (about 3 feet or so, or roughly 900mm) to frame the moon. When framed with a slide holder at that distance, the moon will fill about 1/3 of the height of the frame. So that gives you an idea of what angle of view a 900mm lens will have on a 35mm film camera (or a 35mm full frame DSLR).
Related: What focal length lens do I need for photographing the moon?
Now, if you're talking about a camera with a smaller sensor, such as a 1.5 or 1.6 APS-C crop sensor on modern entry- and mid-level DSLRs, then a 35mm film slide holder no longer works. The framing tool would have to be 1.5 times smaller. In this case, it would be 24 x 16 mm. Using the smaller "1.5 APS-C slider holder" as a framing guide, then you could put it at the lens's focal length ƒ from your eye to judge the field of view size.
Related: Does my crop sensor camera actually turn my lenses into a longer focal length?
This is the easiest way I have found to explain and visualize focal length, without diving into the maths with the thin lens formula and pinhole projection angle of view formula.
Approach it from a different angle. Literally. Most people will understand what an angle and a field of view are. If not, use a compass as a teaching aid. In the worst case, use a sharp tipped compass as a teaching aid :).
I would show them a lens simulator app on a tablet. Some examples are:
https://dofsimulator.net/en/ (which is also useful to explain what aperture or bokeh is)
https://imaging.nikon.com/lineup/lens/simulator/
https://cameraville.co/blog/collection-of-online-camera-simulators-lens-simulators
Focal length is the size of your camera's eye. Giants look farther and don't distinguish close things that much. Dwarves see stuff loom rather when comparatively close, in contrast.
