Why is aperture backwards??

[AlanKlein]

Don't forget us shotguners, the 12 ga. is the diameter of a lead ball that requires 12 balls to make a pound. (about .720 in.) A 20 ga. is the diameter of a lead ball that requires 20 balls to make a pound. (about .620 in.)

I blame that on the Brits.

Then there is the lowly .410 ga. It actually measures .410 in. or so.

The nice thing about the f stop is it is inversely proportional to the speed. Or in plain English, the aperture diameter gets bigger (more light), the shutter speed gets faster. (less light).

ISn't caliber for the diameter of bullets a fraction of the inch? So .50 cal is 1/2 inch. .300 caliber is 3/10ths of an inch???? Of course we now also use metric such as 9mm or 5.52mm.

 

[AlanKlein]

The word stop is used because many lenses had indents at each whole f stop which stops the lenses from being rotated past that point. Of course, some lenses have half stops and thirds stops. Same with shutter speeds stops. Digital lenses don't have hard stops I don't believe for the most part, just the older film type.

Stops are handy because you don't have to look to change stops on the lenses. Just count each one as you turn the lens. This carried over to the way we describe aperture changes as well as ISO and shutter speeds. Add two stops, etc.

 

[Carter Stevenson]

"WOW, so many responses! You guys are awesome, I will review your responses when I get home today!"

 

[johngpt]

"WOW, so many responses! You guys are awesome, I will review your responses when I get home today!"

So cool to see that you came back!
Many of us are accustomed to having a question posted, a good response in attempts to answer, and then never hearing from the person who asked the question.
Kudos to you Carter!

 

[Grandpa Ron]

A quick reply on gauge vs. caliber. Gauge was developed at a time when most firearms were smoothbores shooting relatively large round lead balls. It was easy to measure how many balls of a certain diameter you could get from a pound of lead. All that was need was a scale. Also the historic loading of smooth bores, used wadding and a under bore sized ball or shot. Rifles however require a bit more precision, even when shooting a round ball and especially when shooting an elongated bullet. So caliber dimensions were used to define the size of the bore rather than projectile weight.

Photography also has a history of odd nomenclatures. The one most commonly remembered ASA, ISO and DIN; Just three of the ways to measure film speed. There were dozens of others. Also developing and printing techniques have a long history of competing methods; Calotypes, Daguerreotype, Ambrotypes, Wet plate, Dry plate etc.

Us history buffs dig this stuff. But to answer the OP question, and summarize the afore mentioned posts. The current f number aperture system was developed over time and it is used because it works.

 

[LoveMyDaughter]

Many things are the opposite of what we may expect, Fstop is just one example. Wiring is another, a 2 gauge is larger than say a 6. I always thought about it as speed. F2 is faster than F8 for example. Second place was quicker than 8th place. It drew in light faster to, allowing me a faster shutter speed to freeze hummingbirds, or motocross bike mid jump. Another benefit was lower numbers meant less depth in focus, so it was easier to blur backgrounds, drawing attention to the main subject. Been a long time for me, haven't thought about this much for the past 15 years.

 

[johngpt]

Another backwards seeming scale is sandpaper roughness.
Small numbers mean more coarse, larger numbers mean more fine grit.

Again it has to do with something being divided by something else.
In this case it's the number of sharp particles divided by square inch.

Scales created by dividing seem illogical at first glance. But that's ratios for you.

Edit: And Wikipedia has a good write up of the history of aperture nomenclature under its f-number heading, toward the bottom of the article.

Makes me glad we have come up with a standardized system of nomenclature. Trying to remember how all those older systems relate to one another would give me a headache!

 

[wfooshee]

A common misconception, but F/stops are "ratios" not fractions. A "fraction" is a number that names part of a whole or part of a group. The denominator represents the total number of equal parts the whole is divided into. A "ratio" is a "comparison" of two quantities.

Nitpicking, and going full nerd here for no reason other than that I can, and all in fun. After all, the OP's question has been answered, so veering out into left field shouldn't be too bad a thing!

My math educators taught me that fractions are ratios, with fundamentally no difference. 1/4 is the same as 1:4. A number that can be expressed as a fraction of whole numbers is a rational number, with the word ratio in there. Nevertheless, not all ratios can be expressed as a fraction of whole numbers; pi is the ratio of a circle's circumference to its diameter, but no two whole numbers as numerator and denominator can express its value exactly, so even though it's a ratio, it's an irrational number. Indeed, f:1.4 is an approximation of the actual ratio, since the square root of 2 is an irrational number, and f:√2 is just confusing enough to be useless. Don't even go there for f:11... f: (8*√2).

(Doing Mrs. Lyda proud, here... )

 

[Scott Murphy]

It boils down to a very simple, though not necessarily intuitively obvious, concept. The f/ number is a fraction, not really a ratio as has been mentioned. The formula to calculate it is N=f/D where "N" is the f/ number, "f" is the focal length of the lens in mm and "D" is the diameter of the aperture opening in mm.

For example, for an f/ number of f/8 and a 100mm lens the formula would be 8=100/D. Solving for D you would get 8D=100 so D would be D=100/8 or 12.5mm (roughly 1"). Now if you stop the lens down to f/22 the equation would be 22=100/D so 22D=100 and D=100/22 or 4.5mm (roughly 3/16") .

So as you close down the aperture ring, the diameter of the aperture opening grows smaller, but as you can see the f/ number, N, gets larger. The two are inversely proportional.

Ok, I am normally allergic to math, so now I have broken out in hives!