Apertures and focal lengths

chris321

chris321

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Chris
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I had a thought the other day...... say you had two lenses, both f/2.8, but at different focal lengths. Say a 100mm f/2.8, and a 300mm f/2.8.

Obviously the size of the physical hole the light can get through will be larger for the 300mm lens, so does that mean you could use a shorter shutter speed, as it can detect more light? I'm fairly sure this isn't the case, but I can't quite get my head around why!

The only reason I could think of is that the field of view is narrower, which I suppose means less light can be picked up, but it's a total guess, and it's annoying me that I don't know! Thanks for any help!

Chris
 
chris321 said:
The only reason I could think of is that the field of view is narrower, which I suppose means less light can be picked up, but it's a total guess, and it's annoying me that I don't know! Thanks for any help!

Chris
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Hit the nail on the head.
There is a smaller amount of light in view so the aperture has to be physically larger.
Luckily somebody worked all this out a long time ago and gave us f/numbers to make all this "simple".
 
Wow, I can't believe I got something right! This whole f-number lark really does make everything easier, it's not something I'd even thought of before, but I suppose someone had to 'invent' it....... Good good, curiosity satisfied, thanks a lot! :thumbs:
 
Forgive me if you already know this.

One of the things that confuses people is the halving/doubling relationship between f/numbers, ie f4, passes twice as much light as f/5.6, which also passes twice as much light as f/8, and so on.

The common range is f/1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22. Even though each one passes half or double the amount of light as the f/number either side, the mathematical relationship between them is the square root of 2, ie 1.4.

In this way, 1/125sec at f/5.6 is the same total exposure as 1/250sec at f/4, etc.
 
To add to what Hoppy has said, iirc if you divide the focal length of the lens by the f number you get the size of the front element, so a 200mm f/2.8 lens needs a front element of approximately 71.43mm to allow the correct amount of light in, whereas a 300mm f/2.8 lens would need a 108mm front element to do the same.
 
Flash In The Pan said:
To add to what Hoppy has said, iirc if you divide the focal length of the lens by the f number you get the size of the front element, so a 200mm f/2.8 lens needs a front element of approximately 71.43mm to allow the correct amount of light in, whereas a 300mm f/2.8 lens would need a 108mm front element to do the same.
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Sorry to be picky, but it's not the size of the front element, it's the diameter of the aperture. Much the same thing with longer lenses, but very different with wide angles.
 
also don't forget that at 300mm you should be using a shutter speed of 1/300s or similar to eliminate camera shake....which would be 1/500s on a cropped body perhaps.
 
It is the size of the front lens that limits the aperture. let me know when you pick up a 300mm F1.4 lens.
 
classcams said:
It is the size of the front lens that limits the aperture. let me know when you pick up a 300mm F1.4 lens.
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True, and nobody said that it wasn't, but it's got nothing to do with how the f/number is calculated, even though the two might roughly coincide with long lenses.
 
Thanks a lot guys, I knew the majority of this already but it's good to be told it again just to be on the safe side! :thumbs:
 
I thought the f number was a ratio of the diameter of the front element to the focal length, thus giving the f number.......
 
But it is a ratio, which is why the f number lets in the same amount of light for different focal lengths. The ACTUAL diameter of the aperture changes, but the ratio remains constant. for any given f number.

eg.
50mm lens @ f4 has an actual diameter in the diaphragm of 12.5mm
100mm lens @ f4 the actual diameter of the diaphragm is 25mm

The diameter of the hole in the middle has doubled, but the RATIO of hole size to focal length remains the same.

In order to provide the f number (f4 in this case, but it could be f1 or even f0.9) the front optic has to be of a certain size to allow the required diameter of "hole in the middle".

So, using the same two lenses, 50mm and 100mm, but with an f number of f1 the diaphragm diameters would be:
50mm @ f1 = 50mm
100mm @ f1 = 100mm

and at f0.9 they would be
50mm @f0.9 = 55.5555mm diameter (wider than the lens is long)
100mm lens @ f0.9 = 111.1111mm diameter.

Nikon did make a nocturnal lens once that had a maximum aperture of f1, I can't remember the focal length of it though. Just as a piece of useless factoidal information.
 
Lensflare said:
But it is a ratio, which is why the f number lets in the same amount of light for different focal lengths. The ACTUAL diameter of the aperture changes, but the ratio remains constant. for any given f number.

eg.
50mm lens @ f4 has an actual diameter in the diaphragm of 12.5mm
100mm lens @ f4 the actual diameter of the diaphragm is 25mm

The diameter of the hole in the middle has doubled, but the RATIO of hole size to focal length remains the same.

In order to provide the f number (f4 in this case, but it could be f1 or even f0.9) the front optic has to be of a certain size to allow the required diameter of "hole in the middle".

So, using the same two lenses, 50mm and 100mm, but with an f number of f1 the diaphragm diameters would be:
50mm @ f1 = 50mm
100mm @ f1 = 100mm

and at f0.9 they would be
50mm @f0.9 = 55.5555mm diameter (wider than the lens is long)
100mm lens @ f0.9 = 111.1111mm diameter.

Nikon did make a nocturnal lens once that had a maximum aperture of f1, I can't remember the focal length of it though. Just as a piece of useless factoidal information.
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Yes. But.

My Canon 10-22mm f/3.5-4.5 has a front element measuring 51mm across. But it is not an f/0.2 lens and the size of the front element obviously doesn't change as the lens is zoomed.

The size of the front element doesn't directly affect the f/number but it has a big effect on vignetting. If you compare two lenses like the Canon 50mm f/1.4 and Sigma 50mm f/1.4, the Sigma has a huge lump of glass in the front for the same f/number, but it has less vignetting.

While this doesn't strictly affect the f/number, it is very important since we use the f/number for exposue setting. If a lens vignettes badly, then the amount of light transmitted will only be 'correct' in the centre of the frame. Manufacturers cheat a little with this, as it allows them to produce much smaller and lighter lenses without affecting the f/number, even though it is only correct for exposure purposes in the centre of the image. The camera's TTL metering corrects for this automatically without us even knowing about it.

Without going into to transmission and T/stops vs f/stops, if you take a cheap zoom and note the exposure reading, then fit a high end lens and do the same, the shutter speed is often a bit different, ie higher with the class glass. You could say that the quality lens actually has a wider effective aperture, but I'll leave it there as this has got complicated enough already :)
 
classcams said:
I thought you said dealing with apertures wasn`t complicated
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It isn't. Try asking about depth of field if you want to be REALLY confused!
 
To help eliminate some of the confusion of the mathematics of aperture versus focal length, the following might help.

The strict terminology should be "effective aperture"...this was how it was conveyed when complex lenses came into being but the "effective" bit has slowly been discarded as all lenses are now complex. (This is the "cheat" that Hoppy refers to)
The mathematics work with a simple, single bi-convex optic with a diaphragm behind it.

Bob
 
Having written progs on DOF (normal) DOF (macro) and a full spread sheet for DOF, it`s not very confusing
 
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