Aperture on Lenses

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Briggsy

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What does it mean?

For instance the nifty fifty, f/1.8.

What does this mean? Does it mean it gathers more light so photos taken indoors are higher quality?

Or does it mean that this is the aperture you can go down to with this Lens on your camera?
 
Both.
It is the widest aperture that the lens will go to. The wider the aperture (lower f number) the more light gets through as it's a bigger opening. This means that it can use a faster shutter speed than a narrow aperture whilst still letting the same amount of light through.
The result is that it can use fast shutter speeds even when in poor lighting conditions (e.g. indoors) without introducing camera shake or subject movement.
 
It's the widest aperture you can get with that lens. A wider aperture lets in more light and gives a narrower depth of field.
 
Think of the aperture on a lens as being like the pupil in your eye. The more open it is the more light is let in.

The lower the f number the more open the lens is. So f1.8 will let in a huge amount more light than f22.

The reason for selecting different apertures are many. You might be shooting in low light so need the lens as open as possible, or shooting in bright light and need less light entering the camera so would choose a higher f number.

You can also use the aperture for creativity. The more open the lens the more you will blur the background. So if you want the subject in focus and the background blurred you would choose a low f number. But if you want the subject and the background all in focus, you would choose a higher f number.

Everything is a trade off against something else though!
 
The thing that often confuses about f/numbers is that they are related by the square root of 2, ie x1.4. So f/4 lets in twice as much light as f/5.6, and f/8 is four times as much.

There is a methematical reason for this, but it's not very obvious.
 
Put simply a faster (ie lower F number aperture) will do two things for you, it will allow in more light thus letting you use a faster shutter speed/lower ISO and it will throw the background out of focus, allowing you to use it for creative effects.

So for shooting indoors you will be able to get away with using either a faster shutter speed or lower ISO than you would with a kit lens for instance.

It will not necessarily give you higher quality photographs, that's down to the sharpness of the lens and you using the correct exposure settings for both the subject and the effect you want.
Even then some lenses can display out of focus artifacts (bokeh) that some find un appealing, most preferring the shutter blades of the lens to produce a rounded aperture and thus bokeh, where as cheaper ones tend to be hexagonal and produce less desirable bokeh.
 
This is what I don't get though..

Say you used a f1.8 lens at f5.6 and then used an f2.8 lens at f5.6.

Would the picture you take look the exact same as they're taken at the same aperture? Or would the f1.8 be of higher quality because it has a wider aperture and always lets more light in?
 
Assuming the two lenses are the same focal length (say, 50mm) then the apertures would be identical at f5.6 on both.

Quality also depends on the way the lens is constructed - just because a lens has a bigger maximum aperture doesn't necessarily mean that it's better quality than one with a smaller maximum aperture.
 
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Briggsy said:
This is what I don't get though..

Say you used a f1.8 lens at f5.6 and then used an f2.8 lens at f5.6.

Would the picture you take look the exact same as they're taken at the same aperture? Or would the f1.8 be of higher quality because it has a wider aperture and always lets more light in?
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F/5.6 is 5.6 is 5.6, regardless of lens or camera or anything else. In terms of exposure, a given f/number will allow the same amount of light to fall on the sensor. It's a ratio between focal length and the physical diameter of the aperture, so these other changes are automatically compensated.

And correct exposure is correct exposure. If you allow more light through, you'll just get over exposure and blitz everything.

Of course depth of field changes with different f/numbers, and that makes a big difference. There are also very slight changes in ultimate sharpness at the exact point of focus if you go pixel peeping but good quality lenses are very sharp at all f/numbers.

Basically you use f/numbers to control exposure and depth of field - nothing else.
 
I can see my answer might have confused things - I was talking about physical aperture size. Of course Hoppy is spot on from an exposure point of view - f5.6 is f5.6. :thumbs:
 
Briggsy said:
This is what I don't get though..

Say you used a f1.8 lens at f5.6 and then used an f2.8 lens at f5.6.
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The 1.8 and 2.8 refers to the widest aperature available to you on that particular lens. If you set the 1.8 lens to f/2.8 it should perform exactly the same as your 2.8 lens - but you can still go wider on the 1.8 if you need it whilst the 2.8 lens is maxed out.

EDIT: I should add that some zoom lenses do not have fixed widest aperature through the zoom range. You'll see some listed as 18-55mm f/3.5-5.6 for example. This only means that the widest aperature available to you will vary depending on the focal length. 18mm will give a max of f/3.5 whilst full zoom at 55mm will give a max of f/5.6. That 5.6 is the same 5.6 as Richard mentioned above. Not sure how relevant this is, but feel that the OP is confused about this aspect of certain lenses.
 
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Lets say you had two lenses which are both 300mm. One has a 58mm lens and one is 77mm and you set them both at f5.6, surley the 77mm lens would be faster?
 
I'm just confused about the relevance of the apertures and why it's mentioned in the names of the lenses.

For instance I am a Sony owner and I've read a lot of reviews on the 50mm 1.8. In a lot of the reviews, people state that they rarely use f1.8 as it provides awful bokeh. So I didn't really understand how having a wider aperture means higher price. As you can also get 50mm 1.4 for about three times the price I just wondered what they actually meant, if they provided any other positive apart from being able to set the aperture wider.

By the sound of it, they provide no more quality at any narrower apertures so I don't understand why people would pay more for them.
 
Chr1stof said:
Lets say you had two lenses which are both 300mm. One has a 58mm lens and one is 77mm and you set them both at f5.6, surley the 77mm lens would be faster?
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It is answers like this that confuse newbies. I can only assume you are talking about zoom lens with different settings. :bonk:
 
Chr1stof said:
Lets say you had two lenses which are both 300mm. One has a 58mm lens and one is 77mm and you set them both at f5.6, surley the 77mm lens would be faster?
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Are you talking about the filter size? If so, it has no bearing on the f/number.

Briggsy said:
I'm just confused about the relevance of the apertures and why it's mentioned in the names of the lenses.

For instance I am a Sony owner and I've read a lot of reviews on the 50mm 1.8. In a lot of the reviews, people state that they rarely use f1.8 as it provides awful bokeh. So I didn't really understand how having a wider aperture means higher price. As you can also get 50mm 1.4 for about three times the price I just wondered what they actually meant, if they provided any other positive apart from being able to set the aperture wider.

By the sound of it, they provide no more quality at any narrower apertures so I don't understand why people would pay more for them.
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The maximum aperture (lowest f/number) of a lens is always quoted because it's a vital aspect of the specification. All lenses will run up to f/16 and f/22 at the high end, but no zooms go lower than f/2.8 and most are around f/3.5-4. Only primes can go down to f/2, f/1.4 and even f/1.2.

The difference between f/4 and f/2 is a lot - 4x more light, f1.4 is 8x more than f/4. That's three stops, which is a huge benefit if you want to create shallow depth of field effects, or shoot in very low light.

Not everybody wants or needs these attributes, but attributes they are. Usually very costly ones too.
 
Chr1stof said:
Lets say you had two lenses which are both 300mm. One has a 58mm lens and one is 77mm and you set them both at f5.6, surley the 77mm lens would be faster?
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No.

The amount of light falling onto the focal plane would be the same in both cases and would be determined by the size of the iris aperture.

It's worked out using D=f/N where D is the aperture in mm, f is the f number and N is the focal length in mm, so in both cases it would be:

D = 300/5.6 = 53.6mm

So any light falling on the focal plane is limited by the aperture which is the same for both the 58mm and 77mm lens.
 
Chr1stof said:
Lets say you had two lenses which are both 300mm. One has a 58mm lens and one is 77mm and you set them both at f5.6, surley the 77mm lens would be faster?
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Your confusing aperture with the size of the objective lens.

The size of the aperture is set by the shutter blades, the size of the lens itself has no bearing on it.
 
iancandler said:
Your confusing aperture with the size of the objective lens.

The size of the aperture is set by the shutter blades, the size of the lens itself has no bearing on it.
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I think you meant iris blades, Ian ;)
 
I posted a reply late last night. Now I see there has been a few more posts sent. Now It is only 8am. Don`t you fellas ever go to bed?
 
Chr1stof said:
Lets say you had two lenses which are both 300mm. One has a 58mm lens and one is 77mm and you set them both at f5.6, surley the 77mm lens would be faster?
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classcams said:
It is answers like this that confuse newbies. I can only assume you are talking about zoom lens with different settings. :bonk:
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It was a question of my own, not an answer to the OP :)

I just thought if a lens has a greater diameter it would let more light in, also the the aperture would be physically bigger at say f8 than at would be on a smaller diameter lens as I thought the aperture was a fraction not a fixed size if you get my drift.
Im getting a headache :lol:
 
Chr1stof said:
It was a question of my own, not an answer to the OP :)

I just thought if a lens has a greater diameter it would let more light in, also the the aperture would be physically bigger at say f8 than at would be on a smaller diameter lens as I thought the aperture was a fraction not a fixed size if you get my drift.
Im getting a headache :lol:
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Well there's some truth in that for sure, at least in the principle. It's just that the size of the front element is not always the same physical diameter as the aperture, which is the bit that matters.

Wide angles for example have very much bigger front elements because they need to see a wide field of view. But there is a much closer correlation with longer focal lengths, particularly with primes where the zoom function doesn't confuse things.

The f/number is a ratio, so a given f/number on a long lens is going to be a physically much larger hole than on a wide angle. Using f/2 for easy maths, f/2 on a 25mm lens is an aperture with a diameter of 12.5mm, at 50mm it is 25mm, at 100mm it's 50mm, etc.
 
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