Aperture numbers

[Tom3]

Hi,

I'm having an issue to understand something:

I have two lenses, one is an AF-S Nikkor 18-70 lens, and the aperture listed on the lens is 1:3.5-4.5

However, the camera leaves me choices out of the 3.5-4.5 range.

What is the deal? what really happens when I select an aperture out of this range?

Also, another question, which might show a fundamental misunderstanding from my end: why aren't the blades by the sensor in the camera instead of being in the lens?

 

[laurah]

The aperture numbers listed are the max aperture that can be used at each end of your zoom range, i.e. at 18mm your max aperture is 3.5 and at 70mm your max aperture is 4.5. Using apertures smaller than these (larger number) is fine, the lens is just not capable of going to a larger aperture.

 

[Ptyler22]

Ya, laurah pretty much summed it up.
The smaller the number, the bigger the opening. Meaning more light can get through the lens, with a bigger aperture, and you can have faster shutter speeds. You can select bigger numbers (smaller opening) but you will have to use a slower shutter speed, you might want to do that for more depth of field or if it's really sunny, I am not good at explaining things, but I am sure you will get more responses from people that can explain it better.

 

[Tom3]

yes, I understand the concept of aperture, but I did not know these were the max allowed! that sheds some light!

Now, this brings two questions:
- My second lens is a 18-200 1:3.5-6.3; If I want to make a shot at 100mm, can I make linear math to find the max aperture or does it follow some more complex curve?

- Also, if I set the lens at 200mm, the camera will still let me select a wide aperture, like 3.5; does it make its lighting math as if the lens could really do a 3.5 opening?

 

[ann]

sat the full 200mm the lens will only allow the maxium of 6.3 f stop.


set you camera in aperture priority mode. use the widest opening and the widest angle. i.e. your 18mm wide focal length with a 3.4 fstop. watch the lcd on the top of the camera and rack the lens out to it's longest focal length. You will see at which point the aperature begins to shut down finally reaching 6.3 when you hit 200mm.

this is one big reason some of us don't like these types of lenses and are willing to pay the price for a lens that will maintain the same fstop wide open regardless of focal length.

 

[epp_b]

- My second lens is a 18-200 1:3.5-6.3; If I want to make a shot at 100mm, can I make linear math to find the max aperture or does it follow some more complex curve?

It's not linear. The curve depends on the lens. Use Aperture Priority mode, zoom down to the wide end of your lens, set the aperture as wide as it can be and then zoom back to 100mm. Whatever your camera says is the aperture is the minimum aperture available at 100mm. Same for any other focal length.

- Also, if I set the lens at 200mm, the camera will still let me select a wide aperture, like 3.5; does it make its lighting math as if the lens could really do a 3.5 opening

I'm not sure if that's a statement or a mistyped question. At 200mm, your lens will not do f/3.5. It will be something like f/5.6 or f/6.3. You'll have to check using the instructions I gave you above.

 

[Tom3]

what I meant is that, in manual mode, the camera will let me select 'impossible' choices, such as 3.5 @ 200

but I'm not sure if the camera is aware of the real value when it shows me if the picture is over/under exposed for example.

 

[dhilberg]

Even in manual mode, your aperture choices should still be constrained by the limitations of the lens. In other words, even in manual mode the maximum aperture you can choose on your 18-200mm f/3.5-6.3 @ 200mm should be f/6.3.

What is the make and model of your camera and lens?

 

[Tom3]

I understand the physical limitations of the lens, but I'm wondering what does the camera do internally with its light calculations:

If it expects a 1/3.5 but in reality it's only getting 1/6.3, I would guess the math would be messed up.

I've a Nikon D200 with two lenses: Nikkor 18-70 3.5-4.5 and Sigma 18-200 3.5-6.3

So, I can set the camera to 3.5, set the lens on 200mm: will the camera still think it gets a 3.5 opening and assume the right amount of light will get in, or will it really know it's only going to get a 6.3 and adjust the light computation accordingly?

 

[Jklersy]

If you can set your camera to f3.5 in aperture priority mode with your lens at 200mm then you might want to get your camera looked at. You should also search online to see what is being said about that camera/lens combo because the camera should know what the limitations of the lens are and account for it always.

 

[Ptyler22]

I understand the physical limitations of the lens, but I'm wondering what does the camera do internally with its light calculations:

If it expects a 1/3.5 but in reality it's only getting 1/6.3, I would guess the math would be messed up.

I've a Nikon D200 with two lenses: Nikkor 18-70 3.5-4.5 and Sigma 18-200 3.5-6.3

So, I can set the camera to 3.5, set the lens on 200mm: will the camera still think it gets a 3.5 opening and assume the right amount of light will get in, or will it really know it's only going to get a 6.3 and adjust the light computation accordingly?

I am not totally sure I know what you are trying to say but I might. and the answer is no. Ok so here is a real world test you can do for us to prove what we mean. OK so assuming the d200 has a depth of field preview button, push the depth of field preview button while looking into the lens. Look into the end and then push it while looking into the lens. Then zoom in while still looking into the end of the lens. If you look closely you will see these plates move together toward the center of the lens, making the opening for light smaller. Also what you can do is hold the DOF button down and crank up the aperture all the way to whatever the highest number is you can set the lens to. you will see the actual aperture getting smaller. It's not a calculation, it is a physical hole and the aperture pertains to the size of the hole. I hope this makes sense. If not I can try to explain it better. Or if somebody understands they can try to explain it.

 

[Ptyler22]

And I missed the last part of your question. Yes, it will know in reality it will only get 6.3 and adjust, because that lens isn't physically possible of 3.5 at 200mm, the camera knows what lens it is and every thing about it, so it can figure out that, and sooo much more

 

[Tom3]

ok, it's all clear now!

here's why I was confused:

I thought the camera's software would get out of sync with the lens' real aperture, therefore messing up the light computations.
I assumed that that while the lens would be at 6.3, the camera would still believe it could do a 3.5.

I tried after reading your posts to see if the camera gets the real aperture from the lens and, sure enough, it does.

thanks all for the clarification!

 

[RyanMTaylor]

I was pretty sure I understood aperture.. What is all this light computational talk stuff goin' around? Isn't the aperture just an opening letting in light, not something that it has to 'figure out'?

 

[ksmattfish]

Isn't the aperture just an opening letting in light, not something that it has to 'figure out'?

You got it. Aperture numbers represent a quantity of light, not an actual measurement of the aperture size. If we kept track of actual aperture size, we'd need to do the math in the field depending on what lens we were using, but as long as we know that f/2.8 is twice as much light as f/4 on any lens we're good.