Aperture - are all f stops the same?

[seventythree]

is there some sort of international standard for aperture?

for example would a 50mm lens and a 35mm lens give the same exposure at the same f stop, or would say the same lens across different manafactures let the exactly the same light in at the same f stop ?

cheers, Al

 

[Andy]

I would say they are both the same as its like a measurement. Like a cm is the same on every ruler.

 

[Chaz Photos]

F stop is a maths between the lens in mm and the size of the hole from memery its lens over hole size both in mm so a 50mm lens with a 25mm hole is F2

 

[jontucker]

As Chaz says, the F number is derived by dividing the focal length (in mm) by the diameter of the aperture (also in mm). So a 50mm lens will have a different aperture diameter at f4 than a 35mm lens. You do however, end up with the same amount of light, as in, if you shoot 1/60 at f4, it will be the same exposure no matter what focal length you're using. The thing that will be different depending on the focal length will be the diameter of the aperture.

 

[towershot]

Hmm tricky question, come on you togs. I would guess that a 50 mm lens at f/5 no matter what make like for like would be the same but a opening of f/5 on a 35mm lens would have more depth of field than f/5 in 135mm

or to put it another way,you have a 18-200 zoom , f/8 at the 18 end would have more DOF than at the 200 end .

Now that all sounds funny to me ,so some-one put it simpler please....

 

[seventythree]

cheers, so say sigma vs nikkor 24-70mm f2.8 . they will both let the same light in at the same focal length/apperture.

cheers.

i wasn't considering DOF at all , but now you mention it :shrug:

 

[Freester]

Yes what people say above. An f stop value will let the SAME amount of light in regardless of lens / focal length.

 

[HoppyUK]

F/numbers are a ratio and a given f/number on a camera phone lens passes the same amount of light as the same f/number on a compact, or on a super-telephoto. In order to make this work, the diameter of the aperture changes with focal length, in order to maintain the ratio. Just as described above.

Depth of field changes with magnification, so if you increase magnification with a longer lens, depth of field is reduced. But if you then move back, so that the subject gets smaller and is reduced to the same size framing as before, depth of field remains unchanged.

 

[Goody13]

OK. So this has now got me thinking, although I have not thought this through in detail so my theory may be rubbish! Is that the reason why some zooms have a variable maximum aperture and, if so, for zooms that have a fixed maximum aperture, does the aperture get wider as the focal length increases so the ratio stays the same?

 

[Steve Smith]

F stop is a maths between the lens in mm and the size of the hole from memery its lens over hole size both in mm so a 50mm lens with a 25mm hole is F2

F stop is the focal length divided by the diameter of the aperture. It doesn't matter what measurement system you use (mm, inches, miles, light years, etc.) as long as you use the same units for both measurements.



Steve.

 

[AliB]

Is that the reason why some zooms have a variable maximum aperture

Nope, that's down to cost of manufacture. Generally those with a fixed maximum aperture are fixed at around f2.8 or on longer lengths f4. Those with variable apertures go out to f4 and f5.6. So the variable aperture ones generally let in less light and the optics are not quite of the quality of the fixed aperture ones.

 

[jontucker]

Nope, that's down to cost of manufacture. Generally those with a fixed maximum aperture are fixed at around f2.8 or on longer lengths f4. Those with variable apertures go out to f4 and f5.6. So the variable aperture ones generally let in less light and the optics are not quite of the quality of the fixed aperture ones.

I'm not 100% sure but I think it's actually the 'effective' diameter of the aperture. So, on a lens that's f2.8 through out the entire focal range, the blades of the aperture membrane don't move when you zoom in and out but the element in front (or behind?) the membrane moves making the 'effective' diameter different. My brain might be making that up without telling me though! :bonk:

 

[Chaz Photos]

F stop is the focal length divided by the diameter of the aperture. It doesn't matter what measurement system you use (mm, inches, miles, light years, etc.) as long as you use the same units for both measurements.



Steve.

thats what I said both the same ie in my example both in mm:shrug:

 

[Mike54]

Nothing to add on the technical side, but appreciated the thread and found it quite interesting.

 

[Steve Smith]

I'm not 100% sure but I think it's actually the 'effective' diameter of the aperture.

That's correct. Focal length divided by the aperture diameter is the simplified version and is close enough in most cases.

Focal length divided by the diameter of the entrance pupil is the correct method to work this out.

Entrance pupil is the apparent size of the aperture blade opening as viewed from the front of the lens.


Steve.

 

[Steve Smith]

thats what I said both the same ie in my example both in mm:shrug:

Indeed you did but the implication (as I read it) was that it had to be in millimetres - which it doesn't.


Steve.

 

[Chaz Photos]

The reason some have fixed F-stops for all the range of zoom and other do not:
Is not due to the aperture changing, but due to how the lens zooms.
If a lens zooms and the image size remains constant you have a fixed f-stop but if with zooming the image size increases then you have a loss of light on the film/sensor so an effective change in f-stop size.

 

[Chaz Photos]

Indeed you did but the implication (as I read it) was that it had to be in millimetres - which it doesn't.


Steve.

But anyone know you can use any numbers as long as you use a constant means you get the right answer and as lens are in mm who going to bother to change it to anything else?

 

[Steve Smith]

But anyone know you can use any numbers as long as you use a constant means you get the right answer and as lens are in mm who going to bother to change it to anything else?

With modern lenses, no one. But I have some lenses marked in inches (and some marked in cm). As do a lot of large format users.

Sometimes some of these large format lens elements are put into different shutters which used to house lenses of a different focal length. When this happens, we need to make a new aperture scale for the front. Some people will convert the focal length to mm and measure the entrance pupil in mm too. Others will stick with inches and measure the entrance pupil in inches. Either way is correct and works just as well.

Here is a 6 1/2" lens in a shutter:

If a different focal length set of lens elements was fitted, a different aperture scale would need to be fitted. usually these are attached with a couple of screws to make them easier to change like this:

And here:

is a similar shutter with three scales depending on the focal length used. The focal length can be varied by removing either the front or rear group of lens elements. These are known as convertable lenses. You have to make sure you are reading the correct scale depending on the focal length you are set to.


Steve.

 

[Digitall]

Depth of field changes with magnification, so if you increase magnification with a longer lens, depth of field is reduced. But if you then move back, so that the subject gets smaller and is reduced to the same size framing as before, depth of field remains unchanged.

I think you have this bit wrong. DOF changes with aperture size not the length of lens used. What does change is the compression of the foreground and background. Long lenses tend to compress the distance of anything in front and behind the focussing point, making it look like depth of field.

 

[Chaz Photos]

I think you have this bit wrong. DOF changes with aperture size not the length of lens used. What does change is the compression of the foreground and background. Long lenses tend to compress the distance of anything in front and behind the focussing point, making it look like depth of field.

WRONG changing the length of a lens DO NOT compress the only thing that changes the perspective is the position you take a photo
If you were to take the same photo and fill the frame with the shame it will be different because to get the same shot you have moved with each lens.
Take a shot with two lens from the same spot and both will have the same perspective i.e. if you crop the centre of the wider one to match the long lens it will look the same.
And DOF will change with BOTH

 

[Steve Smith]

WRONG changing the length of a lens DO NOT compress the only thing that changes the perspective is the position you take a photo

Absolutely correct. It is also the same perspective which you can see with your eyes (or to be more precise, one eye) from the same position.


Steve.

 

[HoppyUK]

I think you have this bit wrong. DOF changes with aperture size not the length of lens used. What does change is the compression of the foreground and background. Long lenses tend to compress the distance of anything in front and behind the focussing point, making it look like depth of field.

:thumbsdown:

WRONG changing the length of a lens DO NOT compress the only thing that changes the perspective is the position you take a photo
If you were to take the same photo and fill the frame with the shame it will be different because to get the same shot you have moved with each lens.
Take a shot with two lens from the same spot and both will have the same perspective i.e. if you crop the centre of the wider one to match the long lens it will look the same.
And DOF will change with BOTH

Changing focal length alters the magnification of the image, and that effects depth of field. Moving fowards or backwards alters the size of the image (magnification) and that effects depth of field. If you change focal length, but then move position so that the subject remains the same size in the frame, then the two effects cancel out and depth of field remains the same.

Perspective is a function of shooting distance, not focal length.

Field of view is also a function of shooting distance. Narrow field of view with a long focal length lens reduces the amount of background visible in an image (compared to a wide lens used at a closer distance) and this often has a similar visual effect to shallow depth of field.

 

[HoppyUK]

OK. So this has now got me thinking, although I have not thought this through in detail so my theory may be rubbish! Is that the reason why some zooms have a variable maximum aperture and, if so, for zooms that have a fixed maximum aperture, does the aperture get wider as the focal length increases so the ratio stays the same?

Quite true, evidenced by the fact that zooms do change the size of the aperture mechanically when focal length is changed, to maintain the same f/number.

 

[jontucker]

Quite true, evidenced by the fact that zooms do change the size of the aperture mechanically when focal length is changed, to maintain the same f/number.

As I said above I think it's the 'effective' aperture which is not the actual size of the hole but, as Steve said, the size of it when viewed through the front of the lens (i.e the size of it is optically adjusted). My 24-70 does not change the size of the aperture as you zoom in and out and I can prove it by using video mode (which engages the aperture mechanism) and looking in to and listening to the lens. The aperture blades don't move.

 

[HopefulM]

As Chaz says, the F number is derived by dividing the focal length (in mm) by the diameter of the aperture (also in mm). So a 50mm lens will have a different aperture diameter at f4 than a 35mm lens. You do however, end up with the same amount of light, as in, if you shoot 1/60 at f4, it will be the same exposure no matter what focal length you're using. The thing that will be different depending on the focal length will be the diameter of the aperture.

great explanation, thank you

 

[mrtoad]

they are ratios...not specific diameters but relate to lens diameter fully open...each successive stop reduces the area of the light gathering glass by 1/2

using the formula pi.d^2/4

 

[HoppyUK]

As I said above I think it's the 'effective' aperture which is not the actual size of the hole but, as Steve said, the size of it when viewed through the front of the lens (i.e the size of it is optically adjusted). My 24-70 does not change the size of the aperture as you zoom in and out and I can prove it by using video mode (which engages the aperture mechanism) and looking in to and listening to the lens. The aperture blades don't move.

Jon, that's a 24-70 with constant f/2.8 isn't it? Then no, the aperture won't change physically during zooming. But it changes with all zooms that have a variable maximum aperture.

 

[jontucker]

Jon, that's a 24-70 with constant f/2.8 isn't it? Then no, the aperture won't change physically during zooming. But it changes with all zooms that have a variable maximum aperture.

Ah ok, crossed wires, I thought you meant a constant F number zoom. A quick check with my 18-200mm confirms that it does change the aperture when you zoom in/out. But.... WHY? this seems arse about face to me. Why is it easier (cheaper!) to make a lens that can adjust the aperture blades during zooming than it is to make one that doesn't? Or is the higher cost due to the complexity of making sure the elements in front the aperture membrane are in the right place throughout the focal range?

 

[HoppyUK]

Ah ok, crossed wires, I thought you meant a constant F number zoom. A quick check with my 18-200mm confirms that it does change the aperture when you zoom in/out. But.... WHY? this seems arse about face to me. Why is it easier (cheaper!) to make a lens that can adjust the aperture blades during zooming than it is to make one that doesn't? Or is the higher cost due to the complexity of making sure the elements in front the aperture membrane are in the right place throughout the focal range?

I think the optical 'logic' is that if the a lens like your 18-200 has an aperture big enough to deliver f/5.6 at 200mm, then at 18mm it should be several f/numbers lower.

The practical reality is that this isn't possible when combined with the wide field of view at short focal lengths, but there is some advantage to be had.

Then again, from a practical point of view if you were to attempt this with a lower f/number still, the whole lens would get impossibly big, sharpness would be seriously impaired and severe vignetting would occur.

The top and bottom of it is that the f/ratio is no more than an elementary principle that takes no account of the whole raft of optical problems and compromises that go into designing a lens that has a good zoom range, is sharp across a full frame image, doesn't vignette, is a manageable size and affordable.

 

[SimonTALM]

Ah ok, crossed wires, I thought you meant a constant F number zoom. A quick check with my 18-200mm confirms that it does change the aperture when you zoom in/out. But.... WHY? this seems arse about face to me. Why is it easier (cheaper!) to make a lens that can adjust the aperture blades during zooming than it is to make one that doesn't? Or is the higher cost due to the complexity of making sure the elements in front the aperture membrane are in the right place throughout the focal range?

I thought that in a variable aperture lens the size of the hole doesn't change but the f stop does because of the way the maths work i.e. the size of the hole at f5.6 @200mm = the size of the hole at f4@70mm.

In a fixed aperture lens the size of the hole changes as you go from f2.8 @200 to f2.8 @70mm hence why it is more expensive.

 

[mrtoad]

mechanical restraint...the lens housing that you hold is fixed in diameter
as the zoom does its apparent re-focal-lengthen-ing is is constrained by this so the maximum aperture seems to vary as the 'focal length' does since the opening cant get any bigger

 

[jontucker]

I thought that in a variable aperture lens the size of the hole doesn't change but the f stop does because of the way the maths work i.e. the size of the hole at f5.6 @200mm = the size of the hole at f4@70mm.

In a fixed aperture lens the size of the hole changes as you go from f2.8 @200 to f2.8 @70mm hence why it is more expensive.

Which is what I thought but apparently it's the other way around, or at least it is between my 24-70 (fixed) and 18-200 (variable)!