So... putting aperture on camera?

Just a thought.

What would be the problem with putting the aperture in the camera, between the lens mount and the mirror, instead of in the lens?

You would have to increase the size of the camera, because there are limits to how much sideways the light rays can run between lens and sensor.

Think about a very small aperture opening, sitting centered in front of the sensor. You still need to be able to project light to every part of the sensor. The light rays falling on the center of the sensor hits it at 90 degrees. The light rays at the corner has to move a lot sideways to reach from that opening to the corner for the sensor. And that light ray must continue in that direction on the outside of the aperture too, until it reaches the nearest lens element.

Next thing - having the aperture inside of the lens would require stricter rules for the operation of the lens. Now it isn't enough that it must focus on the sensor surface at a known distance away from the lens mount, and light up a given circle inside the body. You also need new rules for what angles the beams may travel from lens to sensor, which affects what size you must have for the innermost lens element.

Isn't the aperture size focal length dependent?

2.8 on a 400 is physically a fair bit larger than 2.8 on a 16-35.

nellyle wrote in post #17083588
Isn't the aperture size focal length dependent?

2.8 on a 400 is physically a fair bit larger than 2.8 on a 16-35.

The maximum opening of the lens is focal length dependent. But the location of the aperture within the lens decides how large it needs to be based on the magnification of the lens elements in front of, and behind, the aperture.

400mm/2.8 would imply 400/2.8 = 143 mm. But the exit opening of that lens - and the mount diameter - is obviously not 143+ mm. You could see that lens as a 200mm lens with a 2x tele converter on the front. And the 200mm part doesn't need any 143mm aperture.

pwm2 wrote in post #17083605
The maximum opening of the lens is focal length dependent. But the location of the aperture within the lens decides how large it needs to be based on the magnification of the lens elements in front of, and behind, the aperture.

400mm/2.8 would imply 400/2.8 = 143 mm. But the exit opening of that lens - and the mount diameter - is obviously not 143+ mm. You could see that lens as a 200mm lens with a 2x tele converter on the front. And the 200mm part doesn't need any 143mm aperture.

Didn't do too well in math class did you? A 200mm f/2.8 lens must have a 71.4mm wide open diaphragm (200mm / 2.8 - 71.42). Bolting a 2X TC has, as you say, no effect on the aperture. It does affect the focal length, making it 400mm. Also, as expected, the TC'd lens becomes f/5.6 (400mm / 71.42 = 5.6).

If a 400mm lens doesn't have a 143mm aperture, it is not f/2.8.

GeoKras1989 wrote in post #17083739
Didn't do too well in math class did you? A 200mm f/2.8 lens must have a 71.4mm wide open diaphragm (200mm / 2.8 - 71.42). Bolting a 2X TC has, as you say, no effect on the aperture. It does affect the focal length, making it 400mm. Also, as expected, the TC'd lens becomes f/5.6 (400mm / 71.42 = 5.6).

If a 400mm lens doesn't have a 143mm aperture, it is not f/2.8.

Well, given the number of years I spent taking math classes at university, I don't think I did so well.

So please inform me where you see a 143mm exit hole of that 400/2.8 connecting it to the camera.

Ever wondered about the tapered design of the big tele lenses - where the thinner part is thinner than that 143 mm?

Next thing - you forgot one thing. I talked about bolting a tc on the front of a 200mm lens - not bolting it to the back of one. Makes a difference? Yes it does - because a TC to the front of a tele lens requires a big TC, compared to bolting it between lens and body. Why? Because that TC needs to collect a huge amount of light to manage that 2x while feeding a 200/2.8 giving an end result of 2.8.

But it's just a math example - you can debate a 100mm lens connected to the camera, and a TC of more than 2x in front of it. You can always consider a tele lens as the result of multiple components.

Anyway - take your biggest tele lens. Detach from camera. Look at the back of it. Measure the diameter of the glass element. Report back to this thread. And now follow up with the explaining math, since you seem to have done well in school.

The exit orifice at the rear of the lens is NOT the aperture ring.

It doesn't matter where you put the TC. The fact is that the TC changes the FOCAL LENGTH OF THE LENS. It has no effect on the actual size of the aperture.

One step at a time:
200mm lens with f/2.8 aperture = 71.4 mm aperture.

200mm lens with f/2.8 aperture and a 2X TC still has a 71.4mm aperture.

It is now a 400mm lens.

400mm / 71.4 = 5.6.

200mm f/2.8 with a 2X TC is f/5.6, no matter where you put the TC.

This is 4th grade math. Add, subtract, multiply, divide. Nothing special in any of these calculations.

Added:
196 credit hours at Indiana University. Chemistry (bachelor's of science) degree. GPA 3.46/4.00. Minors in math, computer programming, biology, and I believe, physics. Worked with graduate students at IU studying artificial intelligence. While there I wrote the framework for the first, and as far as I know - only, program that could actually learn to play poker. It is still being used in AI research. I am still taking classes. I have already registered for the upcoming fall semester, which will complete my 32nd year of taking college classes. Maybe someday I'll take a second degree.

quadwing wrote in post #17083524
Just a thought.

What would be the problem with putting the aperture in the camera, between the lens mount and the mirror, instead of in the lens?

Not at all clear as to why you want it on the camera. I'm very happy with it left on the lens where it belongs.

GeoKras1989 wrote in post #17083775
The exit orifice at the rear of the lens is NOT the aperture ring.

And why do you think there is any different claim made in this thread?

The size of the opening of the aperture blades depends on where the aperture blades are located. The 143 mm would be relevant if there are no magnification in front of the blades. Each time the aperture blades are moved backwards behind a set of lens elements, then their size can be scaled based on the magnification involved.

That 143 mm is a mathematical concept describing the largest opening the lens must have. But it does not imply that the actual aperture blades must be able to open to that size.

And now we are halfway there. The TC has no effect on the actual size of the aperture. So a TC placed in front of a lens doesn't change the aperture requirement of that lens. A 200mm lens with a 2x TC in front of it, doesn't require a larger aperture. But the TC in front of the 200mm lens must have a huge opening itself to allow a 200mm/2.8 lens to manage f/2.8.

Look at the normal TC used with removable lenses - that is placed between lens and camera. As you notice, it doesn't have any aperture value specified. It just takes the light already collected. Increases the magnification while losing light. So you lose stops while gaining focal length.

But need not have any 71.4 mm aperture blade opening. And most definitely do not have 71.4mm opening into the camera body. So the full pipeline from start to end does not maintain any 71.4mm.

And this part is where you need to sit down and think. The aperture blades - the diaphragm - do not need to be at the front of a lens. And it should be obvious to you that there isn't even room for such a large diaphragm at the back of the lens.

Exept that your math is based on a 2x TC after a 200/2.8 lens.

Try instead a 2x TC with 143 mm lens elements in front of the 200/2.8 lens.
That TC isn't limited to the amount of light collected by a 71 mm opening, but on an opening twice in diameter, collecting four times as much light. So it magnifies the image with a factor two, which should give a two stop loss of light, but has already collected two stops more light.

But then a 4th grade student is quite likely to fail this.

And your error lies in "It doesn't matter where you put the TC." which ignores commutative rules. It very much matters what glass is placed in front of which other glass. This is not a situation where you can just do a * b = b * a.

Follow this link and look at the internals:
http://www.canon.com …seum/tech/repor​t/2011/02/

Notice the size of the large front element.
Notice the location of the actual diaphragm.
Notice the size of the lens elements at that location.
That is not any 143mm lens elements and not any aperture blades opening up to 143 mm.

That 400/2.8 = 143mm of a 400/2.8 is just a concept. But a concept that requires you to scale the diaphragm as it moves backwards and has less and less remaining focal length remaining towards the sensor. The diaphragm is seldom at the front of a lens, and so does not need to be the size you get from that 400/2.8 compuation. Because the effective focal length at that location is not 400mm.

You are introducing way too many totally irrelevant complications into the math.
f/n (n=any number) MEANS Focal length divided by diameter of the aperture diamter (using same units as FL). Stick to that and you'll get a handle on this, eventually.

pwm2 wrote in post #17083869
That 400/2.8 = 143mm of a 400/2.8 is just a concept. But a concept that requires you to scale the diaphragm as it moves backwards and has less and less remaining focal length remaining towards the sensor. The diaphragm is seldom at the front of a lens, and so does not need to be the size you get from that 400/2.8 compuation. Because the effective focal length at that location is not 400mm.

Yeah, this is key. The f-ratio is based on the effective aperture of the primary element. For a diaphragm to modify the aperture, the only place it could be installed and still measure out to the effective diameter... would be in the middle of the primary element on a refracting system. So it would have to be bigger if in front of the primary element, even if touching it, and it has to be smaller behind it.

GeoKras1989 wrote in post #17083893
You are introducing way too many totally irrelevant complications into the math.
f/n (n=any number) MEANS Focal length divided by diameter of the aperture diamter (using same units as FL). Stick to that and you'll get a handle on this, eventually.

Somehow, Canon has already managed to get a handle on this. And they magically didn't install a 143mm diaphragm in the 400/2.8. I'll leave it up to you to figure out how they managed that trick. But it's the same trick that allows the camera mount to have an inner diameter way smaller than 143mm.

GeoKras1989 wrote in post #17083893
You are introducing way too many totally irrelevant complications into the math.
f/n (n=any number) MEANS Focal length divided by diameter of the aperture diamter (using same units as FL). Stick to that and you'll get a handle on this, eventually.

What is wrong with what he wrote?

The in-camera iris diaphragm was done at least once that I'm aware of, that being the 110 film format Pentax Auto 110. That camera actually was one of the few 110 format cameras that offered interchangeable lenses which drove the design to the in-camera diaphragm. The lenses were exceedingly small!

I also seem to remember a Beseler that might have been in-camera but I only saw it in Japan, probably mid to late 1960's.

A lot of folks believe that the f-stop number is the focal length divided by some physical opening in the lens. While this may be true for some theoretical lens, it isn't for most. The f-stop number is the focal length divided by the entrance pupil size. Click on the link and read up on "entrance pupil" and you should understand why the diameter of the iris opening (or the front lens opening or the rear exit hole in the lens) is not the value used to calculate the f-stop number.