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I recently bought my first SLR camera and lens for it and I have questions about how they interact.

My lens attaches on, and I can use the focus on the lens itself, but if I use the auto-focus, does it adjust the lens focus, or is there a secondary lens in the body that gets adjusted?

Same with the aperture settings, is this changing on the attached lens, or is there another lens inside?

If it is changing the attached lens, how do the two connect? I can not see any obvious connection other than the mechanism holding it in place.

I am just curious how they work together.

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4  
For specifics, it depends on the mount system selected by the camera's maker. Different brands use different ways of connecting the camera and lens and controlling the functions of the lens. – Michael Clark Feb 8 at 11:58
up vote 17 down vote accepted

if I use the auto-focus, does it adjust the lens focus, or is there a secondary lens in the body that gets adjusted?

No, there's no secondary lens. The lens attached to the camera contains a motor that moves the lens elements as required by the autofocus system.

Same with the aperture settings, is this changing on the attached lens, or is there another lens inside?

Same answer -- the camera tells the lens to change the aperture; it's not something that can be adjusted in the camera body.

If it is changing the attached lens, how do the 2 connect. I can not see any obvious connection other than the mechanism holding it in place.

If you remove the lens you'll find a row of little gold contacts on the body and a matching set on the lens. When the lens is attached, these contacts mate and form electrical connections that the body uses to control the lens.

Here's a photo from Wikipedia. This happens to be a Canon EF-mount lens, but other manufacturers use similar systems.

lens contacts - By Nebrot - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3336317

Especially on mounts that still support older lenses, there may also be mechanical connections that allow motors in the camera body to power adjustments in the lens. Nikon's F-mount and Sony's A-mount are two prominent examples.

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2  
"The lens attached to the camera contains a motor..." Don't forget screw-drive lenses, where the motor is in the body and lens elements are moved by mechanical means. Autofocus in several Sony A-mount lenses still operates this way. – osullic Feb 8 at 14:35
    
@osullic No argument there. I was focusing more on the meat of question, i.e. whether focus and aperture are adjusted in the body or in the lens, than on providing a complete list of lens mount styles. But it's a good point, so I'll be sure to mention that mechanical connections are also possible. – Caleb Feb 8 at 19:52

As Caleb pointed out, all focus and aperture-related functions (as well as optical zoom when relevant) happen inside the lens, not in the body (in terms of lens components moving at least, not talking about the logic and control).

Nowadays most lenses will have built-in motors to perform these tasks, and are controlled electronically by the body through the contacts visible on the lens and body.

However, some lenses (mostly older lenses, but there are some recent ones, like some Sony A-mount lenses) do not integrate those motors, and instead rely on the body to power the movement of the different parts. You'll then have a mechanical connection for this on top of the electrical contacts.

You can see this contact bottom left on the mount of this Nikon D7000 using a Nikon F-Mount:

Nikon D7000

Image Source: Wikipedia

The same thing on a Sony A-Mount, bottom left:

Sony A-Mount, body

Source: Wikipedia

And the corresponding part on the lens (top left):

Sony A-Mount, lens

Source: Wikipedia

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1  
The contact at lower left is the focus drive screw. The lever at far left of the inside of the mounting flange is the aperture linkage that contacts the aperture lever on the back of Nikon F-mount lenses. – Michael Clark Feb 8 at 11:56
2  
Still several current Sony A-mount lenses have their autofocus operated by a "screwdriver" in the lens mount (as in your Nikon image) that mates to a "screw" in the lens, powered by a motor in the camera body. It's not just "early" lenses that operate this way. – osullic Feb 8 at 14:38
    
Thanks @osullic, updated answer to reflect this. – jcaron Feb 8 at 14:40

Autofocus lenses are always either electrically or mechanically coupled to the body. Since the digital SLR most lenses are now electronically coupled, so that the camera body controls both aperture and focus.

However, lenses with manual focus and aperture controls do not require any electrical or physical communication with the camera body. The focus and aperture mechanisms are physically coupled to the control rings on the lens, while the camera body handles exposure and shutter speed.

Such lenses can be found second hand for very cheap prices, so it's a fun way to get a cheap collection of prime lenses (bearing in mind the optical quality of old lenses is not always great).

If you have an SLR camera, there's not usually any compatibility between mounts – i.e. a Nikon F-mount lens must be used on a Nikon camera. But if you have a mirrorless camera, you can buy affordable adapters to make almost any manual focus lens usable on almost any mirrorless camera.

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lenses with manual focus and aperture controls do not require any electrical or physical communication with the camera body. Even with manual aperture control, the aperture usually only closes to the set f-stop when the mirror goes up. In this case there's a (simple) mechanical coupling to link the diaphragm to the shutter release. – Max Feb 9 at 12:26
1  
@Max the coupling serves to open the aperture to its widest setting, and only close it when taking a photo. If the coupling isn't connected, the aperture ring still works. It's just that the camera cannot over-ride it. – Moriarty Feb 9 at 15:07
    
That's true. I agree that they do not require any communication -- I just meant to point out that even with manual lenses there often is communication for this reason of convenience. – Max Feb 9 at 15:13

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