How do you hold the aperture open?
If I reverse a lens, it's usually a 50/1.8 AF-D, which has an aperture ring. Whether I reverse it directly on the camera (very rarely!) or on another lens (usually a 105 Micro), I use a BR-6 ring.
Here's how reversing a lens works on a Nikon:
But first, let's do an autopsy. Take the lens off the camera. Look at the big hole that you left on the body. You'll need to make note of two things: The aperture actuator (red arrow) and the electrical contacts (blue arrow)
.
The aperture actuator is the mechanism that tell the aperture that the shutter is about to fire and to close down the aperture blades. In this photo, it's in the 'open' position, holding the aperture open so you have a nice bright viewfinder to see, focus and compose with. But when you take a photo, this little lever moves down. (if you turn your camera on now, turn the mode dial to M, and turn off autofocus, you can actuate the shutter and see this little lever do it's thing!) More about this in the next photo.
However, this little lever doesn't
set the aperture. With a G-series lens, the
electronics in the camera do. And like anything electronic, there must be a connection between the camera electronics and the lens. This is accomplished with the arc of electrical contacts at the top of the lens mount (blue arrow). This is how the lens & camera talk to each other, and the lens gets power for it's electronics and focus motor. When you meter a scene and the camera (or you) decide to use f/8, that information is sent through these contacts to the lens. When the shutter release is pressed, the aperture actuator lever (red arrow) moves down, and the lens tells the aperture blades how far to close.
So let's look at the other half of the equation, the matching connections on the lens.
An arc of what looks like little ball bearings (blue arrow) is what mates to the electrical connections in the top of the lens mount on the body. This allows to lens and camera to talk to each other. There's also a little tab (red arrow) that is used to move the aperture blades.
Take any Nikon lens made since 1959 off a camera, and the aperture blades will close to their minimum. To open them up when the lens is on the camera, the actuator on the body pushes up on that little tab on the lens, holding the blades in the lens open (again, so you have a nice bright viewfinder to see with!). When you press the shutter button, several things happen very quickly. The camera sends a message to the lens, saying, "Yo! F/8 and be there!" through the electrical contacts. The lens gets the message, adjusts the blades to be f/8, and the actuator lever in the body drops, allowing the aperture blades to close to f/8. When the shutter closes, the actuator lever moves back up, the blades open to maximum, and you have that nice bright viewfinder again. At all happens so quick most aren't even aware of all of these, and you cannot see it anyway since the mirror has moved up which darkens the viewfinder while the whole process takes place.
Now, what happens when you remove a G-series lens from the camera with the idea to reverse it? Well, you completely lose both the electrical connection of the contacts, as well as the mechanical connection for aperture control. Yes, you can get a reversing ring sized to the filter on your lens, reverse it and it
will mount. But the lens is now at it's minimum aperture since there's no camera-body lever to hold the blades open. Look in the viewfinder, and if you're not in bright light (sunlight), you'll hardly see anything at all.
Now you're probably thinking, "Well, I'll just take my finger and push that little tab on the lens and open the blades. Heh, heh... I'll show
you how to do this!" Truthfully, you can. Push it (
GENTLY!) as far as it will go, and you're now at the maximum aperture for that lens. But now, like all things photographic, you have a choice to make. Do you shoot at the minimum aperture, making focusing and composing difficult? Or do you hold the little tab so you're shooting at the maximum aperture of the lens, which makes it easy to focus and compose, but you have a dismally thin depth of field.
So you have another brainstorm! "I'll just move that little tab
part of the way! Ha!" OK, so take your shot. Did the exposure come out? Since the camera can't really meter now, you're shooting in full manual. So you chimp the shot, make an adjustment and try again. Now the biggest problem with reversing G lenses rears it's ugly head. Where,
exactly, did you hold that little tab? Now this little tab only moves a total of 3/16". And that 3/16" represents the entire scale of apertures available on that lens. So let's say you're using an f/2.8 lens that stops down to f/22. You've got 7 full stops. That works out to............ let's see..... 3/16 divided by 6..... I get 1/32 of an inch for each
full stop. Move that little tab just 1/32" off, and your exposure is now 1 full stop off.
The solution? Either take a whole bunch of photos, trying to keep your finger from moving between frames, guessing and by-gollying, and you might get lucky.
Another solution is to get a lens with an aperture ring. Yep, they're still around, although the manufacturers are moving away from them. And there's millions of 'em scattered all over the planet, some of which come up for sale. Not only is the Nikkon 50/1.8 AF-D a solid prime-lens performer that should be in every serious photographers kit, but it also makes a great lens to reverse... even on DSLRs.
The first thing you'll notice is the aperture ring (green arrow).
When mounted normally on a DSLR, the camera & lens still communicate as always. But when removed from the camera, it is this ring that sets the aperture,
not the camera. And since it's 'old-school' manual, you can set the lens to any aperture your little heart desires. Set it to f/8 and you've got f/8 all day long.
What you see between the lens and camera is a Nikon BR2A reversing ring. I use this one because it's solid machined metal, not plastic or aluminum like some cheap imitations. It has the same F-mount the lens does on one side, and a male 52mm thread on the other. Screw the filter-thread end into the threads of the lens, ....
.. and you can now mount the lens in reverse onto the camera using the adapter ring's F-mount (orange arrow).
This is why it's called reversing... the lens is on 'backwards, with the electrical contacts (blue) and aperture tab away from the camera. But since I have a lens with an aperture ring (green arrow), I don't need the electrical contacts to tell the lens what f/stop to use... I can set it with the aperture ring on the lens!
But, alas.... there's still a problem. Stopping the lens down to f/11 or f/16 still makes for a dark viewfinder. Solution: A BR6 ring!
A BR6 ring allows me to control opening and closing the aperture (manually). The ring emulates the camera lens mount, both with the traditional F-mount as well as that little aperture actuator lever that's inside the camera body (red arrows).
This ring is spring-loaded. So when it's attached to the lens, the spring holds the aperture blades open. If you look close, you can see the lens' aperture tab (red arrow) held by the BR6's actuator lever (pink arrow). As far as the aperture blades are concerned, they can't tell the difference between mounted on the camera body like normal or being controlled by the BR6.
Move the lever down (purple arrow), the actuator mechanism (pink arrow) moves out of the way of the aperture tab on the lens (red arrow). This allows the lens to close down to the aperture chosen with the aperture ring on the lens barrel.
So using the BR2A to reverse the lens physically, and the BR6 to control the aperture, it makes shooting macros with a reversed lens a breeze! Hopefully, this long, wordy post has made reversing a lens a bit more understandable.