As you may have read in my other blog post Using Manual Lenses on your Canon DSLR, the addition of an AF-confirmation chip can make your life easier when working with manual lenses. What’s this all about? I’ll explain this in this blog post.
If you are not clear on when and why you could use these chips, please read my previous blog post first. These chips can basically be added to anything without electronic contacts you attach to the EF(-S) mount of your Canon DSLR. The chip will enable some cool features for you, and give you some limitations at the same time. Who said photography wasn’t about compromises?
If an electronic lens has focus confirmation even during manual focusing, then why doesn’t a manual lens have this? Still not completely sure on this, Canon should be able to create an advanced setting that would enable the focus confirmation even on manual lenses. Anyway, I have not seen any Canon DSLR that can do this (where for example Nikons generally do). So what is the alternative? Add some electronic contacts on the manual lens yourself!
For exactly this reason, some smart guys reverse-engineered Canon’s protocol when talking to electronic EF(-S) lenses. They developed a tiny programmable chip which you can glue onto your manual lens. This chip will fake the lens electronics, and make the camera think a electronic lens is actually attached!
These tiny chips are glued to your manual lens. When positioned correctly, they will do the following tricks for you:
- They will fool the camera in thinking it has an electronic lens attached;
- The chip can often be programmed to report the focal length of the lens;
- The camera will start focus metering again and give you your focus blip and/or flashing metering spots in the viewfinder back;
- Some chips will allow you to correct front/back focussing! If you adjust this, the camera will confirm focus when there is actual focus.
- The chip will report a (fixed) aperture. Normally people suggest to program it with the max aperture, but that may not be smart (see below);
What these chips can do is pretty impressive: My Samyang 35mm f/1.4 lens now has a working focus confirmation when attached to a camera! Also, it reports a 35mm lens. This really helps a lot, and it gets even better as your manual lenses get wider (because manual focusing is harder on wide angle lenses). Beware though, as you stop the lens down, the electronic metering becomes limited as well. Beyond f/6.3 there is metering on my EOS 7D.
AF-chip caveat: light metering
One big caveat to watch out for is light metering when this chip is attached: The camera no longer performs stop-down metering, even though the lens is still a manual lens. So what does this mean? The camera thinks it has aperture control, where in fact it has not. Yes, this will ruin your metering unless performed in a controlled way. The way you meter light with a chip attached (for example in Av mode):
- Set aperture on the camera to what you need;
- Set the aperture on the lens to wide-open;
- press the “*” button on the camera. This will fix the metering;
- Adjust the aperture ring on the lens to match the setting in the camera;
- Press the shutter;
- Your exposure should be properly exposed.
So how flexible is that? Ok I agree, working this way does NOT make me happy. You could of course fall back to manual mode (which I often do), but why does the camera behave the way it does? To understand this, we need to look at how the Canon DSLRs meter light when they are not in stop-down metering mode.
How the Canon DSLR meters light when not in stop-down metering mode
Did it ever occur to you that when you meter light just before a shot, the aperture always stays fully open? Yes, that’s right: The camera measures light that comes in at maximum aperture, then CALCULATES how much light is going to come in when the aperture closes to the desired value, and works from there. So metering light in automatic mode works like this:
- The aperture of the lens is fully opened;
- The camera measures the amount of light falling into the camera;
- The camera calculates the lighting for the desired aperture setting;
- When you take the shot, the camera reduces the aperture in the lens electronically;
- The shot is taken;
- The aperture opens again.
See where things go wrong when the camera THINKS it can set the aperture but in fact cannot because the AF-chip is really just faking the lens being electronic?
Now that you know how electronic light metering is done inside Canon DSLRs, you can probably imagine that a manual lens that fakes being electronic would mess things up, as the aperture is PRESET in the lens:
- The aperture of the lens is set manually;
- The camera thinks the aperture of the lens is wide open, and measures the amount of light coming in;
- The camera CALCULATES the lighting for whatever aperture you have set in the camera;
- As you take the shot, the camera THINKS it reduces the aperture, and acts accordingly. However, the aperture is fixed!;
- The shot is taken, and will be overexposed because the aperture never reduced (unless of course if you shoot the lens wide open);
All this CALCULATING as I describe it, is actually the camera reading the maximum aperture from the lens, and applying its math to this maximum aperture reported by the lens and the amount of light coming into the camera.
It also works the other way round; if you programmed the chip on the lens to report f/3.5, but in fact it is a f/1.4 lens, shooting at f/1.4 will underexpose the images more as you shoot further open.
This explains that the more you stop down a manual lens (that fakes being electronic), the more overexposed the shot will become. The camera basically mis-compensates!
How to overcome the exposure metering-error when using AF chips
How I personally overcome the metering-error in some degree, is by not programming the AF-confirmation chip with the maximum aperture of the lens, but an average one.
Take my Samyang 35mm f/1.4 for example. I hardly ever shoot this lens at f/1.4 (very shallow DOF; only 17cm at 2 meters!). So I almost always stop down the lens to at least f/2.5 (DOF is 30cm at 2 meters. Still not great, but better). I shoot the lens all the way through f/8, and mostly I’ll shoot the lens at f/3.2 – f/4. So somewhere in the middle of all this lies f/3.5.
So I actually programmed my AF-chip to report a value of f/3.5. In this case, the metering is exactly right when I set the lens to f/3.5 manually. When I shoot the lens more open (like f/2.8), the image will slightly overexpose. When I shoot the lens more stopped down (like f/5.6), the metering causes a slightly underexposed result. But in the range I use the most, the light metering is about right. Even better, I tend to open the aperture further in low light, that is where my images will start to be overexposed. And a slight overexposure in low light will actually help my EOS 7D camera in reducing noise (although I obviously need to correct this in post processing). So for me, this really works quite well.
Whenever I shoot very far out of f/3.5, I simply adjust the exposure compensation. At f/6.3 – f/8 I correct to around -1, and at f/2 – f/2.8 I correct to +1. On my 7D this is easily done (in Av mode) using the dial on the back of the camera after you started metering (pressing the shutter halfway).
So where can I get an AF-confirmation chip?
I will post a guide on how to glue the AF-confirmation chip to your manual lens in the near future.