Using Manual focus when autofocus won't lock

Just switched to a Canon 20D from a prosumer, so still learning all the DSLR camera features. Have been playing with the camera in/out of the housing, including low light conditions to get a feel for the autofocus. I have an Ike housing and port with manual focus capability, using a Canon 100mm f/2.8 lens. When fooling around in low light levels, autofocus performance degrades substantially (no big surprise) - but my question is this: How can you leave the autofocus engaged (can't change the switch during the dive), and still use the manual focus for those conditions where autofocus won't lock?

When autofocus doesn't work, I can use the manual focus and achieve sharp focus; but the camera won't always recognize it is in focus, so the shutter won't release until the green focus lock light comes on (I try the half press to attempt autofocus lock again after I achieve a good manual focus, sometimes it works, sometimes it just drives the lens back out of focus). The only workaround I can figure out is to find some other object with higher contrast/better lit to focus lock on, reacquire the object of interest, and then manually focus while keeping the shutter release half pressed the whole time to "keep" focus lock so the camera will allow me to release the shutter. Is this the only way to accomplish what I want, or is there a way to use the * key to allow manual focus, even with the lens switch set to auto?

I have a 350D, not 20D but think the custom function is the same.

If you set Custom Function 4 to value 1, then the * button is the focus lock and the main shutter button always fires when pressed. ie. It wont wait for focus lock, so this is where you can capitalise on lenses that have full time MF override when in AF mode.

I don't have any MF ports with my Ikelite rig so don't use it myself. I prefer to use AF and a Fisheye focus light have have not problems at all getting focus lock.

Hope this helps.

Yes. You can use the * key to lock focus. Dial the menu to custom functions 4, set that to 1 or 3. That will switch the autofocus function to the * key. Autofocus is only engaged when the * key is held down.

I alway set the 100mm macro on auto. In fact I have it taped down to that position so it doesn't get bumped into the manual mode.

I wouldn't recommend fitting a manual focus gear and then using autofocus. If you do the camera or lens motor has to drive the housing gears and it's not built for it.

If we're talking about the Canon 100mm Macro, the AF mechanism is internal, and in AF mode has MF overide so I wouldn't expect this to be a problem.

ie. Any auto focus operation wont move the outer focus ring, hence the moter wont need to drive the housing gears as well.

This is how mine works anyway.

ds

I'm a Nikon user so maybe that's a difference. I had to go and check this out. Some of my lenses moved the focus ring when in autofocus others don't.

most canon USM lenses (if not all the ones we care about under water) have the capacity of being used in autofocus mode with FTM (full time manual) override. that means that you can use autofocus and then fine tune with manual focus.

so you can have a manual focus gear on your housing and use autofocus and/or manual focus if you wish. that's certainly what i do on my canon 100mm macro.

hth/paul

Ken,

The Canon 100mm macro is USM which is like Nikon AF-S lenses.

Nikon AF-S lenses will not rotate the focus collar during autofocus. You can use any of the AF-S lenses with the zoom gear for quick manual focusing instead of zooming depending on how you set it up.

This will apply to the new AFS VR 105mm as well. For Ikelite users this will be provided that you can get a focus gear around the lens and have it still fit in the port. I think a custom port will be required here.

The custom focus port for the Nikon 105mm gets around this by not engaging the linkage while the camera autofocuses. So you can do both AF and MF

If you set Custom Function 4 to value 1, then the * button is the focus lock and the main shutter button always fires when pressed. ie. It wont wait for focus lock, so this is where you can capitalise on lenses that have full time MF override when in AF mode.

 

 

 

I prefer using Custom Function 4 - 2. On this setting you retain AF with the shutter ( which works most of the time especially when using a spotting light) and you have the option to focus lock with the * button for those situations where focus is difficult. Holding the * down locks the focus and the shutter will always fire. You get sharp focus by moving the camera as required or focussing normaly on an object that is the same distance as your intended subject. Thats onother reason why the ergonomic accessability of the * button is so important on a housing.

Andre, very clear, thank you.

And Acropora, if you're out there, does this work on the 5d?

So let's see if I have this right. You set whichever custom fuction it is so that * means "focus lock", so that if the lense is focused, and you use the manual focus override on the lense and press (and hold?) *, the focus will be kept and the shutter will fire, even if the camera doesn't recognize that the image is in focus.

Also, with this custom function setting, if you don't press *, just press the shutter, it will try to focus the normal way, and refuse to fire the shutter if it does not sense the image is in focus.

Am I close? I hope my 5d can do this! This will make macro shooting a bunch easier.

Thanks all.

Taxgeek

On the 5D, I think it's the same with the 20D, If you set CF 4 to 1 or 3 the * key takes over the autofocus function and half pressing the shutter does not engage the autofocus.

In this mode, press the * key once to autofocus, or if you want it to track set focus mode to AI Servo and hold the * key down. You can manual focus any time, probably a bad idea if the AI Servo is running. Pressing the shutter will always take a shot without changing the focus.

Thanks everyone, I was pretty sure it could be done, now I know how

So next question - I will shoot manual quite a bit, but if you do shoot aperture or shutter priority and you have used custom function 4 to set the * button to autofocus, how is exposure set?

Sorry to ask questions that are in the manual, but with a little help from my kids, my manual has temporarily (I hope) disappeared.