ND filter and auto-focus

[rpcrowe]

I suppose that I could try this with my own camera/lens tomorrow but, I am throwing it out tonight...

My 7D camera will only autofocus when the maximum aperture is at least f/5.6. It will not focus when the maximum aperture of the lens is f/8 or smaller.

If I were to use a 2 or 3 stop ND filter, wouldn't that effectively cut my maximum aperture of an f/5.6 lens down to f/11 or f/16. Would the lens focus effectively?

I am thinking about this because of the need to shoot at a relatively slow shutter speed (1/80 or 1/100 second) in order to prevent propellers at airshows from being frozen.

The day I shot the planes of Fame Airshow was quite dull, yet I had trouble using a slow shutter speed and a wide f/stop. I was generally shooting around f/16 to f/22 which did not allow me to knock the BG OOF even though I was shooting at 400mm at times...

[NorthernFocus]

All you can do is give it a try, Richard. In my experience with my Nikon gear as long as the camera reads a lens aperture that is within spec it will try to focus. It can't differentiate ND filters fitted from low ambient light entering the lens. On all of my current bodies AF functions when I can barely see what I'm aiming at. I suspect Canon will do the same.

[Black Pearl]

My Fuji AF's with a 10 stop filter on so can't see you having a problem - so long as there's contrast for it to get lock on.

[Dave Humphries]

Hi Richard,

I agree with Robin and Dan,

I suspect the aperture specification is more about the effective Depth of Field* the AF system has to cope with than the amount of light hitting the sensor.

Although the lens auto-focuses when wide open, if, due to magnification introduced by say, a tele-convertor (coupled with loss of light also), it then may struggle or fail to AF.

* If effective aperture is too narrow, there will be so much DoF that it will be difficult for the electronics to judge where 'best' focus is. Heck, just try focusing a lens manually with when stopped down and you'll see what I mean.

That's my working theory anyway

Since this doesn't apply in your situation - and you'll likely have nice contrasty subjects against a different and far more distant background - I'd expect it to work, although I've not tried it myself.

Cheers, Dave

[Inkanyezi]

My 7D camera will only autofocus when the maximum aperture is at least f/5.6. It will not focus when the maximum aperture of the lens is f/8 or smaller.

If I were to use a 2 or 3 stop ND filter, wouldn't that effectively cut my maximum aperture of an f/5.6 lens down to f/11 or f/16.

A filter will not change the size of the aperture, unless it is a cutout bokeh filter, which is quite a different animal.

Autofocus, erroneously, falsely, called "phase detecting", uses triangulation for focusing, and the base for triangulation is two patches on the entrance pupil of the lens. When the lens is stopped down with the diaphragm, these patches will eventually fall outside the entrance pupil, impeding "phase detect" focusing stopped down. Also, when using a lens with too small maximum aperture, the patches where the AF sensors aim may fall outside the exit pupil of the lens, impeding PDAF.

When you mount a filter, less light will hit the optical system, but its physical size of diaphragm and entrance/exit pupils will not change. The two patches, from which light is directed to the AF sensors, will still fall within the pupils of the lens. If there is enough light, the AF system will focus.

[DanK]

I think the answer is to try it and see what happens. Even though light will be falling on the AF sensors, the ability of the camera to focus accurately will depend on the amount of light and the contrast in the area of focus. For dense NDs, I focus first and then add the filter. However, if you are using a lens with external focus and a screw-in filter (as I do), you have to be careful not to nudge the extended lens barrel when putting the filter on.

[William W]

My 7D camera will only autofocus when the maximum aperture is at least f/5.6. It will not focus when the maximum aperture of the lens is f/8 or smaller.

If I were to use a 2 or 3 stop ND filter, wouldn't that effectively cut my maximum aperture of an f/5.6 lens down to f/11 or f/16.

No. The ND Filter just makes the scene uniformly darker.

Would the lens focus effectively?

Probably yes, nearly always. I can get a 7D to AF very well in a reasonably dark room, provided I use Centre Point AF and have a good contrast edge.

WW

[rpcrowe]

Thanks for the replies. For some reason I always suspected that the focusing problems with lenses that have a max aperture smaller than f/5.6 was due to the amount of light transmitted. Now I realize that my assumption was incorrect.

I will first try autofocusing with an ND filter by following relatively fast moving cars. Then there is a small airport not very far from my home where I can try the AF with an ND filter.

My next airshow will be at Marine Corps Air Station (MCAS) Miramar, CA in the Fall. However most of my subjects will be jet aircraft which are relatively easy to capture since I can crank up my shutter speed as fast as the ISO and light will allow.

[Inkanyezi]

Airplanes, as well as birds, often choose a position where the background is rather smooth. Mostly a small aperture is no problem, except for maybe dust on the sensor.