I took a few pictures on a bright sunny day, and my thought was to get motion blur in the background.
So I had an exposuretime of between 1/60 and 1/30, ISO 100 and a polarizing filter.

Because of the sun I had to step down the aperture to F/13 - F/16.
This made the background more visible than I wanted.

I currently only own a 10 stop ND filter, and I know I can't use that :-)
But if I buy a 3-4 stop filter that should give me a aperture about F/4 - F/6, but can I see through the filter, can the camera focus?
As I said I only have a 10 stop now and I have to do everything prior to putting it on, will a 3-4 stop filter do the same?

I think I still need the polarizing filter, so filter stacking is probably needed.
I would like a filter that makes the aperture lower and making the background less visible, but still using optical viewfinder and autofocus.
Does that exsist with my settings and requirements?

Camera Eos 6D, Sigma 50 mm 1.4 Art.

F/16 1/30 ISO 100 enter image description here

F/16 1/30 ISO 100 enter image description here

F/13 1/60 ISO 100 enter image description here

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    You might also consider using a longer lens. A (short) tele would separate the subject from background more than your 50mm – Jindra Lacko May 16 '18 at 9:40
  • @JindraLacko maybe. I haven't tried. I only have one larger lens and that is the Canon 100-400 L (old version, about 2005-ish). I know a tele will blur out the background better, but at F/13-16? – Andreas May 16 '18 at 9:43
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    There is no substitute to testing, but in theory a longer lens will separate the subject better than a normal one. There will be other issues (such as tracking your fast moving subject - ask the bird on a wing photographers) but in principle it should work. – Jindra Lacko May 16 '18 at 9:49
  • @JindraLacko True! Will try it out some day :-) – Andreas May 16 '18 at 9:51
  • Just to clarify your intent - you're pretty comfortable using 1/30 - 1/60 doing the panning shots of skydivers landing. You want to stick to that shutter speed, and thinks that that a ND of 3-4 stops would allow you to open up the aperture, helping to blur the background more, right? FYI your polarizer is probably taking 2 stops of light in itself, but I think you already know that. – Calyth May 16 '18 at 19:52

Since you were shooting at f/16 and around 1/30 or 1/60 s. (ISO 100) the scene that day was around 13 EV (see an Exposition Value table here https://en.wikipedia.org/wiki/Exposure_value#EV_as_an_indicator_of_camera_settings).

Using an ND4 filter would take you to a ~9 EV situation, so in terms of autofocus you would not have any problems, since EOS 6D is capable of focusing even at -3EV situations (as Canon states in their webpage) as long as you use the central AF, which is the most precise.

ND filters would enable you to stop down [edit:] widen the aperture, so you can get a more blurred background. However, this blur is what we know as bokeh and it is different from the motion blur you mention at the beginning of your question.

Obviously, a wider aperture will make the background look more blurred but if you are looking for that motion blur you mention, you should practice a technique called 'panning'. And when 'panning', the piece of equipment that will help you the most is probably a lens with image stabilization (Sigma's HSM, Tamron's VC, Canon's IS...), though it is not indispensable at all.

'Panning' (https://en.wikipedia.org/wiki/Panning_(camera)) is a difficult to master, but very funny technique consisting of shooting with slow speeds (1/10, 1/15, 1/30, 1/50 s., for example) and following a fast-moving subject as you expose the photo, so it will look sharp (or sharp enough at least) while the rest of the scene is motion-blurred in the direction of the movement. You will need SERVO autofocus, slow shutter speeds (go for S or M modes) and a nice, stable position while shooting.

  • "ND filters would enable you to stop down the aperture, so you can get a more blurred background." This is the wrong way round - stopping down gives you a less blurred background. – Philip Kendall May 16 '18 at 9:27
  • The pictures do have panning motion blur. Open them in full size and you will see. My goal is to have both bokeh and panning motion blur. The reason for this is to blur out the building in the background more and then add the motion blur to give the image "speed". I think that will help to seperate the subject more from the background. Thank you so much for the technical part of the answer. I had no idea about that part! – Andreas May 16 '18 at 9:28
  • @PhilipKendall true, but I think we understand what he meant. It is confusing to step down is a higher value and a smaler hole. I believe it's a honest simple mistake. – Andreas May 16 '18 at 9:29
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    @PhilipKendall You are right, my mistake. I meant to widen the aperture, not to narrow it. – pgg66 May 16 '18 at 9:41
  • @Andreas Right. I assume you are somewhat familiar with panning. In that case, just the first part of my answer applies. If already shooting at lowest ISO and not willing to speed up the shutter, the only way to widen the aperture is to use ND filters, indeed. And you should be able to focus and see through the viewfinder using ND4 or so. – pgg66 May 16 '18 at 9:41

The autofocus needs a sufficiently large aperture to work. Focussing is done at the maximum aperture, 1.4 in your case, and adding a neutral density filter does not change that. Nor does a filter change the effective aperture.
You seem to want something in the order of 1/30 s with F5.6 at ISO 100. My camera (Sony A-58) has no trouble focussing at an indicated exposure of 4s @ F5.6 /100 ISO (indoors in a rather dark room).

You want a certain aperture value and shutterspeed as lowest ISO. Your metering tells you there's too much light for what you want, and suggests an optimal value for the aperture (supposing you use S mode with fixed ISO). From that, it's easy to calculate what ND filter you need.

Keep in mind that the polariser also absorbs part of the light, iirc 2-3 stops. But why do you think you need the polariser? If it's for the sky, with panning you run the risk of getting large differences in sky colour between images (depending on the angle over which you pan and your initial orientation).

In the images you show, you are at F13-F16. Suppose you are at F16 and want to go to F5.6. That's 3 stops, so you need a 3-stop ND filter (designations vary between brands) in addition to your polariser. I'd try a 4 stop without the polariser. The 10-stop filter would have put you at < F1.0, impossible with your lens.

You will still have the background, but with the larger aperture it should be blurred more.

  • Just a quick comment as I'm about to walk away from my computer. I'm shooting in manual mode. But I only change the aperture between the pictures so bottom line I could have used shutter priority. But I didn't. I actually didn't think about it until I read your answer. – Andreas May 16 '18 at 6:57
  • Now that I had some time to think about my descision on manual mode, if I would use S mode the camera will use the light meeter to evaluate the exposure. Depending on tilt and sun the exposure could be different from what I wanted. That is why I used manual, I took a test shot first and figured out what could work. I want the polariser mostly for getting more blue and green in the image. But will the camera be able to focus through a ND3 or ND4? – Andreas May 16 '18 at 7:36
  • Using "S" mode was just an example. Of course you can do it in manual as well, but if I can use automation, I will (one less thing to think about). – remco May 16 '18 at 8:36
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    @Andreas - not necessarily...It may be a change, but you can set up the buttons to separate autofocus and exposure lock. This way, you can use Shutter Priority mode and "lock in" the exposure settings before, say, panning into the sun (or lock in a shade setting if the subject will pan into the shade). Give or take more work than it deserves but, the option is there for you to play with :-). – Hueco May 16 '18 at 21:44

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