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I want to do long exposure photography but I am not willing to shell the dollars for a good 100mm filter kit.

I am now considering a cheaper solution using circular filters. I want to be able to shoot without vignetting at 17mm with my 17-55mm Canon APS-C lens. The thread diameter is 77mm.

What filter diameter should I purchase in order to be able to stack two filters without producing vignetting? ("the largest the better" not a helpful answer).

The combinations I am planning to use are

  • single stopper (4 or 10 stops)
  • one stopper plus one circular polarizer

The stoppers are 5.5mm thick. The circular polarizer I am currently looking at is 7.5mm but I'd rather use one that's 4.5mm thick. My filters would thus have max thickness of 10mm to 13mm.

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    How do you plan to attach the filters to the lens? – Blrfl Oct 13 '17 at 22:13
  • By "circular filters" you mean "circular polarizing filters", right? I've edited with that assumption. – mattdm Oct 14 '17 at 13:31
  • But there's another assumption I'm unclear on. A good circular polarizer will not be meaningfully (if at all) cheaper than a good ND filter. A larger circular polarizer will probably be much more expensive. Why not just get one filter meant for the task at hand? – mattdm Oct 14 '17 at 13:33
  • @mattdm I don't think Philippe was necessarily referring to circular polarizers per se. In 1st paragraph, he said he wasn't willing to buy a 100mm filter kit, as in 100mm × 100mm square filter kit. Instead he just wanted to buy screw-on (round) filters. Probably should have said round rather than circular. – scottbb Oct 15 '17 at 4:34
  • @scottbb Maybe? I guess I'm voting for "unclear what you are asking". – mattdm Oct 15 '17 at 5:26
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According to a FOV calculator, 17mm gives a 76.2 degree diagonal FOV for Canon APS-C sensors. That's an angle of 38.1 degrees either side of straight ahead.

Sin(38.1) = 0.617+ so for each mm forward, a light ray at the edge of the FOV goes sideways by a bit over 0.617mm.

You probably need to add a few mm for a step up ring to attach the bigger filters to the camera, so lets say 3mm for that, giving you a range of 13 to 16mm.

13mm gives 8.02mm extra needed on each side = 93.04mm, and 16mm gives 9.87mm extra needed on each side = 96.75mm

Since standard sizes are 82, 86, 95, and 112mm, you'd probably want to go for 95mm (which might give you slight vignetting on the corners with thicker filters - it depends whether the original 77mm is actually needed, or whether that's just the closest standard size over the actual requirement), or 112mm.

[Edit - just realised I got the trig wrong - sideways distance should be tan (= opposite / adjacent) rather than sin (= opposite / hypotenuse).

tan(38.1) = 0.7841 so for each mm forward, a light ray at the edge of the FOV goes sideways by a bit over 0.7841 mm. This gives 10.193 mm per side for 13mm thickness , = 97.387mm, and 12.546 mm per side for 16mm thickness = 102.09mm.

However, the original 77mm is presumably chosen to work with a single filter so if you reduce the effective thickness to compensate for that, you're somewhat better off - if you knock 3mm off (which is probably conservative) you're back to 92.682 mm for 10mm thickess so 95mm is probably OK ]

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    And when the OP sees the price of most quality 95mm filters they'll likely realize why most shooters using similar lenses use 100mm filters. – Michael C Oct 14 '17 at 13:07
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Search the web for gelatin neutral density filters. They are inexpensive and plus the quality is high. Get a 10 stop square gel and tape it over your camera lens. Try this, you will like it!

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    YES; but, gelatin filters are not as cheap as they used to be! You might want to get and use a filter holder to preserve your gelatin filters instead of tape. – Stan Oct 14 '17 at 15:00

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