Ok, I know that we shouldn't stack filters...but sometimes, it's either necessary (e.g. ND + CPL) or practical (e.g. you have a UV filter mounted on your lens, and want to screw in an ND or CPL as quickly as possible to get a shot). So, I set up a series of tests to determine the degree of vignetting that would result from stacking multiple filters on a few lenses.


The test setup was a flat, white target with even overhead illumination (corners were checked with a light meter and were within a few lux). Since I don't have Imatest (which Bryan uses for the vignetting tool - and I keep hoping they release the Mac version they've had planned for quite some time!), I used the simple [url="http://www.bobatkins.com/photography/technical/lens_vignetting.html]quantitative method suggested by Bob Atkins[/url], taking a metered exposure, then taking sequentially reduced exposures (by 1/3 EV), and comparing the luminance in the corners at the metered exposure with the luminance at the center of the reduced exposures to determine the EV lost to vignetting at the metered exposure.


RAW images were captured in manual mode, ISO 200, with the monochrome Picture Style (with peripheral illumination correction OFF), then converted to TIFF files using DPP. For each lens and combination of filters, images were collected over a 4 EV range (varying shutter speed only) at the maximum aperture for the lens, and 2-stops narrower than max. Quantitative analysis was done with a macro script in ImageJ (I can't imaging trying to do it manually for the 600+ images!), with 4 central and the 4 extreme cornerareas(64x64 pixels)averaged for each image.


Three zoom lenses were tested at the wide end (EF 16-35mm f/2.8<span style="color: #ff0000;"]L II on the 5DII, EF-S 17-55mm f/2.8 IS on the 7D, and EF 24-105mm f/4<span style="color: #ff0000;"]L IS on the 5DII), with the folllowing combinations of stacked filters (all B+W):
  • No filter
  • XS-Pro
  • F-Pro
  • XS-Pro + F-Pro
  • 2x F-Pro
  • XS-Pro + 2x F-Pro



B+W mount thicknesses are as follows:
  • Slim mount - 3 mm
  • XS-Pro mount - 3.4 mm
  • F-Pro mount - 5 mm
  • Slim CPL - 5 mm
  • Standard CPL - 7 mm



From that, you can determine what you can safely stack, e.g. XS-Pro + F-Pro would equate to an XS-Pro UV with a Slim CPL, etc.


I also tested two fast primes (EF 35mm f/1.4<span style="color: #ff0000;"]L and EF 85mm f/1.2<span style="color: #ff0000;"]L II, both on the 5DII). Since I don't have a 72mm XS-Pro filter, I used 1, 2, or 3 stacked F-Pro filters for the primes.


In the composite images below, all images are crops of the upper left quadrant (to reduce the size of the final composite image).
<div>Here are the results:</div>
<h3>EF 16-35mm f/2.8<span style="color: #ff0000;"]LII</h3>
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<div>The 16-35mm II has a fair amount of native optical vignetting, but in this test could tolerate a standard (F-Pro) filter without additional optical vignetting. Vignetting worsened with a stacked XS-Pro + F-Pro (meaning a standard CPL would increase vignetting), and 2 F-Pro filters resulted in mechanical vignetting.</div>
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<h3>EF-S 17-55mm f/2.8 IS</h3>
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<div>The 17-55mm has moderate native vignetting, and even though the image circle is matched to the sensor format, even 2 stacked F-Pro filters did not cause any additional vignetting, although with more than that, mechanical vignetting became evident wide open.</div>
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<h3>EF 24-105mm f/4<span style="color: #ff0000;"]LIS</h3>


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The 24-105mm has substantial native vignetting, shows very slight additional vignetting wide open with2 stacked F-Pro filters, and mechanical vignetting with more.



<h3>[b]EF 35mm f/1.4<span style="color: #ff0000;"]L[/b]</h3>





The 35mm prime has moderate vignetting with no filter, and even 3stacked F-Pro filters (15mm of mount thickness) does not increase the vignetting.



<h3>EF 85mm f/1.2<span style="color: #ff0000;"]LII</h3>





The 85mm prime has some vignetting with no filter, and even 3stacked F-Pro filters (15mm of mount thickness) results in only a very slight increase in vignetting wide open; stopped down, there is no detectable vignetting without or with filters.


Thanks for looking!


--John



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