Confused about variable ND filters for video
I have some questions about variable ND filters when shooting video. I have never used an ND filter before.
I am using the Sony a6500, currently I have the Sony 18-105 f/4 and Sigma 30 f/1.4 lenses, and I would like an ND filter solution that fit both lenses.
I have been reading around, and am now a bit confused. Apparently, variable filters cause x markings and vignetting when used at wide angles or at the minimum and maximum densities, so I have some questions about using the filter with my lenses.
On bright sunny days if I´m shooting at 18mm f/4 on the 18-105 at shutter 1/50 or 1/250, would the variable ND filter cause any problems (X markings or vignetting)?
And would I be able to go up to f/1.4 on the sigma on the same conditions and settings?
Would a variable ND filter lower the sharpness of my footage?
I was thinking of buying a 72mm filter that will fit the Sony 18-105, but can I use step rings to use the filter on the Sigma that is 52mm without lowering the quality?
The filter I have been looking at is a Hoya NDx3-NDx400 https://www.amazon.com/gp/product/B007RLWORM/ref=as_li_tl?ie=UTF8&tag=peter...
I want to use a variable filter cause I´m shooting a lot of run and gun, and it seems like a cheaper solution. However should I use a fixed filter instead, which density would you recommend?
This really depends on the variable ND filter you purchase. Most Variable NDs will create the x pattern when at their darkest setting, but this really isn't an issue if you treat it right. If your ND filter says it covers 8 stops just think of it as covering 6 stops and never go past the point where it starts to x mark.
Variable NDs are a great way to go for run and gun filming. The cheaper ones will create a color cast and soften the image a bit, but if you get a good one, this isn't much of an issue.
For the quality vs price best bang for your buck purchase, I would recommend either the Tiffen variable or the Genustech Eclipse.
I use the Genustech every day for run and gun filming and the quality is very good.
This is a video filmed with a GH4 and G85 using the Genustech Eclipse if you want to check out the quality. The filming was on the fly run and gun with 100% natural light:
I'll echo Ben - you only get the X when you turn them too far. It's an artifact of the polarisation (a vari ND is two polarising filters stacked. If they were perfect filters, they'd go completely black at one point (90 degrees). They don't go black, instead you see the weird X pattern. The more money you pay (to generalise wildly), the further you can go (i.e. the greater the range of density).
Similarly, the more money you pay, the less the filter degrades the image (sharpness, and colour cast).
Buying one and leaving step down rings on the smaller lens is a great way to save cost. If time is critical, however, look at getting a filter for each lens. I'd think about getting them all in the largest size anyway so they're completely interchangeable. Try to get the same brand too, so the any colour shift is consistent across your lens/filter range.
Another alternative to multiple filters is to get some of the magnetic step down rings, so you don't have to unthread the filter, just pull them on and off to swap between lenses. They're not cheap either, however.
I'd say vari ND is the best way for changing your exposure on the fly, allowing you to keep the aperture you want. The worse thing about them is that it's hard to know exactly what stop your at, as the markings on them are cosmetic at best.