Correcting Focus When Stacking Filters

[AdamD]

Hi all, here's my next installment of technique questions. This one is about correcting focus, when you stack multiple filters onto a lens.

Ok. I'm going to post a photo (see below). You'll immediately see that it's not exposed correctly, but I want you to look past that and zoom in a bit. You'll see the focus is off. The waterfall rocks are very soft and notice the morning dove on the fence, you can barely tell it's a dove!!

When I had this image focused, everything looked perfectly sharp, even the dove! AFTER I shot it, everything was sharp (except the dove, she flew away). But when I developed it, you can see, it's not very good. I checked the negative and it matches the image here. So something went wrong.

I believe my problem was filter stacking. In this case, I wanted to get a little wild. I used the following: Red, ND2, CP and a 6-stop ND (100x100). That's 12-2/3 stops in all. I was trying to get the waterfall to blur but you can't see that because I didn't get the exposure right.

I did a little digging and asked a few questions and I believe my problem is that with all those filters, I moved the focus point off the film plane.

So, if that's correct, how the heck are you supposed to refocus after stacking all those filters? I can't even see the image on the GG. My understanding is also that you should focus using stacked UV filters to achieve the same focal distance, but I don't own any UV filters for my LF lens, let alone 3 of them.

When technique do you use to ensure proper focus when stacking filters?

210869

[AdamD]

And you can see here where I got it right. But in this case I waited for better light and stacked fewer filters. This was had a Red and 6-stop ND (that square filter).

210870

[Drew Wiley]

I never stack filters; that helps. Otherwise, use a good loupe and practice with something appropriately lit till you can actually see the effect in the groundglass. A piece of reflective aluminum tinsel out in the scene hit by a laser pointer or other bright light might be a good place to start.

[Michael R]

I can’t tell from the posted images if the whole thing is soft or if it is out of focus.

Someone can correct me if I’m wrong but stacking filters on the front of the lens should not require refocusing. You only need to focus with a filter in place when it is behind the lens.

Maybe stacking all those filters simply degraded sharpness because one or more of them is crap. If they aren’t coated, stacking filters can also add flare which reduces contrast.

[AdamD]

Those are interesting ideas.

I can say this, individually each filter is good. I also noticed that the uploaded images are hard to tell of it's soft or out of focus like Michael R was saying. I can tell looking at it, it looks way too soft and must be out of focus to me....

Thanks for the tip!

[ic-racer]

Try focusing with the filters in place.

[AdamD]

IC-RACER, that's exactly what I can't do. This light is too dark after stacking all those filters to get the shot.

[Kiwi7475]

This doesn’t make sense other than the filter stack degrading the quality due to its impact to the MTF (aberrations, etc.) because they’re not “perfect”. Otherwise, if you placed a perfect glass in front of a lens focused to infinity, there would be absolutely no effect on imaging aside from some stray light considerations.

Often times having a resin filter (grad filter) can cause some very slight degradation, often not obvious at all, and so do other filter types. By themselves you most likely won’t be able to tell unless you have an MTF bench. But if you stack and stack you can add up to a substantial amount of aberrations.

The experiment would be to take multiple shots of a flat target (eg USAF 1951 chart or equivalent) as you add one more filter at a time, then one again at the end without them to make sure nothing has moved. That’ll give you an empirical limit of how and what filters you can combine.

[Jim Noel]

If your filters were each optically flat, and absolutely clean, they should make no difference in focus. BUT unless you paid a rather high amount for each filter i doubt that any of them were absolutely optically flat, or that they were meticulously clean. Therefore fuzzy, indistinct image.
If I use ND filters, they are single gels so there is no optical interference and are behind the lens eliminating flare.
These are leftover habits from when I thought every image had to be very sharp. Since I have decided that images look better if they see as the eye sees this ultra sharpness is not important to me, except perhaps in one tiny area.

[ic-racer]

IC-RACER, that's exactly what I can't do. This light is too dark after stacking all those filters to get the shot.
Maybe more time under the dark cloth to adjust your eyes.

[Nodda Duma]

Jim Noel is spot on. Theoretically, filters should contribute no optical power and therefore would not affect the image. In the real world, consumer-grade filters do have an impact... not focus per se, but degradation of the wavefront and all sorts of other effects.

If you want to be able to do what you want to do, buy filters that you can review the specs for, and ensure that the tolerances on wedge, wavefront error, and surface flatness are all minimized. If the filter you have/are looking at doesn’t specify these, then assume it’s not a good enough filter. Otherwise, you’ll need to figure out how to focus with them on to at least minimize that contribution.

[Bernice Loui]

The Cinema folks stack multi filters all the time. ND filters being most common with color correction (film), soft matt or similar and ... They all go into slots at the back of a Matt Box specifically designed for this.. Most common problem with filter stacking is increased flare, reduced light transmission and all related to disrupting the optical path. In theory, if the filters are optically perfect, there is no difference, in reality NO filter is optically perfect and real world trade offs must be made when using any filter (s) in front of the lens or in the back of the lens.

With this said.

Why 12-2/3 stops in an attempt to produce glow from moving water? What was the exposure time?

It is possible the out of focus could be due to movement in the camera, subject or ? due to the ? exposure time.

ND filtering down 12-2/3 stops is not needed to achieve glowing water effect from moving water.

Suggestion, don't rely on filters alone to achieve an expressive image.



Bernice






I believe my problem was filter stacking. In this case, I wanted to get a little wild. I used the following: Red, ND2, CP and a 6-stop ND (100x100). That's 12-2/3 stops in all. I was trying to get the waterfall to blur but you can't see that because I didn't get the exposure right.

I did a little digging and asked a few questions and I believe my problem is that with all those filters, I moved the focus point off the film plane.

So, if that's correct, how the heck are you supposed to refocus after stacking all those filters? I can't even see the image on the GG.

When technique do you use to ensure proper focus when stacking filters?

210869

[Doremus Scudder]

I agree with the others that a focus shift do to stacking filters is "theoretically" not your problem, however, stacking so many filters introduces a lot of other possible causes for image-degradation.

The first that comes to mind for me is the extra-long exposure you're going to have. Sure, you wanted to blur the waterfall, but you likely also blurred the mourning dove as it probably wasn't sitting absolutely still for the entire exposure.

Long exposures also show more degradation from camera movement and negative pop/movement.

The accumulated imperfections in each of the filters, which wouldn't degrade an image appreciably when used alone, add up. If you make a practice of stacking filters, invest in the best filters your money can buy.

Lots of uncoated surfaces in front of the lens = lots of flare. That can reduce contrast appreciably, especially in the least-dense areas of the negative.

And, lastly, fiddling around mounting all those filters after focusing is almost certain to shift focus a bit. Too bad you can't check focus after mounting the filters. Maybe add them all but the strong ND filter, check focus, and then carefully add the ND filter?

Best,

Doremus

[Drew Wiley]

More air/glass interfaces, and not sealed ones, but isolated surfaces greatly more subject to haze, dust accumulation, humidity fogging, physical damage, etc. It all adds up. People working on movies sets were expected to be real pros, but they certainly weren't shooting large format frames either! I have chatted with a few, and it amazed me how much they really knew about film, lenses, and metering. But any momentarily individual projected cinema frame is not intended to be examined up close or pontificated about like our large format framed prints. Just the sheer cost of LF film these days, esp color, would seem incentive enough to keep the light path as clean as possible. But if someone happens to like the look of vaseline over the lens, or aphids crawling between filters, it's not illegal.

But if water can't be distinctly blurred by a 1 or 2 sec exp, it sounds more like a swamp than a waterfall. Sometimes I've wanted to slow down an exposure dramatically in relation to a particular lens without shutter, where I use a simple lenscaps exposure method instead, and might additionally want grass blurred by the wind while things like rocks and textured tree bark remain totally crisp. A small f-stop, med speed film, and basic 47 deep blue or 29 deep red contrast filter will easily slow things down enough for that kind of project.

[Alan Klein]

Unless I missed it, you never mentioned what aperture you used. What was it? Where did you focus? What was the DOF?

[AdamD]

Unless I missed it, you never mentioned what aperture you used. What was it? Where did you focus? What was the DOF?

Alan, you didn’t miss anything...
I shot this at f/16 for 8 seconds in full on Arizona sun. I focused on the rock waterfall.

This was more of an experiment to see if I could use a bunch of filters to get the shutter slow enough to blur the water.

I’m gathering from your collective answers that the focus error was more likely user error as opposed to filter stacking error. Which is a good thing. I can say this, I probably should have added a stop more light or maybe 2/3 more instead of the calculated value.

As always, I would never have picked up a LF camera had it not been for this forum!! So thank you!

[John Olsen]

If your filters were each optically flat, and absolutely clean, they should make no difference in focus. BUT unless you paid a rather high amount for each filter i doubt that any of them were absolutely optically flat, or that they were meticulously clean. Therefore fuzzy, indistinct image.
If I use ND filters, they are single gels so there is no optical interference and are behind the lens eliminating flare.
These are leftover habits from when I thought every image had to be very sharp. Since I have decided that images look better if they see as the eye sees this ultra sharpness is not important to me, except perhaps in one tiny area.

I agree with Jim. The few mm's of filter can't have any effect on your focus as long as they're in front of your lens. It's just too many surfaces and too many chances for slight uncleanness to build up fuzziness. Now, if they were behind your lens, then every mm of glass would require 1/2 mm of focal correction.