Fine Tuning SX-70 Exposures with a Variable Neutral Density Filter

Note: There is a video version of Using the Zone System with Polaroid SX-70 Film, Part 1 at the bottom of this post.

It would be nice to adjust exposures in ½ EV increments with my MiNT SLR670-S camera. Unfortunately, the camera can only make full shutter speed adjustments, one EV at a time; there is no ability to select in-between speeds. While most manually-controlled cameras allow f-stops to be set in half or third stop increments, the original SX-70 cameras that MiNT refurbishes have a ‘fixed’ f/8 aperture. 

All too often, I will take a meter reading only to find that the best exposure lies between two shutter speeds, for example between 1/125s and 1/250s at f/8. Because I can’t change the aperture, I’m forced to choose the lower shutter speed, which will result in a slight over-exposure, or the higher shutter speed, which might be darker than I wanted.

Because I’m shooting Polaroid SX-70 integral film, which has a very limited latitude, especially in the highlights, these seemingly minor compromises in exposure can have a major impact on the quality of the final photograph.

A side note about EVs: I’ll be using the term a lot in what follows.  EV (exposure value) is a neutral term that describes one full f-stop difference or one full shutter speed difference. For example EV+1 describes changing an aperture setting from f/8 to f/5.6 or the shutter speed setting from1/500s (second) to 1/250s. If an exposure meter indicates an EV of 13 for a given subject, with a 100 ISO film the photographer could use an exposure of f/8 at 1/125s or f/2.8 at 1/1000s. Both will result in the same EV 13 exposure.

But all is not lost; here’s a workaround that allows a -½ EV setting. I mount a variable neutral density (VND) filter over the lens, preset it to -1½ EV, set the shutter speed to -1 EV which will result in a result in a net -½ EV exposure change. For example, if the desired shutter speed lies between 1/125s and 1/250s at f/8, the camera would force me to choose one or the other with the result being lighter or darker than pre-visualized. With a VND filter set at 1½ EV placed over the lens and a 1/60s shutter speed, I get the exposure I wanted. Let’s call it 1/125-and-a-half second at f/8.

To do this, the VND filter needs to be calibrated and mounted over the camera lens. Here’s how to do it.

You will need an SX-70 camera that allows manual exposure control such as MiNT Camera’s SLR670-S. This modified vintage SX-70 folding camera and its attached Time Machine allows user control of shutter speeds.

You also need a custom filter holder to hold the VND filter over the camera’s lens. I recommend Retrograde Engineering’s SX-37 Filter/Lens Adapter, a 3-D printed plastic clip with a metal 37mm filter holder. It’s a simple piece of kit yet so useful that it should be in the camera bag of anyone using a folding Polaroid SX-70 camera.

MiNT SLR670-S with SX-37 Filter/Lens Adapter attached.

37mm filters can be attached directly to the filter mount but there is a much wider range of 40.5mm filters available so I use a 37-40.5mm step-up ring. Note that filters larger than 40.5mm are likely to interfere with the hinged flap on the folding camera. The popular 49mm filter size is a definite no-go.

There are a number 40.5mm variable neutral density filters available. I use an inexpensive Vivitar 40.5mm Variable NDX Filter with an advertised 1-to-10 f-stop reduction range.

Clockwise from top: SX-37 Filter/Lens Adapter, 37mm-40.5mm step-up ring, Vivitar 40.5mm Variable NDX Filter.

You also need a light table. I use an Autograph Lightpad. If you don’t have one, try finding a piece of translucent white plastic and light it from below. Note that light panel apps for iPads or iPhones will not work; their screens incorporate a polarizing filter which will interfere with VND filters, which also use polarizers.

Place the filter on the light table and outline the area with masking tape. Defining a specific area helps ensure that it will be getting consistent light readings.

Outlined measuring area on the Artograph LightPad A930.

Start by taking a lightmeter reading of the masked-off area (with the filter removed) and note the EV setting. Use a spot meter (such as a Pentax Spotmeter V or a spotmeter app) to get a good reading inside the tight 40.5mm circle. The free Lumu Light Meter app works well for this project; it allows a precise EV reading. In the app’s General Settings, select ‘Continuous Measuring’ and, in Spot Meter Settings, select ‘Show EV.’ Make a note of the EV reading. 

Taking an EV reading with the Lumu Light Meter app.

Now mount the VND filter on the SX-37 Filter/Lens Adapter and tape the adapter to the light table so the filter is directly over the masked-off area.

The assembled filter mount, step-up ring and VND filter temporarily taped to the light table.

All VND filters come with an uncalibrated scale and the Vivitar filter is no different. There are no specified 1-stop or 2-stop markings. To calibrate it, carefully attached a thin strip of masking tape over the filter’s ‘scale’ on the rotating ring of the filter. It’s easier to do this while the filter and camera mount is taped to the light table. Carefully attach ½” masking tape over the ring, stretching from the ‘max’ to ‘min’ marks on the filter (the actual words/symbols used may vary depending on filter brand). With a sharp knife, trim the excess tape using the filter’s edge as a guide.

Next, take a meter reading while slowly rotating the filter ring to get a reading that is exactly 1.0 EV lower than the bare light table reading. Once a -1.5 EV reading is established, carefully (without accidentally rotating the filter ring) mark the point on the masking tape of the filter using a fine-tipped permanent marker. On the Vivitar VND filter, there is a small triangle imprinted on the filter’s fixed ring and the -1.5 mark should be exactly under this triangle.

Measuring the -1.5 EV setting.

I like to mark each point with a red permanent marker and, after all the points are marked, annotate each point (1.5, 2, 2.5, 3) with a very fine black marker.  

Now locate the -2.0 EV location using the same approach and mark the point on the filter. Locate darker -2.5 EV and -3.0 EV settings on the filter and mark these as well. Note that my Vivitar filter manages a maximum 5½-stop range—far short of the advertised 10-stop range—and I could not eke out a -1.0 EV setting. More expensive VND filters such as the 40.5mm B+W XS-Pro Digital ND Vario filter may do better.

My calibrated VND filter.

That’s it. You can now take photos at calibrated half-EV settings!

Here’s how it works in practice: 

  • Take a meter reading of a subject and, if the ideal exposure lies between two shutter speeds, let’s say between 1/250s and 1/500s for this example, attach the 37X filter holder and mount your calibrated VND filter.
  • Set the VND filter to the 1.5 setting.
  • Taking note of the slower of the two shutter speeds bracketing the ideal exposure, set the shutter speed on your 670SLR-S to an additional shutter speed slower. For this example, the slower of the two shutter speeds is 1/250s, so set the camera shutter speed to 1/125s.
  • If you want to take a bracketed set of photos, set the VND filter to its 2.0 setting and take a second photo at -½ EV. Then remove the filter/filter holder, set the shutter speed to the slower shutter speed (1/125s in this example) and take a third photo at +½ EV.
  • Lastly, to avoid damaging your camera, remove the filter holder before folding it!

Here’s two examples of the VND in action:

Photo 1. Metered (ideal) exposure at 1/250s
Photo 3. -½ EV at 1/125s and VND filter set to -1.5 EV
Photo 2. -1 EV at 1/500s

This interior photo was taken in a contrasty window sill location with a bit of fill light. In this case, the first exposure taken at 1/250s was exactly what I wanted. The second photo taken at 1/500s (-1 EV) was way too dark. A third photo was taken with my VND set to 1.5 EV at 1/125s. Laying out the three photos side-by-side shows that the VND is giving me a credible ½ EV exposure difference. Although I still prefer my first exposure, the -½ EV photo, taken with my VND filter, is a viable alternative, certainly more viable than the -1 EV exposure.


Photo 1. 1/125s with VND filter set to -1.5 EV
Photo 2. 1/125s with VND filter set to -2.0 EV

My exposure readings for this lake scene suggested an ideal exposure straddling 1/250s and 1/500s. Using my VND set to 1.5 and a 1/125s shutter speed, I was able to achieve my ideal exposure. As a back-up, I took a second shot with my VND set to 2.0 at the same shutter speed. I prefer my first metered exposure, but the second -½ EV exposure is also a viable alternative.


Polaroid integral film is a finicky material. Getting the correct exposure—the exposure you envisioned—is always challenging. Tones easily slip in and out of range. Highlights can go from washed out to dull grey within a one-stop range. And, once the photo is ejected, there is no means to massage the result. Fine-tuning your exposure with a calibrated VND increases your chances of walking away with a good looking photo.  

So I hope you find this helpful. Let me know in the comments, one way or the other. You’ll also find a resource list down there for the equipment I used.  Finally, you’ll find the video version of this topic below. 

In the meantime, take a walk, click that shutter and make some Polaroid art.           

Resource List

MiNT Camera SLR670-S: https://mint-camera.com

Retrograde Engineering SX-37 37mm Filter/Lens Adapter: available on eBay at https://www.ebay.com/itm/124515912384, Etsy at https://www.etsy.com/ca/shop/RetrogradeENG, and Brooklyn Film Camera at http://www.brooklynfilmcamera.com/accessories/sx-37 

Vivitar 40.5mm Variable NDX Filter (at B&H Photo): https://www.bhphotovideo.com/c/product/944338-REG/vivitar_viv_vndx_40_5_40_5mm_vari_ndx_filter.html

Asahi Pentax Spotmeter V: check eBay for used meters.

Lumu Light Meter app (iOS only): Available on the Apple App Store: https://apps.apple.com/us/app/lumu-light-meter/id730969737 

Artograph LightPad light tables: https://www.artograph.com

More From Me:

Check out my SX-70 YouTube videos at https://www.youtube.com/channel/UC7d0LyjlePCsIFkZfaZjgZQ

My review of the SLR670-S: https://walkclickmake.com/2021/03/24/the-polaroid-folding-sx-70-on-steroids-a-review-of-mint-cameras-slr670-s/

Instagram: https://www.instagram.com/dfirman/

Firmangallery portfolio and store: https://www.firmangallery.com

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