I have a Yashica Electro 35 film rangefinder camera. I usually load some higher sensitivity film so I can easily take pictures indoors, but now in summer I ran into trouble shooting outside on several occasions. There was simply too much light. I figured that I could use an ND filter to deal with this; the problem is, Electro 35 has an exposimeter on the body rather than through the lens (unlike, e.g. Minoltas). This means that even with an ND filter, my exposimeter will still measure the scene without it.

So my idea is that I could just lower the ISO setting on my camera to compensate for the drop in light caused by the ND filter. The problem is I have no idea how to calculate proper values and I didn't seem to find anything online.

Is this idea total nonsense for some reason, or is it there a way to get to the numbers? How strong of an ND filter would be needed to successfully work with ISO 800 film, but have the camera think it was ISO 100?

  • Why not just use the proper ISO film for the light conditions outside?
    – Alaska Man
    Jun 11, 2017 at 14:49
  • @Alaskaman Maybe because he want's to use the entire roll and his camera isn't set up to make properly unloading/reloading a half exposed roll of film very practical? Or maybe because he's also shooting indoors intermittently throughout the roll?
    – Michael C
    Jun 11, 2017 at 17:09
  • @Michael Clark Maybe? I was trying to elicit a reason from the OP with out guessing at maybe to determine if he understood that changing films is possible and better. I manually roll my half exposed rolls back and mark them as "shot to X number" when i want to use the film that is BEST for the situation.
    – Alaska Man
    Jun 11, 2017 at 17:28
  • 1
    @Alaskaman As Mark said. I started to consider this especially as I visit this event every year where I end up taking pictures in frequently changing indoors and outdoors. I just started experimenting with film this year, so it occurred to me that I should have some solution prepared for this. :) I am also not sure if I even can move film forward several frames in the Yashica, in case I wanted to unload/reload half exposed film. ND filter sure beats constant changing of the film anyway.
    – Delltar
    Jun 11, 2017 at 18:00
  • @Delltar Those of us who shot film for many years before anything else existed in the consumer market have been there, too. Just as many roll film shooters forgot the flexibility that sheet film shooters had 50 years earlier (just read some attempts to explain Adams' zone system by those who had never shot anything other than roll film), digital only shooters have no concept of the limitations of the same emulsion and development times on an entire sequence of images in a single piece of film.
    – Michael C
    Jun 12, 2017 at 5:56

3 Answers 3


The number of stops between the ISO values is just the "number of doublings/halvings" between them. In mathematical terms, the number of stops is log₂(ISO A / ISO B).

In your case, relative to ISO 800,

  • ISO 400 would be 1 stop slower film
  • ISO 200 would be 2 stops slower film
  • ISO 100 would be 3 stops slower film, than ISO 800.

So if you set your camera to ISO 100, a 3-stop ND filter with ISO 800 film would meter and expose as expected.


While your idea is definitely not nonsense, you don't really need to adjust ISO. From light meter's recommendation just open to larger aperture, or slow shutter down by required amount.

Your example of reducing 800 to 100 would require a ND8x filter.

ISO 800 to ISO 100 is a drop of 3 stops (800 -> 400 = 1 stop, -> 200 = 2 stops, -> 100 = 3 stops ), which requires ND8x

ND2x = 1 stop ; ND4x = 2 stops ; ND8x = 3 stops ; ND16x = 4 stops ; ND32x = 5 stops, and you can probably figure out the rest of the sequence.

  • 1
    Thanks for the answer! Problem is Yashica Electro 35 is basically aperture priority automatic only, so I have no way of setting my own values. ;)
    – Delltar
    Jun 11, 2017 at 13:53

The labeled values of ND (neutral density) filters are called "filter factors". How do we use filter factors to calculate a revised exposure setting?

A. The filter factor is a multiplier. We multiply the shutter speed by the filter factor to discover a revised exposure time that compensates for the density of the filter. Example: The shutter speed without filter factor is 1/400 of a second. We mount an ND4. The math is 1/400 X 4/1 = 4/400 = 1/100 of a second. We have calculated the revised exposure time.

B. We divide the ISO by the filter factor and re-set the exposure meter to this value. The chip logic of the camera or the readings of a hand-held meter make the necessary compensation. Thus if the ISO is 400 and we mount an ND-4 the ISO revised setting is 4/100 = 100.

C. We count on our fingers by 2's. We count 2 - 4 - 8 - 16 - 32 etc. Say we mount a ND4 we count to 4, that's 2 fingers. This tells us to compensate 2 f-stops. If the filter factor is 16, then we count 4 fingers. This method tells us to stop down 4 f-stops.

Caution: Most modern cameras measure scene brightness through-the-lens. That means, if you add a filter, the light energy reduction is seen by the sensor and the chip logic of the camera applies the needed correction. Your input is not required. However, it is desired because you can take the camera off "automatic". In this case, it is you that decides if the compensation will be shutter speed or aperture or a combination of both.

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