# How to calculate under or over exposure using these values TTL meter (Lux), Shutter speed and ISO/ASA

I would like to get into film photography as a hobby, I own two 35mm film Cameras which both have broken TTL meters, they are common Fujica bodies and I would like to replace the (battery damaged) TTL meter system with a new sensor and microcontroller as a project.

My aim is to use the inbuilt indicator needle +/- to indicate if I am over or under exposing the film. The issue is I cannot for the life of me find a formula or work one out.

I have found two answers that have helped but not enough to work out a formula that works.

What is the difference between luminance and illuminance?

Is there a formula to calculate ISO according to Shutter speed?

The light is measured through the prism and that appears to need an extra value called reflected-light meter calibration constant. I cannot find this for Fujica but I've tried using the Pentax value which is 14.

To be able to calculate EV I would need f/stop, however, I can only access the following values.

1. Shutter speed
2. ISO/ASA
3. LUX

It must be possible to obtain if I am over or under exposing the film from these values as the cameras did it before they broke. It would be fantastic to find out what the formula/calculation would be? It might be a simple case of my Google-Fu failing me due to lack of knowledge.

• You do not calculate the ASA, the film you put into the camera has a specific ASA. The light meter makes its calculations based on that ASA, you tell the light meter what the ASA is by setting the ASA dial. If the light meter is broken then is it repairable or Have you considered getting a hand held light meter? Jan 30, 2021 at 14:02
• The inbuilt indicator needle ( +/- ) only knows what +/- is from Information it obtains from the light meter. How is your light meter damaged? Why do you think you need this work around? Why do you think this is an alternative to a proper functioning light meter? Jan 30, 2021 at 14:08
• @AlaskaMan Thanks for the reply. I can set the ASA according to the film that I am using and set the shutter speed. The issue is that even with power neither camera responds to the changing values, both have had battery damage which is why replacing them seemed the best option. I am able to tap into the ISO/Shutter speed dial using the microcontroller/workaround. I am also able to control the indicator needle via the microcontroller. The issue is that I am unable to find a formula to tell me to change my shutterspeed according to the amount of light/lux the sensor is exposed to. Jan 30, 2021 at 14:36
• The bottom line is you have to be able to measure the light in the scene in order to calculate the shutter speed and aperture needed based on the ASA of the film you’re using. What sensor are you referring to, Does that mean the light meter? I’m still unclear as to what damage your camera has, if it does not have power than how is it actuating the shutter. Jan 30, 2021 at 14:42
• @AlaskaMan Ah, replacing the sensor in the prism/viewfinder assembly would then make it possible to measure the light in the scene through the lens. I will work at it from that angle! thanks for your help. Jan 30, 2021 at 15:26