Feline light metering

Cats have excellent night vision, because the pupils of their eyes can open up so wide, to let more light in. They can also see well in bright daylight because the pupils shrink and let less light in. This is exactly the same as the aperture on a camera, and it got me thinking: can I look at my cats’ eyes and use the size of the pupils to judge the correct aperture setting to photograph the cat? It might (I said might) one day prove useful if I want to photograph the cat with a camera that lacks a light meter.

First I need to do an experiment to collect some data. I need to photograph a cat in various lighting situations. I’ll end up with a collection of pictures of cat eyes in various states, and from the EXIF data saved in each picture I’ll be able to calculate the exposure value (EV) which is a single number that represents the combination of camera settings, and compare it with the diameter of the cat’s pupil.

where

• N is the relative aperture
• t is the exposure time (shutter speed) in seconds
• S is the ISO speed

We have two cats. Lou Lou has larger eyes but is quite camera shy. Mittens has smaller eyes but is more playful and more willing to do as instructed (as far as cats ever are). I decided to go with Mittens to make it a bit easier.

I need one picture of the target cat with a ruler held against her eye so I can measure the width of the whole iris. From that (roughly fixed) measurement, it should be quite easy to judge the diameter of the pupil from every other photograph without having a ruler in the picture by comparing with the iris.

Trouble is, cats don’t really like having things shoved in their face and Mittens kept on trying to kill the ruler. Nonetheless, I think the width of her whole iris is about 15mm. This shouldn’t vary in different lighting conditions, although sometimes the eyelid covers the edge. I’ve shown this measurement on the diagram below with the red line. The green arrow shows the width of the pupil, measured at 90 degrees from the longest measurement of the pupil and taken at its widest point. In this case the pupil is about 7mm wide.

The camera decided to expose this picture at ISO1600, 1/100, f/3.5 which gives it an exposure value of 6.3. That’s my first data point 🙂

Now I’ll collect pictures of Mittens in different lighting conditions, measure the pupil diameter and calculate the EV. In the end I took nearly 80 pictures from the midday sun through to dusk, and chose 39 of them to be included in the calculations. I excluded ones were I couldn’t easily measure both eyes, and I excluded pictures that would have caused to many identical results (I was a bit snap-happy with the camera). I heard that there is a shortage of cat pictures on the internet, so I thought I’d post some representative shots of Mittens being subjected to experimentation.

I decided to measure the diameter of both eyes in each picture and use the mean diameter to plot against the EV. I can measure the diameter of the pupil to the near half-millimetre and taking the mean might help with accuracy a bit. You might have gathered by now that this is pretty unscientific.

The results are reasonably well clustered around the line. There are some outliers. There are several explanations for this:

• The cat is looking in a different direction from the camera. The cat’s eyes and the camera’s sensor are measuring the brightness of different things.
• The cat was photographed in front of different backgrounds which could have caused the camera to over- or under-expose differently in each setting. I should really have used spot metering and metered off the fur between the cat’s eyes, but she wouldn’t sit still enough!
• Cats’ pupils dilate more when they are frightened or threatened. It’s entirely conceivable that the sound or proximity of my camera alarmed the cat and caused her pupils to dilate.

Other observations

• I was surprised that the cat’s pupils didn’t get larger in low light. I was expecting to see very little of the iris at all when the EV was below 4. Maybe Mittens is faulty.
• Mittens’s right pupil (as you face her) usually measured 0.5mm larger than her left pupil. Again, maybe she is faulty.

According to the spreadsheet, the equation of the line is

where D is the diameter of the pupil. Rearranging,

So next time you want to photograph a cat but you don’t have a light meter on you, just remember that equation. Couldn’t be simpler – just don’t forget to calibrate your cat beforehand.