How to measure a Giant Peacock Moth in Photoshop

Last night I was surprised to see a huge bug flying near the house. A bug, at least that’s what I thought at the time. Then I realised it was a kind of butterfly, but not a normal one because I’ve never seen butterflies flying during the night, and definitely not any of this size. After about 30 seconds flying around the light above the main door of the house, and hitting it several times, it landed on the wall just beside the door. It stayed there, completely still, for a long time, at least 1 hour, and I decided to take some photos of it.

Natalie has a book about all the animal species which live in the region (she has a book for almost everything) and found exactly the same butterfly under the bugs chapter. It was a Saturnia pyri or Giant Peacock Moth, which lives in Europe (seen from May to August) and is the largest moth on the continent.

Using the tripod and a long exposure was an option but the only light available was the dimmed bulb which was on the top of the wall meaning the subject was in the darkness. So I used a Nikon SB910 camera off while I tried to get really close to the moth with the Nikon 24-70.

Giant Peacock Moth

Gian Peacock Moth Owl's eyes

The good thing is that it landed a few centimetres away from a thermometer we have hanging on the wall. The book says they are usually more than 120mm wide so I wanted to check how big this one was, and the thermometer was the perfect tool for measuring it as it’s scaled. The length of the moth measures roughly from -4°C to +19°C. I measured that and it was 6.1cm.

From here you can follow the same steps for measuring anything on a photo:

1. Open the photo in Photoshop and create 2 horizontal guides and 2 vertical guides framing the subject. This is optional but helps with the measuring.

Measuring Distance Photoshop

2. Go to Photoshop>Preferences>Units and Rulers and make sure Units for Rulers are in pixels.

3. Go to Image>Analysis>Ruler Tool and measure your reference distance. In my case this is the height of the moth and was 1265 pixels.

Measuring Distance Photoshop

4. Go to Image>Analysis>Set Measurement Scale>Custom. Here you set up your reference distance telling Photoshop that 1265 pixels are 6.1 cm.

Measuring Distance Photoshop

5. And last, Go to Image>Analysis>Ruler Tool and drag the ruler over what you want to measure, but make sure you tick Use Measurement Scale so it uses your reference. And that’s it.

Measuring Distance Photoshop

As you can see in the image W: 14.58 cm meaning that the Giant Peacock Moth which visited us last night was a huge one!

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