# Receptive Field and ConvNets

So I was reading this paper: https://arxiv.org/pdf/1409.1556.pdf

VERY DEEP CONVOLUTIONAL NETWORKS FOR LARGE-SCALE IMAGE RECOGNITION Karen Simonyan∗ & Andrew Zisserman+ Visual Geometry Group, Department of Engineering Science, University of Oxford

and at a point it mentions: "It is easy to see that a stack of two 3 × 3 conv. layers (without spatial pooling in between) has an effective receptive field of 5 × 5; three such layers have a 7 × 7 effective receptive field."

I don't understand how these effective receptive fields are calculated in relation to the convolutions / convolution units.

• Please add a complete citation for the paper. – gung - Reinstate Monica May 10 '16 at 1:10
• @gung I kinda added it now, not sure if you meant that though. – Pf Spf May 10 '16 at 1:13

If the size of the filter is an odd number of pixels ($n$) then it will increase the receptive field by $n - 1$ pixels.