Error when using rgb2gray in a for loop.

I have four different .tif images in my folder. I want to use a for loop to load all of the images into Matlab then convert them to grayscale and then perform frame averaging on all for and display them in a montage. But I keep getting an error MAP must be a m x 3 array.

only one section of my code:

for i = 1:4
    img{i} = imread(sprintf('PShot%d.tif',i));
Igray = rgb2gray(img);
end

I believe my issue is that the variables are in a cell which is messing with the rgb2gray function but im not sure. I'm fairly new to Matlab so this maybe a trivial question.

Answer 1

[Walter Roberson]

nfile = 4;
img = cell(nfile,1);
Igray = cell(nfile,1);
for i = 1:nfile
    img{i} = imread(sprintf('PShot%d.tif',i));
    Igray{i} = rgb2gray(img{i});
end