diff --git a/core/preproc/filter_norm.m b/core/preproc/filter_norm.m
index ba31633d031ccaecea5c20f91064c639d988ce76..1bd8d5735d4054c9cd86dc4ff16ab2d648137030 100644
--- a/core/preproc/filter_norm.m
+++ b/core/preproc/filter_norm.m
@@ -21,4 +21,4 @@ function [im_filt,nvpair] = filter_norm(im, nvpair)
     sz              = nvpair.size;
     sz				= sz + mod(sz+1,2);     % round to nearest odd integer
 	[minv, maxv]	= minmaxfiltnd(im, sz);
-	im_filt			= (im - minv) ./ max(maxv-minv, 1/max_gain);
\ No newline at end of file
+	im_filt			= (im - minv) ./ max(maxv-minv, 1/nvpair.max_gain);
\ No newline at end of file
diff --git a/core/preproc/image_preprocess.m b/core/preproc/image_preprocess.m
index 86baf43050dcc9307cf1aa2c67824703c437492c..ed679ca2fbe1e9fd7df8f19f084257a5216e3fe7 100644
--- a/core/preproc/image_preprocess.m
+++ b/core/preproc/image_preprocess.m
@@ -15,37 +15,41 @@ function im_filt = image_preprocess(im, filters)
 	
 	for i = 1 : n_filters
 		filter	= filters{i};
+        ftype   = filter.type;
+        % convert to name-value pairs
+        % need to remove type field as this is not an argument
+        filter  = rmfield(filter, 'type');
         args    = namedargs2cell(filter);
-		if strcmp(filter.type, 'gaussian')
+		if strcmp(ftype, 'gaussian')
             im_filt     = filter_gaussian(im_filt, args{:});
-		elseif strcmp(filter.type, 'custom')
+		elseif strcmp(ftype, 'custom')
             im_filt     = filter_custom(im_filt, args{:});
-		elseif strcmp(filter.type, 'ssmin')
+		elseif strcmp(ftype, 'ssmin')
             im_filt     = filter_ssmin(im_filt, args{:});
-        elseif strcmp(filter.type, 'ssbg')
+        elseif strcmp(ftype, 'ssbg')
             im_filt     = filter_ssbg(im_filt, args{:});
-		elseif strcmp(filter.type, 'lmax')
+		elseif strcmp(ftype, 'lmax')
             im_filt     = filter_lmax(im_filt, args{:});
-		elseif strcmp(filter.type, 'levelize')
+		elseif strcmp(ftype, 'levelize')
             im_filt     = filter_levelize(im_filt, args{:});
-		elseif strcmp(filter.type, 'median')
+		elseif strcmp(ftype, 'median')
             im_filt     = filter_median(im_filt, args{:});
-		elseif strcmp(filter.type, 'norm')
+		elseif strcmp(ftype, 'norm')
             im_filt     = filter_norm(im_filt, args{:});
-        elseif strcmp(filter.type, 'norm2')
+        elseif strcmp(ftype, 'norm2')
             im_filt     = filter_norm2(im_filt, args{:});
-        elseif strcmp(filter.type, 'mednorm')
+        elseif strcmp(ftype, 'mednorm')
             im_filt     = filter_norm2(im_filt, args{:});
-		elseif strcmp(filter.type, 'clip')
+		elseif strcmp(ftype, 'clip')
             im_filt     = filter_clip(im_filt, args{:});
-        elseif strcmp(filter.type, 'transpose')
+        elseif strcmp(ftype, 'transpose')
             im_filt     = filter_transpose(im_filt, args{:});
-		elseif strcmp(filter.type, 'invert')
+		elseif strcmp(ftype, 'invert')
             im_filt     = filter_invert(im_filt, args{:});
-		elseif strcmp(filter.type, 'null')
+		elseif strcmp(ftype, 'null')
             im_filt     = filter_null(im_filt);
 		else
-			error('unrecognised filter type %s', filter.type);
+			error('unrecognised filter type %s', ftype);
 		end
 	end
 end