Changing the heading, subheading and caption fonts

Question

I am using the scrbook class document and observed that the heading, subheading and caption fonts are different from the normal text font style.

Along-with the scrbook settings, I have caption package included, which helps me changing the caption font style only. But I'm not sure how can I change the heading and subheading fonts. Once I know the normal text style, I can copy the same style into caption package.

But how can I make them all (heading, subheading, caption and normal text) of same style, but of different sizes? However, the chapter titlefont style should remain the same.

MWE:

\documentclass[
fontsize=11pt,   % Schriftgröße
DIV=12,          % Seitenaufteilung
BCOR=5mm,        % Bindekorrektur
ngerman,         % für Umlaute, Silbentrennung etc.
Inhaltsverzeichnis ein
]{scrbook}

\usepackage[textfont={small,sf},labelfont={small,bf,sf},labelsep=endash,format=hang,justification=justified]{caption}
\usepackage{titlesec, graphicx, geometry, titlecaps}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\begin{document}
 %Chapter Headers%
 \titleformat{\chapter}[display]{}   {\normalfont\Large\scshape\titlecap{\chaptertitlename}\enspace\scalebox{1.8}{\thechapter}\filright} {8ex}{\Huge\bfseries\sffamily\filleft}[{\titlerule[0.5pt]}]
 \titlespacing*{\chapter}{0pt}{30pt}{20pt}

\chapter{Fruits} 
\section{Apple}
The most of the fiber in apples is soluble fiber...
\begin{figure}[h!]
\caption{Apple Apple  Apple  Apple  }
\end{figure}
\subsection{Apple}
Most of the fiber in apples is soluble fiber...
\end{document}

Answer 1

You can use \setkomafont{<section>}{<font>} to adjust the font to suit your needs. See Change size of section, subsection, subsubsection, paragraph and subparagraph title.

In terms of the settings for caption, remove the use of sf in the font selection:

\documentclass[
  fontsize=11pt,   % Schriftgröße
  DIV=12,          % Seitenaufteilung
  BCOR=5mm,        % Bindekorrektur
  ngerman,         % für Umlaute, Silbentrennung etc.
  Inhaltsverzeichnis ein
  ]{scrbook}

\usepackage{caption}
\captionsetup{
  textfont={small},% Removed sf
  labelfont={small,bf},% Remove sf
  labelsep=endash,
  format=hang,
  justification=justified
}
\usepackage{titlesec, graphicx, titlecaps}

%Chapter Headers%
\titleformat{\chapter}
  [display]
  {}
  {\normalfont\Large\scshape\titlecap{\chaptertitlename}\enspace\scalebox{1.8}{\thechapter}\filright}
  {8ex}
  {\Huge\bfseries\sffamily\filleft}
  [{\titlerule[0.5pt]}]

\setkomafont{section}{\normalfont\Large\bfseries}
\setkomafont{subsection}{\normalfont\large\bfseries}

\titlespacing*{\chapter}{0pt}{30pt}{20pt}

\begin{document}

\chapter{Fruits} 
\section{Apple}
The most of the fiber in apples is soluble fiber \ldots

\begin{figure}[h]
  \caption{Apple Apple Apple Apple}
\end{figure}

\subsection{Apple}
Most of the fiber in apples is soluble fiber \ldots

\end{document}

_{titlesec is not suggested for use with KOMA-script.}

Answer 2

You can use \addtokomafont{disposition}{<font settings>} or \setkomafont{disposition}{<font settings>} to modify or change the presettings for all sectioning levels. To change the size of single section level use \addtokomafont{<section level>}{<size command>}.

Note that it is not recommended to use titlesec together with a KOMA-Script class. This package breaks some KOMA-Script features.

Here is a suggestion without titlesec and titlecaps. There is also no need to load package caption, if this package is only used to change the font of the captions and their labels.

\documentclass[
DIV=12,          % Seitenaufteilung
BCOR=5mm,        % Bindekorrektur
%ngerman,         % für Umlaute, Silbentrennung etc.
]{scrbook}[2015/10/03]

\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}

% font presettings for all section levels
\setkomafont{disposition}{\normalfont\normalcolor\bfseries}

% formating the chapter titles
\KOMAoptions{chapterprefix}
\RedeclareSectionCommand[
    beforeskip=-30pt,
    afterskip=20pt,
    innerskip=8ex,
    font=\Huge\bfseries\sffamily,
    prefixfont=\normalfont\Large\scshape
]{chapter}

\renewcommand*\raggedchapter{\raggedleft}
\renewcommand*\chapterformat{\chapapp\enspace\scalebox{1.8}{\thechapter}}
\renewcommand*\chapterlineswithprefixformat[3]{%
    {\raggedright#2}#3
    \par\vspace*{\dimexpr-\parskip-5pt\relax}\rule{\textwidth}{.5pt}%
}

% settings for captions and captionlabels
\addtokomafont{caption}{\small}
\addtokomafont{captionlabel}{\bfseries}
\renewcommand*\captionformat{~--~}

\begin{document}
\tableofcontents
\chapter{Fruits} 
\section{Apple}
The most of the fiber in apples is soluble fiber \ldots

\begin{figure}[h]
  \caption{Apple Apple Apple Apple}
\end{figure}

\subsection{Apple}
Most of the fiber in apples is soluble fiber \ldots
\end{document}