How to increase length of minor and major ticks of a frame?

Question

Following is the mathematica command I am using to obtain a contour plot(attached). I see that the length of minor and major ticks are very small. What is a good command to increase the length of both ticks?.

Table[ContourPlot[
  f[\[ScriptCapitalD], {x, y}], {x, 55, 80}, {y, 15, 25}, 
  PlotRange -> Full,  
  PlotLegends -> 
   BarLegend[Automatic, LabelStyle -> {FontSize -> 32, Black}], 
  ContourStyle -> Thickness[0.004],  ColorFunction -> GrayLevel, 
  Axes -> True, 
  FrameLabel -> {Style["x (m)", 32], Style["y (m)", 32]},  
  Contours -> 10, LabelStyle -> Black, FrameStyle -> Thickness[.005], 
  AxesStyle -> Black, ContourStyle -> Black, ContourShading -> None, 
  FrameTicksStyle -> Directive[FontSize -> 12], 
  BaseStyle -> {FontSize -> 21}, 
  AspectRatio -> 1/2.5 ], {f, {PDF, CDF}}]

Answer 1

Though not clearly written in MMA help, FrameTicks accept function in a similar way as Ticks do. Hence the following code can be tweaked to do the job.

ClearAll[fticks]
fticks[min_, max_]:=Module[{division = Subdivide[min, max, 10]}, Table[If[EvenQ[i], {(*pos*)division[[i]], (*lbl*)i,(*size*) {.1, 0},(*col*)Red}, {(*pos*)division[[i]],(*lbl*)i,(*size*){.05, 0},(*col*) Blue}], {i, Range@Length@division}]]
Plot[Sin[t], {t, 0, 2 Pi}, Frame -> True, FrameTicks -> fticks]

which gives

You can tweak the code to get what you want. Read help documentation for Ticks to see more use cases.

Answer 2

Update 2: As noted by @ercegovac in a comment, using @Carl's fix in this answer, the approach in my original answer below also works in Version 11:

Show[cp, FrameTicks -> fticks]

Update: Using a modification of the function tickF from this answer to create a custom ticks function:

ClearAll[tickF];
tickF[s_ : 1][div1_, div2_ : -1] := (If[div2 == -1, 
    Thread[{#, #, s {.02, 0}}, List, 2] &@ FindDivisions[{#1, #2}, div1], 
    Join @@ MapAt[Join @@ # &, {Thread[{#, #, s {.02, 0}}, List, 2] &@#[[1]], 
      Thread[{#, "", s {.01, 0}}, List, 2] & /@ #[[2]]} &@
       FindDivisions[{#1, #2}, {div1, div2}], {2}]]) &

Example:

ft = (tickF[3][8, 5][## & @@ #] & /@ PlotRange[cp]);
Show[cp, FrameTicks -> {{ft[[2]], ft[[2]]}, {ft[[1]], ft[[1]]}}]

Original answer:

The following approach works in version 9:

cp = ContourPlot[Evaluate[Re[Product[x + I y - (a + I b), {a, -2, 2}, {b, -2, 2}]]],
  {x, -3, 3}, {y, -3, 3}, ImageSize -> 400];

Get the FrameTicks using AbsoluteOptions and post-process to increase the tick lengths:

fticks = FrameTicks /. AbsoluteOptions[cp, FrameTicks]; 
fticks[[All, All, 3]] = 5 fticks[[All, All, 3]];

Use fticks as the option value for FrameTicks in Show:

cp2 = Show[cp, ImageSize -> 400, FrameTicks -> fticks];
Row[{cp, cp2}]