cap.read returns black image after adjusting focus

[Typschke]

Hello,

I have this program to get the best focus for my image:

import cv2 as cv
import time

cap = cv.VideoCapture(0, cv.CAP_DSHOW)

cap.set(cv.CAP_PROP_FRAME_WIDTH, 3264)
cap.set(cv.CAP_PROP_FRAME_HEIGHT, 2448)
cap.set(cv.CAP_PROP_BRIGHTNESS, -12)
cap.set(cv.CAP_PROP_CONTRAST, 50)
cap.set(cv.CAP_PROP_SHARPNESS, 100)
cap.set(cv.CAP_PROP_GAMMA, 100)
cap.set(cv.CAP_PROP_WHITE_BALANCE_BLUE_U, 6500)
cap.set(cv.CAP_PROP_BACKLIGHT, 0)
cap.set(cv.CAP_PROP_EXPOSURE, -5)
cap.set(cv.CAP_PROP_AUTOFOCUS, 0)
cap.set(cv.CAP_PROP_FOCUS, 240)

time.sleep(0.3)

focus_found = 0

for z in range(240, 280, 2):
 #Setting Focus
 cap.set(cv.CAP_PROP_FOCUS, z)
 time.sleep(0.3)

 #Read Data from Camera and convert to Grayscale
 ret, data = cap.read()
 gray = cv.cvtColor(orig_data, cv.COLOR_BGR2GRAY)

 ...
 #Code to determine the focus. Setting variable focus_found ether to 0 or 1
 ...

 #Break if focus  is found
 if(focus_found):
  break
  

This works now for 2-5 steps for "z", but then i only get a complete black image from cap.read() even though "ret" is returning True. Do you know what i'm missing?

[sahilprajapti-ai]

HI @Typschke
solve error code

import cv2 as cv
import time
import numpy as np

Open camera (DirectShow on Windows)

cap = cv.VideoCapture(0, cv.CAP_DSHOW)

Camera settings

cap.set(cv.CAP_PROP_FRAME_WIDTH, 3264)
cap.set(cv.CAP_PROP_FRAME_HEIGHT, 2448)

cap.set(cv.CAP_PROP_AUTOFOCUS, 0)
cap.set(cv.CAP_PROP_AUTO_EXPOSURE, 1)

time.sleep(1)

best_focus = 0
best_score = -1

for z in range(240, 280, 2):
# Set manual focus
cap.set(cv.CAP_PROP_FOCUS, z)

# Flush old frames (VERY IMPORTANT)
for _ in range(6):
    cap.read()

time.sleep(0.8)

ret, frame = cap.read()

# Validate frame
if not ret or frame is None or frame.mean() < 1:
    print(f"Black frame at focus {z}")
    continue

# Convert to grayscale
gray = cv.cvtColor(frame, cv.COLOR_BGR2GRAY)

# Focus metric: Variance of Laplacian
score = cv.Laplacian(gray, cv.CV_64F).var()

print(f"Focus={z}, Score={score:.2f}")

# Keep best focus
if score > best_score:
    best_score = score
    best_focus = z

Set camera to best focus

cap.set(cv.CAP_PROP_FOCUS, best_focus)

print("\nBest focus found:")
print("Focus value:", best_focus)
print("Focus score:", best_score)

cap.release()
cv.destroyAllWindows()