Image FFT and Lens MTF: Evaluating Final Image Quality

In summary, the FT multiplies the frequency information from the object with the lens MTF to create the final IFFTed image. You need to sample the object at the same frequency as the lens MTF to achieve accurate results.
  • #1
Neels
4
1
Hi, I am trying to determine the final image on a detector having a 600x600 mm viewed object and the known MTF of a lens. So when I FFT the object, thus creating frequencies -128/600 : 128/600 for a 256x256 image, how does the multiplication of the transformed object and the MTF "keep track" of the frequencies or the size of the input object? Because I can change the object to 60x60 mm and that would not affect the multiplication. How then do I evaluate the correctness of the final IFFTed image? thanks
 
Science news on Phys.org
  • #2
Neels said:
Hi, I am trying to determine the final image on a detector having a 600x600 mm viewed object and the known MTF of a lens. So when I FFT the object, thus creating frequencies -128/600 : 128/600 for a 256x256 image, how does the multiplication of the transformed object and the MTF "keep track" of the frequencies or the size of the input object? Because I can change the object to 60x60 mm and that would not affect the multiplication. How then do I evaluate the correctness of the final IFFTed image? thanks

If I understand you correctly, shrinking the object scale will result in larger spatial frequencies- the FT has a scale factor essentially corresponding to object size (or feature size, if you prefer). Because the MTF is constant (in your case), the smaller the object, the more attenuated those larger spatial frequencies are, resulting in more pronounced blurring.
 
  • #3
Thanks Andy - yes, I understand that there will be more pronounced blurring, but if I FFT the object image I don't know how to handle the frequency information when the transform is multiplied with the lens MTF or OTF.
 
  • #4
Neels said:
Thanks Andy - yes, I understand that there will be more pronounced blurring, but if I FFT the object image I don't know how to handle the frequency information when the transform is multiplied with the lens MTF or OTF.

Goodman's book has all the information you need. In brief, calling 'h' the point spread function and H the OTF (|H| is the MTF), 'o' the object field and 'O' the transform of object field, 'i' the image *intensity* and 'I' the transform of i, and letting '×'/'⊗' represent the convolution/autocorrelation integral, there are two cases, depending on whether or not the imaging system is phase-sensitive:

Incoherent imaging: i = |h|2×|o|2; I = [H⊗H][O⊗O]
Coherent imaging: i = |h×o|2; I = HO⊗HO

Does that help?
 
  • #5
Yes, but I guess my issue is more about the frequency values that apply - I have a measured lens MTF with a cut-off at 20 lp/mm. The object I am viewing is 600x600 mm - so do I need to sample (i.e. use nr of pixels) on the object to get the same order of max frequency before I perform the ⊗?
 
  • #6
Neels said:
Yes, but I guess my issue is more about the frequency values that apply - I have a measured lens MTF with a cut-off at 20 lp/mm. The object I am viewing is 600x600 mm - so do I need to sample (i.e. use nr of pixels) on the object to get the same order of max frequency before I perform the ⊗?

I don't understand your question.
 
  • #7
I believe I have complicate things a bit - but managed to sort it out. Thanks
 
  • Like
Likes Andy Resnick

1. What is an Image FFT?

Image FFT stands for Image Fast Fourier Transform. It is a mathematical algorithm used to decompose an image into its frequency components. This technique is commonly used in image processing to analyze the quality of an image and identify any distortions or blurriness.

2. How is Image FFT used to evaluate final image quality?

Image FFT can be used to analyze the frequency response of a lens, which is a measure of how well the lens reproduces the details of an image. By comparing the FFT of an original image with that of the final image, we can determine the degree of distortion or blurriness introduced by the lens, and thus evaluate the final image quality.

3. What is Lens MTF?

Lens MTF (Modulation Transfer Function) is a measure of the contrast transfer capabilities of a lens. It represents the ability of a lens to reproduce fine details in an image. A higher MTF value indicates better contrast and sharpness in the image.

4. How is Lens MTF related to Image FFT?

Lens MTF is directly related to Image FFT. The FFT analysis of an image can provide the spatial frequency response of a lens, which can then be used to calculate the MTF. This allows us to determine the quality of a lens and its impact on the final image.

5. What factors can affect the final image quality?

The final image quality can be affected by a variety of factors, including the quality of the lens, the resolution of the camera, the lighting conditions, and the subject itself. Other factors such as camera settings, image processing techniques, and environmental conditions can also play a role in the final image quality.

Similar threads

Replies
10
Views
1K
  • Engineering and Comp Sci Homework Help
Replies
1
Views
2K
  • Introductory Physics Homework Help
Replies
5
Views
966
  • Introductory Physics Homework Help
Replies
7
Views
2K
  • Programming and Computer Science
Replies
1
Views
1K
  • Introductory Physics Homework Help
Replies
4
Views
1K
Replies
13
Views
3K
  • Introductory Physics Homework Help
Replies
1
Views
2K
Replies
13
Views
16K
  • Electrical Engineering
Replies
1
Views
872
Back
Top