# Coloc2: calculating the approximate PSF

Tags: #<Tag:0x00007fb88426d8a8> #<Tag:0x00007fb88426ccc8>

#1

Hi Stephanmg,

I am also analizing some images using Coloc2 but I don’t know how calculate the approximate PSF. It would be nice if you could tell me how can I do it.

Thank you very much.

Best
Alfonsa

Colocalization analysis
#2

This is info I got from the Coloc2 expert @chalkie666… If images are collected properly the PSF should be close to 3 pixels. To be sure, you can calculate it knowing the optical resolution d = lambda / 2*NA. (NA is the second number written on the objective lens, after the slash after the magnification number.) Then divide d by the pixel spacing in the image and that is the approximate PSF size.

You could also use this tool from SVI: Nyquist Calculator… as ultimately, the psf depends on many factors: the magnification, objective lens, camera binning, immersion oil, mounting medium, …

eta

#3

Hello Etarena,

Thank you so much for the info. I still have some questions, it would be nice if you could help me:

1. Is lambda the value of the laser wavelength???
2. If I use the Nyquist Calculator I do not obtain a value for the PSF, I obtain 3 values in X, Y and Z, Which of these values ​​is the one that I have to use as the value of the PSF in coloc2?

Thank you very much and sorry for the inconveniences,

Best
Alfonsa

#4

Hello Etarena,

This is Alfonsa again, I forgot asked you if the “pixel spacing” is the “pixel size”…

Thank tou so much
Best
Alfonsa

#5

Good day,

there is no pixel size but only a pixel spacing however, because this fact appears to be a big secret, most often both expressions are used in the same sense.

Regards

Herbie

#6

Hi Herbie,

Thank you!!!

Best,
Alfonsa

#7

Yes.

What are the values that you calculate in the end? Can you provide me with an example image so I can take a look myself (make sure it’s in the original formate)?

If you are worried - you can just use @chalkie666’s calculation and you will be fine. But the size that you put in for the PSF is in pixels… so keep that in mind.

eta

#8

Good morning Etarena,

I’m sorry to bother you and I thank you very much for your help.
I attach the image of a bead, of which I am trying to calculate the value of the PSF to put it in the “PSF” section of the plugin Coloc2.
I did:
d = lambda / 2*NA;
d=488/2(1.4)=174.2;
Then I divided d by the pixel spacing (0.041um = 41nm);
d/pixel spacing = 174.2/41 = 4.2.
PSF = 4.2

Using the Nyquist Calculator I obtain 3 values

as you can observe in this image I obtain 3 values x: 43nm, y: 43nm, z: 130nm; and I don’t know which value is the PSF value I need to Coloc 2.

thank you very much for helping me and sorry for the inconvenience.

Best
Alfonsa

#9

here goes the bead image (It looks like I forgot sending it in my previous post)
Thanks!!

#10