> For the complete documentation index, see [llms.txt](https://knowledge.illumina.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://knowledge.illumina.com/instrumentation/general/instrumentation-general-reference_material-list/000006325.md).
# Focus images on the MiniSeq and NextSeq 500/550
**Background**
At the beginning of Read 1 on the MiniSeq and NextSeq 500/550, the instrument determines the best Z position (distance between the objectives and flow cell) where the clusters are well-focused. During this process, the instrument takes and saves a series of Focus Images.
Many of the images are black with a laser dot and contain in the file name nomenclature, "Laser\_AF\_zpos... method\_AutoFocus". These images are laser only and thus clusters are not excited and not seen.
However, some focus images show clusters and have in the file name nomenclature, "Red\_IM\_zpos... method\_AutoFocus\_BestFocus". These images show visible clusters when the Z position is correct, allowing for proper focus of the clusters on the flow cell.
Technical Support can evaluate Focus Images to diagnose clustering issues (failed to cluster/ underclustering and overclustering) or an unstable focus model.
**Location of Focus Images:**
* MiniSeq: D:\Illumina\MiniSeq Sequencing Temp\[Run Folder]\Images\Focus
* NextSeq 500/550: D:\Illumina\NextSeq Control Software Temp\[Run\_Folder]\Images\Focus
**Example Focus Images:**
| **Clustering** | **Phenotype** | **Example Image** |
| ------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Normal cluster density
\~220 k/mm2
| Standard clustering |  |
| Ultra low cluster density | All cameras disabled
at cycle 1
Visible background between clusters
White edges due to increased intensity
|  |
| Very low cluster density | All cameras disabled
at cycle 1
Visible background between clusters
|  |
| Low cluster density | Low intensity or high G base calling
Visible background between clusters
No white edges
|  |
| High cluster density | No white edges
Visible fold-like artifacts
| 
|
| No clusters | White edges indicate increased intensity to compensate for initial lack of clusters |  |
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| :--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------: |
| *For any feedback or questions regarding this article (Illumina Knowledge Article #6325), contact Illumina Technical Support* [*techsupport@illumina.com*](mailto:techsupport@illumina.com?subject=Question%2FFeedback%20Regarding%20Illumina%20Knowledge%20Article%20#000006325%20-%20Instrumentation%20\&body=Dear%20Illumina%20Technical%20Support,%0D%0A%0D%0A)*.* |
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