ToolSahla, Tools Sahla

What is the difference between a zoom factor of 2 and a DPI setting when converting a PDF to images?

The result is the same scale, only written differently. A zoom of 2 means two pixels per point of the page, which is exactly 144 DPI, because DPI is turned into a zoom by dividing it by 72. The practical difference is that the DPI menu has five fixed values, while the zoom field accepts anything from 0.1 to 6.

Published · 3 min read

One scale, two units

A PDF page is measured in points, 72 to the inch. Choosing DPI asks: how many pixels do I want per inch? Choosing zoom asks: how many pixels per point? Both land on the same number internally. The tool turns 150 DPI into a zoom of 150 ÷ 72, and uses a zoom you type exactly as it is.

Worked example

An A4 page of 595.28 × 841.89 points at zoom 2 gives a 1191 × 1684 px image.

The same page at zoom 1 gives 596 × 842 px, one pixel per point, identical to 72 DPI.

At zoom 3 it gives 1786 × 2526 px, the equivalent of 216 DPI, which is not on the DPI menu.

Sizes are always rounded up, which is why 595.28 points at zoom 1 comes out as 596 px rather than 595.

When DPI is the better choice

  • When you think in print terms: the printer asks for 300 DPI, so pick it from the menu with no maths.
  • When familiar steps are enough: 72, 96, 150, 200 and 300 cover most screen and print needs.
  • When a colleague or designer talks in DPI, so you both use the same language.

When zoom is the better choice

  • When you need a value between the menu steps, such as 1.75 for a huge page that almost breaks the browser limit, or 2.5 for something a little sharper than 150 and lighter than 200.
  • When you want a plain multiple of the page's point size: 1 for exactly one pixel per point, 2 for double.
  • When you want a tiny preview image, since zoom goes down to 0.1 while the lowest DPI on the menu is 72.

Switching between them

  1. In PDF to images, find How to set the resolution, with the options DPI and Zoom factor.
  2. Choose Zoom factor. The DPI menu disappears and a number field appears, set to 2 by default.
  3. Type your value; a comma or a point both work as the decimal mark.
  4. Watch the size line, which updates with every number you type.

To convert by hand: zoom = DPI ÷ 72, and DPI = zoom × 72. So a zoom of 2.5 is 180 DPI.

Common questions

Is zoom 2 sharper than 150 DPI?

Slightly lower, in fact: 150 DPI is a zoom of about 2.08. The difference is small and the two images look almost identical.

What happens if I type 7 as the zoom?

The maximum is 6, so the field shows an error and the conversion does not start until the value is in range.

Does the zoom affect JPEG quality?

No. Zoom only sets the number of pixels. JPEG quality is a separate slider that sets how hard each image is compressed.