Open an image compression tool, and right in the center of the interface is a slider:
Quality: 80. You stare at it for three seconds—75 or 85? Would dragging it to 60 make it blurry?
In the end, you didn't move it and just clicked "Compress".
If most people end up stopping at the default value, is this slider actually helping you, or is it pushing the decision that the tool should make onto you?
The Numbers on the Slider Actually Don't Have a Unified Meaning
"Quality 80" sounds like "keeping 80% of image quality," but it's not.
- It is not a percentage. JPEG quality is just a coefficient used to scale the quantization table; the difference in image quality between quality 80 and quality 40 is definitely not "half."
- The same number is not comparable across different encoders. Writing 80, libjpeg, MozJPEG, Photoshop, and various phone encoders produce different image quality and file sizes. Photoshop's "Quality 8" and an online tool's "80" are even two different scales.
- It is even less comparable across different formats. 80 for WebP, 80 for AVIF, 80 for HEIC—behind them are completely different encoding principles. AVIF also has speed, WebP has method, and these parameters affect the result as well, yet they are often hidden in "Advanced Options" or simply not provided at all.
So when a tool presents you with a 0–100 slider, what it is actually asking is: "Please choose a suitable quantization coefficient for this encoder, this format, and this image." The vast majority of users cannot answer this question, and they shouldn't be the ones to answer it.
Even If You Understand It, One Slider Can't Tune a Batch of Images
Assume you are a professional user who understands the meaning of each parameter. The problem still exists: the optimal parameters are different for every image.
- A blue sky, a white wall—lots of smooth gradients; a slightly lower quality will cause banding or blocky artifacts;
- A forest, a grassy field—dense details; the same parameters show almost no loss;
- A screenshot—sharp text edges; using JPEG compression causes edge blurring—it is actually better suited for PNG.
Drag in 300 images, but there is only one slider. You either set it based on the most demanding image, wasting space on the other 299 images; or you set it based on the average, and some images will be over-compressed. A global slider is inherently in conflict with batch processing.
What the Slider Actually Does: Shifts Responsibility to You
This is the point that is least often stated.
When the compression result is unsatisfactory—blurry, or not compressed small enough—users' first reaction is usually, "I didn't tune my parameters," rather than "this tool isn't good." The slider makes the tool never wrong in any situation; the person who dragged the slider is always the one who is wrong.
Thus, users start testing repeatedly: compress once, open the comparison, adjust a bit, compress again. An action that was originally "drag and drop to finish" turns into a parameter-tuning experiment with no standard answer. For those who just want to make their folder smaller, this is pure burden.
A good tool should make the decision, not package the decision as "freedom" and hand it to you.
Removing the Slider Means the Tool Has to Do More Homework
Removing the slider isn't laziness—quite the opposite. The tool must complete the work that was originally pushed onto the user. This is exactly the path we chose when building ImgZilla: parameters are fixed, and there is no quality slider on the interface.
But "fixed" does not mean "one-size-fits-all":
- Each format is tuned separately. JPEG, WebP, AVIF, HEIC, and GIF each have their own set of parameters, selected within the visually lossless range; PNG and SVG go for true lossless compression, with not a single pixel changed.
- Process according to the real format. The file header is read to determine the actual format, so files with wrong extensions are processed according to their real format and won't be mis-processed.
- Don't compress if it can't compress. Files with a compression rate of less than 0.4% are marked as "minimized" and kept as-is, without deducting from the free quota. Images that have already been seriously optimized won't be repeatedly re-encoded.
- Results are verifiable and reversible. Built-in comparison window (
⌘D) with split-screen and zoom to actual pixels for side-by-side review; the original image goes to the trash by default, and if you're not satisfied, you can "put it back."
The only judgment you need to make is: Take a look, keep it if you're satisfied, or restore it if not. This is much easier to answer than "75 or 85."
There are also real-world test data: out of 32,188 camera-direct JPEGs, an average of 76.8% was saved, and a batch of JPGs that had already been compressed on websites was still compressed by another 46.8% (see the ImgZilla Test Data series). This shows that fixed parameters don't mean being conservative—the parameters of most images originally were much higher than truly necessary.
When You Actually Need a Slider
Saying "there shouldn't be a slider" doesn't mean the slider has no value. Some people do need it:
- Need original images for post-processing. ImgZilla's effective quality for JPEG is around quality 70 and uses 4:2:0 chroma subsampling, which removes EXIF and other metadata. This is sufficient for browsing, sharing, and going online, but if this image needs to be color-graded or re-exported later, you should keep the original, or use a tool that allows you to manually specify high-quality parameters (see ImgZilla Test Data (III): Photographer Perspective).
- Have clear volume targets. For example, "must be less than 200 KB per image," requiring parameters to be calculated backwards from the target.
- Need to convert formats or change dimensions. ImgZilla does not convert formats or change resolution; for these needs, please use more flexible tools like Squoosh or Zipic.
So a more accurate statement is: for the vast majority of scenarios where people just want images to get smaller without getting ugly, the slider is superfluous. Those who truly need it know very well what they need.
Summary in a Table
| Tools with Quality Slider | ImgZilla (Fixed Parameters) | |
|---|---|---|
| User Decision | Choose a parameter for each format/batch | Take a look at the result, keep it if satisfied |
| Batch Processing | One parameter applies to all images | Each format is tuned separately; uncompressible ones are skipped automatically |
| What if Compressed Badly | Retune parameters and compress again | Restore from Trash |
| Suitable For | Professional users with clear parameter requirements | People who want to compress images without studying encoding |
Good tools shouldn't hand you a multiple-choice question. It should answer it correctly for you, then give you a chance to change your mind at any time.
ImgZilla currently supports macOS (12.3 and above) and can be downloaded from the Mac App Store. The free version allows compressing 10 images per day, and only successful compressions count toward the quota—drag a folder in, don't adjust any parameters, and decide after seeing the results.
}