Question: I've heard claims ranging from *"higher-quality lenses only make a noticeable difference for scientific applications and super-huge prints,"* to *"even the difference between high-quality and uber-high-quality lenses can be seen with the naked eye on 6x10 prints"*

Before I sell my house so I can take amazing pictures from a cardboard box, I'd like to actually see the difference. However, all the examples I've found compare photos not only from different lens, but also with different cameras/scenes/photographers!

**Are there any examples of the same scene, with the exact same settings on the same camera, taken with multiple lenses (of differing quality/price)?** Multiple comparisons with different scenes/cameras *(especially cameras of different qualities)* would be preferable, to eliminate any variables. I'd also prefer a non-biased source *(so, not the lens-manufacturer)*.

I'd also be interested in comparing telephoto to zoom+teleconverter *(of the same quality, focal-length etc.)*, or comparing zoom to fixed-length... but perhaps those are asking too much :)

Answer: 
Sure; this is exactly what what <http://photozone.de> does in lens reviews. The sample images aren't always exactly the same but show similar subjects, but the technical analysis is all done for each lens on the same camera (for each brand).

For example, the sample images for Pentax's [70mm DA Limited](http://www.photozone.de/pentax/127-pentax-smc-da-70mm-f24-limited-review--test-report?start=2) and earlier [77mm FA Limited](http://www.photozone.de/pentax/128-pentax-smc-fa-77mm-f18-limited-review--test-report?start=2) show statues in the same garden from roughly the same viewpoint, but they're taken in different days and without mechanical precision. However, the *technical* analysis, done on an optical bench and shown on the previous pages are done in a controlled environment.

If you want super-identical images, reality limits that to still-life studio setups; other Lens review sites like [SLR Gear](http://slrgear.com/reviews/showcat.php/cat/2) will have those; see for example the the Pentax [40mm DA Limited](http://slrgear.com/reviews/zproducts/pentax40f28/zz_sampleindex.htm) and [43mm FA Limited](http://slrgear.com/reviews/zproducts/pentax43f19/zz_sampleindex.htm). You can see both a studio still-life and a test target. **These images, by their nature, are pretty dull and can't really tell you the whole story of the lens.** 

The Digital Picture site linked to in other answers also shows test target images, without even a studio still-life. You may find DPReview's [Lens Comparator Widget](http://www.dpreview.com/lensreviews/widget/Fullscreen.ashx?reviews=36,51&fullscreen=true&av=1,1&fl=50,55&vis=VisualiserSharpnessMTF,VisualiserSharpnessMTF&stack=horizontal&&config=/lensreviews/widget/LensReviewConfiguration.xml?4) similarly helpful; it gives a colorful presentation of results and you can hover over parts of the image for actual crops from the test target. But really, I can't see much advantage of staring at test targets yourself over looking at the crunched data and reading what the reviewers are saying.

As I re-read your question, though, I think that what you're *actually* asking may be best covered by **[What characteristics make a good lens good?](https://photo.stackexchange.com/questions/25572/what-characteristics-make-a-good-lens-good)**, which goes into what exactly the practical, visible differences are between higher-quality lenses and what you might want to look for in any samples. Once you know what you're looking for, it's not really important for the test images to be exactly identical.
I'm not sure you'll find one site that compares all lenses with one realistic scene, but a few lens review sites I've found interesting in the past include

* [Ken Rockwell's Camera and Lens Reviews](http://www.kenrockwell.com/tech/reviews.htm) Has reviews of high-end Canon and Nikon lenses, and he's not afraid to tell you to buy the cheaper models if he doesn't see any real difference in image quality (and he offers photographic evidence to make his case).
* [Camera Labs' lens reviews](http://www.cameralabs.com/lenses/Latest_DSLR_Lens_reviews.shtml) Reviews and round-ups between mostly mid-range Canon and Nikon zooms.
* [Digital Photography Review - lenses](http://www.dpreview.com/lensreviews/) Technical reviews of lenses, which offers a comparison feature which comares lens sharpness rather than side-by-side scene images.

Pentax lenses don't feature hugely on these sites, but they might illustrate whether top-end lenses really offer enough extra quality to justify their considerably higher price. And Ken Rockwell's site will probably just tell you to save your money and spend it on going out to find things worth photographing anyhow.