Frequently Asked Questions
Q. How many copies of a lens do you test for a review?
Results are normally derived from a single lens. It's not practical for us to test multiple samples of a lens as a matter of rule - it would simply take far too long.
Q. So how do you know if you have a good copy?
Our analysis software provides us with more information than reaches the reader of the review. We measure the sharpness across the four diagonals of the frame, before averaging the results for the final data. This shows us whether there is any significant asymmetry, which is a good predictor for whether we have a 'good' or 'bad' copy. When we have a clearly problematic lens, we reject it and request a second copy for test.
Q. Can I compare results obtained using different test cameras?
Strictly speaking, sharpness results should not be directly compared between tests carried out using different DSLR bodies, as the camera itself influences the results. Most notably, results get unreliable at higher spatial frequencies due to the differing effects of the test cameras' anti-aliasing filters.
This shouldn't make a huge difference for cameras with similar pixel counts, and as most of our test bodies are in the 12-14Mp range, the results should be broadly comparable. But it will have a more significant effect the further apart two test cameras are in resolution; for example, if you're comparing data between 12Mp and 24Mp cameras, then any data in the 'blue region' of the sharpness display should be considered to be not directly comparable.
However, all of the other results (chromatic aberration, distortion, and falloff) should be directly comparable regardless of the camera body used, and we’d stress that these are equally as important as sharpness in evaluating the overall performance of a lens.
Q. Will I really see the difference between lenses if I don’t print large or pixel peep?
If you normally view photos full screen on your computer (as opposed to pixel-peeping at 100%), or only print relatively small (6” x 4” or 7” x 5”) you won’t see a huge effect from differences in sharpness, or much colour fringing from chromatic aberration. However, you will still see the effects of distortion and falloff no matter how you view your pictures.
Q. Are your MTF50 numbers in line widths / picture height or line pairs/picture height?
Line widths per picture height.
When we first developed our lens data widget, we made a mistake. We calculated the MTF50 data in line widths per picture height, but labeled the y-axis as line pairs/picture height, and included a dotted line marked 'Nyquist Frequency' at the appropriate point for each camera on that scale.
For a number of reasons, we came to realise that the numbers were mislabeled in the presentation widget, and on going back though our calculations discovered the source of the error. This obviously meant we had to relabel the graphs, and as a direct consequence the 'Nyquist' line has gone too (it would now be off-scale for all of the test cameras we've used). It's important to understand that this change is not about the measured values, and only affects presentation of those values (the scale of the graph) - it doesn't affect the actual MTF50 calculations.
The change to correct the error in the presentation widget doesn't affect the lens review conclusions themselves as these are based on extensive testing, comparison and analysis in and out of the studio, and not on the absolute values presented by the graphing widget.
Apologies to our readers for any confusion caused.
Q. Why doesn't the MTF50 sharpness data in lens reviews match the resolution measurement in camera reviews?
Quite simply, these are different methods measuring different things. The sharpness data in lens reviews is presented as MTF50 (i.e. the spatial frequency at which image contrast is reduced to 50%); this is determined by calculation from a slanted-edge target, using camera raw files subjected to standardized processing with no additional sharpening. In contrast, in camera reviews we're looking at 'resolution', which is a visual assessment of the point where individual lines in the test target can no longer be distinguished. These two tests are therefore fully expected to give different results.
Q. I can't see the lens review widget in my web browser, but get a blank space instead. What's the problem?
The widget requires Adobe Flash Player to operate, so you may need to update to the latest version or uninstall/reinstall if you're having problems. If you're using Flash blocking software (and given the number of bad flash-based sites in the web we can forgive you for this), you'll need to add www.dpreview.com to your list of allowed sites.
Q. I'm looking at a new lens review and the widget doesn't load. Instead it's just showing 'loading CSS...'
Your browser has failed to load the latest version of a configuration file needed to view the test data. You'll need to close the page, clear your browser's cache and try again. Here's how to do it in some of the more popular browsers:
- Internet Explorer: Tools > Internet Options. 'General' tab, under 'Browsing History', click the 'Delete...' button and check 'Temporary Internet Files'
- Firefox: Tools > Clear Recent History. Check 'Cache' and click 'Clear Now'.
- Safari: Click on Cog icon at top right corner, 'Reset Safari'. Check 'Empty the Cache', click 'Reset'.