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Canon 50mm and 40mm lenses vs 50mm Cassar S

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#1 JMC

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Posted 18 April 2019 - 15:10

Unlike most folks on here I am mainly a Canon user. I've been using a Canon 40mm pancake lens as my 'go to' autofocus lens for UV, as it seemed to me to give a reasonable image and not rob as much light as my other lenses. But I haven't really done any systematic test of that. So I thought I would rectify that.

The aim was to test the following 4 lenses;
Steinheil Cassar S 50mm (known as a good UV lens)
Canon EOS 50mm f1.8
Canon EOS 50mm f2.5 Macro (mk1)
Canon EOS 40mm f2.8 pancake

All tests done with my ACS converted EOS 7D mk1. Settings - ISO400, f11, 1/100s, shot in RAW. Images include 20% diffuse reflectance standard.

Lights - 2 x Bowens GM500 flash heads, each 35cm from the flower, each set at 45 degrees from the camera axis. Full power. Uncoated glass tubes. Small reflectors on each flash head. No protection in front of the bulbs.

White balance in DarkTable using the diffuse reflectance standard. Exported from DarkTable as Jpegs, then cropped and resized in Photoshop. No further processing.

Exposure values for diffuse reflectance standard done in RawDigger, when opening the files as RAW composites.

The results.......

Images - all same settings.

Steinheil Cassar S 50mm
Attached Image: IMG_8520 Cassar S f11a.jpg

Canon 50mm f1.8
Attached Image: IMG_8521 50mm f1pt8 f11 iso400a.jpg

Canon 50mm f2.5 macro mk1
Attached Image: IMG_8522 50mm macro f11 iso400a.jpg

Canon 40mm f2.8 pancake
Attached Image: IMG_8523 40mm pancake f11 iso400 a.jpg

And how much light actually gets through each of these? The channel data from the RAW composite files in RawDigger is as follows.

...................................Red.......Green.....Blue.....Green2....Average
Cassar S....................9707......4365......4035.....4576........5671
50mm f1.8..................4427......1914......3355.....1985........2920
50mm macro..............3375......1482......3063.....1521........2360
40mm pancake..........5663......2443......3865.....2540........3628

Overall it was as expected. The Cassar S let the most light through, and the 40mm pancake was the best of the Canon lenses tested here. What amazed me is that on average it let less than 1 stop less light through than the Cassar S. The 50mm macro lens was the worst in terms of final image brightness. This matches my previous observation comparing it with the 40mm pancake. The 50mm f1.8 prime was a bit better than the 50mm macro, but worse than the pancake.

There's more to be gleaned from the relative ratios of Red, to Green to Blue, as these change with wavelength - that may come later if I have time.

I'm quite impressed with my little 40mm pancake lens for UV, especially as it retains autofocus if needed (although to be fair I focussed all these manually). Plus it is a 52mm filter thread (in fact as were all the Canons tested here).

Edited by JMC, 18 April 2019 - 15:18.


#2 dabateman

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Posted 18 April 2019 - 17:58

Its totally unrelated to the test. But I find it interesting that the green channels are different. I may need to do a better controlled study of mine using histogrammer or shell out for Rawdigger. Visually from my channel isolation test, they look the same, for my Em1. Which had the interesting consequence, that I could use the isolated green channels to run a median filter and cut out noise, as the noise is different due to there diagonal spacing. Only negative is the resolution drops by 4.

Edited by dabateman, 18 April 2019 - 18:00.