The speed of a lens is often referred to how fast it is when shooting wide open. So a F1.4 lens is considered faster than a F2.8 lens, meaning the F1.4 lens will be brighter and easier to get critical focus. This test is between a Nikon Non AI 50mm F1.4 lens, a Takumar 50mm F1.4 lens and Mamiya 50mm F4.5 lenses. With the Nikon and Takumar being F1.4 at wide open they are both considered to be much faster than the Mamiya, however the Mamiya lens is a 6x7 format lens with the others being only 35mm. That gives the Mamiya a few advantages. To me, speed is not just about how fast it is at wide open, but if all lenses were set to the same aperture does one lens allow for a faster shutter speed? I think that is a more interesting question because that would allow a lens to use a higher f-stop for greater depth of field, a faster shutter to catch better action, or better lower light ability.
Most lenses do not perform well shooting wide open (unless you've got something amazing like a Zeiss Otus 50mm F1.4, but if you have, then I seriously doubt you'd be bothered reading this article) and as digital sensors get more and more sensitive they show more and more short comings with lenses when shooting wide open. For example the Nikon 50mm F1.4 lens in this test shoots fine, wide open on a Nikon D50 (6 megapixel), but on the D800E (36.3 mega pixel) it shows chromatic aberration and loss of sharpness.
The Scope of the Test was to maintain constant exposure for each photo and then review the settings. So to achieve that the histogram was used (in camera) to get each photo as equal as possible while leaving the lens set to F8. This was better than just using matrix, center weight or spot metering because of the differences between each lens.
All photos displayed here are low resolution samples taken with a Nikon D800E (36.3 mega pixel). All photos have been adjusted in photoshop, however all have had the exact same filter (and settings) applied to maintain consistency.
The photo did come out a fraction darker than the other two lenses and the camera shutter speed was 1/10th of a second.
This lens showed some good depth of field behind the focal point, but not quite as much as the takumar and still showed some good sharpness. It was able to achieve a shutter speed of 1/13th of a second. So it is a little faster than the Takumar.
In 6x7 format, 50mm is considered a wide angle lens and as such you can see the retro-focus design. Retro Focus design drastically impacts on lens symmetry (PMF) and usually results in having an impact on the depth of field and that is exhibited in the results.
Focus was performed with the lens wide open then it was stopped down to F8 to take the photo. This lens was able to achieve a shutter speed of 1/20th a second.
Basically it means that for the same shutter speed as the Takumar achieved (1/10th), I could set the Mamiya lens to F11 (1 stop more Depth of Field). It's very hard to complain about being able to gain another stop, or to have that extra shutter speed in low light situations.
Even though this Mamiya lens exhibited less depth of field than the Takumar or Nikon, that is something which you can overcome by other means, however inherent sharpness can not, so being able to retain fine detail holds a higher priority. Speed is also beneficial in that the faster the speed you can use the easier it is to eradicate vibration and movement which will both effect sharpness, so it is a win/win for the Mamiya.
I make no guarantees about the scientific nature of these tests. These are my observations after photographing a real world object in my studio setup. These tests were performed to help me analyze how each lens performs in a normal shooting situation (not a lab or on a test rig), so that I can then use the best lens for the assigned task.
* Coming soon * Next comparison will be 100mm or equivalent as that will move away from retro-focus design. The only slight issue will be that there is no 100mm lens for Mamiya, it's 90mm or 110mm versus Takumar 100mm and Nikon 105mm.