Steve Spencer
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 Re: Pros/Cons of Samyang 50mm 1.4 ii over Sony 55mm F1.8 | |
joelRichards wrote:
Steve Spencer wrote:
joelRichards wrote:
Steve Spencer wrote:
joelRichards wrote:
I also tested these two lenses (the Zony 55 and 50) on both the R3 and the A9. They behaved as I (and you) described.
I think we’re missing something in our communication because what you shared, while new to me, confirms what I’ve been saying. The 55 has a higher rate of transmission than the Zony 50 and may also with the Samyang.
I think our exchange has gotten a bit off topic but just in case someone stumbles upon it later in a search, I’ll respond one more time: Even if the measured T value changes with the camera a lens with a 1.4 aperture that measures as 1.8 is losing much more light than a lens with a 1.8 aperture that is measuring as 1.9. I never said the Zony 55 let in more light than the 50 wide-open, I only said the difference was much less than the expected 2/3 stop. At smaller apertures the 55 will let in more light because the apertures of both lenses are the same and the 55 transmits more light relative to it’s aperture—your numbers also confirm that.
What are we disagreeing about?
I hate to stick my nose in here, but I will to hopefully clarify things. It seems to me that what you are QuietOC are disagreeing on is the usefulness of DXO transmission testing. T-stop is a property of the lens and should have nothing to do with the camera and yet DXO transmission ratings differ on different cameras. That certainly suggests the DXO transmission testing is suspect. Earlier you said the DXO transmission testing was something they do well. DXO's own data suggests otherwise as they don't get the same number when they use different cameras.
I think this means that while getting the T-stop of a lens is a great idea that we can't trust DXO to provide these numbers reliably and that getting the real T-stop that isn't influenced by the camera on which the lens is tested may not be easy. The best bet at getting the T-stop is if the company offers the lens in a cine version in which they report the T-stop. As an aside when the F-stop is 2/3rd of a stop wider, then the T-stop is not going to be less than a third of a stop difference. This should have been the first clue that DXO's measurements are suspect, but the difference with different camera just confirms there is something amiss with DXO's transmission testing.
How DXO is screwing up is hard to decipher. One possibility is they aren't measuring transmission just in the center (by definition T-stop is transmission in the center and not in the periphery of the image) and if they are making this mistake their transmission number are going to be affected by vignetting and they will not reflect T-stops. This still wouldn't explain the differences between camera unless the properties of the camera sensor (e.g.FSI vs. BSI, or micro lenses) are affecting vignetting which they might be. Another possibility is that DXO is just sloppy and makes lots of mistakes in reporting their values for lenses. There is some evidence for this explanation as Bill Claff at Photons to Photos has documented a huge number of mistakes that DXO has made in reporting their ratings for cameras. A third possibility is that DXO is just testing a small number of lenses (e.g., 2) and they get different ratings because there is variation in the lenses. This would be problematic for the DXO ratings because the ratings would be affected by random variation in the manufacture process and would therefore tell us very little about the specific lens we buy. I don't think there is nearly as much variance from lens to lens as this explanation would suggest, but I can't rule out this interpretation. A fourth explanation is that the DXO ratings are affected by other factors related to the camera that aren't controlled well and one possibility there is dust on the sensor cover glass. If DXO does not clean the testing camera well, or only intermittently then their transmission number could simply reflect not just the transmission of the lens but also how much dust has built up on the testing camera. Of course these four explanations don't exhaust the possibilities for how the DXO results might be flawed and more than on of the explanations might be affecting the results but what we can know is the DXO measurements should not be taken as a clear indicator of a lens' T-stop. They are a rough estimate at best.
Wow, we're really off topic here. Actually if you go back to my first comment my point was just that the actual transmission of the Zony 55 is very good and if the OPs was really trying to meaningfully improve exposure values (as opposed to DoF), the upgrade might not be worth it. They should try to hunt down the actual transmission values of the other lenses they were looking at. It doesn't seem like my point turned out to be relevant to the OP anyway.
Regarding DXO Mark, I'm not going to defend them. I clearly acknowledged the "many faults" in their testing. True, I wasn't aware of the inconsistencies regarding T-stop their measurements. If anyone read my comment as a blanket endorsement of DXO Mark I publicly apologize. Perhaps you or someone else would like to share a more reputable database of tested/verified lens transmission values?
Regardless, I don't think it invalidates my point, which may relevant to those stumbling on this thread later, that hoping for a better exposure value isn't as simple as upgrading from a 1.8 to 1.4 lens. Different lens designs have different transmission levels and the FE55 does exceptionally well in this regard. Almost as well as most 1.4 lenses wide open and potentially better when both lenses are stopped down.
I don't think there is any good testing of transmission values and DXO doesn't test transmission of the lens. It tests transmission using the lens and camera. I agree it would be great to have someplace to examine real transmission value, but I don't think DXO is that and I don't know of another place that does it either.
With regard to the OP's question I don't think we can say the 55 f/1.8 has excellent transmission values. The only evidence for that supposed high transmission value is DXO and we have already established their measurements of transmission are not of just the lens and kind of suck.
I actually think we can just go with the measured aperture. An f/1.8 lens is going to let in about 2/3rds of a stop more light than an f/1.4 lens and there really isn't any evidence to question that. Certainly, the poor measurements at DXO that measure more than just the lens don't let you do that. And in fact with wide aperture lenses I bet rounding of the f number is at least as big a source of differences in transmission than the physical properties of the lens (after all an f/1.351 and an f/1.449 both round to f/1.4 and that difference is more than a fifth of a stop). I doubt the actually physical changes allowing light through are nearly as big as that rounding difference we tolerate all the time. So within the bounds of rounding that we tolerate all the time I doubt differences in actual light passing through really make a difference, but we won't know until real tests of transmission (as opposed to what DXO do) are made.
So, in my view, I would expect the Samyang f/1.4 to have about a two thirds of a stop advantage in letting in more light. You of course are welcome to see it differently, but I just don't think the DXO measurements are a solid basis on which to see it differently.
Again, moving away from a DXO debate, if you read my posts I've tested the Zony 55 vs the 50 and the 55 does, in my objective testing, have a higher transmission value. I think it isn't unreasonable to believe the same might be true for the Samyang. I thought that might be of value to the OP. I didn't realize it would upset so many people.
Believe me or don't. I'm not trying to sell you anything.
I guess what I am saying is that I can't see how anyone testing one copy of each lens on their own without a lab can hope to precisely measure transmission in the center of the lens (that's where the T-stop is measured). It seems like you used a camera to meter and that is going to test a lot more than just the center and I don't see how it can precisely tell us what you claim it does. It is not that I am not believing you. I just think what you are reporting is a lot more of a subjective impression that an objective test. And my subjective impression is the opposite of yours. In my experience using both the ZA 55 f/1.8 and ZA 50 f/1.4, the faster lens does indeed have about 2/3rds of a stop higher transmission in the center. The difference in our results may be copy variation, it may be bias, or it may just be it is hard to measure transmission precisely without a lab. I just wanted to weigh in and say my experience is different from yours and I don't think the ZA 55 f/1.8 has particularly high transmission.
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