Steve Spencer
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 Re: Pre-order: Sony FE 20mm f/1.8 G lens | |
chez wrote:
Steve Spencer wrote:
chez wrote:
Steve Spencer wrote:
chez wrote:
Steve Spencer wrote:
pdmphoto wrote:
Steve Spencer wrote:
pdmphoto wrote:
KarmaKramer wrote:
Oye...the technical jibber jabber... just take photos already! Are you a photographer or an analyst?!
arneh wrote:
Fred Miranda wrote:
arneh wrote:
Fred Miranda wrote:
FE 20/1.8 G: It's almost a stop slower than the 24/1.4 GM
Do we know the T-stop of the 24 GM?
Bright lenses usually lose more light than slower lenses, so the actual light loss may be less than 2/3 stop. I'm guessing 1/3 to 1/2 stop difference is more likely.
I'm sure the 20/1.8G will be a great lens but you are stretching now! 
Maybe I am. But that's why I was interested to hear if anyone actually know the T-stop of the 24. I admit I don't have much data for T-stops. But from looking at dxomark which do measure it, all of Sony's F1.4 lenses they have measured have T-stops of 1.5 or 1.6, while all Sony F1.8 lenses have a 1.8 T-stop.
So I'm not convinced the difference is "almost one stop" in light gathering ability.
How does actual T-stop differ from what most everyone else here brings up like rendering, sharpness, corner sharpness, color, vignetting, handling, build, af noise, ..... you name it?
T-stop is simply the amount of light per unit of time that goes through the lens. It is basically unrelated to the other factors you mentions.
If you shoot a lot wide open, actual T-stop can matter more than the small differences between some of those other factors.
For some applications (e.g., video) that is absolutely true, but for other applications personally I find all the other things you mentioned much more important. I can turn the ISO up a third of a stop almost all the time in my shooting and the slight decrease in transmission matters little.
I purchase fast lenses firstly to be able to shoot in dimmer light...so for me the t-stop matters a lot if it is much higher than the theoretical f-stop. I wish manufactures would publish this value.
I shoot quite often at ISO 12,800 so increasing the ISO is not the desired answer.
I don't think anyone has shown, however, that the difference between T-stop and f-stop is much different between fast lenses and slower lenses. Even the one example in which T-stop was measured by the Chinese site the difference between the difference in T-stop and f-stop between the faster lens (f/1.4) and the slower lens (f/1.8) was considerably less than a third of a stop (about a 1/6th of a stop as measured). So by any reasonable standard that is a very small difference and in fact my argument has been it is so small that I have not seen any one show that we can measure T-stop precisely enough to even measure this difference reliably.
Said another way, if the the difference between T-stop and f-stop is pretty much the same for all lenses, then f-stop, which is a physical measure and can be measured precisely, tells us basically the same thing and T-stop. Only if the difference between T-stop and f-stop varies quite a bit (and T-stop can be measure accurately and fairly precisely) does T-stop provide any additional information.
DXO publishes their t-stop values and they do vary from lens to lens, some more drastic than others, some are bang on their f-stop rating. If one lens is rated at t-stop 1.8 and another much larger heavier lens is rated at t-stop 1.6...then that matters to me and I need to take that into my decision as to whether the more expensive and heavier 1.4 lens is warranted.
On the DXO site there are f2.8 lenses that have a t-stop of 3.2...quite a difference IMHO.
For example the Tamron 24-75 f2.8 lens has a t-stop of 3.3...so you really are not getting a 2.8 lens as far as low light shooting goes.
But what we don't know is how precise and how reliable those DXO numbers are. Do they have the precision in their measurements to tell the difference between 3.3 and 3.1? If they tested 30 lenses what would be the confidence interval around their T-stop measurement? Would it be .05 or would it be .4? We don't know any of that which makes it quite ambiguous what these numbers mean. Of course, a T of 2.8 vs. 3.3 is just over a third of a stop, but if their measurement is precise to even a sixth of a stop (which I highly doubt it is even that precise) then the lens might only be giving up a sixth of a stop versus the nominal f-stop which would not be bad at all and a lens measuring T of 2.8 could also be giving up a sixth of stop it is just the margin of error happened in different directions which is not only possible but will happen with one test of one lens. So unless they can test with more than a sixth of a stop precision and they start testing multiple lenses it is quite possible that a lens they rate with a T-stop of 2.8 is no different from a lens they rate as having a T-stop of 3.2.
Why are you doubting the precision of DXO measurements? Seems to me the consumer grade lenses have more variance away from their f-stop which would be what I expect. Also seems to me zooms vary more than primes...again it's what I would expect. What are you basing your negativity around. From what I see in the data, it appears to line up with what I'd expect.
I guess you are a glass half empty guy.
I doubt the precision of DXO measurement for two reason 1) they don't report to more than one decimal place. In science you report to one decimal place beyond where you have precision. So by simply reporting to one decimal place if they are following typical scientific reporting procedure they are telling us they do not have precision to one decimal place. They might have precision to even .2 or even .15, but it could also be precision to .9 or .95 (it is likely more precise than that). Still if they had precision below .1 then they should be reporting to the second decimal place (e.g. 2.95). Suggesting precision is at best .1 and that means in comparing any two lenses at least a .2 swing could happen just by lack of precision (with the error going in opposite ways). 2) They only test one lens rather than multiple lenses. We therefore no nothing about how testing procedure and copy variation would affect T-stop. My guess is copy variation alone may not allow much precision and multiple copies would be needed to get any sort of precision and a report on the range of T-stops one could expect across multiple copies of a lens. When you test only one copy, however, you know nothing about copy variation.
For these two reason we need to take DXO measurement as like a very imprecise measure of T-stop.
Finally let me add I am also appalled by the numerous errors that Bill Claff has found in DXOs reporting of their scores. See the large number of errors for example in his audit of DXOs sports scores:
http://www.photonstophotos.net/Charts/Sports.htm
Such terrible sloppiness hardly instills confidence in DXOs testing and reporting of their tests.
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