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DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!
  
 
Tariq Gibran
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p.5 #1 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


Lee Saxon wrote:
Well, I'd assumed the FE55 was around half a stop slower than advertised, like most. It actually being 1.8 makes me feel a lot better about how large it is.


I don't think we can say the actual lens has a T-stop of 1.8 and an F-stop of 1.8. For one thing, it's impossible from my understanding of T-stop since glass lenses can't achieve 100% light transmittance:
"A lens with a T-stop of N projects an image of the same brightness as an ideal lens with 100% transmittance and an f-number of N"..."Since real lenses have transmittances of less than 100%, a lens's T-stop is always greater than its f-number".
http://en.wikipedia.org/wiki/F-number#T-stop

What's going on here is that DxO is not giving us a true T-stop for the lens but rather an "effective" T-stop when the lens is used with a specific camera/sensor. So, the sensor/ camera is where the "voodoo" is occurring to skew that T-stop rating on DxO in this case since the T-stop can't really equal the F-stop.



Jan 31, 2014 at 05:05 AM
snapsy
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p.5 #2 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


Tariq Gibran wrote:
I don't think we can say the actual lens has a T-stop of 1.8 and an F-stop of 1.8. For one thing, it's impossible from my understanding of T-stop since glass lenses can't achieve 100% light transmittance:
"A lens with a T-stop of N projects an image of the same brightness as an ideal lens with 100% transmittance and an f-number of N"..."Since real lenses have transmittances of less than 100%, a lens's T-stop is always greater than its f-number".
http://en.wikipedia.org/wiki/F-number#T-stop

What's going on here is that DxO is not giving us a true T-stop for the lens but rather an "effective"
...Show more

Perhaps the 55 FE's actual f/stop is faster than its nominal f/1.8 rating.



Jan 31, 2014 at 05:11 AM
Tariq Gibran
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p.5 #3 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


snapsy wrote:
Perhaps the 55 FE's actual f/stop is faster than its nominal f/1.8 rating.


That's a calculated number so it should be accurate.
The actual formulae is at my previous link for T-stop.



Jan 31, 2014 at 05:18 AM
snapsy
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p.5 #4 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


Tariq Gibran wrote:
That's a calculated number so it should be accurate.

Yes but calculated to what precision?



Jan 31, 2014 at 05:24 AM
Tariq Gibran
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p.5 #5 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


snapsy wrote:
Yes but calculated to what precision?


I think it extremely less likely (really impossible) that Sony has taken a real F1.4 or 1.5 lens and rated it at 1.8, particularly when we know from the link you provided on page 1 directly from DxO that "DxO Mark measurements are based on an assessment of the effective T-stop for every camera-lens combination." The camera is the highly probable variable here, not the F-stop.



Jan 31, 2014 at 05:32 AM
snapsy
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p.5 #6 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


Tariq Gibran wrote:
I think it extremely less likely (really impossible) that Sony has taken a real F1.4 or 1.5 lens and rated it at 1.8, particularly when we know from the link you provided on page 1 directly from DxO that "DxO Mark measurements are based on an assessment of the effective T-stop for every camera-lens combination." The camera is the highly probable variable here, not the F-stop.


How about f/1.7?

Regarding the sensor - it can only reduce the effective transmission, not increase it, so that wouldn't explain how a > T/f1.8 lens was measured as T/f1.8 by DxO.



Jan 31, 2014 at 06:04 AM
Tariq Gibran
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p.5 #7 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


snapsy wrote:
How about f/1.7?

Regarding the sensor - it can only reduce the effective transmission, not increase it, so that wouldn't explain how a > T/f1.8 lens was measured as T/f1.8 by DxO.


The evidence that it's extremely likely to be related to the sensor/camera body and not the actual F-stop is to be found looking at the behavior of other specific lens T-stop changes when used on different camera bodies at DxO...the only variable being the camera body/sensor. For instance, the same Otus 55/1.4 ZE has a T-stop of 1.4 on a Canon 5D but only a T-stop of 1.7 on a Canon 450D/ Rebel Xsi at Dxo. We know that Otus does not have an actual T-stop of 1.4 if the F-stop is 1.4..and I completely don't buy the idea that Zeiss avoided calling the Otus an F1.2 lens if that was the actual calculated F-stop (which would be an easy calculation for them).

There are many attributes of a camera that could influence what DxO calls "effective" T-stop - mount, mirror box, shutter accuracy, mis-stated sensitivity, sensor design/ toppings and so on. I don't know what caused the inflated T-stop but I think the evidence strongly points to something in the camera body rather than Sony marketing not taking advantage of calling the FE 55 a F1.5, 1.6 or even 1.7 lens rather than an F1.8 lens when the F-stop is known to them (if one is suggesting they are being conservative with stating the F-stop). That just doesn't make sense to me.




Jan 31, 2014 at 01:50 PM
Dpedraza
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p.5 #8 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


Tariq Gibran wrote:
I was going to say they should have tested the FE 55 on the A7. That would have been very helpful.

The Otus might actually test worse on the A7r, depending on the silly adapter variable!

Well that is true but you cannot use the FE 55 on a D800/E.

I'd be interested to see the results if they used an otus on the A7R vs the FE 55




Jan 31, 2014 at 02:58 PM
snapsy
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p.5 #9 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


Tariq Gibran wrote:
The evidence that it's extremely likely to be related to the sensor/camera body and not the actual F-stop is to be found looking at the behavior of other specific lens T-stop changes when used on different camera bodies at DxO...the only variable being the camera body/sensor. For instance, the same Otus 55/1.4 ZE has a T-stop of 1.4 on a Canon 5D but only a T-stop of 1.7 on a Canon 450D/ Rebel Xsi at Dxo. We know that Otus does not have an actual T-stop of 1.4 if the F-stop is 1.4..and I completely don't buy the idea
...Show more

Again, these are examples of sensors losing transmission. You're implying the 55FE is T/1.8 due to some type of transmission gain in the A7r sensor. That's not possible.



Jan 31, 2014 at 03:01 PM
Tariq Gibran
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p.5 #10 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


snapsy wrote:
Again, these are examples of sensors losing transmission. You're implying the 55FE is T/1.8 due to some type of transmission gain in the A7r sensor. That's not possible.


So if at ISO 50, as stated by the manufacturer, we have a measured ISO of 73 (the case with the A7r), that is a case that would give a higher T-stop due to a camera variable. Similarly, if 1/60 sec via the new shutter in the A7r is really 1/45 sec., that's another case that would inflate the T-stop due to a camera variable. Maybe the lack of the AA filter could have the influence of transmitting more light than a sensor that has an AA filter? I don't know but there do seem to be a few places, as above, that a camera variable could result in the T-stop being overstated.



Jan 31, 2014 at 03:21 PM
 

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snapsy
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p.5 #11 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


Tariq Gibran wrote:
So if at ISO 50, as stated by the manufacturer, we have a measured ISO of 73 (the case with the A7r), that is a case that would give a higher T-stop due to a camera variable. Similarly, if 1/60 sec via the new shutter in the A7r is really 1/45 sec., that's another case that would inflate the T-stop due to a camera variable. Maybe the lack of the AA filter could have the influence of transmitting more light than a sensor that has an AA filter? I don't know but there do seem to be a few
...Show more

DxO measures actual vs nominal ISO in their sensor tests so that shouldn't influence their t/stop results.



Jan 31, 2014 at 03:37 PM
Tariq Gibran
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p.5 #12 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


snapsy wrote:
DxO measures actual vs nominal ISO in their sensor tests so that shouldn't influence their t/stop results.


The thing is that we don't know the specifics of how they test for T-stop - or at least I could not find the actual methodology. Do you have a reference for it?

All we know is that it's not the actual - traditional - T-stop but an "effective" T-stop that does include the camera as a variable. We also know that a real T-stop must be slower than the actual F-stop (a number that the manufacture knows) AND we know there are examples via DxO that show T-stops that equal F-stops. Your suggestion seems to be that in all these cases (T-stops = F-stops), the lens manufacturer has understated the real F-stop (the lens is actually faster than the manufacture is indicating). I don't think there is a sales benefit to a company understating the F-stop from a sales/ marketing perspective so I sill say the evidence is that it's highly less likely that is what is transpiring.



Jan 31, 2014 at 03:48 PM
snapsy
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p.5 #13 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


Tariq Gibran wrote:
The thing is that we don't know the specifics of how they test for T-stop - or at least I could not find the actual methodology. Do you have a reference for it?


http://www.dxomark.com/About/In-depth-measurements/DxOMark-testing-protocols/Light-transmission



Jan 31, 2014 at 04:04 PM
Steve Spencer
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p.5 #14 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


Well it still seems possible that the A7r's shutter is slightly slower than it is rated and this would inflate the t-stop.


Jan 31, 2014 at 04:39 PM
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p.5 #15 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


Their test may or may not be a joke, but their reporting of the results is. A one number score for sharpness really tells you very little about how the lens performs in terms of sharpness. A DXO comparison between lenses -- as they present them -- gives me very little useful information. I still would not know why I would want one lens over the other.


sebboh wrote:
pretty much all lens tests are a joke if you're asking the question: "is this a good lens?" or "is lens A better than lens B?". nevertheless pretty much all lens tests provide useful information if you look at the actual data and know what you are looking for.

at this point i don't think their is any question that the FE 55/1.8 is an outstanding lens in the ways most people are interested in cross the frame sharpness and contrast.





Feb 01, 2014 at 01:40 PM
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p.5 #16 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


Lotusm50 wrote:
Their test may or may not be a joke, but their reporting of the results is. A one number score for sharpness really tells you very little about how the lens performs in terms of sharpness. A DXO comparison between lenses -- as they present them -- gives me very little useful information. I still would not know why I would want one lens over the other.



Just like with their sensor scores, you can look at several charts and graphs that plot resolution, distortion, etc.



Feb 01, 2014 at 02:39 PM
Lotusm50
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p.5 #17 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


Yes, dig down deep you can find additional info. But their reduction of information to a single number in virtually all their presentations is essentially useless -- and worse, its potentially misleading to average bloke who is minimally informed.


douglasf13 wrote:
Just like with their sensor scores, you can look at several charts and graphs that plot resolution, distortion, etc.




Feb 01, 2014 at 03:04 PM
douglasf13
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p.5 #18 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


Lotusm50 wrote:
Yes, dig down deep you can find additional info. But their reduction of information to a single number in virtually all their presentations is essentially useless -- and worse, its potentially misleading to average bloke who is minimally informed.




I agree. There's never been much reason to bother with their single scores, sensor or lens.



Feb 01, 2014 at 04:43 PM
briantho
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p.5 #19 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


briantho wrote:
I'm not saying the FE has bad bokeh, but I'm pretty sure my Contarex 50mm f2 Planars have smoother bokeh from what I've seen so far. When I get my FE55 I'll do a comparison.


A bit late, but here is a quick WO comparison of the Contarex Zeiss Planar 50mm f2 and the Sony Zeiss FE 55mm f1.8. I realize I need to do more tests.

The comparisons are at the Sony MFD, the last image is to show the Contarex MFD, which is much closer than the Sony's. Advantage Contarex when it comes to MFD.

At first glance the sharpness looks to be equal, but at 100% you can see that the 50 year older lens doesn't have the same micro contrast as the Sony, but it's a very slight difference.

When it comes to CA, considering the 3 aspherical elements in the Sony, and 0 in the Contarex, it's a bit of a mistery that the Contarex seems to handle CA better, at least in this shot. Both lenses are well controlled though.

As bokeh is subjective, I'm not going to comment on it.

Full sizes available.on flickr.

FE55:



C50:



FE55 100% crop center sharpness:



C50 100% crop center sharpness:



FE55 100% crop CA:



C50 100% crop CA:



C50 MFD:





Mar 29, 2014 at 12:38 PM
AhamB
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p.5 #20 · DxOmark reviews Sony FE 55mm 1.8 ZA: Outstanding!


briantho wrote:
When it comes to CA, considering the 3 aspherical elements in the Sony, and 0 in the Contarex, it's a bit of a mistery that the Contarex seems to handle CA better, at least in this shot. Both lenses are well controlled though.


Aspherical elements are used to correct SA, not CA.

The bokeh is a day and night difference (much softer on the FE55 despite its higher contrast). Interesting to see how the focus transition of the Contarex goes from sharp to double lines immediately, whereas the FE55 is about as smooth as it can get. The stronger LoCA of the FE55 may have something to do with the higher contrast. I've seen more old lenses that don't have much CA, just general haziness.

Thanks for sharing your test results.



Mar 29, 2014 at 04:08 PM
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