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11-03-2010, 08:49 AM   #1
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Does DxO explain the F2.4 in the new DA L 35mm?

Not sure if this has been discussed already, apologies if so, but have people here on PF discussed this open letter to the industry from Luminous Landscape?

An Open Letter To The Major Camera Manufacturers

In it, with the empirical help of DxOMark labs, they discover that CMOS sensors are physically limited in accepting the light given by large aperture lenses, and consequentially, digital camera manufacturers stealthily increase the sensor ISO over the set ISO to compensate. worse than any noise gain of this sensitivity boosting is that you don't gain any DoF blur by using these extreme aperture lenses, the light rays are just too off-angle to reach modern CMOS sensor wells.

They show a graph where the 'secret' ISO compensation kicks in below F2.8, slowly gaining the sensor sensitivity up as the aperture size increases. the true sensor ISO becomes 1:1 by F2.8 in these tests, but they were testing Canon and Nikon cameras which have no F2.4 lenses.

as many noted, the optical formula of the new DA 35mm superficially looked identical to the old F2 FA 35mm. many wondered why it "lost" a half stop of light. I'm betting that in making this lens a truly "Digitally Optimized" lens, Pentax tuned the aperture and back focus to get the most direct CMOS compatible light without any background sensor ISO increase. In other words, at F2.4 the given ISO is the true ISO of the sensor. anything lower in number than that is effectively lost and compensated by ISO gain.

11-03-2010, 08:56 AM   #2
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I think the only way to test this would be to use them on a film camera, see if the metering gives the same exposures. But my guess is that they made the lens elements smaller.
11-03-2010, 09:18 AM   #3
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Ouch. moved to General Photography?

I'm sorry but shouldn't this at least be moved to DSLR Lens Discussion?
11-03-2010, 10:47 AM   #4
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Without poring over the article, I am either very skeptical of the article or of your summation of it.

Right off the bat, I have a hard time respecting a "technical" article written by someone who can't even spell "lose" correctly.

Claiming that there's no DoF change is patently wrong. The original article implies that there may be "some" but that part sounds like conjecture. Obviously DoF changes - as proven by everyone who's ever done the usual round of test photos at different apertures from any fast lens, including F1.2 ones.

Furthermore - automatic ISO changing? How about when using a manual-aperture lens, like the Cosina F1.2? Or any fast M42, like the 55mm F1.8 or 50mm F1.4? The camera has zero idea what aperture the lens is set at. The photos from these turn out just fine, and I believe at the same shutter speed as when using an auto-aperture lens, which would indicate that it is using the same ISO.

11-03-2010, 11:34 AM   #5
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QuoteOriginally posted by Groucho Quote
Furthermore - automatic ISO changing? How about when using a manual-aperture lens, like the Cosina F1.2? Or any fast M42, like the 55mm F1.8 or 50mm F1.4? The camera has zero idea what aperture the lens is set at. The photos from these turn out just fine, and I believe at the same shutter speed as when using an auto-aperture lens, which would indicate that it is using the same ISO.
Without reading the article, you're not getting it. It's basically saying that modern day lenses do not produce the f-stop they claim, and the manufactures bump the ISO to compensate, and build that into the number. i.e. ISO 800 may actually be ISO 950 or so. Different variables for different cameras. It's a constant for the camera, and is not based on the lens at all.

Anyway, this has all been discussed in like four different threads.
11-03-2010, 04:19 PM   #6
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if you see the title of the thread, i was stating the reasoning as a likely reason for the half-stop 'loss' on the 35mm DA L compared to the FA 35mm
11-03-2010, 04:29 PM   #7
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There is a thread on the open letter where I post results with an f1.4 lens on three generations of sensor. The results are the same and the whole theory breaks down if you consider that there is no difference between CMOS and CCD and two of the sensors tested are also found in nikon and Sony cameras that were reported problematic

I think the bigger issue is that fast lenses are sometimes not quite as fast as advertised

11-03-2010, 04:33 PM   #8
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QuoteOriginally posted by Lowell Goudge Quote
There is a thread on the open letter where I post results with an f1.4 lens on three generations of sensor. The results are the same and the whole theory breaks down if you consider that there is no difference between CMOS and CCD and two of the sensors tested are also found in nikon and Sony cameras that were reported problematic

I think the bigger issue is that fast lenses are sometimes not quite as fast as advertised
sorry could i bother for you for the link? thanks
11-03-2010, 04:37 PM   #9
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Just go a few posts down in this forum. There are two threads discussing this
11-03-2010, 07:06 PM   #10
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QuoteOriginally posted by Rory Quote
Without reading the article, you're not getting it. It's basically saying that modern day lenses do not produce the f-stop they claim, and the manufactures bump the ISO to compensate, and build that into the number. i.e. ISO 800 may actually be ISO 950 or so. Different variables for different cameras. It's a constant for the camera, and is not based on the lens at all.

Anyway, this has all been discussed in like four different threads.
If I'm understanding you correctly, not quite; The point of the article is that DSLR sensors do not capture all of the light from super fast lenses. a Canon EF 50mm f1.2 is exactly as fast as an SMC Pentax 50mm f1.2, but either lens will be slower on digital than film. the f-stop, as in the ratio of aperture to focal length, stays the same, but the t-stop is decreased by the sensor, not the lens.
11-03-2010, 07:13 PM   #11
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QuoteOriginally posted by Super A-wesome Quote
If I'm understanding you correctly, not quite; The point of the article is that DSLR sensors do not capture all of the light from super fast lenses. a Canon EF 50mm f1.2 is exactly as fast as an SMC Pentax 50mm f1.2, but either lens will be slower on digital than film. the f-stop, as in the ratio of aperture to focal length, stays the same, but the t-stop is decreased by the sensor, not the lens.
Right. And the manufactures are countering this by boosting ISO gain artificially to compensate the T-stop. So says the article.
11-04-2010, 04:15 AM   #12
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QuoteOriginally posted by Rory Quote
Right. And the manufactures are countering this by boosting ISO gain artificially to compensate the T-stop. So says the article.
Except that when I measured this on pentax bodies using. K50F1.4 there is no significant difference in exposure when shooting at either F1.4 and 1/640 or F2.0 and 1/320

If the DXO letter was correct in it's facts I should have measured it. What I did measure was vignetting of my 50 mm when wide open. This was less than 1 stop center to corner wide open and gone by F2.0
11-04-2010, 08:36 AM   #13
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QuoteOriginally posted by Lowell Goudge Quote
Except that when I measured this on pentax bodies using. K50F1.4 there is no significant difference in exposure when shooting at either F1.4 and 1/640 or F2.0 and 1/320

If the DXO letter was correct in it's facts I should have measured it. What I did measure was vignetting of my 50 mm when wide open. This was less than 1 stop center to corner wide open and gone by F2.0
Yeah, their study didn't include Pentax; although, they invited Pentax to explain. I didn't quite get that.
11-04-2010, 01:24 PM   #14
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QuoteOriginally posted by Rory Quote
Yeah, their study didn't include Pentax; although, they invited Pentax to explain. I didn't quite get that.
But the point is there is nothing to explain. The only thing I found was vignetting wide open and less than 0.1 ev change in the center of the frame from F1.4 to f8 meaning that the whole thin is BS because two of the three cameras I tested share sensors with nikon and Sony
11-04-2010, 01:34 PM   #15
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QuoteOriginally posted by Lowell Goudge Quote
But the point is there is nothing to explain. The only thing I found was vignetting wide open and less than 0.1 ev change in the center of the frame from F1.4 to f8 meaning that the whole thin is BS because two of the three cameras I tested share sensors with nikon and Sony
I don't think this is explainable through typing.

What I gathered out of the article, which admittedly has holes in it, is that ISO100 on the Canons they tested was not a true, scientifically measured ISO100. It was bumped so that a lens that at f1.4 or f16 has the same equivalent, effective T-stop on digital.
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