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03-23-2010, 01:26 AM   #1
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Is the DA 70mm F2.4 Ltd not really F2.4 and is actually F2.8?

I own the DA 70mm and have recently read a blog post comparing the DA 70 and FA 77 and the author mentioned:

Stopped at f/2.4, the FA 77 seems to expose about 1/3 stop faster than the DA 70set to the same aperture and the DOF is narrower, which makes me think that the DA 70 is closer to f/2.8 than the claimed f/2.4 wide open.

Digital SLR and Lens Review: By popular demand: Pentax DA 70mm Limited compared to FA 77mm

Does this means that DA 70mm is not really F2.4 and is actually F2.8? If this is truely the case, isn't Pentax abit misleading when they label the lens as F2.4 when its not?

03-23-2010, 01:42 AM   #2
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Better test

How about testing DA*50-135mm at 70mm f2.8, then DA70mm f2.8 with same shutterspeed. They should have equal brightness.

Now use DA70mm @F2.4 and set to same shutter speed. If indeed the DA70mm is a true F2.4 the picture will be brighter.
03-23-2010, 02:26 AM   #3
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I would like to do so but I do not have the DA*50-135mm.. Anyone who has both lenses care to test this?

QuoteOriginally posted by howieb101 Quote
How about testing DA*50-135mm at 70mm f2.8, then DA70mm f2.8 with same shutterspeed. They should have equal brightness.

Now use DA70mm @F2.4 and set to same shutter speed. If indeed the DA70mm is a true F2.4 the picture will be brighter.
03-23-2010, 04:41 AM   #4
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My DA 70 exposes the same as the DA 50-135 at 70 mm and f2.8. I don't believe that the aperture is wrong. The only thing that I would believe about it is that it isn't quite 70 mm long (maybe 67 or 68?), but I still love the lens. Still, definitely half stop faster than the 50-135 at 70 mm.

03-23-2010, 09:52 AM   #5
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Some time ago when the Samyang 85/1.4 was first released, there was some talk on that is was 'darker' than other 85/1.4 offerings, in that case ascribed to less clear glass elements. I'm not saying that this is the case here, but not all lenses transmit the same amount of light with the same size aperture. The technical term here is 'transmission stop' I believe, this is the way to compare exposure. (But if it had to do with T-stops, I'd guess the DA70 would perform better than the DA50-135, on account of the fewer lens elements.) T-stops do not influence the DOF, but the DOF effect could be ascribed to the different focal length of the FA77. I'd say DOF is very difficult to measure absolutely anyway.

I hope this helps
03-23-2010, 10:07 AM   #6
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I do not own the Da 70mm, but have seen this kind of result when testing other lenses. There is definitely something going on with variations of light getting to the sensor, at the same focal lengths & same aperture, between different lenses.
03-23-2010, 12:45 PM - 1 Like   #7
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Right - differences in glass transmission can come into play. Also, DOF would be shallower on the FA77 because it's longer. I agree the DA70 may or may not be *exactly* f/2.4, but it's pretty clearly brighter than my f/2.8 lenses (none of which are 70mm) by something that closely resembles half a stop. Of course, f2.4 isn't the precise figure for half a stop between f/2 and f/28, either, although it's the figure Pentax traditionally uses. But mathematically, f/2.4 *is* a little less than half a stop better than f/2.8 (the actual halfway point would be f/2.3784...)
03-23-2010, 09:05 PM   #8
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f2.4 is f2.4, nothing to compare there, f2.4 is the measure of the geometric diaphragm, other thing it the T stop, that's the one who tells you exactly how much light reach the sensor. they are close enough to use just de f stop, but they are not the same because the differences in glass transmission.

(sorry for my english!)

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