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10-21-2010, 11:41 PM   #1
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Is the 50mm A1.7 faster than F1.7?

I've been taking lots of pics of with my da21 lately. I really like it a lot but it sucks real bad for indoor candids, unlike my a1.7.Even with the 50mm at f4 and the 21mm at f3.2, the 50mm is brighter and more colorful. With very bright lights or the sun


Last edited by troglodyte; 10-21-2010 at 11:47 PM.
10-21-2010, 11:47 PM   #2
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as a light source, the two lenses tend to look rather equal. But for candle type lighting, even when f-stops are the same, my A1.7 is like a blowtorch. What gives? My da21 looks muted in those situations.
10-22-2010, 06:20 AM   #3
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I;ve come to realize that different lenses let in different amounts of light at the same aperture. It Aperture has to do with the size of the opening, but light is adsorbed and reflected when it passes through each element, and different optical designs have different numbers of elements / different coatings etc.
10-22-2010, 11:38 AM   #4
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Paperbag,
Then is there a light transmission rating for apertures for lenses? Even though I love my da21 I need a lens that's between 21 and 35mm that has the same light transmission qualities as my A1.7 at f3.2-4 even. Manual wouldn't be a problem.


Last edited by troglodyte; 10-22-2010 at 11:51 AM.
10-22-2010, 11:58 AM   #5
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The problem is the da21 doesn't show full advantage of it's light transmission as you call it until f5.6 and keeps it until f11. While I can get very worthy pics out of my a1.7 from f2.2. It's very hard because this isn't in a chart for a lens nor reviews
10-22-2010, 02:53 PM   #6
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I believe there are simply too many variables, including sensor sensitivity, for this to be really worth doing. Better would be to simply try the lenses out and compare, but it can feel like splitting hairs.

"Some lenses, mainly those used in motion cinematography and not still photography, have apertures rated in T-stops. T-stops indicate the actual or absolute amount of light being passed through (transmitted) the aperture diaphragm. They differ therefore from f-stops, which indicate the aperture size relative to the focal length. (in a sense, T-stops are absolute and f-stops are relative)."

I.e., as you change focal length f-stops become relatively smaller as you go wide and larger as you go long. Now two 28mm lenses may transmit different amounts of light at f4 due to a differing number of elements, but additionally, you might have a problem if you go wider, as f4 is actually smaller. Correct me if I am wrong, but it seems that around 50mm is a "sweet spot" where you have the ability to made the front element quite large due to the normal field of view (i.e, light does not need to bend as much as a wide-angle). Additionally the design allows for super-wide apertures. I've heard that 85mm 1.4's are essentially light-vacuums, but good luck using them indoors comfortably!

Compare the front element of your 50mm and your 21mm and you might see what I mean.

In any case, I would suggest that it is the focal length's fault here, and maybe not transmittance. I would go for a narrower focal length, maybe 35 would be a good compromise for indoor work. I know there is the FA 35 2.0, and the new DAL 35 2.4. If you can, try them out before you commit.

And please keep the 21, it's supposed to be a wonderful lens, albeit not exactly a low-light wonder .

Hope this all is accurate!
10-22-2010, 04:12 PM   #7
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paperbag,
I used to fool around with my grandmother's Argus rangefinder with a 35mm lens. So the 21 is like home for me as a focal length on my k-7. But metering, the A1.7 feels more natural once stopped down. Maybe I could live with a 24mm but any more
10-22-2010, 04:16 PM   #8
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feels like I'm a voyeur or birding. If I'm on a hike, I love bringing along my ISCO 135mm. BTW, no plastic fantatics for me. If I wanted that I'd be just another Nikon whore buying that garbage at Costco or this old dude who was talking to me and saying

10-22-2010, 04:19 PM   #9
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how alike we were with our SLR cameras but he never heard of Pentax and I didn't want to get into who and what my camera was compared to his Rebel with plastic mega zoom to my all metal/rubber body and metal/glass lens.

Last edited by troglodyte; 10-22-2010 at 04:33 PM.
10-22-2010, 04:24 PM   #10
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Yes, the T-stop. Thats always pissed me off! The K-7 uses T values for metering. But AFAIK, Pentax lenses use f values. So how that translates to your picture is beyond me. That's why I asked the title. If the A or DA lenses are rated different. It seems
10-22-2010, 04:29 PM   #11
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once the k-7 goes over iso 640 and dependind if you push or pull with the shutter your results get better. But no matter what even when the A lens is stopped down a whole stop or more the DA still can't catch up for colors in low light!
10-22-2010, 11:11 PM   #12
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While I'm not sure I totally agree with your distaste for plastic lenses (as that severely limits you), you do have a few options.

Manual focus:

The A 20mm 2.8 MIGHT fit the bill. I'm not sure about this and have never tried it myself. However reviews state that it is pretty sharp wide open, and that is pretty much the best aperture you are going to get in that focal length.

Any of the 28mm 2.0's might also fit the bill, being slightly brighter for low light, but a little longer than you want.

Autofocus:

The FA* 24mm 2.0 might work.... it looks like a very nice lens.

Finally you might want to consider the FA 31 1.8 LTD if money is no object - I have never heard anyone complain about the lens for wide angles / indoors and it is plenty fast.

These are the only metal primes in the general range you want that I can think of.

The absolute ideal for you, IMHO, would be the A 50 1.2... on film. But that's a completely different ball of wax.
10-23-2010, 12:03 AM   #13
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Paperbag,
My ideal if Pentax came out with a FF digital would be the FA31.
I guess if I had unlimited funds, Leica would be my best fit with all its ultra wide sub f2 lenses.
You know anything about m42? Olympus couldn't have been the only one back then
10-23-2010, 12:08 AM   #14
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with a smallish Zuiko 21mm f2 prime. I'd be all over the fa*24. But, bazooka lenses aren't my thing!
The reason I dislike plastic so much is it feels so wrong. That's why I got the k-7. It was the cheapest all magnesium alloy and rubber body out there at
10-23-2010, 12:11 AM   #15
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at the time. Plus, if I wanted to mate the body from canon or nikon with similar glass not plastic element lenses, I'd have to move to Asia and sell some of my body parts first.
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