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10-01-2011, 10:58 AM   #1
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Pentax Q crop factor

The Pentax Q lens specification for the 01 prime lens states that it is equivalent to a 47 mm lens in the 24x36mm format.

I don't understand that claim, unless some other of their data are wrong.

Here is my math:

Sensor size: 1/2.3" diagonally which equals 11 mm.

The diagonal of the 24x36mm format is 43.3 mm

The Q crop factor is thus 43.3 / 11 = 3.9

The 01 lens has a focal length of 8.5 mm, which would then correspond to 8.5 x 3.9 = 33mm on a full frame camera.

A 33 mm equivalent lens is more suited as a walk around lens than a 47 mm equivalent, so it would make sense were my math correct.

What am I missing?

10-01-2011, 01:25 PM   #2
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The Pentax Q sensor is a 1/2.3", not a 2.3" size. Thus, it has a 7.7mm diagonal and the crop factor against a 35mm frame is 43.3/7.7 = 5.6, yielding the 47/48mm equivalence for the 8.5mm lens.

All the above is rounded and subject to calculation error, hence the 47/48 result quoted.
10-01-2011, 01:40 PM - 1 Like   #3
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QuoteOriginally posted by shades Quote
Sensor size: 1/2.3" diagonally which equals 11 mm.

What am I missing?
1/2.3" is not it's size, but a name. The names used has a long tradition back to when tubes was used and 1/2.3 would indicate the tube's outer diameter and not the image size.

The sensor in Q is 7.7mm in size.
10-01-2011, 03:22 PM   #4
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Since we have the Q under review, I just took two indoor shots, one with the Q, and one with the FA* 28-70 set to 31mm (as per the EXIF) on a K-7. The field of view of the two shots are identical. 31mm on APS-C corresponds to 47 mm on full frame.

So StigVidar nailed it, 1/2.3 " is just a name which has no (obvious) bearing to the actual sensor size.

10-01-2011, 05:08 PM   #5
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Pentax Q's sensor size is 6.17 x 4.55 mm. Hence a 7.67 mm diagonal and a 5.64 crop factor.
10-01-2011, 08:55 PM   #6
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QuoteOriginally posted by Mistral75 Quote
Pentax Q's sensor size is 6.17 x 4.55 mm. Hence a 7.67 mm diagonal and a 5.64 crop factor.
Also note that the Q's sensor's area is ~28mm^2, and APS-C size is 419mm^2. Call those 3cm2 vs 42cm2. That's 14x difference. FOURTEEN TIMES! So, comparing pixel density (a noise culprit) in 12mpx sensors, the Q's is 14x as dense as a Kr's. FOURTEEN TIMES! Or did I figure this wrong? Guide me, o numerate ones.
10-02-2011, 04:52 AM   #7
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QuoteOriginally posted by RioRico Quote
Also note that the Q's sensor's area is ~28mm^2, and APS-C size is 419mm^2. Call those 3cm2 vs 42cm2. That's 14x difference. FOURTEEN TIMES! So, comparing pixel density (a noise culprit) in 12mpx sensors, the Q's is 14x as dense as a Kr's. FOURTEEN TIMES! Or did I figure this wrong? Guide me, o numerate ones.
I think you're wrong about APS-C, your size is for the film, sensors are smaller. AFAIK the K-r has a 23.7 x 15.8 sensor. That's 374mm^2. Assuming you're right about the Q sensor size, that's closer to 13 times larger for the K-r.

Your big mathematical mistake is thinking 10mm^2 = 1cm^2. If you square the unit you square the difference too, 100mm^2 = 1cm^1. But this doesn't invalidate your results in this case, you're still basically right.
12-18-2012, 10:41 AM   #8
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q with 200mm lens

if i interpret all this from you various photomathaticians , if i use a 200mm lens via adapter, then the q would give a 200 x 5.6 = 1120mm. is that the general consensus now? 5.6 crop factor ? so even a 24mm lens would be a 135mm ?

12-18-2012, 11:54 AM   #9
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QuoteOriginally posted by dh4412 Quote
is that the general consensus now? 5.6 crop factor ?
Correct as far as it goes; a 200mm on the Q gets you the same field of view as something like 1120mm on "full frame" (bearing in mind the differing aspect ratios). Personally, as a Pentax DSLR user, I find it more helpful to think in terms of the Q->APS-C crop factor (around 3.7), because that's what I do with my K-mount lenses -- put them on the Q or the K-5.

I'm more interested in what the Q gains us, practically speaking; IMO it is quite a bit less than the "crop factor". You can crop a K-5 file more heavily than a Q file and still get a good result. And of course the shooting experience is different, with the Q generally quite a bit less responsive, plus no AF with adapted lenses.

Video is another story, though. I think macro and ultra-tele video will turn out to be where that crop factor gives the biggest advantage.
12-18-2012, 11:55 AM   #10
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QuoteOriginally posted by dh4412 Quote
if i interpret all this from you various photomathaticians , if i use a 200mm lens via adapter, then the q would give a 200 x 5.6 = 1120mm. is that the general consensus now? 5.6 crop factor ? so even a 24mm lens would be a 135mm ?
I've been using 5.5 as the crop factor just because it is easier for my brain to do the conversions. What's .1 between friends?
But yes, 200mm crops to 1100 equivalent, 300mm crops to 1650 equivalent.
12-18-2012, 12:26 PM   #11
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Just to throw another wrinkle into all of this brain pain, my reference FOV is really APS-C since I've been shooting Pentax DSLRs for over 7 years now, and haven't even looked through a 35mm VF for over 10 years (though I shot 35mm film SLRs for decades before going digital). To get things straight for my reference, I use 3.6x (compared to APS-C) as the Q "crop factor". Not as impressive, as the numbers aren't as big, but it seems to make more sense to my brain. . .I think

Scott
12-18-2012, 06:19 PM   #12
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According to the Silkypix Raw converter that came with the Q the 55-300 zoom at 300 = 1674. Interesting that the software reports this at all.
thanks
barondla
12-18-2012, 09:08 PM   #13
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My calculations indicate that the Q has 10x the pixel density of the K-5. Crop factor would be based on image diagonal which is a ratio of 3.7 vs APS-C. Round numbers.

I think that the pixel density is a real hidden gem of this camera, providing a capability that doesn't exist elsewhere. Noise scales with pixel area so the Q will have higher noise than other cameras and diffraction is a serious problem at fairly fast focal ratios. But, if you can work within those limitations, the higher pixel density is a real asset for some applications, basically macro and telephoto. The K-mount adapter is what makes this exciting to me.
12-18-2012, 11:29 PM   #14
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even with the da 40, being a small lens, you'd have a possible hand-held, lightweight 200mm ?
12-19-2012, 07:21 AM   #15
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QuoteOriginally posted by dh4412 Quote
even with the da 40, being a small lens, you'd have a possible hand-held, lightweight 200mm ?
I'd prefer the 06 zoom for this -- once you put the DA 40 on a K->Q adapter, it's no longer so small.
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