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04-04-2013, 03:31 PM   #1
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Pentax Q macro

Here are a couple of boring shots of my stainless steel ruler taken with the Q and then the K-5. The lens used was the Tamron Adaptall-2 SP 72B, 90/2.8 macro at it's MFD (slightly more than 1:1). Contrary to popular belief there is no increase in the magnification ratio, the image magnification is all due to the crop factor:

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Pentax Q's sensor size is 6.17 x 4.55mm, as you can see in the image, there is slightly more than 6mm shown on the ruler.

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Pentax K-5's sensor size is 23.4 x 15.6mm and the image shows slightly more than 23mm.

04-04-2013, 06:10 PM   #2
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QuoteOriginally posted by excanonfd Quote
Contrary to popular belief there is no increase in the magnification ratio, the image magnification is all due to the crop factor:
Well, we might say the crop leads to an increase in apparent magnification, but of course you're right that the magnification ratio is, by definition, independent of sensor size. What the Q gives is an increase in usable resolution, hence an advantage over cropping from a lower-resolution sensor. This is not to be sneezed at. For one thing, it means you can get an equivalent image (compared to that from a larger sensor) with lower magnification and hence greater working distance.
04-04-2013, 08:27 PM   #3
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QuoteOriginally posted by baro-nite Quote
Well, we might say the crop leads to an increase in apparent magnification, but of course you're right that the magnification ratio is, by definition, independent of sensor size. What the Q gives is an increase in usable resolution, hence an advantage over cropping from a lower-resolution sensor. This is not to be sneezed at. For one thing, it means you can get an equivalent image (compared to that from a larger sensor) with lower magnification and hence greater working distance.
You are right of course, I should have shot and posted this with the opening thread: An equivalent FOV of the Q vs the K-5 at 1:1 magnification - to achieve the same with the Q, the sensor plane is a good 0.5m away from the subject.
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04-05-2013, 01:11 AM   #4
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QuoteOriginally posted by baro-nite Quote
Well, we might say the crop leads to an increase in apparent magnification, but of course you're right that the magnification ratio is, by definition, independent of sensor size. What the Q gives is an increase in usable resolution, hence an advantage over cropping from a lower-resolution sensor. This is not to be sneezed at. For one thing, it means you can get an equivalent image (compared to that from a larger sensor) with lower magnification and hence greater working distance.
. . . and with greater working distance comes deeper DOF for a given aperture on a given lens and a flatter angle for on-camera flash for handheld shooting.

Personally, I'm planning on trying shorter FL dedicated macros for a much lighter and more compact kit. I shoot a Sigma EX 180 f3.5 Macro with an F 1.7x AFA on my DSLRs, but will probably go with either my D-FA 100 f2.8 or my A 50 f2.8 for handheld work with the Q for critters.

Scott

04-05-2013, 04:23 AM   #5
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QuoteOriginally posted by snostorm Quote
and with greater working distance comes deeper DOF for a given aperture
I'm no longer convinced of this. The extra resolution cuts both ways here. I did a test with my new 10x rig on the K10D and then compared that to using the same rig at 3.5x on the Q. Images were roughly equivalent, and DOF was about the same.

[Edit]: I should note that another factor here is that while I used the same nominal aperture in both cases, the change in magnification meant that the effective aperture was much larger (i.e., smaller f-number) with the Q shots. This particular rig is fixed aperture so I can't stop down, but even if I could I wouldn't want to, because of diffraction. That is, by reducing both magnification and sensor size to keep an equivalent field of view, but keeping nominal aperture constant, I get a similar degree of diffraction softening. So what I am saying is that practically speaking there is no DOF advantage.

Last edited by baro-nite; 04-05-2013 at 04:30 AM. Reason: Addendum
04-05-2013, 12:57 PM   #6
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Hi baronite,

I'm talking about use with dedicated macro lenses only, not extreme macro setups with ext tubes. reversed lenses, or diopters which alter Min Focusing Distance and/or magnification. Longer working distance will always give you deeper DOF, and the Q's crop allows us to maintain subject to frame size relationships from larger sensor formats while using longer working distances with dedicated macro lenses that have the ability of attaining focus at any point from MFD to infinity.

Let's say we have a 6mm subject. At 1:1 on an APS-C, it will fill @ 1/4 of the frame horizontally. With the Q, it will almost fill the frame horizontally at 1:1, but back off so it only fills 1/4 of the frame, giving you the same subject to frame proportions as the APS-C shot, and you will have deeper DOF for a given aperture because of the greater distance from the lens and sensor plane.

Scott
04-05-2013, 03:36 PM - 1 Like   #7
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QuoteOriginally posted by snostorm Quote
Longer working distance will always give you deeper DOF
At the same aperture, yes. But on the Q you can't stop down very far before diffraction kills detail.

I'm very curious about this so did a quick test. Using the Sigma 70/2.8 macro, I did some 1:1 shots on the K10D, and then some shots with the same lens on the Q, trying to keep the subject the same size similar, relative to the size of the sensor. Didn't get it exactly but close enough for this test, simply meant as a rough comparison. These are 50% crops from the K10D and ~44% crops from the Q. If you're on a wide monitor, make the window wide and you can view pairs side-by-side.

Wide open and the difference in DOF is indeed dramatic:

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Stopping down to f/16, the K10D is still reasonably sharp, whereas the Q is total mush:

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Now here's f/5.6 and f/8 on the Q. I think the f/5.6 shot compares well to f/16 on the K10D in both DOF and resolution. f/8 on the Q is significantly softer.

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Again, this is a rough comparison. But it is enough to convince me that the DOF advantage on the Q is pretty much completely countered by diffraction. So the Q's main advantage for macro is working distance, and the ability to use ordinary lenses for near-macro without extension or diopters. I repeat, this advantage is not to be sneezed at.

Last edited by baro-nite; 04-05-2013 at 05:40 PM.
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