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02-04-2015, 10:48 PM   #1
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Full Frame and Crop Mode

I get that everything is speculation at this point, since we don't know how Pentax's design will work, but are there benefits to using an APS-C lens on a full frame camera in crop/compatibility mode?

I get that it doesn't make the best use of the full frame sensor. The biggest attraction for me for full frame is the superb high ISO performance it offers. Would that still be retained in compatibility mode? Looking at pixel density, a full frame sensor at 36MP has 33% more pixels than a 24MP sensor but nearly twice the area. The pixel densities are:

16MP APS-C on K-5: 43.752 px/mm^2
24MP APS-C on K-3: 65,641 px/mm^2
36MP FF on K??: 41,666 px/mm^2

If ISO performance is tied to pixel density heavily, then it would mean the FF sensor would only do a bit better than the K-5. (Of course, implementation and the sensor matter *a ton*.) I assume that the performance would still hold in crop mode, no?

02-04-2015, 10:54 PM   #2
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If the full frame sensor is 36MP, in crop mode it'll be 16MP. If it's literally a larger version of the k-5 sensor, then it'll be like a k-5 in crop mode. Case in point, the D800 and the D7100.

The use of the crop mode is just to allow users who own DA sensors that do not cover the full 35mm sensor to use their lenses without making their lenses completely obsolete.
02-04-2015, 11:07 PM   #3
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I think you mean 24 MP in crop mode? 36 / 1.5 = 24?
02-04-2015, 11:10 PM   #4
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I made a typo, not D7100, D7000. I'm pretty sure the D800 crop mode outputs 16MP. It's like 1.5x1.5 = 2.25, 36MP/2.25 = 16MP.

02-04-2015, 11:39 PM   #5
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QuoteOriginally posted by JinDesu Quote
It's like 1.5x1.5 = 2.25, 36MP/2.25 = 16MP.
No that is incorrect. We are dealing with a 1.5 crop sensor, Relative to full frame. 36 / 1.5 = 24. The IQ and noise should be similar to a K-3, In crop mode. The main benefit will be a wider field of view, at a higher resolution. a 50mm lens will act like a 50mm lens. A lens with a shorter focal length will give you a wider view. However a longer focal length will not give you the same magnification as a crop sensor. And that is the main drawback to a FF.
02-04-2015, 11:55 PM   #6
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QuoteOriginally posted by JinDesu Quote
The use of the crop mode is just to allow users who own DA sensors that do not cover the full 35mm sensor to use their lenses without making their lenses completely obsolete.
?? I don't see the DA lenses being in any way obsolete on a FF body, with or without built in crop
mode. Cropping can be done in PP, which I would plan to do any way, with or without crop mode
built into the camera. And some of those DA lenses purportedly fill a FF frame just fine.
02-05-2015, 12:41 AM - 1 Like   #7
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QuoteOriginally posted by promacjoe Quote
We are dealing with a 1.5 crop sensor, Relative to full frame. 36 / 1.5 = 24. The IQ and noise should be similar to a K-3
A FF sensor has 36 mm x 24 mm = 864 mm˛. An APS-C sensor has 23.5 mm x 15.6 mm = 366.6 mm˛. The APS-C sensor thus covers only 42.4 % of the FF sensor's area. Assuming that the FF sensor has 36 MP, the cropped picture will therefore have 36 x 0.424 = 15.26 MP. The FF will be comparable to the K-5 in crop mode, not to the K-3.
02-05-2015, 12:45 AM - 1 Like   #8
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QuoteOriginally posted by promacjoe Quote
No that is incorrect. We are dealing with a 1.5 crop sensor, Relative to full frame. 36 / 1.5 = 24.


Pixel density is constant throughout the sensor surface. Total "megapixel count" is the product of sensor area and pixel density. The 1.5 crop factor refers to linear dimensions, not to areas, so the ratio of the areas of an FF and APS-C is 1.5x1.5=2.25. The scaling factor is thus 2.25, yielding a 16MP resolution for a 36MP FF sensor operating in APS-C "crop mode" .

Someone already replied while i was writing


Last edited by simon_199; 02-05-2015 at 12:51 AM.
02-05-2015, 12:53 AM   #9
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If any doubt remains, why not look up an existing example?

Officially the D800 has a 36.1MP sensor with 25MP 1.2x crop mode and 15.4MP DX crop (Nikon speak for APS-C) mode.
02-05-2015, 01:12 AM   #10
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QuoteOriginally posted by digitalCG Quote
If any doubt remains, why not look up an existing example?
There is no doubt, there's just correct calculation and incorrect calculation To be comparable to the K-3 in crop mode, a FF Pentax must have 56.6 MP - now that would be something
02-05-2015, 01:47 AM   #11
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From the camera manuals:
DX crop mode in the 24MP Nikon D610 results in a 3936 X 2624 pixel image - 10.3MP.
DX crop mode in the 36MP Nikon D810 results in a 4800 X 3200 pixel image - 15.4MP.
02-05-2015, 02:21 AM   #12
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And it feels likely that the FF will use the same sensor as the D810.
02-05-2015, 02:44 AM   #13
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The only reason to use crop mode on the new FF would be if it's your only body. I'm sure most of us who buy it would keep an APS-C body anyway.
My K-3 will remain the natural environment for the long lenses. Wide to portrait length is the preserve of the larger sensor.
02-05-2015, 02:55 AM   #14
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in another thread it was already mentioned that it will be the 36 mp sensor
02-05-2015, 04:15 AM - 1 Like   #15
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QuoteOriginally posted by MadMathMind Quote
a full frame sensor at 36MP has 33% more pixels than a 24MP sensor
36MP is 50% more than 24MP.

QuoteOriginally posted by MadMathMind Quote
If ISO performance is tied to pixel density heavily
But it's not. It's tied to sensor area.
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