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10-13-2021, 03:50 AM   #1
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Pentax K1 and APS-C lenses

I have a Pentax K1 MkI and a few APS-C lenses left over from my K3 MkII. If I use my 55-300mm DA APS-C lens on the K1 in Crop or Auto Mode the image fills the sensor but the camera drops to 15megapixels. If I select Full Frame mode I get vignetting. It seems to me that if I were to use the lens in Full Frame mode then crop afterwards, I will get the full detail of the 32 megapixel sensor in the cropped image rather than 15mp. That seems too simple. The sensor must surely use all the pixels in the centre crop section whatever the setting. Any suggestions?

10-13-2021, 04:18 AM   #2
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If you use 1:1 mode, it will almost fully cover the usable pixels. The center crop on K1 is designed to be usable with all DA lenses, so likely it has the center APS-C Dimensions which happens to have 16MP resolution. If the K1 was 50MP, you'd have around 24mp of an aps-c in that dimension.


It doesn't matter to be honest. Just shoot with your 55-300 and crop later in post. Ignore the vignetting you'll get used to it soon.
10-13-2021, 04:28 AM   #3
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So others may be able to explain better than I can but the K1 in crop mode is just reading the center part of the sensor, its not downgrading the picture quality to 16mp.

If you use the crop factor lens in full frame mode and then crop out the vignetting you will get a different size crop for each lens because every lens has a different amount of vignetting.

The 55-300 may have a larger usable area than some other lenses, but the camera doesn't really know this it just knows you have a crop factor lens on it.
Also you may be be more or less picky about the vignetting than other people so how much you crop will be different than others.


Pentax just decided to use the center (apsc size) part of the sensor because that is what the lens's are designed for.
10-13-2021, 04:30 AM   #4
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QuoteOriginally posted by gwoodward Quote
the image fills the sensor but the camera drops to 15megapixels
15 megapixels is the size of the cropped area on the sensor. The resolution of the camera hasn't "dropped". The pxels are the same size but there are fewer of them because it is a smaller area.

10-13-2021, 05:04 AM - 1 Like   #5
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QuoteOriginally posted by gwoodward Quote
If I use my 55-300mm DA APS-C lens on the K1 in Crop or Auto Mode the image fills the sensor
In crop mode the whole of the sensor is not used, only an aps-c-sized portion of it. That portion has 16MP

QuoteOriginally posted by gwoodward Quote
It seems to me that if I were to use the lens in Full Frame mode then crop afterwards, I will get the full detail of the 32 (36MP) megapixel sensor in the cropped image rather than 15mp
If you crop in PP your final image MP count will depend on how much you crop. If you crop to the same size as the aps-c mode image would have given you, you still only end up with 16MP.

Think of it this way...... The magnification of the lens is identical in both modes. Let us say there is a house in the centre of your scene. That house may cover 5MP on the sensor. It makes no difference whether you use FF or aps-c mode, the house will still be covering 5MP

Last edited by pschlute; 10-13-2021 at 05:10 AM.
10-13-2021, 06:22 AM - 4 Likes   #6
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Don't overthink it, just use the APS-C lenses and enjoy the results.
10-13-2021, 06:59 AM - 4 Likes   #7
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QuoteOriginally posted by robgski Quote
Don't overthink it, just use the APS-C lenses and enjoy the results.
and - Turn Off Crop Mode (unless you are too lazy to do some cropping in PP). Pixels once lost can never be recovered, but saved ones can always be tossed later.

10-13-2021, 10:50 AM   #8
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I generally use APS-C lenses in FF mode like others have said, and crop later. However, depending on the lens, you need to be a bit careful to avoid expecting to use more of the FF area at a longer focal length (vs using a shorter FL and cropping like APS-C mode would have done anyway). In many cases, the vignetting is greater in the final image than the optical viewfinder would suggest.

For example, the DA55-300 RE PLM at 300mm (and longer FLs in general) has a fair bit of vignetting, while 200mm cropped at 1.5x will have basically none, at the cost of lower resolution. So the 300mm FF shot may realistically need to be cropped to a narrower field of view which may eliminate part of the image you want to keep, if the vignetting is distracting to you.

The DA*60-250 doesn't exhibit this behaviour extremely enough to be an issue. The DA*50-135, 20-40, 18-135, and 50-200 also require considerable cropping.

If it matters to you, one disadvantage of FF mode is that lens corrections will not be applied, except for DA lenses identified by Ricoh as fully FF compatible (such as the DA*300). The files are also bigger, burst rate lower, and buffer smaller. None of these bothers me enough to use APS-C mode with DA lenses!
10-13-2021, 03:29 PM   #9
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Thanks guys. As it happens I tried the 55-300mm DA lens today in Full-Frame (FF) mode on my K1 and there was no perceivable vignetting. The images were ok on screen (1920x1080 monitor) but a bit soft at 100%, but then it was only 109 second hand! Thanks again and be snappy.
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