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07-31-2020, 08:53 PM   #1
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Question about compatibility of APSC-designed lenses with K-1

Greetings; new member here, but four-decades-long Pentax owner, both film and APS-C digital, with about a dozen Pentax lenses.
I recently bought the Pentax DA 20-40mm f/2.8 Limited zoom, but the Ricoh/Pentax website says that it's an APS-C lens (vs. the DFA 24-70mm f/2.8 lens, which it says is designed for FF cameras as well as APS-C). I'm planning to buy a K1 II camera soon, and am wondering what happens when one of these APSC-designed lenses is put on the K1. What problems are there? thanks for the patience, as I'm sure this has come up previously in this forum (but I didn't find it in a quick search)..

07-31-2020, 08:58 PM - 1 Like   #2
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Welcome to the forums!
It will vignette (i.e. your corners will be black or you will see a black circle surrounding the image). You can avoid this by using the crop mode.
07-31-2020, 09:05 PM   #3
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Welcome aboard. The link below will answer most of your questions........................................I hope.

Pentax K-1 Lens Compatibility Guide - Articles and Tips | PentaxForums.com
07-31-2020, 09:10 PM   #4
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As far as I know, by default the camera will use up to 15MP downsampling when cropping APS-C when DA lenses are connected. Optionally, you can enable the use of the entire sensor area.
I heard SMC PENTAX DA 50MM F / 1.8 seems to be without vignette. While this may not be the case..


Last edited by Martin Stu; 07-31-2020 at 09:26 PM.
07-31-2020, 09:51 PM - 2 Likes   #5
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See also this article for tests of each DA lens:

Full Frame Coverage of DA Lenses: Comprehensive Test - Gear Guides | PentaxForums.com

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08-01-2020, 05:46 AM   #6
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In the end , if the image looks good, you've found a good lens.
I own a K-1, but use the DA 21, DA 40 LTD , DA 40 LTD, and DA 18-135 regularly, and often without using crop mode, because the vignetting is minimal to nonexistent with those lenses, and I usually compose knowing I will crop in post.
08-01-2020, 06:11 AM - 1 Like   #7
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QuoteOriginally posted by cometguy Quote
Greetings; new member here, but four-decades-long Pentax owner, both film and APS-C digital, with about a dozen Pentax lenses.
I recently bought the Pentax DA 20-40mm f/2.8 Limited zoom, but the Ricoh/Pentax website says that it's an APS-C lens (vs. the DFA 24-70mm f/2.8 lens, which it says is designed for FF cameras as well as APS-C). I'm planning to buy a K1 II camera soon, and am wondering what happens when one of these APSC-designed lenses is put on the K1. What problems are there? thanks for the patience, as I'm sure this has come up previously in this forum (but I didn't find it in a quick search)..
@BruceBanner showed some images and commented on using this lens in K-1 FF mode. You may look for these posts.

08-01-2020, 11:45 AM   #8
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Welcome to the forum cometguy. Have you been able to catch a glimpse of Neowise? Members have posted some great shots.
08-02-2020, 12:22 PM - 2 Likes   #9
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QuoteOriginally posted by Apet-Sure Quote
Welcome to the forum cometguy. Have you been able to catch a glimpse of Neowise? Members have posted some great shots.
Yes, I've been able to see the comet on numerous nights. I only have a K10D (but am getting a K-1 Mk II soon) so I don't have the astrotracker feature and I just bought an iOptron ProGuider Pro clockdrive to mount my camera on (but it arrived in bright moonlight so haven't had a chance yet to really test it -- though it tracks stars well (I've tested it in moonlight, but no good pics of the comet).
Attached is a photo that I took of the comet before moonlight, perhaps 10 days ago, taken with my K10D camera and my DA 12-24-mm f/4 lens on a tripod (no tracking), exposure 20 seconds wide open; this photo is interesting because I caught a meteor up near the bowl of the Big Dipper, as well as the comet just above the central tree (taken overlooking the Atlantic Ocean from Gloucester, Massachusetts).
Attached Images
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PENTAX K10D  Photo 

Last edited by cometguy; 08-02-2020 at 01:18 PM.
08-02-2020, 11:04 PM   #10
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Nice, @cometguy . You'll find that on the K-1 that DA12-24 works from 17mm onward.

BTW, if you still use your film cameras, that's a good way to test your lenses for yourself.
08-03-2020, 02:06 PM - 1 Like   #11
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QuoteOriginally posted by clackers Quote
Nice, @cometguy . You'll find that on the K-1 that DA12-24 works from 17mm onward.

BTW, if you still use your film cameras, that's a good way to test your lenses for yourself.
I do still use my LX occasionally. So the K-1 sensor is supposed to be the same size as the area exposed on 35-mm film?
08-03-2020, 02:19 PM - 1 Like   #12
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QuoteOriginally posted by cometguy Quote
I do still use my LX occasionally. So the K-1 sensor is supposed to be the same size as the area exposed on 35-mm film?
Yes the full frame sensor is the same size as the slr negative

QuoteQuote:
Sensor Size 35.9 x 24.0 mm (35mm full frame)
Read more at: https://www.pentaxforums.com/reviews/pentax-k-1-review/specifications.html#ixzz6U5p7h4mU

Last edited by aslyfox; 08-03-2020 at 02:25 PM.
08-12-2020, 04:44 PM   #13
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QuoteOriginally posted by cometguy Quote
Yes, I've been able to see the comet on numerous nights. I only have a K10D (but am getting a K-1 Mk II soon) so I don't have the astrotracker feature and I just bought an iOptron ProGuider Pro clockdrive to mount my camera on (but it arrived in bright moonlight so haven't had a chance yet to really test it -- though it tracks stars well (I've tested it in moonlight, but no good pics of the comet).
Attached is a photo that I took of the comet before moonlight, perhaps 10 days ago, taken with my K10D camera and my DA 12-24-mm f/4 lens on a tripod (no tracking), exposure 20 seconds wide open; this photo is interesting because I caught a meteor up near the bowl of the Big Dipper, as well as the comet just above the central tree (taken overlooking the Atlantic Ocean from Gloucester, Massachusetts).
re: Your review of the DA 20-40 Ltd, did you update your K10D's firmware to version 1.31?

Pentax K10D and HD Pentax-DA 20-40mm F2.8-4 ED Limited DC WR - Compatibility | PentaxForums.com

Downloads - Ricoh
08-16-2020, 09:24 PM - 1 Like   #14
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What I don't get is that the pixel size in the KP is 61% smaller than the pixel size in the K-1. That means better resolution in the KP than in the K-1. Why doesn't Pentax use the same pixel size in the K-1, at least? I've not been able to find pixel size in any Ricoh literature or on the Pentax Forums specs for these cameras, so I calculated it myself. In astronomy imaging, pixel size is advertised on all cameras because it is extremely important. So the APS-C crop in the K-1 has lower (worse) resolution by a large amount (1.6x) compared to the full-size APS-C sensor in the KP. I want to know why, in a camera that costs twice as much.

---------- Post added 08-16-20 at 09:26 PM ----------

QuoteOriginally posted by luftfluss Quote
I have not yet had time to update the firmware, but it's on my list (low priority because I rarely use autofocus; I prefer to manually focus). I modified my review to reflect the comments I've received about my lack of autofocus with my non-updated K10D.
08-17-2020, 02:58 PM   #15
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QuoteOriginally posted by cometguy Quote
That means better resolution in the KP than in the K-1
Sure the KP has more pixels per area than the K1/K1II. But they are two different cameras. I have yet to find the limits of my K1 resolution in 4 years of use. More MP by itself is a goal the major manufacturers stopped chasing a few years ago.

QuoteOriginally posted by cometguy Quote
Why doesn't Pentax use the same pixel size in the K-1, at least?
Price. The K3 was released a couple of years before the K1. The K3 had a 24MP sensor. If Pentax were to have made the K1 with the same sensor as the K3, but cut from a larger slice of the "wafer" so as to make it FF, it would have been over 50MP. It would also have been maybe 4 times the price at launch. The bigger the sensor you cut from the "wafer", the more expensive it becomes, and it is not a linear curve. It is an exponential one, due to wastage. The K1 with a launch price of £6,000 would not have been the same camera.
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