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11-07-2020, 02:39 AM - 1 Like   #1
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Hello and a couple of observations

Hello,

I'm a Pentax dslr owner since 2007 or so, just upgraded to the kp and getting back into things after a lot of things going on the past 5 years or so. I had been aware of these forums for a while, but finally subscribed.

Observation #1. I've recently been picking up some cheap used older or manual lenses. Not all are great, but I've been in the process of doing some testing with some charts to test sharpness and see what I want to use regularly or where the strengths and weaknesses are.

I was really shocked to find out how much different the magnification is for lenses of supposedly the same (or similar focal lengths). For example, I tried the Tamron Adaptall-2 80-210mm set at 80mm lined up on a chart. I thought, hmm, now I should compare with this
SMC Pentax-FA 28-90mm to see if it's as bad as the reviews say. I didn't move the tripod and the pentax set at 90 was significantly wider angle than the Tamron at 80! I could expect small variations, but this was a crazy difference and the supposedly longer one was wider angle. What's going on there?

Observation #2. I've been wanting to pick up a decent 28mm prime for cheap-ish, so I ordered a Vivitar 28mm f2.5 and a Chinar 28mm f2.8 off ebay or similar sites for cheap (I think less than $25 each, including shipping).

I kind of like the Vivitar since it gives everything a kind of creamy look and the condition was absolutely pristine with really no detectable wear at all and came in its original box from probably 35 years ago or something. Like I've received new items in worse condition. But it's a little soft if you look at details closely with that 24 megapixel sensor. Still, it could be fun to use in the right situations.



The Chinar seems like the worst lens I own. The best I can get from it looks slightly out of focus in medium to small details across all f-stops. Idk if my copy is bad or it has been disassembled or messed up or if they're all like that. But it's really borderline unusable. I don't see problems in the glass, scratches or fungus or anything, though. It also flares like crazy. Idk why it's so bad, really. I wish it were better.

I'd rather find an inexpensive 28mm that feels a little more all-purpose than the creamy Vivitar or the unusable Chinar. Any suggestions?



Oh and a random photo from a 135mm Tele-lentar with dual aperture rings I picked up. I was very confused until I figured out how that worked . I think that lens is pretty good for the price I got it for.



11-07-2020, 03:24 AM - 1 Like   #2
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Welcome Scott! Enjoy your KP - it's a great camera.

Now to your first query. Focal length is measured at infinity focus. Focussing considerably closer (like one does at a test chart) can change things significantly.

As for your 28mm lenses, the Pentax A 28/2.8 can be had pretty cheaply. It is optically identical to the highly regarded F 28/2.8, which I have owned and found to be a brilliant lens.
11-07-2020, 03:29 AM - 1 Like   #3
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Welcome to the forums!
So, for your first observation, I guess your test chart is set up fairly close to the lenses, right? If that is the case, keep in mind that a lens' focal length is given when focused at infinity - if you focus closer the length may change depending on the lens; this is known as "focus breathing". Some lenses exhibit a very strong breathing effect, some less.
11-07-2020, 05:53 AM   #4
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Oh, thanks. I hadn't thought about the focal length being measured at infinity and that making such a difference. Yeah, I'm testing in a medium size room and at 80-ish mm, I'm only a few yards away. Also, one at the start and one at the end of its zoom.

I'm really liking the kp and find it gives me an extra stop of sensitivity before it's too noisy compared to my old k-x. The shake reduction also gives me something extra, I think. In reality liking it so far.

I'll keep an eye out for the A 28/28 and see if I can pick one up used.

Oh, and I got my photos reversed. The Vivitar is second and the tele-lentar first. :-)

11-07-2020, 07:29 AM - 1 Like   #5
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QuoteOriginally posted by tealbadger Quote
Oh, thanks. I hadn't thought about the focal length being measured at infinity and that making such a difference. Yeah, I'm testing in a medium size room and at 80-ish mm, I'm only a few yards away. Also, one at the start and one at the end of its zoom.

I'm really liking the kp and find it gives me an extra stop of sensitivity before it's too noisy compared to my old k-x. The shake reduction also gives me something extra, I think. In reality liking it so far.

I'll keep an eye out for the A 28/28 and see if I can pick one up used.

Oh, and I got my photos reversed. The Vivitar is second and the tele-lentar first. :-)
If you cannot find a Pentax, Tokina has made a number of very decent 28/2.8 lenses, and the Cosinas should also be fairly good! The Cosina (also branded as Exakta) 24/2.8, if you'd like something a tad wider, is also very well regarded: Cosina 24mm f/2.8 MC MACRO Lens Reviews - Cosina Lenses - Pentax Lens Review Database
11-07-2020, 08:39 AM   #6
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Is “focus breathing” a norm? It effectively changes the perspective of a photo. So how can I know if and how much my lens is affected??
Of course, welcome !

Last edited by Michail_P; 11-07-2020 at 08:59 AM.
11-07-2020, 08:52 AM - 1 Like   #7
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welcome to the forums

you will find friendly members and a lot of knowledge here

If interested in " experienced " equipment be sure to check out the forums' marketplace " buy/sell "

it can be sorted by country:

The Pentax Marketplace | Buy & Sell Pentax Cameras and Lenses (United States) - PentaxForums.com

I found my Kino Precision Japan Kiron 28mm F2 MC P/KA on the marketplace

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you might consider getting the E book for the KP:

Pentax eBook Downloads - Now on PF! (purchase here for instant access) - Page 7 - PentaxForums.com

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FYI:

Pentax 28mm primes:

https://www.pentaxforums.com/lenssearch/?minfl=28&maxfl=28&inproduction=0


Last edited by aslyfox; 11-07-2020 at 09:03 AM.
11-07-2020, 08:56 AM   #8
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QuoteOriginally posted by tealbadger Quote
Oh, thanks. I hadn't thought about the focal length being measured at infinity and that making such a difference. Yeah, I'm testing in a medium size room and at 80-ish mm, I'm only a few yards away. Also, one at the start and one at the end of its zoom.

I'm really liking the kp and find it gives me an extra stop of sensitivity before it's too noisy compared to my old k-x. The shake reduction also gives me something extra, I think. In reality liking it so far.

I'll keep an eye out for the A 28/28 and see if I can pick one up used.

Oh, and I got my photos reversed. The Vivitar is second and the tele-lentar first. :-)
Welcome to the forums, and a quick note about posting, please observe the heading for your posts this one probably should be under discussion threads!
11-07-2020, 02:33 PM - 1 Like   #9
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Welcome from Rome, Italy.
11-07-2020, 06:21 PM - 1 Like   #10
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Welcome to the forum from Berlin!
11-07-2020, 09:22 PM - 2 Likes   #11
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Thanks for the welcomes. I'm happy to be here.
11-08-2020, 05:56 AM - 1 Like   #12
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Welcome aboard, enjoy your time here
11-08-2020, 06:32 PM - 1 Like   #13
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Welcome to PF from Tennessee. Looks like you’re question was answered. I hope you’ll write some reviews and I look forward to reading them!
11-16-2020, 02:16 AM - 1 Like   #14
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QuoteOriginally posted by tealbadger Quote
I was really shocked to find out how much different the magnification is for lenses of supposedly the same (or similar focal lengths). For example, I tried the Tamron Adaptall-2 80-210mm set at 80mm lined up on a chart. I thought, hmm, now I should compare with this
SMC Pentax-FA 28-90mm to see if it's as bad as the reviews say. I didn't move the tripod and the pentax set at 90 was significantly wider angle than the Tamron at 80! I could expect small variations, but this was a crazy difference and the supposedly longer one was wider angle. What's going on there?
I don't know about the difference in length of the two lenses, but the FL would be from the front of the lens, so you'll get an exact picture at infinity. On a test chart two or three meters away, you may see the kinds of deviations you describe.

Another thing is focus breathing.

Very few reviews actually test the focal length of a lens. DPReview did this for a very short while (i.e. just a few lens tests), and if memory serves, the deviations found from the advertised FL were generally within 2mm.

QuoteOriginally posted by tealbadger Quote
The Chinar seems like the worst lens I own. The best I can get from it looks slightly out of focus in medium to small details across all f-stops. Idk if my copy is bad or it has been disassembled or messed up or if they're all like that. But it's really borderline unusable. I don't see problems in the glass, scratches or fungus or anything, though. It also flares like crazy. Idk why it's so bad, really. I wish it were better.
I think the hunt for a lens that's less sharp than a pinhole is still on - maybe your copy qualifies.
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