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03-13-2019, 02:45 PM   #1
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A Lenses

Good Day All,

This is one of the moments in life when you just completely lose any thought; a brain fart. I'm probably asking a question most will think is obvious, but seriously, my brain is just not functioning this week.

I have a K-3 Limited. One of my lenses is a Sigma 20mm 1: 1.8 that I use for star shots and landscape. My cam was flashing F-- today and I realized the lens was not set to "A" It got me thinking, and that can just be a bad thing some days because I just overthink. Obviously, as a Sigma lens, I have to put it on the A setting, which prohibits from using the varied f-stops on the lens. Knowing that, is there any disadvantage to utlizing A settings on varied lenses vs using lenses (more often than not, Pentax lenses) that allow you to pic specific settings. In short, is the A (automatic) setting as reliable as the standard setting?


I've never really found discussions on this and was wondering. I do like Pentax, I only have Pentax cams, but have found not all Pentax lenses are great. I've had some under-perform compared to some Sigmas and others which often requires you use the A setting.

Thanks

03-13-2019, 03:08 PM - 2 Likes   #2
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Hello and welcome

Any K-mount lens, whether Pentax, Sigma, Tamron or otherwise, with a dedicated aperture ring and "A" setting on it allows you to set the aperture from the camera's control wheel by setting the ring to the "A" position. Then, from within any of the P, Sv, Tv, Av, TAv, M and B modes, you select the aperture of the lens using the control wheel on the camera. For lenses without a dedicated aperture ring (e.g. Pentax DA lenses), they operate as those with a ring set to the "A" position.

I don't know what Pentax and Sigma lenses you've been using. Generally, I would say most modern Pentax lenses are great. Older models vary, but are mostly OK-to-excellent. Sigma lenses are, in my experience, much more variable in performance depending on whether they're primes, zooms, and the era in which they were developed and released. Generally, I prefer Pentax lenses. But there are exceptions, such as the DA*16-50, where the Sigma equivalent - in this case, the 17-50 f/2.8 HSM - is arguably better optically.

The latest Sigma lenses can be very good indeed, but those that are compatible with Pentax K-mount sometimes have inconsistent or unreliable auto-focusing, as Sigma reverse-engineered Pentax AF operation when designing its lenses, rather than paying for a licence to use the official specification

Last edited by BigMackCam; 03-13-2019 at 03:15 PM.
03-13-2019, 03:13 PM - 1 Like   #3
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The aperture ring is obsolete (on DSLRs). Unless you are reverse mounting the lens or something like that, then simply leave the aperture ring in the "A" setting and then set the aperture using the e-dial (or automatically).

Adam
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03-13-2019, 03:56 PM - 1 Like   #4
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If you have a lens with aperture ring and "A" setting then use the "A" setting. Taking the ring out of "A" severely restricts what exposure modes can be used with a modern DSLR. Basically you are stuck with Manual and need to use stop down metering. In any other mode such as Av , the camera will only use wide open aperture irrespective what you set on the lens.

03-13-2019, 04:14 PM - 2 Likes   #5
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QuoteOriginally posted by Posse96 Quote
Obviously, as a Sigma lens, I have to put it on the A setting, which prohibits from using the varied f-stops on the lens.
I'd like to address only this sentence of yours.

QuoteOriginally posted by Posse96 Quote
Obviously, as a Sigma lens, I have to put it on the A setting,....
The A setting is available on many lenses, not only Sigma. It simply means that you can use your camera to set the lens aperture, you don't have to set it on the lens.

QuoteOriginally posted by Posse96 Quote
....which prohibits from using the varied f-stops on the lens.
The camera is still capable of using all the varied f-stop settings. Nothing is prohibited, provided you ensure you use your camera in one of the modes where changing the F-stop value is possible.
03-13-2019, 05:02 PM   #6
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QuoteOriginally posted by Posse96 Quote
In short, is the A (automatic) setting as reliable as the standard setting?
Good question. Much depends on the aperture ring implementation as well as the aperture actuator implementation for a particular lens. If both are well-calibrated and robust, the two methods of setting aperture are fully equivalent in terms of accuracy and precision for the aperture settings supported by both*. Notice the emphasis on the word "If". The camera does not check the accuracy or precision of its aperture controller against the actual behavior of the lens**. Likewise, even if a lens aperture ring was properly calibrated at the factory, there is potential for falling out of spec with age and wear. It might be good to note that even wide-open, many (most? all?) prime lenses fail to deliver a true f/1.4, f/2, or whatever at infinity and get progressively "slower" at closer planes of focus. Similar problems occur with zooms at different focal lengths. These failings apply to both ways of setting the aperture opening.

Does all this leave us in the weeds? Probably only if we are using an external meter or are a little a**l-retentive about rightness of settings. TTL metering provides compensation for attenuation due to focal length and/or extension for close focus. Also in our favor is that differences from the expected aperture diameter are usually quite minimal at all but the narrowest settings. If you have doubts and your lens has an aperture ring, you can do comparison shots of an evenly-lit blank wall for a given f-number.


Steve

* Most aperture rings support at most 1/2 stop increments and often not for all stops. In a similar vein, if the camera is set to 1/3 stop increments and the aperture ring has no settings that correspond to a 1/3 stop, there is no point for comparison.

** Probably not a bad idea for a future feature. After all, we fine-tune AF, why not aperture control?
03-13-2019, 07:37 PM - 3 Likes   #7
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QuoteOriginally posted by Adam Quote
The aperture ring is obsolete (on DSLRs). Unless you are reverse mounting the lens or something like that, then simply leave the aperture ring in the "A" setting and then set the aperture using the e-dial (or automatically).
I respectfully disagree. It is sometimes very useful to set the exposure in M mode, and then depending on the subject/lighting very quickly change the aperture setting on the lens to adjust the exposure (and thus almost instantly get the shot). It requires you to have a reasonable sense of exposure/estimating the change needed, and comfort using the aperture ring--but these are likely normal for many of us--especially coming from the (slide) film era.

Last edited by dms; 03-13-2019 at 07:52 PM. Reason: fix typo
03-14-2019, 03:11 AM - 2 Likes   #8
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QuoteOriginally posted by Adam Quote
The aperture ring is obsolete (on DSLRs).
The most recent Pentax lens release (HD FA 35mm f/2) has an aperture ring.

03-14-2019, 07:53 AM - 1 Like   #9
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QuoteOriginally posted by Paul the Sunman Quote
The most recent Pentax lens release (HD FA 35mm f/2) has an aperture ring.
Very mischievous of you, Paul!

03-14-2019, 10:33 AM - 2 Likes   #10
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Bring back the aperture ring, and the A setting.

I shoot nearly 100% in manual mode. I use the camera to meter the scene, except for night time and low light shots, which I usually guess at.

Most of my lenses are manual lenses, and I have been leaning towards the ones with the A setting. I like the A setting, because I can set the aperture using the thumbwheel on the camera, and see what aperture I am at in the viewfinder.

I prefer having the aperture ring so when I am using a bellows or extension tubes, or shooting with the lens reversed, i still have control over the aperture.

I know, there is an adapter that adapts K mount lenses to Nikon, that has an aperture ring to manually control the aperture, and if one removes the optics it has to adjust for infinity focus when used as designed, it facilitates using lenses without aperture rings.

*sigh*

Guess I just am nostalgic about the aperture ring.
03-14-2019, 11:00 AM - 1 Like   #11
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QuoteOriginally posted by Racer X 69 Quote
Guess I just am nostalgic about the aperture ring.
Me too. I prefer the lens controls to be on the lens and the camera controls to be on the camera. With a non-crippled mount, this works very nicely, even on a "modern" camera (example: Pentax MZ-S) and if such a mount were on the K-3 successor, I would be the happiest of clams with the option of both e-dials and aperture ring.


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03-14-2019, 12:22 PM - 2 Likes   #12
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There may be a trend in the industry. The recent, highly rated and very expensive Sony GM primes (24 1.4, 85 1.4 and 135 1.4) have aperture rings and, of course, the A position.
03-14-2019, 01:04 PM   #13
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I doubt that a manually set aperture set via an aperture ring is necessarily more accurate than one set using an "A" setting and letting the camera set the aperture, that is, if you set f5.6 on an aperture ring, it's unlikely it is more precisely and exactly f5.6 than the aperture the camera sets when using the "A" setting and selecting f5.6 via the camera. And if the lens is set to anything other than infinity, chances are the aperture won't be exactly f5.6 regardless of whether the lens is unit focusing or internal focusing. AND ALSO, the human eye can barely detect a 1/3 f-stop change, so if either the ring or the camera sets f5.6 within plus or minus about 20%, it's nearly certain that you could not tell the difference.
03-14-2019, 09:22 PM - 1 Like   #14
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Good Evening,

I thank you all, fantastic feedback. Of course, I wake up in the middle of the night, starting to realize what you all conveyed. I have to shake my head at myself as I work in DNA Forensics and Biometrics. I can build a $5 Mln ABIS (Automated Biometric ID System), and yet, there are times when, as I noted, the obvious just escapes me. As most pointed out, I awake in bed; "you idiot, you're using the cam dial to set the settings" and it just went from there.

I agree with those who highlighted variations in lenses and quality. I will say, I've been very pleased with the Pentax SMC 15mm f/4.0 DA ED AL Limited Wide Angle. Yet was so so with the Pentax DA 55-300mm f/405.8 ED lens. My goal now is to find a good landscape lens. I've been making the move back to prime lenses and find I wasn't satisfied with a large range zoom like the 55-300mm as photo clarity and quality didn't see that great.

I accept all recommendations on suggestions for a solid landscape lens.

Again, thanks everyone. I really appreciate the patience of a guy who, as I say; train went into the ravine (my explanation of a lost thought).
03-15-2019, 07:06 AM   #15
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QuoteOriginally posted by Posse96 Quote
Good Evening,

I thank you all, fantastic feedback. Of course, I wake up in the middle of the night, starting to realize what you all conveyed. I have to shake my head at myself as I work in DNA Forensics and Biometrics. I can build a $5 Mln ABIS (Automated Biometric ID System), and yet, there are times when, as I noted, the obvious just escapes me. As most pointed out, I awake in bed; "you idiot, you're using the cam dial to set the settings" and it just went from there.

I agree with those who highlighted variations in lenses and quality. I will say, I've been very pleased with the Pentax SMC 15mm f/4.0 DA ED AL Limited Wide Angle. Yet was so so with the Pentax DA 55-300mm f/405.8 ED lens. My goal now is to find a good landscape lens. I've been making the move back to prime lenses and find I wasn't satisfied with a large range zoom like the 55-300mm as photo clarity and quality didn't see that great.

I accept all recommendations on suggestions for a solid landscape lens.

Again, thanks everyone. I really appreciate the patience of a guy who, as I say; train went into the ravine (my explanation of a lost thought).
The Pentax A28mm (three different versions) should work well.

I have the Sigma 28mm f2.8 Mini Wide and have used it quite a bit since I picked it up. There is an A version of this lens I would like to try.
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