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04-09-2017, 08:13 PM   #1
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A lenses versus M lenses -- what is the difference to the photographer?

I just slapped an M 28 f/2.8 lens onto my K-1 and I noticed it seems to be metering for me... no green button pushing... the shutter speed changes as I change the lighting indoors (simply by walking my camera around my living room).

Thinking it was just a K-1 advantage, I tried the same lens on my much older K-5 II to see it behaves in the same manner!

For some strange reason, I was under the impression the camera wouldn't lift a finger to read the lighting in the scene (and thus adjust settings) unless I pressed the green button.. but that is not the case.

So what advantage does an A series lens have over an M series? Just the ability to adjust aperture from the body (and subsequent recorded EXIF data)?

04-09-2017, 08:25 PM - 1 Like   #2
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With the M lens, it will meter and adjust the shutter speed, but only using the lens's wide-open aperture. You need to use the green button to get it to work with other apertures.

How to use/meter Manual & M42 Lenses on all Pentax DSLRs (K-1, K-3, K-5, K-30, etc) - PentaxForums.com
04-09-2017, 08:27 PM - 1 Like   #3
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The A lens an communicate its aperture electronically. They don't seem as well made as the M series - a lot more plastic. The A 50/1.7 has had mechanical problems.
04-09-2017, 08:34 PM - 1 Like   #4
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QuoteOriginally posted by mee Quote
I just slapped an M 28 f/2.8 lens onto my K-1 and I noticed it seems to be metering for me... no green button pushing... the shutter speed changes as I change the lighting indoors (simply by walking my camera around my living room).

Thinking it was just a K-1 advantage, I tried the same lens on my much older K-5 II to see it behaves in the same manner!

For some strange reason, I was under the impression the camera wouldn't lift a finger to read the lighting in the scene (and thus adjust settings) unless I pressed the green button.. but that is not the case.

So what advantage does an A series lens have over an M series? Just the ability to adjust aperture from the body (and subsequent recorded EXIF data)?
Unless you're always shooting wide-open in Av mode, M lenses require you to stop-down meter (press the green button to set the correct shutter speed) and you can only use them in M mode (and Av if and only if you're shooting wide-open).

A lenses allow you to use all the camera's shooting modes.


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04-09-2017, 08:43 PM   #5
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QuoteOriginally posted by Adam Quote
Unless you're always shooting wide-open in Av mode, M lenses require you to stop-down meter (press the green button to set the correct shutter speed) and you can only use them in M (and Av if you're shooting wide-open).

A lenses allow you to use all the camera's shooting modes.
I was a little confused by this... first I see now the camera refuses to jump into any other mode beyond M and Av -- even if you set it to Tv (for example), the camera reports Av mode!

Second.. even in Av mode, I set the lens to f/2.8 take a shot.. 1/15s f/2.8 ISO 800 looks good. I then adjust the aperture to f/22 and take another shot at the same scene in the same lighting. resultant shot still looks good and still reading 1/15s ?!

Then I realized the camera is still using f/2.8 regardless of the aperture selected on the lens. Only until I changed to Manual M mode and pressed the green button did the camera actually read the scene correctly and adjust the setting.

Ok that makes more sense... a little subtle what the camera was doing there.

Thanks, all, for the responses. Learned something new tonight.
04-09-2017, 09:16 PM - 1 Like   #6
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QuoteOriginally posted by arnold Quote
The A lens an communicate its aperture electronically. They don't seem as well made as the M series - a lot more plastic. The A 50/1.7 has had mechanical problems.
An A 50/1.7 was kitted with the Super Program I purchased in 1984. From day one, the aperture ring hung up between f/8 and f/11, and that matters to me because most of the time I was using apertures in that range. I used that camera, mostly with just that lens, for eleven years. I checked just now. The aperture ring still hangs up between f/8 and f/11, and the lens still works just fine.
04-10-2017, 07:49 AM - 1 Like   #7
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QuoteOriginally posted by reh321 Quote
The aperture ring still hangs up between f/8 and f/11, and the lens still works just fine
Bit of a contradiction there.
04-10-2017, 07:55 AM   #8
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QuoteOriginally posted by dsmithhfx Quote
Bit of a contradiction there.
No, I do have to wiggle the ring a bit, but I always get to the setting I want, and this 30+ year-old lens may be the sharpest lens I own.


Last edited by reh321; 04-10-2017 at 07:57 AM. Reason: add to thought
04-11-2017, 07:10 AM   #9
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Mee , it seems that you have auto iso on, which is a great pentax feature and others are including it too.
With a M lens, you may choose any mode and it will continue metering adjusting iso ranges, until you manually change speed and/or aperture. You can even close to f22 and choose a very high speed and your pentax will adjust iso until its limit , probably 25600 or 51200. after that you get to see the ISO led blinking , which means that the exposure is beyond limit.

---------- Post added 04-11-17 at 07:14 AM ----------

"Then I realized the camera is still using f/2.8 regardless of the aperture selected on the lens."

This is curious
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