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10-02-2018, 06:30 PM   #1
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Difference between Av mode and M mode on fixed aperture lens.

I'm planning to get a lensbaby sol 45mm lens for its special effect, when checking the compatibility to the camera ( K1 / K5), it shows it need to stop-down meter in M mode.

As this lens have fixed aperture (f 3.5) , so what's the difference if I use Av mode (wide open as its also f3.5) and the M mode ? Some of the mirror lens also have fixed

aperture, so it s/b the same as the lensbaby lens, thanks.

10-02-2018, 08:07 PM   #2
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I shoot my lensbaby in AV mode and have since the K10D. There’s nothing to stop down and since I have the aperture disks, whichever one is in is wide open per the camera.
10-03-2018, 02:32 AM   #3
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Yes, there is no point using M mode, leave it in Av and it should work like any manual aperture lens.
10-03-2018, 07:43 AM   #4
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QuoteOriginally posted by micl161 Quote
As this lens have fixed aperture (f 3.5) , so what's the difference if I use Av mode (wide open as its also f3.5) and the M mode ?
The short answer is that the difference is the same as with any other manual aperture lens and that there is a difference.

That being said, yours is a good question that leads directly into the realm of unverified (wild) claims by people like me. My personal experience with Pentax dSLRs (post-2009) as well as others on this site is that stop-down metering using the optical viewfinder is more consistent in M mode (green button or EV scale) than Av. Yes, there is a difference and often the difference is notable.*

Another difference for stop-down metering is between the optical viewfinder and live view with live view holding a distinct edge. My practice in recent months has been to get my exposure settings using M mode and the green button in live view and then shoot using the optical viewfinder in M mode. The settings should remain good until either the subject or light changes.**


Steve

* Sometimes as much as 1-2 stops. A good way to test is with a light gray or white blank wall in good natural light (e.g. north facing at noon) filling the frame. Actual behavior depends heavily on the lens being used and the aperture setting. A proper exposure will yield a centered histogram for settings measured at all apertures.

** This approach is very old-school, but works quite well in actual practice.


Last edited by stevebrot; 10-03-2018 at 08:15 AM.
10-03-2018, 07:57 AM   #5
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QuoteOriginally posted by stevebrot Quote
Yes, there is a difference and often the difference is notable.*
Just off the top of my head as I recall the difference between Av and green button M on my K10D can be as much as 2/3 a stop.
10-03-2018, 08:13 AM   #6
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QuoteOriginally posted by Not a Number Quote
Just off the top of my head as I recall the difference between Av and green button M on my K10D can be as much as 2/3 a stop.
I never did the comparison on the K10D while I had it, probably because stop-down metering on that camera was such a crap shoot overall.* Significant improvements were made with the K-7 in 2009 and it was quickly noted by some users on this site that Av mode and M mode generated different meter readings with Av being not much better than on the K10D and K20D. Again, much depended on the lens and aperture being used. When I got my K-3 in 2014, I was thrilled with the stop-down performance with K-mount lenses (M mode) and less thrilled with M42 lenses in Av mode. After a year or so, it finally occurred to me that the green button might work with manual aperture lenses in M mode (silly me ) in the same manner as a K-mount lens wide open. I am not much of a live view shooter, so another year or so passed before I tried stop-down metering in live view and found it to offer the best performance overall and is now my go-to solution for lenses that I know are problematic.


Steve

* How does 2+ stops underexposed with some lenses at maximum aperture sound?
10-03-2018, 11:25 AM   #7
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As with the other posters, there are (or can be) differences between Av and M metering on the same camera with the same lens, but ... not all fixed-aperture lenses affect all cameras in the same manner, so there must be camera model specific differences (which I've not bothered to try and tabulate).


I've got four Pentax DSLR's, two six megapixel, a K-5 16MP and a new K-70 24MP and the only rule I can guarantee across all bodies is to use Manual mode and the "Green Button" with anything other than automatic lenses.


After that, make a few trial comparative exposures and if they "look the same" then you'll probably be OK, but if they don't either remember the differences and compensate or use Manual


YMMV !
10-03-2018, 07:58 PM   #8
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QuoteOriginally posted by stevebrot Quote
hat being said, yours is a good question that leads directly into the realm of unverified (wild) claims by people like me. My personal experience with Pentax dSLRs (post-2009) as well as others on this site is that stop-down metering using the optical viewfinder is more consistent in M mode (green button or EV scale) than Av. Yes, there is a difference and often the difference is notable.*
Its always good to have somebody's experience as reference so you know what you most likely will see, thanks.

---------- Post added 10-03-18 at 08:04 PM ----------

QuoteOriginally posted by kypfer Quote
I've got four Pentax DSLR's, two six megapixel, a K-5 16MP and a new K-70 24MP and the only rule I can guarantee across all bodies is to use Manual mode and the "Green Button" with anything other than automatic lenses.
Your rule looks simple, but just thinking if I'll 'waste' all those A- setting lenses. Of course the final result says all..

10-04-2018, 12:13 AM - 1 Like   #9
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QuoteOriginally posted by micl161 Quote
Your rule looks simple, but just thinking if I'll 'waste' all those A- setting lenses. Of course the final result says all..
I don't think there's any "waste" involved. Automatic lenses were introduced both to speed up the whole photo-taking process and to assist those who struggle with the basic concepts.


I think most of us who use the older lenses do so either because of a nostalgia for "the way things were", a sense of achievement -"I took that with no auto-anything and it still looks good!" or the unique rendition of some of the older lens formulas. Cost may also be a factor, of course


If you really feel the need for masochism, simply take your A-setting lenses off the A-setting and use manual with them as well
10-04-2018, 08:42 AM   #10
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I chimp all the time whether its green button m mode or Av and the reason is pretty simple. While the exposure may be correct according to an algorithm, it may not look very good in practice. Shooting mixed groups of dark and light people, for example, always takes a bit of touch to get right. Beyond that, the issue can be lens by lens. My Tokina AT-X 90mm tends to overexpose whether in Av (which I use for wide open) or green button manual, so there is always a preshoot and adjustment. For the OP's lens in question, we are probably talking pretty meditative photos where there is time to get exposure exactly right, so Av vs M probably doesn't matter.
10-04-2018, 06:29 PM   #11
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M + green button should give better readings since it's closing to the actual aperture. Some old lenses can have a lot of play in the aperture mechanism and vary greatly at each actuation, specially at smaller apertures (tiny play equals great difference in area covered).

10-04-2018, 06:41 PM   #12
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QuoteOriginally posted by torashi Quote
M + green button should give better readings since it's closing to the actual aperture. Some old lenses can have a lot of play in the aperture mechanism and vary greatly at each actuation, specially at smaller apertures (tiny play equals great difference in area covered).
The OP's lens has a fixed aperture, so that's not an issue.
10-04-2018, 08:50 PM   #13
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QuoteOriginally posted by kernos Quote
The OP's lens has a fixed aperture, so that's not an issue.
true, but it’s still a good post from torashi considering that the conversation drifted to manual lenses in general
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