Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
07-14-2009, 02:48 AM   #1
Forum Member




Join Date: Jul 2008
Location: South Africa
Posts: 62
M and A lenses exposure accuracy

Hi,

Is my understanding correct in that one gets a more accurate exposure across the F range a lens when using a M lens and pressing the green button (K200D) each time you change the F stop, than you get with an A lens across the F range.

Greg

07-14-2009, 03:19 AM   #2
Site Supporter
pschlute's Avatar

Join Date: Mar 2007
Location: Surrey, UK
Photos: Gallery
Posts: 1,157
QuoteOriginally posted by gregmoll Quote
Hi,

Is my understanding correct in that one gets a more accurate exposure across the F range a lens when using a M lens and pressing the green button (K200D) each time you change the F stop, than you get with an A lens across the F range.

Greg
A K200D owner will need to confirm, but I would be surprised if that is correct. The A lenses (with the aperture ring set to A) should expose accurately in all modes and at all aperture settings (NO green button needed).

The M lenses do have a tendency to progresively overexpose as you stop-down with the K10 + K20 due to the type of focus screen those cameras have. This was NOT a problem with my old *ist-DS, and may or may not be a problem with your K200D.

Either way the A lenses should NOT expose less accurately than the M lenses.
07-14-2009, 03:33 AM   #3
Forum Member




Join Date: Jul 2008
Location: South Africa
Posts: 62
Original Poster
Thanks,
I read somewhere that the A lens lever indicating the F stop to the camera was either in linear or log format and that the newer cameras read the F setting in the opposite format to that of the lens and therefore there is a cumulative error ??. I'm having difficulty in explaining myself :<)

Greg
07-14-2009, 05:09 AM   #4
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,399
I think you will find the A lenses will expose much more accurately than the K lenses or M lenses, because the metering knows exactly what is attached.

As pschlute stated the K10/K20 have big problems with this, My *istD was much better, and I have not yet tested my new K7.

what I do find, and this is not a pentax issue but probably more every lens manufacturer's problem, is that there are errors in the aperture mechanism, I have at least 2 lenses that exhibit a slight gradual increase in exposure as you stop down, even in the A setting. To me this indicated perhaps an error in the lens since it is repeatable on both my *istD and K10D

07-14-2009, 06:32 AM   #5
Veteran Member




Join Date: Sep 2008
Location: New Jersey, USA
Posts: 337
With my K200D, I definitely get better metering with my A lenses (for example, A 50/1.7 and Vivitar Series 1 105mm Macro), as opposed to my M lenses (for example, M 28/3.5, M 85/2). Not that I am particularly experienced with manual lenses, but I simply have never been able to master my M lenses, even though they are capable of producing outstanding images. My A lenses, however, are really quite easy to use and get accurate (or close to accurate) exposure.
07-14-2009, 06:49 AM   #6
Forum Member




Join Date: Jul 2008
Location: South Africa
Posts: 62
Original Poster
I wonder why the A lens exposes better than an M, one would think that because with an M you have to measure the exposure with the green button, which actually closes down the diaphragm to the selected F stop to take the reading, it would take an exact measurement. I appreciate that the focus screen could have an effect on the reading. As I understand it, the A lens takes the reading when you shoot the shot ? All these observations refer to using the camera in manual M mode and the A lens set to "A"

Greg
07-14-2009, 08:12 AM   #7
Inactive Account




Join Date: Dec 2008
Location: Ames, Iowa, USA
Photos: Albums
Posts: 2,965
I think there are many variables at play. Here's a metering series for a Tamron lens used in A mode, followed by a metering series for a Soligor lens used in preset mode* (ie. the lens was stopped down before metering.) The graphs show the brightness across the photo for the f-stops indicated (255 full scale - vertical scales adjusted so they can be directly compared).


As you can see exposure varies for both type lenses as f-stop changes. My tests show the degree of such variation depends on the lens, on the metering mode, on the position of the moon, etc... IIRC +.3EV compensation was used for the A lens and +.7 for the preset lens.

I don't know how a K type lens would fare with the Green Button because I don't have one to try.

Dave in Ames


* This was done using a K100D which thought the preset lens was an A type (the A pin was shorted).

The In-viewfinder f-stop was set to the lowest value when using the preset lens so the camera did not try to close the aperture at all ( the camera thought the lens was wide open and would remain & so calculated the exposure based only on the light measured.)

Last edited by newarts; 07-14-2009 at 08:19 AM.
07-14-2009, 08:13 AM   #8
Veteran Member




Join Date: Sep 2008
Location: New Jersey, USA
Posts: 337
gregmoll,

Let's say you are comparing an A 50/1.7 to an FA 50/1.4 (I have both). I can put each one on my K200D, set the camera on AV mode and set my desired aperture on the camera, set the aperture on both lenses to A (meaning aperture is controlled by the camera, not the lens), and each lens should be metered in the same manner. The A lens has the same functionality as my FA lens, except for lacking autofocus. That is why they are more valuable.

My non-technical analysis of the M lenses is that Pentax has given us the option of continuing to use this old glass that has wonderful optical qualities, but that wasn't made with the digital age in mind. So the green button tries to mimic what the camera can do as far as exposure is concerned, "talking" to a lens that it can't quite communicate fully with, and the results are simply not going to be as consistent as when the camera and the lens are communicating more fully (like the A lens, FA, DA, etc.). Mind you, some of the more experienced members here seem to be able to do great things with the K and M lenses, it just may take a bit more effort and skill than the A lenses.

07-14-2009, 08:27 AM   #9
Site Supporter
pschlute's Avatar

Join Date: Mar 2007
Location: Surrey, UK
Photos: Gallery
Posts: 1,157
QuoteOriginally posted by gregmoll Quote
I wonder why the A lens exposes better than an M, one would think that because with an M you have to measure the exposure with the green button, which actually closes down the diaphragm to the selected F stop to take the reading, it would take an exact measurement. I appreciate that the focus screen could have an effect on the reading. As I understand it, the A lens takes the reading when you shoot the shot ? All these observations refer to using the camera in manual M mode and the A lens set to "A"

Greg
This is quite simple really. The A lenses (with the aperture set to A) behave the same as any F; FA or DA lens: the camera knows what aperture has been selected and meters accordingly. All this happens while the aperture is wide open, BEFORE you press the shutter. When you press the shutter,the lens stops down to the selected aperture and the shot is captured. There is no metering "as you take the shot".

With the M lenses the camera has no idea what aperture will be used and so you have to take a meter reading by stopping down with the green button, BEFORE you take the shot. This is quite different to open-aperture metering that the camera has been designed to use. With the older DSLRs this worked quite well, but the focus screens were designed differently starting with the K10 to be "brighter". This had the effect of fooling the meter at certain apertures when using the green button metering method, and thus you get exposure inaccuracies.

Those of us that use M and "K" lenses rely on the histogram to ensure correct exposure.
07-14-2009, 08:50 AM   #10
Forum Member




Join Date: Jul 2008
Location: South Africa
Posts: 62
Original Poster
Thanks, I guess I'll have to live with the exposure variation, what I do at the moment is take a quick shot of the scene and check the histogram. Why I asked the question is because most of my lenses are M series and I'm going to buy a 200mm and 300mm to complete the line-up. I think I will stick to the M's so that my pre shot mental check-list is the same for all my lenses.

I had forgotten that you can set the aperture of an A lens in the camera under Av mode, never use it.

Thanks for all the help.

Whilst waiting for replies I've just cut down a ME Super screen and it works like a charm.

Thanks,
A more educated Greg
07-14-2009, 09:44 AM   #11
Site Supporter
pschlute's Avatar

Join Date: Mar 2007
Location: Surrey, UK
Photos: Gallery
Posts: 1,157
Greg

With the A lenses, you should use them on the A setting and use the camera to set the aperture.

I am pleased you are using the M lenses, as I am a great fan of them too.
07-14-2009, 09:59 AM   #12
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,399
QuoteOriginally posted by newarts Quote
I think there are many variables at play. Here's a metering series for a Tamron lens used in A mode, followed by a metering series for a Soligor lens used in preset mode* (ie. the lens was stopped down before metering.) The graphs show the brightness across the photo for the f-stops indicated (255 full scale - vertical scales adjusted so they can be directly compared).


As you can see exposure varies for both type lenses as f-stop changes. My tests show the degree of such variation depends on the lens, on the metering mode, on the position of the moon, etc... IIRC +.3EV compensation was used for the A lens and +.7 for the preset lens.

I don't know how a K type lens would fare with the Green Button because I don't have one to try.

Dave in Ames


* This was done using a K100D which thought the preset lens was an A type (the A pin was shorted).

The In-viewfinder f-stop was set to the lowest value when using the preset lens so the camera did not try to close the aperture at all ( the camera thought the lens was wide open and would remain & so calculated the exposure based only on the light measured.)
what are the "spikes in your graph between F stops?

also, your tamron exhibits the same type of behavior as minbe, exposure drifting upwards as you stop down,
07-14-2009, 10:01 AM   #13
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
QuoteOriginally posted by gregmoll Quote
I had forgotten that you can set the aperture of an A lens in the camera under Av mode, never use it.
Whether you use the lens in Av mode or not, with an "A" lens, you should definitely use the aperture ring in the "A" position. Otherwise you lose the full time meter reading, you lose flash metering, you lose multi-segment metering, you lose the aperture in the EXIF, etc. You are still welcome to use "M" mode or any other mode you like, but you are definitely better off in all respects leaving the aperture ring in "A". Not to mention the fact that some A-lenses have fragile aperture rings; the less you mess with them, the better.

Since M lenses require M mode, and you seem to value consistency (as I do), I'd recommend simply using M mode all the time. This makes the experience of using M lenses almost identical to using other lenses, except that aperture is set on the lens instead of the body. But the same procedure is followed otherwise.

Do note that one potential difference in metering between M lenses and auto-exposure lenses is that only the latter support multi-segment metering. So you'll potentially get different results for the same scene for that reason alone. Again, in the interest of consistency, I find it most useful to just stay in center-weighted metering all the time.

To address some other points:

- Yes, in theory, stop down metering would yield perfect results - but only if the focus screen was designed for that. It isn't, and more obviously so with the K10D and K20D than the K200D. I actually ran a full series with my M135/3.5 doing a Green button meter reading at each aperture settings, and got exposures that were all within a quarter stop of each other, which is as good a result I get with DA lenses. The K10D and K20D don't do so well here.

- As for the bit about the aperture lever working differently (linear versus quadratic or whatever), I think that has more to do with issues in trying to convert an M lens to work like an A. If you are using the aperture ring at all with a DSLR, the aperture lever is basically just an on/off switch - it isn't proportional to anything. So there shouldn't be any difference between M and A lenses if you are using the aperture ring, as far as my thinking tells me.
07-14-2009, 10:02 AM   #14
Forum Member




Join Date: Jul 2008
Location: South Africa
Posts: 62
Original Poster
And the price is more reasonable. :<).

Now I must find a 200mm and 300mm, I'm thinking of getting them from Henry's, may be a bit more expensive but less chancy than an eBay purchase.

Greg
07-14-2009, 10:06 AM   #15
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
KEH.com is also a reliable source for used lenses. The M200/4 is very common and cheap.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
exposure, k-mount, lens, pentax lens, range, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
accuracy of K-x metering manual / M42 lenses? kuuan Pentax DSLR Discussion 7 01-22-2010 01:01 PM
Exposure meter with M lenses. blwnhr Pentax DSLR Discussion 18 07-30-2009 02:06 PM
K-7: Exposure accuracy and high ISO? Fototim Pentax DSLR Discussion 0 06-21-2009 06:34 PM
K10, split-screen, LF80 & LL60 : Focus and exposure accuracy dlacouture Pentax DSLR Discussion 24 11-18-2008 06:20 AM
K20D exposure metering accuracy Andrews Pentax DSLR Discussion 3 05-18-2008 07:18 AM



All times are GMT -7. The time now is 08:49 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top