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07-14-2009, 10:13 AM   #16
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Thanks Mark,

One of the things I like about the M lens is that I can lock the aperture to the depth of field or sweet spot I want on the lens without worrying whether or not I've inadvertently turned the wrong knob/button and changed the setting.

Greg

07-14-2009, 10:14 AM   #17
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QuoteOriginally posted by newarts Quote
I think there are many variables at play...
An understatement. Probably most of the participants in the thread know this, but I have to mention that if the camera is set to matrix metering for KA lenses, it will switch to center-weighted when an M lens is mounted. Dave is fooling his camera so it doesn't do this for his Soligor, but not everyone is aware of the metering switch. Focus screens are different for some series of cameras, and those affect metering. A few Pentax and some third-party lenses might have an M-type mount but an aperture that moves like a KA lens. (I suspect the Adaptall lenses are like this.) And not every 30 year old lens is working exactly like it was designed.

My point is, you can measure accuracy with your own lenses and cameras, but applying those results beyond that won't work well.
07-14-2009, 11:01 AM   #18
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I'll also second the comment on there being many variables at play. I tend to think of my K200D with an M lens attached as having an additional hurdle to clear for a well exposed shot. I've noticed that stopping an M lens down proportionally increases the odds of a poorly exposed shot which is easily recognized on the LCD afterward. I simply adjust settings and reshoot. I find this process quite engaging and it's nice to turn a real aperture ring on a lens.

It is possible camera software could be configured to recognize every lens Pentax made along with the resulting aperture steps of those lenses to interpolate perfect exposures. In relative terms, exposure values could be mapped for the given shutter speeds and such but you would lose the seamlessly variable shutter speeds now possible. And you might have a list of lenses you would have to scroll through to select the lens you're using.

Aw, shucks, my K200D and M lenses work great as it is!
07-14-2009, 11:16 AM   #19
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QuoteOriginally posted by Lowell Goudge Quote
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,

The spikes are just an artifact of the technique I used. Each region between the spikes is a full screen width of a photo at a particular f-stop.

A photo of my laptop's blank screen was taken at a series of increasing f-stops. These photos were assembled side by side with a small white gap between, then the whole strip was greatly reduced in size. This yields a "test strip".

The test strip was then loaded into ImageJ and an intensity plot across the strip's with was made. That plot is what my posting shows. I should have described the experimental process better; sorry,

Dave

07-14-2009, 11:24 AM   #20
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QuoteOriginally posted by Just1MoreDave Quote
... Probably most of the participants in the thread know this, but I have to mention that if the camera is set to matrix metering for KA lenses, it will switch to center-weighted when an M lens is mounted. Dave is fooling his camera so it doesn't do this for his Soligor, but not everyone is aware of the metering switch. ....
Right, I fooled my camera into thinking all lenses are A type lenses (and set to A on the aperture ring). Therefore I can use matrix, center weight, or spot metering.

The results I showed were actually the result of a test series where I tried to figure out which of matrix, CW, or Spot was best with my Chinese split-prism screen. CW seems to win on my K100D.

I've been considering going back to an LL-60 focusing screen - maybe Lowell will comment on whether the potential gain in exposure precision out-weighs the loss of split-prism manual focusing?

Dave
07-14-2009, 11:27 AM   #21
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QuoteOriginally posted by newarts Quote
The spikes are just an artifact of the technique I used. Each region between the spikes is a full screen width of a photo at a particular f-stop.

A photo of my laptop's blank screen was taken at a series of increasing f-stops. These photos were assembled side by side with a small white gap between, then the whole strip was greatly reduced in size. This yields a "test strip".

The test strip was then loaded into ImageJ and an intensity plot across the strip's with was made. That plot is what my posting shows. I should have described the experimental process better; sorry,

Dave
I suspected as much.

I don't look at side vs center, although probably I should, I just use the central ~10% of the image and measure average greyscale
07-14-2009, 08:52 PM   #22
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Unless I had just loads of time, before I'd meter an M lens with a histogram shot, I'd use the green button at F/4-5.6 and adjust shutter and aperture to what I wanted keeping the same EV. Every K and M lens I've tried is metered fairly well at F/4 or F/5.6.
07-15-2009, 04:46 AM   #23
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QuoteOriginally posted by GeneV Quote
Unless I had just loads of time, before I'd meter an M lens with a histogram shot, I'd use the green button at F/4-5.6 and adjust shutter and aperture to what I wanted keeping the same EV. Every K and M lens I've tried is metered fairly well at F/4 or F/5.6.
Yep , this is how I start to get a meter reading, and then go from there. After a while of course, you begin to set the shutter speed yourself without the green button. At this point you realise you have learned something about exposure.

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