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01-27-2011, 10:57 PM   #1
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Dumb question...

I was all sorts of confused today when I was going over a new (to us) ZX-10 i bought for my wife today... I thought the camera was displaying the wrong aperture, but it turns out it was changing just a few stops when I zoomed in/out- on one zoom setting it would be correct based on what was displayed on the aperture ring on the lens and then when the zoom would change it would change a few stops (I cant remember if it was correct when zoomed in or out). Why does it do this?

Sorry for the dumb question...

01-28-2011, 01:17 AM   #2
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Ok all the usual remarks about there being no such thing as a dumb question aside, I actually had to look this up for the term. Better zoom lenses will be 'constant' aperture, i.e., where the minimum aperture will be say f2.8 at all zoom lengths. Cheaper zoom lenses will vary with focal length from the minimum aperture of say f3.5 at say 50mm to f5.6 at say 150mm. These are termed 'variable relative aperture' lenses. (Had to look that up.)

This isn't an issue for prime lenses because the focal length doesn't change. After I learned about this with my kit and other early lenses, I started getting constant aperture lenses whenever I could to avoid dealing with this issue. It kinda sneaks up on you. These variable relative aperture lenses do this because it's harder and more expensive to make lenses that are constant through their zoom range.
01-28-2011, 04:00 AM   #3
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QuoteOriginally posted by Paleki Quote
I was all sorts of confused today when I was going over a new (to us) ZX-10 i bought for my wife today... I thought the camera was displaying the wrong aperture, but it turns out it was changing just a few stops when I zoomed in/out- on one zoom setting it would be correct based on what was displayed on the aperture ring on the lens and then when the zoom would change it would change a few stops (I cant remember if it was correct when zoomed in or out). Why does it do this?

Sorry for the dumb question...
the one part of this I don’t understand is the part where you say: "it would be correct based on what was displayed on the aperture ring.” for your ZX-10, wouldn’t the lens have to be in ‘A’ mode, thus not showing an actual aperture on the aperture ring? which lens were you using by the way? the information on the front of the lens is a dead give away to being ‘variable aperture’ which essentially is just as blackcloudbrew described. it will say something like 3.5-5.6 or similar. basically meaning that its maximum (read: widest) aperture will vary through the focal length range of the lens.
01-28-2011, 08:38 AM   #4
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QuoteQuote:
the one part of this I don’t understand is the part where you say: "it would be correct based on what was displayed on the aperture ring.” for your ZX-10, wouldn’t the lens have to be in ‘A’ mode, thus not showing an actual aperture on the aperture ring?
In metered manual mode, or in aperture priority.

01-28-2011, 09:00 AM   #5
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QuoteOriginally posted by Paleki Quote
In metered manual mode, or in aperture priority.
in any mode, how can the body know what aperture its set at if the ring is not on A? im confused. the ZX-10 must work differently than im thinking. the body will display the aperture on the LCD display even if the lens isn’t set for automatic aperture control?
im sure the camera can meter properly with open aperture metering since it has the full (non-crippled) KAF2 mount, (using non-A lenses) but how does the camera know and display the aperture on the LCD without engaging the auto aperture of the lens? I would think that without the ‘A’ engaged it wouldn’t be able to display any aperture setting but would just adjust the shutter speed based on the selected aperture on the lens (while staying wide open for focusing of course). based on what you described and my assumption on how the camera works (ive never used a pentax AF film body) the only thing that should vary on the LCD screen is the shutter speed, as the widest aperture changes through the focal range. I ask again, what lens were you using? I want to make sure im understanding this right, because either ive severely misunderstood something or you aren’t describing something right.
01-29-2011, 01:02 PM   #6
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On my ZX-5n my SMC-A will not display the aperture while in Av mode. But, when using a Promaster zoom (FA style) in Av mode it does show the aperture and it does show it varying as I zoom.
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