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Old Jan 12, 2007, 2:28 AM   #1
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I have a Tamron 28-200mm lens that has an aperture ring. I already set the camera to allow non-A settings. I'm somewhat flustered by the control I can have over the aperture, as I've never used manual lenses before.

With the lens set to A, I can adjust the aperture through AV, but the maximum aperture decreases as I increase the zoom, say from f/4.0 to f/5.6. Now if I set the aperture through the ring, could I leave it at f/3.6 all throughout the zoom range? If so, what is the disadvantage of doing so?
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Old Jan 12, 2007, 3:38 AM   #2
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Hi Illuminati,

there is a great explanation by Tom LaPrise about aperture and zoom in the
"Steves Forums > Digicam Help > Tips & Tricks > Digital (Aperture) Fstop Query" section.

I've quoted him here below

[line]
OK--first, the aperture vs. zoom thing:

Aperture setting is effectively the lens diameter divided by the focal length. (You're not changing the actual diameter of the lens, though, when you change the aperture setting; you're changing the diameter of an opening of an iris inside the lens assembly.)

You're probably aware that an aperture setting is written as f/some number, with f being the lens' focal length setting. For f/3.5, the iris is set so that the effective diameter of the lens is your focal length divided by 3.5. At f/8, the opening will make your effective lens diameter focal length divided by 8. As you can see, since it's a fraction, if you make the denominator smaller, the iris opening gets bigger.

There is a physical limit to how big that iris opening can get because the parts that make up the iris (often, metal or plastic "leaves" that pivot at one end and overlap) have to fit inside the lens barrel. The smallest number shown for your available aperture settings represents that limit.

Since the aperture setting is a ratio of effective lens diameter (controlled by the iris diameter) to focal length, as you zoom in and increase the focal length, to keep the same aperture setting, the iris has to open up to compensate. When the iris is open as far as it can be, a longer focal length will result in a smaller aperture setting (larger number, i.e. f/5.6 instead of f/3.5) because the iris can't widen any further as the focal length gets longer.

If you're at a medium focal length and set the aperture to a medium setting (let's say f/5.6), as you zoom out to wide angle (shorter focal length), the camera will automatically make that iris opening smaller to keep the aperture at f/5.6; as you zoom in (longer focal length), it will make the opening larger to compensate. Once you reach wide-open or fully closed, though, the camera will not maintain the aperture setting at f/5.6.


With that out of the way, to blur the subject while keeping the background focused, you could aim at the background, press the shutter halfway to focus, keep the button pressed halfway to keep that focus setting, then aim at the subject and press the shutter the rest of the way. If your subject is a lot closer to you than the background is, the subject should be pretty blurry, especially if you use a large aperture (low f/ number, like 3.5).
[line]
Leaving the iris max open and increase the zoom and the aperture will still go down.

Ronny
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Old Jan 12, 2007, 3:49 AM   #3
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The Tamron 28-200mm has an aperture of f3.5 @ 28mm & f5.6 @ 200mm so if you set the lens aperture manually to f3.5 & zoom out to 200mm it will assume f5.6 due to the mechanics on the lens etc.

So, IMHO, I would set the lens to 'A' & allow the aperture to be set using the camera controls in whatever mode you choose to use.
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Old Jan 12, 2007, 11:07 AM   #4
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I agree with Carbells.Setting the ring to Aand letting the camera control the aperture is much easier than using the ring on the lens to control aperture. Also, i'f you leave the lens set to A there is no need to change the menu setting. The Aperture ring allowed setting is only needed if you want to use the lens to adjust aperture.
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Old Jan 12, 2007, 11:13 AM   #5
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ronnie, i'm mixed up now... lol

as ron said, at your smaller MM zoom the lens has enough room to make the ap at 3.5
when you zoom to 200mm even tho the ap is wide open at 3.5 the size of the hole of the ap is only large enough to let in the equivalent amount of light equal to a f.5.6 opening. this esoteric crap kills me. best thing to do is learn what the aperture will do when taking a shot.

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Old Jan 12, 2007, 11:33 AM   #6
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Thanks for the response everyone. It cleared up my confusion about the setting. Guess there really is no need for non-A if the lens has the A setting.
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Old Jan 12, 2007, 12:51 PM   #7
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correct, but i sure love my non-A lenses....

roy
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Old Jan 12, 2007, 3:13 PM   #8
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I will add that I have no problem using lenses in non-A mode, even if they have an A setting. I currently only have two lenses that even have an A setting.

Of course, I do have a background shooting with these same lenses on a manual focus, manual exposure film camera, so I am used to manipulating the Aperature Ring.

Even on the K10D, I primarilly use my non-A lenses in fully manual mode, since that gives me access to the camera's meter, which I can't get when using the lenses in AV mode.

Paul

P.S. I'm actually more annoyed at lenses that don't have an apperature ring. When I use those on my old Ricoh, I have no adjustment of the apperature at all.


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Old Jan 12, 2007, 6:51 PM   #9
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have to agree with you paul. i'm very compfortable with manual lenses.
there are many times tho that i have wished for AF. especially when using the tak500mm f4.5. it's almost impossible to get action shots with it.
adj focus, pan, take shot = 3 hands every time.. at least i can use it in Av.

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Old Jan 12, 2007, 8:37 PM   #10
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pbender wrote:
Quote:
I will add that I have no problem using lenses in non-A mode, even if they have an A setting. I currently only have two lenses that even have an A setting.

Of course, I do have a background shooting with these same lenses on a manual focus, manual exposure film camera, so I am used to manipulating the Aperature Ring.
Makes sense - it's all what you're used to. My only non-AF lens is a 50mm A 1.7, so for me it's much easier to leave the ring in the A position. That way the 50mm works just the same as the other lenses except for the manual focusing.

Quote:
Even on the K10D, I primarilly use my non-A lenses in fully manual mode, since that gives me access to the camera's meter, which I can't get when using the lenses in AV mode.{/Quote]
I need to pull out my newbie card here. I don't understand what you mean by not having access to the camera's meter with the lens on the A setting. I can use all of the modes on the K100 this way, and they seem to function the same way they do for my DA lenses. Would I gain some more options by using the aperture ring?


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