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02-26-2010, 01:26 AM   #1
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Lens F/stop Limitation Question

I'm a confused regarding the f/stop of a lens and what aperture setting I can use to shoot.

For example, I have a 50-200mm F4-5.6 wr lens for my K7.

Am I limited to shooting at an aperture setting between 4 & 5.6 or can the shots be taken at apertures higher or lower than the 4-5.6 range?

I've been playing around outside of the green mode and it appears that I can set the aperture to values lying outside of the lens rating in most other modes. I realize that the iso and shutter speed changes to keep things balanced.

How does the camera know what the lens is rated for (f/stop wise) and how does this rating affect the shot (being that I can adjust aperture to an outside the range f/stop)?

These digital DSLR lens are different than my old 35mm lens whereby I can set the aperture manually but only within the range of the particular lens.

I've been reading a k7 book by Burian that goes into details about lens but I'm still confused about the f/stop issue.


Last edited by seachunk2; 02-26-2010 at 03:54 AM.
02-26-2010, 01:29 AM   #2
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QuoteOriginally posted by seachunk2 Quote
I'm a confused regarding the f/stop of a lens and what aperture setting I can use to shoot.

For example, I have a 50-200mm F4-5.6 wr lens for my K7.

Am I limited to shooting at an aperture setting between 4 & 5.6 or can the shots be taken at apertures higher or lower than the 4-5.6 range?
It's actually still the same like 35mm film lenses. It's just that these zoom lenses are "variable aperture" and gets less wide as you zoom. "50-200 / 4-5.6" means that at 50mm, the range is f/4 - f/32; at 200mm, the range is f/5.6 to f/32.

Cheers
02-26-2010, 01:47 AM   #3
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At power up, lens and camera communicate so the camera knows which lens and the 'capabilities' like focal length and apertures.

PS In the old days, there were variable aperture zoom lenses as well (e.g. 35-105/3.5-4.5)

Last edited by sterretje; 02-26-2010 at 01:58 AM.
02-26-2010, 04:10 AM   #4
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Alright, I think I'm beginning to understand.

So, for example, if I zoom to 200mm the variable aperture goes to 5.6 on its own.

However, if I opt to shoot at f11, the aperture of the lens can't physically get any smaller because it's limited to the 5.6 diameter opening so the "smart" camera adjusts the iso and shutter speed (and also incorporates all other user dictated tweaks such as ev compensation) so that you have a good exposure.

Does that sound about right?

02-26-2010, 05:39 AM   #5
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QuoteQuote:
However, if I opt to shoot at f11, the aperture of the lens can't physically get any smaller because it's limited to the 5.6 diameter opening so the "smart" camera adjusts the iso and shutter speed (and also incorporates all other user dictated tweaks such as ev compensation) so that you have a good exposure.
The aperture can't go smaller or bigger because you told the system to use that aperture.
02-26-2010, 05:52 AM   #6
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Reading your question and your response to the answers others have given it appears you are getting yourself more confused.
The F4 aperture is the widest available when the lens is at 50mm but you can set the lens with the wheel to F22 if you wish to - the camera will compensate appropriately according to the camera modes you have chosen.
When you zoom out to 200mm, the widest the lens aperture can physically open to is F5.6 but you can still narrow it down to F22 electronically with the dial and the camera will compensate as before.
The F4 to F5.6 is not the only range of the lens aperture, it is just the widest open it can go through the range of zoom offered.
Given this basic understanding now go back and read the other answers as they are all correct once given this understanding of lens nomenclature.
02-26-2010, 06:04 AM   #7
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QuoteOriginally posted by seachunk2 Quote
Alright, I think I'm beginning to understand.

So, for example, if I zoom to 200mm the variable aperture goes to 5.6 on its own.

However, if I opt to shoot at f11, the aperture of the lens can't physically get any smaller because it's limited to the 5.6 diameter opening so the "smart" camera adjusts the iso and shutter speed (and also incorporates all other user dictated tweaks such as ev compensation) so that you have a good exposure.

Does that sound about right?
If you tell it to use f/11, it won't use f/5.6, it will use f/11.

If you use Av mode or HyperProgram, you determine aperture. The camera will determine shutter speed and ISO if you set it to auto ISO. Exposure compensation defaults to zero; the camera won't change this automatically, but if you change it, shutter and/or ISO will change as a result.

If you use TAv mode, you determine aperture and shutter. The camera will determine ISO. Exposure compensation defaults to zero; the camera won't change this automatically, but if you change it, ISO will change as a result.

If you use M mode, you have to setup everything; there's no exposure compensation in M mode because there's no automation.
02-26-2010, 06:23 AM   #8
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Thanks for the replies.

I'm starting to understanding what you're all telling me.

I also did a google search under variable aperture and I'm coming across numerous write ups that also discuss this topic.

You've all put me on the right track. Much appreciated.

We just got a nice dose of fresh snow here in NY this morning & the office is closed.

Great day to head out and experiment!


Last edited by seachunk2; 02-26-2010 at 01:04 PM.
02-26-2010, 06:41 AM   #9
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QuoteOriginally posted by seachunk2 Quote
Thanks for the replies.

I starting to understanding what you're all telling me.

I also did a google search under variable aperture and I'm coming across numerous write ups that also discuss this topic.

You've all put me on the right track. Much appreciated.

We just got a nice dose of fresh snow here in NY this morning & the office is closed.

Great day to head out and experiment!
My friend's flight was cancelled today.

And if you're just starting to play around with it, I would really stay away from shots with a lot of snow in them. All of that white is tricky to deal with.

Stick with medium and close-ups of subjects where white isn't the dominant shade, especially since you're trying to nail your aperture usage.
02-26-2010, 07:09 AM   #10
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Rethinking about what I wrote earlier (post #3), it might work slightly different. Note that this is a addition/correction post #3

Numbers tailored to your 50-200.
Every time that you turn the dial to change the aperture, there will be communication from the camera to the lens and the lens will confirm to the camera. The camera will remember the selected aperture. So assume you have selected f/4 at 50mm and start zooming. When metering is active, the lens will constantly tell the camera the aperture that it will use. Getting to 200mm, the lens will tell the camera that it will use f/5.6 for the aperture as it does not support f/4 at that focal length. Zooming back, it will go back to f/4 at 50mm. As long as you don't 'touch the dial', the camera will remember the f/4. If you have selected f/8 at 200mm and next select f/5.6 and start zooming towards 50mm, the camera remembers the f/5.6 and the lens will not report a different value back at 50mm as f4 is supported and you actively selected it.

The aperture that is reported by the lens is the value that the camera will use for its calculations.

When the camera is powered on, it will instruct the lens to use the aperture that was selected the last time. If you used the 18-55 at f/3.5 or a FA31 at f/1.8 before changing the lens to the the 50-200mm, the camera tells the lens to use f/3.5 or f/1.8. The lens will reply with the closest aperture that it supports (f/4 at 50mm or f/5.6 at 200mm. As long as you don't 'touch the dial', the camera will however remember f/3.5 or f/1.8 so when you but the previous lens back on it will fall back to the selected aperture.

PS a similar thing happens at the other end; some lenses go up to f/22 and others go up to f/32.

EDIT: tested with FA31LTD and DA18-55 on a K10D

Last edited by sterretje; 02-26-2010 at 07:15 AM.
02-26-2010, 07:27 AM   #11
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QuoteOriginally posted by seachunk2 Quote
... if I opt to shoot at f11, the aperture of the lens can't physically get any smaller because it's limited to the 5.6 diameter opening ...
Apertures are confusing at first. See, f/5.6 looks like a smaller NUMBER than f/11, but it's a larger / wider APERTURE, a larger fraction. Aperture is a fraction, the ratio of the focal length to the widest opening. So an f/2 means the opening is 1/2 the focal length (FL), and f/11 means it's only 1/11th of the FL. The SMALLER the f-number, the WIDER the opening -- and more light gets in. So f/4.5 is much wider than f/11, just as 1/2 is much larger than 1/7.

So when a zoom lens is rated as f/4.5-5.6, that means that at the shortest zoom, the widest possible aperture is f/4.5, and at the longest zoom, the widest possible aperture narrows down to f/5.6. If you have set the aperture to f/11 or f/16, or any other setting narrower than f/5.6, the camera stays at that narrower setting (LARGER F-NUMBER).

The aperture number is also called an f-stop. Setting a narrower aperture is called stopping-down. Whatever the widest possible f-stop, you can always stop-down as far as the lens allows (maybe f/22 or f/32, not alway desirable).
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

What effect does this have on shooting? A larger, faster f-stop lets in more light, so you can use a faster shutter. But it also reduces depth-of-field (DOF), the distance from the lens where stuff is in focus. When you shoot wide-open with any lens, at its widest f-stop, you have to be more careful about focusing. You might aim at a face, and the face will be sharp, while everything nearer or further is blurry. This is great when you want to emphasize a certain part of the picture.

A smaller, slower f-stop lets in less light, so your shutter speed is slower. But your DOF is increased -- more of what's in front of you is in focus. If you're shooting a deep landscape, and you want everything in focus, crisp and sharp, from the nearest flower to the farthest mountain, you use a smaller f-stop.

This is especially so on wide-angle lenses. I can put a slow cheap 21mm lens on my camera, set the f-stop to f/8, set the manual focus to 6 feet -- and my DOF is 3 feet to infinity! I don't have to refocus on anything further than a yard away! This is great for shooting in crowds, and in crowded places. I don't even have to raise the camera to my eye! Yeah, I like sneaky street-shooting.
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

All this stuff is related. Depth-of-field (DOF) is a result of aperture (f-stop) and focal distance (camera to subject). A wider lens or a slower f-stop mean thicker DOF, more is in focus, but you need good light or a slower shutter speed. A longer lens or a faster f-stop means thinner DOF, be more careful of your focus, but you can shoot in less light or with a faster shutter. Using the camera is all about making compromises, the trade-offs between focus and f-stop and shutter speed, and learning how to exploit all the possible combinations. Have fun.
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