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09-15-2008, 05:16 PM   #1
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Opening aperture

This is probably a dumb question, but I would like to get some very shallow depth of field shots and can't figure out how to open my aperture to more than 3.5 or 4.0. I am shooting in manual.
Thanks,
Karen

09-15-2008, 05:23 PM   #2
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You need a lens that supports an aperture that large. Chances are your lens does not (a physical limitation). The larger the aperture (aka faster lens), the more expensive the lens (generally rule).

The kit lens only goes to f/3.5 at the widest setting.
09-15-2008, 05:25 PM   #3
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I thought that was probably my problem but I was hoping I could over ride it somehow!
Thanks!
09-15-2008, 05:28 PM   #4
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What lens are you using Karen?

09-15-2008, 05:40 PM   #5
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I have a kit lens, DA 16-50 (which I had put away for a while but just pulled out to play with) and the one I've been shooting the most with lately is an 18-250.
What do you suggest for portraits?
Thanks!
09-15-2008, 05:44 PM   #6
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Well, of those three, to 16-50 would give you the more shallow DOF as it goes down to f2.8.
09-15-2008, 10:18 PM   #7
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Look into the FA 1.4/50

Its not expensive, it is shape, its has a very shallow depth of field, and it works well for portraits.

Check out some of the threads in the lenses section. Overall, I would say it is a very popular lens.

Eric
09-16-2008, 12:53 PM   #8
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QuoteOriginally posted by karenw217 Quote
I thought that was probably my problem but I was hoping I could over ride it somehow!
Any time you buy a lens, you should pay attention to what the maximum aperture is. It usually says this right in the full name of the lens, like:

DA* 16-50 f/2.8 ED AL [IF] SDM

The f/2.8 is what you are looking for. Sometimes this is abbreviated, like DA*16-50/2.8.

Many zooms have a maximum aperture that varies from the wide end to the long end, like DA 18-55 f/3.5-5.6.

Needless to say, all else equal, lenses that support a larger maximum aperture are usually more expensive than lenses with a smaller maximum aperture.

As for "override", that is physically impossible. AMaximum aperture means using the entire piece of glass. Anything smaller than that is done by closing off part of the lens, but you can't make it physically bigger than it is! If the lens is f/4, that as big as the glass actually is.

09-16-2008, 02:07 PM   #9
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You can try getting closer to the subject to get a shallower DOF.
09-16-2008, 04:44 PM   #10
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QuoteOriginally posted by bentax Quote
You can try getting closer to the subject to get a shallower DOF.

This is true, get close to your subject, and have the background fairly far away, and you'll get the "shallow DOF" look, even with the kit lens.
09-16-2008, 07:00 PM   #11
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QuoteOriginally posted by bentax Quote
You can try getting closer to the subject to get a shallower DOF.
This is true but keep in mind that the closer you get to your subject, especially if it is a 'head-and-shoulders' shot, you risk distortion i.e. the 'big nose and little ears' effect.

If you use a zoom at the longer end, it reduces the depth-of-field while minimising any distortion.
09-17-2008, 05:10 AM   #12
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QuoteOriginally posted by karenw217 Quote
What do you suggest for portraits?
Thanks!
Since you are shooting Manual, try to pick a M50 f/1.7, great lens and dirt cheap. You can find them on fleabay. Check the lens review database for more information on this lens.

Cheers
Nish
09-17-2008, 12:09 PM   #13
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QuoteOriginally posted by Marc Sabatella Quote
As for "override", that is physically impossible. AMaximum aperture means using the entire piece of glass. Anything smaller than that is done by closing off part of the lens, but you can't make it physically bigger than it is! If the lens is f/4, that as big as the glass actually is.
Not quite. An f/4 lens will have a maximum aperture opening that is 1/4 of the focal length (assuming that it is a prime, a zoom will have a variable opening to match the focal length). If it opened to the physical size of the lens, you'd have an f/1.

If you look at a zoom lens with a variable aperture (like the kit lens), the aperture doesn't actually change size as you zoom. What happens is that because the aperture is the same physical size, the effect is a smaller f/ number. For example: 50-100mm zoom lens, with max aperture of f/2 at 50mm. When you zoom in to 100mm, the aperture is still 25mm (50/2=25mm). That same sized opening is now an f/4 (100/4=25mm). Variable aperture zooms are simpler to make because they don't account for the zoom length at the long setting.

Fixed aperture lenses on the other hand (like the 16-50 f/2.8) will adjust the aperture size to adjust for zooming. 50mm f/2 is 25mm, while 100mm f/2 is 50mm. More technically complicated to make and the barrel needs to be larger to account for the extra size needed by the aperture blades when zoomed into the longer focal lengths.

The same thing works in reverse with the minimum aperture.

These sizes also are not necessarily the real sizes of the aperture, as the blades can be placed inside the lens where the path of the light is constricted, so they can use smaller mechanics to shrink the overall size of the lens.
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