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Constant aperture lenses
Do they always have a constant apertue in all zoom ranges and if a lens has a constant aperture of f2.8 doesnt this mean that it will always produce images with a narrow depth of field?..

Comments (6)

Ziggy25 wrote:.

Do they always have a constant apertue in all zoom ranges and if alens has a constant aperture of f2.8 doesnt this mean that it willalways produce images with a narrow depth of field?.

"constant aperture" in this context is a misnomer. It's constant maximum aperture. Minimum aperture is typically f/22 or smaller (larger numerically). You can choose any aperture in this range..

If you shoot at (say) f/8, you will get the same DOF at the same focal length whether the lens is f/2.8 max aperture, f/3.5-5.6, or whatever..

Seen in a fortune cookie:Fear is the darkroom where negatives are developed..

Comment #1

Ill explainIf you have for example a 70-200 F2.8 lens you can have F2.8 at 70 and 200..

But you can also change the aperture to F4, F5.6 F8 etc. But the largest you can go to is that lenses max F stop EG F2.8What they mean by constant is your aperture does not change when you zoom..

For example of non constant the EF-S 17-85 lens can have an aperture of F4 at 17mm but at 85mm the largest aperture is F5.6 you can still change the aperture to F8 etc..

Lenses with constant aperture are nice to have so when you zoom in you can have a nice shallow DOF or allow a lot of light to hit the sensor.Sparky_caGo take photos ..

Comment #2

Aha it makes sense now. I always thought that with a constant aperture lens you cant change the aperture but obviously you can..

Thanks..

Comment #3

Ziggy25 wrote:.

Aha it makes sense now. I always thought that with a constantaperture lens you cant change the aperture but obviously you can..

There are some lenses that really are fixed aperture. For example, mirror lenses:.

Http://www.photozone.de/3Technology/lenstec5.htm.

Seen in a fortune cookie:Fear is the darkroom where negatives are developed..

Comment #4

To answer the second part of your question, if you keep the aperture at one setting (say f2.8), keep the lens focused at the same subject, and change the zoom, the depth of field will change as you change the zoom. For example, say you have a Canon Digital Rebel XT with a 24-70mm f2.8 lens. If you focus on a subject that is 10 feet away with the lens at the wide end of the zoom (24 mm) and the aperture wide open at f2.8, you will have a depth of field of 6.13 feet. If you zoom the lens out to 70 mm, keeping focus on the subject 10 feet away and the aperture at f2.8, the depth of field will drop to 0.65 feet..

Check out this online depth of field calculator: http://www.dofmaster.com/dofjs.html..

Comment #5

EdwardR wrote:.

To answer the second part of your question, if you keep the apertureat one setting (say f2.8), keep the lens focused at the same subject,and change the zoom, the depth of field will change as you change thezoom. For example, say you have a Canon Digital Rebel XT with a24-70mm f2.8 lens. If you focus on a subject that is 10 feet awaywith the lens at the wide end of the zoom (24 mm) and the aperturewide open at f2.8, you will have a depth of field of 6.13 feet. Ifyou zoom the lens out to 70 mm, keeping focus on the subject 10 feetaway and the aperture at f2.8, the depth of field will drop to 0.65feet..

Check out this online depth of field calculator:http://www.dofmaster.com/dofjs.html.

Thanks I didnt think of that...

Comment #6

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