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dSLR lens aperatures
Hello, I am just getting into purchasing my first dSLR. however I have a couple quetions about the apertures on the lenses..

They are advertised as say 18-55mm f5.6-8. does that mean there's only fstops of 5.6 to 8? isn't that kinda limiting? I even see some that just say 5.6. does that mean it only has that one aperature? or does the digital camera control all that in the body?..

Comments (17)

Oscar Mach wrote:.

Hello, I am just getting into purchasing my first dSLR. however Ihave a couple quetions about the apertures on the lenses..

They are advertised as say 18-55mm f5.6-8. does that mean there'sonly fstops of 5.6 to 8? isn't that kinda limiting? I even see somethat just say 5.6. does that mean it only has that one aperature?or does the digital camera control all that in the body?.

No those are just the maximum apertures. so a 18-55 f/5.6-8 would have max aperture of f/5.6 at 18mm and max aperture of f/8 at 55mm. Hope that makes sense..

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Comment #1

Oscar Mach wrote:.

Hello, I am just getting into purchasing my first dSLR. however Ihave a couple quetions about the apertures on the lenses..

They are advertised as say 18-55mm f5.6-8. does that mean there'sonly fstops of 5.6 to 8? isn't that kinda limiting? I even see somethat just say 5.6. does that mean it only has that one aperature?or does the digital camera control all that in the body?.

The listed value pertains only to the maximum aperture. With some zoom lenses, the maximum aperture will be wider at the wide end of the zoom than at the long end of the zoom. This helps to reduce cost as well as size and weight..

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Comment #2

Oscar Mach wrote:.

Hello, I am just getting into purchasing my first dSLR. however Ihave a couple quetions about the apertures on the lenses..

They are advertised as say 18-55mm f5.6-8. does that mean there'sonly fstops of 5.6 to 8? isn't that kinda limiting? I even see somethat just say 5.6. does that mean it only has that one aperature?.

No - the numbers are merely the widest aperture the lens is capable of. In the case of something like 18-55m f5.6-8, it would mean that the lens is capable of going as wide as f5.6 at 18mm and f8 at 55mm, from those points the lens can be stopped down further (ie, f11, f14, etc). Generally f5.6 will be the higher number (ie, F3.5-5.6 is more likely for an 18-55 lens) - if you find a lens that has a number higher than 5.6 it's probably not even worth it (there are some exceptions, but even then 6.3 is about the limit). You can find lenses that go as wide as F1.2 but they are very expensive..

When a lens just has one number, ie F5.6 - it means one of two things: It's a fixed focal length (ie, a 400mm F5.6), or it's a fixed aperture zoom (70-200F2.8) that is capable of the same wide aperture at the wide and telephoto end..

Generally lenses with lower numbers are better and more expensive (ie, Canon has two 70-200 F2.8 lenses - a standard for $1100 and an image stabilized version for $1700), wheras Canon also offers a 70-200 F4 for about $600, or $1100 for image stabilized. The F4 version allows only half as much light in as the 2.8 version...

Comment #3

The f-stop quoted is the maximum. Cheaper zoom lenses have a different maximum f-stop at either end of the zoom range. It's easier to make them this way. The maximum aperture varies as the lens is zoomed..

If the aperture is quoted as f5.6-8 then the max aperture would be f5.6 at the wide end and f8 at the telephoto end...

Comment #4

Ah thank you very much for all the replies. I understand it now . 1 more thing though, how do I know the minimum aperture? are all lenses equipped with a standard range?..

Comment #5

Typically F22-F32 is the smallest aperture. It varies from lens to lens, but it should be listed in any spec sheet for a given lens..

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Comment #6

Typically F22-F32 is the smallest aperture. It varies from lens tolens, but it should be listed in any spec sheet for a given lens..

But with an APS-C crop DSLR (i.e 99% of DSLRs) you should not use anything above f/16 because of diffraction. The precise cut off point will vary with the density of pixels on the CCD. On my Nikon D80 it is f/13..

Diffraction means that the aperture "hole" has got too small to give a good path to the pixels on the CCD so the photo loses sharpness..

Chris Elliott.

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Comment #7

Smaller f/ initilal number - more light reaches the sensor with fully opened diaphragm..

F/1.4 lens is much faster (doesnt reduce light so much) as f/2.8 which is much better then with initial f/5.6(slow lens)..

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Comment #8

Another newbie question I always wondered about. When you change the aperture in DSLR body to f5.6 while the attached lens is fixed aperture zoom (eg. f2.8 50mm - 135mm), what is the aperture used to shoot the image?.

Thanks,Robert..

Comment #9

RobertS wrote:.

Another newbie question I always wondered about. When you change theaperture in DSLR body to f5.6 while the attached lens is fixedaperture zoom (eg. f2.8 50mm - 135mm), what is the aperture used toshoot the image?.

I don't think such a lens exists because it would be useless..

Usually a 50 - 135 mm f/28 is f/2.8 from the widest to the narrowest angle, or all the way through the zoom range. And they cost a fortune. Have a look at the Olympus 35 - 100 mm which has f/2 available all the way through the zoom range for a idea of what I consider a fortune....

Or else they are trying to deceive you and it's f/8 at the far end and not much use. And they've "forgotten" to mention it..

If puzzled the best thing is to search for the maker's website and then down load the specification and (better still) the instruction leaflet. They will usually tell you all you want to know..

ITOH, when setting (say) f/2 at the camera when it's only got f/35 available the request will be ignored and you'll get f/35 regardless..

Regards, David..

Comment #10

"Another newbie question I always wondered about. When you change the aperture in DSLR body to f5.6 while the attached lens is fixed aperture zoom (eg. f2.8 50mm - 135mm), what is the aperture used to shoot the image?".

Answer 5.6.the lens is f2.8, that is what you are looking through when you are not taking the picture. when in aperature priority you set aperature to 5.6 nothing changes in the viewfinder but when you shoot the aperature goes to 5.6 and the metering system givs you a shutter speed that will give a normally bright pic at that preset fstop...

Comment #11

GaryDeM wrote:.

"Another newbie question I always wondered about. When you change theaperture in DSLR body to f5.6 while the attached lens is fixedaperture zoom (eg. f2.8 50mm - 135mm), what is the aperture used toshoot the image?".

Answer 5.6.the lens is f2.8, that is what you are looking throughwhen you are not taking the picture. when in aperature priority youset aperature to 5.6 nothing changes in the viewfinder but when youshoot the aperature goes to 5.6 and the metering system givs you ashutter speed that will give a normally bright pic at that presetfstop..

Of course the camera should have a DOF Preview Button (Depth of Field) to show you what the image does when stopped down to the selected aperture. In a tiny viewfinder the most noticeable thing is that the image gets considerably darker, but you should see more things pop into focus if you step down far enough...

Comment #12

Thanks for the answers. I'm still a little confused, however, but let me see if I got this straight..

So back to using the 50 - 135mm f2.8 lens, the f2.8 on lens is about how bright the lens is, which in this lens it's constant throughout. But by stopping down the aperture on the camera body to f5.6, it is the aperture on the camera body that, combined with the focal length selected in the zoom range, determines the DOF. In zoom lens with variable aperture, the brightness of the lens will depend on the focal length selected but the aperture used is what the camera body is set to..

Hope this sounds right. BTW, it's funny because I've never had formal education on aperture/focal length/shutter speed until I had to explain to my spouse who was taking a photography class & I couldn't intelligently explain (the textbook didn't do a good job either). This, after shooting for more than 25 years on 35mm SLR as hobbyist - I just knew through experience of what setting to get the desired result but I could never (and didn't care to) explain..

Thanks,Robert..

Comment #13

"So back to using the 50 - 135mm f2.8 lens, the f2.8 on lens is about how bright the lens is, which in this lens it's constant throughout. But by stopping down the aperture on the camera body to f5.6, it is the aperture on the camera body that, combined with the focal length selected in the zoom range, determines the DOF. In zoom lens with variable aperture, the brightness of the lens will depend on the focal length selected but the aperture used is what the camera body is set to.".

The camera body has NO aperature, it simply controls the lens' aperature. if you have a 50mm f1.4 lens you are always looking through the f1.4 with the optical viewfinder. thatnks to metering the camera decided that to take a pic in the conditions that you have it needs 1/250sec and f8.0. when you close the shutter the camera fires at 1/250sec and closes the aperature down to f8.0 while the pic is being taken. when complete the mirror which swung away for the picture taking returns and you are again looking through f1.4..

If you have a zoom lens 50mm-150mm f2.8-5.6. that means that at variing zoom positions the available widest aperature will vary from 2.8 to 5.6. at all mm the lens can close down to the smallest say f22.0, but the widest available will be 2.8 to 5.6 depending on the mm selected..

To add more fun, you might like to know that there are 2 kinds of zoom lenses, varifocal zooms and true zooms..

Truezoom lens means that when you focus at a certain distance and go from one end of the zoom range to another the subject does not go out of focus, ever..

Varifocalzoom lens means that when you focus on a subject at a certin distain distance and go from tht mm to ANY OTHER you have to refocus because the focus changes..

A true zoom lens is more expensive than a varifocal lens. though after focusing they take the same pic and you would not know which was which...

Comment #14

A 50-135mm f2.8 throughout all focal lenghts zoom lens would be a top-shelf product with a very large objective element. It simply refers to the fact that the widest aperture available (2.8) remains constant throughout the zoom range. This says nothing at all about any of the other available (smaller) apertures at a given focal legth in the range..

Hence, in addition to having f2.8, it would also have f4, f5.6, f8, f11 etc...(whatever).

In cheaper zoom lenses, the f stop represented by the widest available diaphragm aperture diminishes in direct proportion to the focal length the lens is extended to, since the f-number is the fraction:.

Focal length/diameter of the aperture in the diaphragm, at the time..

With a fixed maximum aperture diameter (determined by the diameter of the objective element of the lens), the further you zoom out, the more times the diameter of any given aperture will fit into the focal length..

(This is actually much more simple than it sounds on screen.)..

Comment #15

Got it!!! I learned a LOT today. Thank you for sticking with me. Greatly appreciate it. Now I need to save up to get that 50-135 f2.8..

Robert..

Comment #16

I think most points of this thread have already been covered, but I thought I'd point out that an 18-55mm f/5.6-8 sounds like a really shitty lens, well below even kit lens standards, what company makes that one?..

Comment #17

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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