Aperture question

Discussion in 'Photography Beginners' Forum' started by Judobreaker, Mar 20, 2012.

  1. Judobreaker

    Judobreaker TPF Supporters Supporting Member

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    I've been wondering about the aperture for a while.
    This is probably the one thing in a lens my knowledge is the lowest about. :p

    I know aperture is simply the actual opening in the lens and the aperture blade shape affects the bokeh and all.
    The real question lies in the amount of light going through.
    I know the openings are standardized (calculated by dividing the focal length through the pupil opening according to Wikipedia).
    This means f/4 always illuminates the sensor with the same amount of light.

    Now I've been looking to get a new (tele)lens and I've been unsure what to do.
    I've been looking at it rather logically.
    I will probably not shoot wide open a lot so I can live without f/2.8.
    A 300mm at f/8 will give me about .5m DOF which I think is quite nicely and most lenses are sharpest around f/8 (from what I gathered in various reviews).
    So, my conclusion was: Get a nice long lens with an aperture like 4.5-5.6 and go nuts.

    Now here comes my problem.
    A friend of mine said I'd be way better off getting a 2.8 lens.
    He said that the wider my maximum aperture, the better the lens would be at gathering light.
    According to him a 2.8 would also gather more light at f/8 than a similar lens with a lower widest aperture.
    This seems illogical to me...
    He really stresses this point but I'm not sure how much I should believe this.

    I've seen some nice looking lenses for <1000, would that 2.8 aperture really be worth spending way over that 1000?
    They're really quite a lot more expensive... xD


     
  2. Overread

    Overread has a hat around here somewhere Staff Member Supporting Member

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    Light gathering isn't just about your exposure, it also affects two key properties:

    1) Viewfinder image - when focusing and composing a brighter maximum aperture means more light and thus a much clearer image.

    2) More light for the AF sensors - all Af is done with the aperture blades wide open, if that is f2.8 that means those sensors have a lot more light to work with and thus can offer much more improved accuracy and response.

    Furthermore its important to realise that optical quality also generally improves with wider max apertures. That isn't to say so much that wider makes the lens better, but that lens manufacturers put their best glass into wider maximum aperture lenses.
    A 70-300mm f4-5.6 will have slower AF motors and a generally lower grade of optics than a 300mm f4, which then again will be lower grade (though not by as big a jump) to a 300mm f2.8.

    In addition higher end lenses have improved sharpness and optical clarity even when wide open. Yes around f8 they'll still perform their best, but they'll also deliver very good quality results wide open. This not also means that you can work in dimmer and dimmer lighting whilst still shooting and not having to raise your ISO to exceptionally high levels.

    Furthermore something like a 300mm f2.8 can easily take a 2*TC to give you a 600mm f5.6 lens which still delivers good optical performance (and if closed down to around f7.1/f8 a very good level).


    A lot of this varies lens by lens so the above is a generalist explanation.
     
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  3. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    Short answer: Yes, it's is better to have a lens with a larger maximum aperture.

    Let's look at your 300mm example. If you are using a 300mm lens, you will want a fast shutter speed to prevent blur from camera shake. There is a rule of thumb that tells us that you should be using a shutter speed of at least 1/300. Many say that the rule should be adjusted for the crop factor (1.5X), or that it should be doubled. So if you want to get the most out of a 300mm lens, you should probably be using a shutter speed of 1/450 to 1/600.

    Now consider how much light you would need to have, so use a shutter speed like that, along with an aperture of F8, or even F4. Quite a bit. You could raise the ISO (will probably have to) but the higher you go, the more digital noise you have to put up with.

    So by having the option of a large maximum aperture, it makes it easier to get a faster shutter speed...which is especially important with a longer lens.

    With long lenses, they typically get used wide open 90% of the time, because a fast shutter speed is so very important.

    You DOF is also determined by the distance to the subject. The farther away your subject is (focus point), the deeper the DOF.

    [​IMG]
     
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  4. 2WheelPhoto

    2WheelPhoto TPF Noob!

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    Buy the f2.8 lenses for a plethora of reasons. They're best zooms and not only because of the fastest zooms.

    Thank me for the tip later.
     
  5. dxqcanada

    dxqcanada Been spending a lot of time on here!

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    So what he is saying is a 300mm f/2.8 vs a 300mm f/4.5 ... when both lenses are set to f/8.0 ... the 300mm f/2.8 will expose the film/sensor to more light ??
    The 300mm f/2.8 lens elements will "gather" more light ... but it still has to pass through the aperture.
     
  6. Tiberius47

    Tiberius47 No longer a newbie, moving up!

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    That's true, but remember that the lens doesn't actually stop down the aperture until you open the shutter. While the camera is metering and focusing, it's doing it with the aperture wide open. This lets in more light for autofocus and metering, as well as giving you a nice bright viewfinder image.

    Plus, f2.8 lenses tend to be more expensive because they have better quality glass.
     
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  7. KmH

    KmH In memoriam Supporting Member

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    The lens apeture f-stop is a fraction. It is a ratio realtive to the focal length of the lens.

    A 100 mm lens set to f/4 has lens aperture diameter of 25 mm.
    A 200 mm lens set to f/4 has a lens aperture that is 50 mm wide.
    A 50 mm lens set to f/4 has a lens aperture that is 12.5 mm wide.

    Next lets consider what a stop is. A stop is a doubleing or a halving.
    Doubeling or halving the lens aperture diameter does not double or halve the amount of light the aperture lets through. The lens aperture opening area has to be doubled or halved.
    Since it's the area of the lens apertue that is invloved, to calculate a full stop change in lens aperture we have to multiple or divide using the square root of 2, which is: 1.41421356237309504880168872420969807856967187537694807317667973799...

    For our purposes 1.4142 is sufficient. So.....to halve the amount of light a lens set to f/1 lets in (1 x 1.4142 = 1.4142), we stop the lens down to f/1.4. To halve the amount of light a lens set to f/1.4 lets in (1.4 x 1.4142 = 1.97988), we set the lens to f/2. (Notice the rounding off.)

    2 x 1.4142 = f/2.8
    2.8 x 1.4142 = f/4
    4 x 1.4142 = f/5.6
    5.6 x 1.4142 = f/8

    The concept of a stop can also be applied to shutter speed and ISO. However, shutter speed and ISO are not about area so times or divide by 2, not the square root of 2.
    Most DSLR come set by default to allow adjusting the exposure settings in 1/3 stop increments. Most also let the user change the increments to 1/2 stop or full stop steps.

    Few fast, or wide aperture, lenses deliver their sharpest focus when the lens aperture is used wide open. Most need to be stopped down 2 or more full stops to their sharp focus 'sweet spot' of aperture range. As lens aperture gets smaller still, focus sharpness again starts dropping off from the effects of diffraction.
     
    Last edited: Mar 21, 2012
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  8. Judobreaker

    Judobreaker TPF Supporters Supporting Member

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    Some solid arguments in favor of a larger maximum aperture in here. ^^


    Hmm... I kind of overlooked the TC. That might actually be a pretty good idea.
    So a 2x TC would bring me down 2 stops?
    I really should consider this... ^^


    Makes sense. I didn't really think it was that big a deal to be honest.
    Good thing I asked this. xD


    Yeah, I forgot to mention the distance. I took 12m, seemed like a nice distance to me. :p


    Yeah, that's what I thought too. xD


    A good point. I hadn't thought about the metering actually...


    Thanks for all the replies so far.
    This'll help me loads. :D
     
  9. Diffuser

    Diffuser TPF Noob!

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    That was all great info to read.

    So your friend was right in a way, but just didn't explain it clearly to you ;-)
     
  10. Overread

    Overread has a hat around here somewhere Staff Member Supporting Member

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    Yep a 2*TC takes away 2 stops; a 1.7TC takes away 1.5 and a 1.4TC takes away one stop. With something like a 300mm f2.8 you can easily use any of the TCs to increase the range as you need. Of course the higher the value of the TC the more degradation you'll see to the optical quality. On many lenses a 1.4TC has very negligible effect; a 1.7 is a bit more middle of the ground whilst a 2*TC is generally best only used on the best of lenses. Further whilst you retain auto focus all the time, once you go past an effective aperture of f5.6 on a lens (or lens + TC) the performance of the AF will drop more noticeably on all but the top range of lenses (canon side you actually lose AF once the lenses maximum capable aperture goes smaller than f5.6 barring on their 1D line bodies).


    It would help if you gave some idea of the subjects, situations, lenses and budget that you're considering. Granted by the direction of the conversation we can guess a fair bit, but some idea of what your thinking helps as well. You might well be better served considering a native longer lens (although costs do go up) and there might be some older nikon glass that would be good quality, but cheaper which might be worth considering as well.
     
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  11. Judobreaker

    Judobreaker TPF Supporters Supporting Member

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    Ok, I'm taking the following in consideration for this lens:

    - The subject will mainly be wildlife, though I'll probably also use it for other things.
    - Price range is preferred but not limited around &#8364;1,000 (if there's real good arguments to spend 2k I'll just wait a while longer).
    - I usually prefer new over used (I hardly ever get something second hand).

    Taking the TC idea into consideration the Nikon 80-200 f/2.8 AF-D and the Nikon 70-200 f/2.8 AF-S VR II both look really nice.
    With a 2x TC that would give me a maximum of 400mm which I think should be enough for my needs, it's a huge upgrade from my 105mm lens and I'm not afraid of sneaking further up to animals.
    These lenses also should be nice for portraiture.

    The 70-200 has VR II which is of course very nice. The downside however is that it's almost twice as expensive as the 80-200 and optically they're almost equals (according to Ken Rockwells reviews). Also the 70-200 isn't really 200mm at close focus (I think I read it's about 135mm).

    Then again, when looking at the 80-200, I'm not really sure how that one is going to work with TCs.
    Ken mentions Nikon doesn't make converters that will auto-focus with this lens. I'm not too afraid of manual focus so far (I always manual macro) but I haven't got any experience with telelenses so that might be different in use.

    I'm leaning towards the 80-200 at this point because of the rather large price difference.
     
  12. Overread

    Overread has a hat around here somewhere Staff Member Supporting Member

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    Putting a 2*TC onto a zoom lens is asking a lot of the zoom. There are few that can really take a 2*TC and still deliver good optical and general performance.
    I don't know the 80-200mm that well for Nikon, but honestly even if the difference is small I'd still go for the 70-200mm VRII if you want to use a 2*TC. If its like its Canon counterpart you should get good optical performance through the range, though best will be at around f7.1/f8; wide open will be softer, usable, but not as crisp. And as said you get a stunning 70-200mm lens for general portrait work and shooting.
    I wouldn't worry too much about close up focal lengths, many lenses reduce their effective focal length when focusing closer and its an area where things get a little vague (since its not a property most people know about and one that is also not reported on that often).

    That gives you a 140-400mm lens which is a great start for wildlife work.

    That said there are a few other options:

    Sigma 120-300mm f2.8 OS (not the original without OS). Expensive, much bigger and heavier (in fact this is heavier than most 300mm f2.8 primes). However you can use a 1.4TC for a great 420mm at the long end or a 2*TC for a strong 600mm capable lens.

    300mm f2.8 prime - I don't know the Nikon range all that well so this is more generalist, but a bit lighter than the zoom, more expensive; but there might be older nikon versions that are suitably good performers to consider as well. Again you can push to 600mm if needed.

    400mm f2.8 - 500mm f4 - etc.... - getting well into the serious money here and into heavier and bulkier lenses. Fantastic optics but expensive; and oft lighter options (eg 300mm f2.8 or 70-200mm +2*TC are still kept around for lighter options)

    200-400mm - I hear great things about this lens and I know a few have swapped over to Nikon just to get this lens; that said it is, again, a very expensive option.



    Sadly wildlife is one area of photography where going cheap on the lenses is hard to do; long range quailty optics are sadly expensive to produce. Of course you can save a bit if you've very good field craft, but typically you'll be wanting at least 300mm as a starting point for wildlife - and longer is generally helpful.
     
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