high f number or low iso?

Discussion in 'Beyond the Basics' started by darich, Mar 15, 2007.

  1. darich

    darich TPF Noob!

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    I've just read on another thread that most lenses are sharpest around their middle range of apertures eg f8 or f11. I agree that this is the case.

    My 2 L lenses both have a constant aperture of f2.8 and i use them at that end quite a lot. But in order to get the sharpest image from my lenses, i should set my aperture to around f8 as previously mentioned.
    The problem is then the light. At f2.8 light isn't so much of an issue. But at f8 it is. So i could easily increase the ISO to compensate.
    I know there are times i'd want f2.8 and others i'd want f8 so i'm talking about general, run of the mill shots on my 24-70 L lens.

    The 5D has incredibly low noise even at 1600 but the question i have is this:

    Would i be better to shoot at f2.8/3.2 and ISO 100 or get a sharper image at f8/f11 but have higher ISO eg 1000 or 1200??

    if i get a chance i'll try a comparison over the next day or so and post them but what do you guys think?
     
  2. Big Mike

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

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    I would think that using the lower ISO would be the priority for getting the best image quality...but that's only an opinion.

    Maybe if it was a choice between turning up the ISO on a great camera like the 5D or using a cheap lens wide open...then it would be harder...but good quality lenses (Canon L) are usually not that bad when used wide open. Some of them, like the telephoto lenses, are designed to be used wide open so are really sharp at F2.8 or F4.

    Personally, I only think of using F8 (for quality) when there is enough light to still get a shutter speed that I can use (or when using a tripod)...but DOF is usually more of a priority. Actually, now that I think of it...I do use F8 for quality when shooting with my studio lights...but I also use ISO 100 and would change the aperture before changing the ISO.

    I'd be interested in seeing your comparison tests.
     
  3. Digital Matt

    Digital Matt alter ego: Analog Matt

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    On high quality lenses with wider max apertures, the sweet spot extends much wider than f/8. I would guess that on your 24-70 2.8, it's probably sweet at f/4 already, and definitely f/5.6. That's why we pay all that cash for fast glass. I hardly ever use my 50 f/1.8 wide open. It looks fantastic from f/3.5-f/11 though.
     
  4. darich

    darich TPF Noob!

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    thanks guys
    it's quite sunny here just now so i might be lucky and get a few sample shots tonight f8 and ISO approx 1200 and f2.8 with ISO at 100 or L (50)
    i might also try f4 or f5.6 as matt said he reckoned they'd be really sweet.

    i also have a 50mm f1.8 - i might try that as a comparison against the L quality zoom lenses.

    Cheers!
     
  5. I assume you're talking about hand-held shots, but some subjects (esp. landscape and architecture) benefit from using a tri-pod so that you can get a longer exposure at a tighter aperture but still using a low ISO.
     
  6. darich

    darich TPF Noob!

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    Yes - i'm talking about handheld shots. I use a tripod when going for good landscapes so i can get large DOF and low ISO.
     
  7. RMThompson

    RMThompson the TPF moderators rock my world!

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    This is what I dont get.

    We want the fastest glass, i.e. f/1.4 - f/1.8 but then a lot of people reccomend never using them that wide. If I am going to set my glass at f/5.6, why not use a f/3.3 lens?!

    explain?
     
  8. shingfan

    shingfan TPF Noob!

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    the sweet spot (so called) is usually 2 stops or more from the largest aperture........say if you compare a f1.8 lense and a f5.6 lense both set at f5.6....the f1.8 lense would usually yeild better result....that's all
     
  9. Digital Matt

    Digital Matt alter ego: Analog Matt

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    Nobody ever said you shouldn't use a lens wide open. Good glass is sharp wide open. It's "sharper" when stopped down a stop or 2 from it's max however. Every lens is. So if you are trying to maximize your sharpness, and you don't mind adding a bit more depth of field, it's cool to stop down a few. We pay for expensive 2.8 and 1.8 glass because, we don't have to stop down as far to get that amazing sharpness.

    Why not use an f/3.3 lens? #1 because one doesn't exist. #2 because it's not going to be as sharp at f/5.6 as a 2.8 will most likely.
     
  10. Big Mike

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

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    That's a good question...but it should be easy to answer.

    Firstly, it's nice to have the option to use a lens at a large aperture. This thread is more about ultimate image quality...sure the lens is better at F8 but that doesn't mean that we always shoot at F8. If the light is low, sometimes the only real option is to shoot at F1.8. Or sometimes we want that really shallow DOF that comes with shooting at F1.8.

    Secondly, lenses are usually best when stopped down at least one stop from their max and get better down to F8, F11 etc. So if we have a zoom lens that is F3.5-5.6...then at the long end, F5.6 is the max and probably isn't all that sharp. Even F8 is only one stop closed from the max. However, if we had an F2.8 lens, F5.6 is two stops closed and F8 is three stops closed down from the max...which puts it right in the sweet spot.

    *edit* They beat me to it. :D
     
  11. Jeremy Z

    Jeremy Z No longer a newbie, moving up!

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    Hi David,

    In fact, some high quality lenses that are designed to be fast, such as constant-aperture f/2.8 lenses, are sharper wide open than they are when stopped down.

    What Digital Matt says is usually true. But there are lenses out there that actually get worse the more stopped down they are, but these are usually primes.

    For me, I'll take a sharp picture, but with a bit of grain over a motion-blurred picture with fine grain any day. Since you also have the fast lens and good noise reduction going for you, it is a no brainer.

    Now if you take a picture, and later want to sell it or enlarge it and it has too much noise, the noise can be all but eliminated with software made specifically for this purpose. Not so with a blurred image. With a blurred image, you're SOL.
     
  12. Don Simon

    Don Simon TPF Noob!

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    I'm with Mike... I would prioritise low ISO. Now if we were talking 'kit' lenses or other cheap and slow zooms, definitely I would not shoot below f/8 unless I could avoid it. When a kit zoom has a max aperture of f/3.5-5.6 I consider it effectively an f/8 lens because the difference in sharpness is that obvious. But here we are talking about quality glass which is designed to be used wide-open or near to wide-open. I would be very surprised if your f/2.8 L lenses weren't about as "sweet" as you could reasonably want at say f/4.

    RMThompson, if it's a good fast lens you shouldn't have to stop it down to f/5.6. As Matt said, with a few exceptions most lenses are sharper a couple of stops down from the max aperture. With an f/1.8 lens that's obviously not as far as f/5.6. Personally I like using an f/1.4 lens; now if it were necessary to stop down to f5.6 to get really good results from it then I would be mad to spend the money on such a lens. Once I hit about f/2.8 it's about as sharp as I could want, and f/4 is really sweet. Going to f/8 doesn't really give me any extra benefit, except it gives me greater DOF. But even though my lens performs best at f/2.8 and beyond, it will still do a good job even wide-open; it just won't be all that sharp. In fact for me the bigger problem with a really wide aperture is probably not lack of sharpness but the shallow DOF which requires much more precise focusing to obtain that sharpness.
     

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