PDA

View Full Version : 2 stops from Full Open?


10dedfish
06-02-2011, 05:27 AM
Hey everyone,

I have read in more than one place that most lenses are sharpest at 2 stops from full open. My question is, what is 2 stops? Is it the difference between F1.8 and F4? Or is it 2 changes in Fstop from full open? For example, If I was using F1.8, and used the dial on my canon to close the iris down and it clicks 2 times.

Thanks

Matt

disneyboy2003
06-02-2011, 06:48 AM
I have read in more than one place that most lenses are sharpest at 2 stops from full open. My question is, what is 2 stops? Is it the difference between F1.8 and F4? Or is it 2 changes in Fstop from full open? For example, If I was using F1.8, and used the dial on my canon to close the iris down and it clicks 2 times.


The difference between f/1.8 and f/4 is a little more than 2 f-stops, so your initial assumption was correct. Technically, though, 2 f-stop difference from f/1.8 is actually f/3.6.

Traditionally, f-stops have been measured in increments of http://upload.wikimedia.org/math/b/f/c/bfc552de0f2e3353d96542d0e6382405.png. Thus, whole f-stops have been:

... 1.4 / 2 / 2.8 / 4 / 5.6 / 8 / 11 / 16 / 22 / 32 ...

Canon's dSLR, however, allows you to change the aperture in 1/3 f-stop increments. So your 2 "clicks" of the dial from f/1.8 will actually decrease the aperture by a total of 2/3 stop. Your camera should, thus, display an aperture of f/2.2 as a result of your 2 "clicks". (check to see if this is true!)

(btw, your dSLR might also have the option of changing aperture by 1/2 f-stops instead of 1/3 f-stop increments. This option is probably buried deep in the menu options. If you set your camera to change aperture by 1/2 f-stops, then 2 "clicks" of the dial will change your aperture by 1 full f-stop.)

Hope that helps.

photo_chick
06-02-2011, 08:59 AM
Just to add to what's already been said, some DSLR's (like my 50D) do give you the option to set aperture, shutter speed and ISO to whole stop increments. I have mine set this way because it makes it easier for me to shift the exposure value around in my head when I need to.

Now the whole 2 stops down thing... it's not a hard and fast rule, but lenses are generally at their softest wide open. And many are sharper if you stop them down even farther than just 2 stops.

Set up a controlled shot on a tripod (focus test charts work good for this). Change the aperture for each shot (and the shutter speed to keep the exposure value consistent) keep the ISO the same so the noise is consistent. Compare the shots and find where the sweet spot is on your lens. It's probably more than two stops down.

Bstanley
06-02-2011, 10:48 AM
I haven't actually run an experiment but I strongly suspect that it really depends on what 'wide open' means on a particular lens.

For example I have a Canon EF 70-200 F4L IS telephoto. I doubt that it gets significantly sharper when you stop it down. IMHO it's not just the 'L' that makes it sharp at F4, it's the F4 itself...

I also have the Canon EF 50 F1.8 prime. I find that it is very soft at F1.8, but by F2.2 it's hugely improved and by F2.8 it's a marvel. IMHO it's the ability to open to 1.8 that softens it.

I suspect the farther away from 'Easy 8' you are the exponentially harder it gets to build a sharp lens.

A kit lens that only opens up to F5.6 (at 55mm) may be as sharp as it gets within a single stop or less...

10dedfish
06-02-2011, 10:53 AM
Wow, thanks everyone for your responses. It does make sense that the more narrow your aperture the more sharp your image. But, thats why I try to ask these question here rather than looking in a book. Experience will always trump over book knowledge.

You guys still rock!:cool1:

Matt

boBQuincy
06-02-2011, 11:23 AM
Wow, thanks everyone for your responses. It does make sense that the more narrow your aperture the more sharp your image. But, thats why I try to ask these question here rather than looking in a book. Experience will always trump over book knowledge.

You guys still rock!:cool1:

Matt

Narrower is better but only to a point. There are two main factors working here: aberrations and diffraction. Aberrations get better as we stop down, diffractions get worse. Depending on sensor size diffractions can begin to deteriorate image quality by f/11 or so (for APS-C). Because of this many lenses will give best image quality around f/8, where aberrations are well controlled but diffraction is not yet an issue.

I can confirm from experience that f/22 loses sharpness, trying to get waterfall motion blur in daylight (now I have a 1.8 ND filter for that).