Whats the sweat f number?

chiffs

chiffs

Messages
437
Name
Sean
Edit My Images
Yes
What is the sweat f number for my lens

Kit 18-55mm is

50mm f1.8 II

EFS 55-250mm

Body is a 500d if that makes any difference

Want to do some trials of this over the weekend

Uses for the lens are as follows

Kit lens, landscapes, some portraits and general dogs body lol

50mm for portraits, mainly 7 month old daughter inside with poor light.

55-250 for wildlife in the garden and have used it a couple of times at zoos for tigger, larry the lion, daffy the duck and all there friends.

No speedlight yet for any indoor stuff.
 
Totally depends upon the situation really? Two stops down from wipe open is normally where you get the sharper images, but this is only a generalisation. Some lenses are just as sharp wide open so it makes little difference.

Too general of a question to be honest? Will more depth of field bring more to an image or ruin it by adding unnecessary detail. Maybe on a landscape a stop or two down will be preferable than to a portrait where you want to isolate the subject?
 
From experience with the 18-55 kit lens I found it to be good at around f5.6 - 8
On the 50mm prime it was good at f2 - 2.8 and beyond, at f1.8 it can be quite soft.

Can't answer for the 55-250 though as i've never used that lens.
 
I get quite sweaty the bigger and heavier the lens I use.

Oh, wait....

I see what you mean, sweet spot... The 55-250, Use it wide open, f/4-5.6, it's very sharp all the way through.

50mm from f/2.2 I'd say and the kit, I can't remember ever getting that sharp (I had an old non IS one though)
 
Last edited:
strumstrum said:
I like the 50mm 1.8 at f4, I think it gives a good balance of focus/DoF for portraits..

if you are using your kit 18-55 for candid portraits then you want to be slapping it on a tripod and shooting f11+
Click to expand...

You don't want to go above f11, with any lens on a crop, you'll start causing diffraction... F11 - f13 is about the limit.
 
My 18-55 is best at f8 and the 55-250 f10 or 11, depending on light.Sorry unable to help with the rest.
 
killwilly said:
My 18-55 is best at f8 and the 55-250 f10 or 11, depending on light.Sorry unable to help with the rest.
Click to expand...

55-250 at f/11? It's sharpest way before then!
 
Softening due to diffraction only becomes a limiting factor for total sharpness when using a sturdy tripod, mirror lock-up and a very high quality lens.
 
EdBray said:
Softening due to diffraction only becomes a limiting factor for total sharpness when using a sturdy tripod, mirror lock-up and a very high quality lens.
Click to expand...

Got to disagree with you there Ed. On a crop format camera, which lowers the diffraction threshold by about one stop compared to full frame, it's very visible at f/16 and really bad at f/22, with any lens, any time. Agree with Jim.

Edit: good link showing the sweet spot here, on a Canon 50 1.8 http://www.dpreview.com/lensreviews/canon_50_1p8_ii_c16/page3.asp If you slide the f/number, you can see how the blue area (sharpest) spreads from the centre as the f/number rises and aberrations are reduced. Then it peaks around f/5.6, the sweet spot, before diffraction begins to take its toll and gets progressively worse. See also how the edges of the frame continue to get better after the centre has peaked and starts to drop. Note that 'worse' in this context doesn't mean bad - sharpness up to around f/11 will be damn good. If you're shooting at f/11 and diffraction is the only thing that's holding sharpness down, then you'll have some pretty damn sharp pictures - which is Ed's point I think.

On that test you can also switch to the same lens on full frame and see how that moves the sweet spot up to about f/8, which is because there is roughly only half the magnification involved with the larger fomat, which equates to about one stop more tolerance. All decent quality lenses peak around f/5.6 on crop format, and f/8 on full frame. A really good lens might do it a bit lower and reach a higher peak (eg Canon 100L macro on that site) and a lower quality lens will peak a bit higher and at a lower peak, but the big difference between the best and worst lenses is at the edges and corners of the frame, which are sometimes never decently sharp (eg consumer grade kit zooms).
 
Last edited:
Indeed. And due to the crop sensor, and the smaller space it needs to sharpen, even on a half decent lens the edges should be sharp by f/11 so there's very little point in going over that as you'll just start losing valuable light.
 
HoppyUK said:
Got to disagree with you there Ed. On a crop format camera, which lowers the diffraction threshold by about one stop compared to full frame, it's very visible at f/16 and really bad at f/22, with any lens, any time. Agree with Jim.

Edit: good link showing the sweet spot here, on a Canon 50 1.8 http://www.dpreview.com/lensreviews/canon_50_1p8_ii_c16/page3.asp If you slide the f/number, you can see how the blue area (sharpest) spreads from the centre as the f/number rises and aberrations are reduced. Then it peaks around f/5.6, the sweet spot, before diffraction begins to take its toll and gets progressively worse. See also how the edges of the frame continue to get better after the centre has peaked and starts to drop. Note that 'worse' in this context doesn't mean bad - sharpness up to around f/11 will be damn good. If you're shooting at f/11 and diffraction is the only thing that's holding sharpness down, then you'll have some pretty damn sharp pictures - which is Ed's point I think.

On that test you can also switch to the same lens on full frame and see how that moves the sweet spot up to about f/8, which is because there is roughly only half the magnification involved with the larger fomat, which equates to about one stop more tolerance. All decent quality lenses peak around f/5.6 on crop format, and f/8 on full frame. A really good lens might do it a bit lower and reach a higher peak (eg Canon 100L macro on that site) and a lower quality lens will peak a bit higher and at a lower peak, but the big difference between the best and worst lenses is at the edges and corners of the frame, which are sometimes never decently sharp (eg consumer grade kit zooms).
Click to expand...

odd jim said:
Indeed. And due to the crop sensor, and the smaller space it needs to sharpen, even on a half decent lens the edges should be sharp by f/11 so there's very little point in going over that as you'll just start losing valuable light.
Click to expand...

I agree diffraction has an effect but it is not the B all or end all, there are usually many other parameters that affect sharpness in images other than diffraction, such as subject movement, camera movement and the big one. how much the final image will be enlarged. Makes you wonder why manufacturers make crop factor lenses that go beyond f11 when it would no doubt be cheaper to make without.
 
Last edited:
You must log in or register to reply here.
Back
Top