Should a lens be sharp at all F stops?

[dixielandcandles]

Using my Tamron 28-75XR Di 2.8, and using the tip of someone's nose as the focal point (center focusing in program mode, parameters set to 0, hi-jpg), I tested (several times) F2.8, F4, and F8. It seems that the tip of the nose was tack sharp at F8, but slightly softer at F4, then softer at F2.8. Is this normal? I understand my DOF will be super shallow at 2.8, but shouldn't the actual focal point be tack sharp through all apertures? I emailed Tamron and they asked me to send the lens to them for calibration but will take a month to get it back. Your thoughts?

[Jon]

Just my $0.02, but you should consider redoing the test with something a little more precise than the tip of someone's nose. One idea is to have a ruler stretched across the field of view with a contrasty object at one point along it (say the 1 foot mark). Remember, you've got better DOF at f/8 than at f/4 or f/2.8. You may be focussing on the tip of the nose and seeing a problem with the lens, or body, being slightly off, or you may be focussing on a slightly different spot and seeing the effects of DOF.

[roanjohn]

No.

Ro1

[AzzKicker]

Its perfectly normal. Even Canon 'L' Glass is not sharp wide open.

[dixielandcandles]

Ok, thanks.... soooo, does this mean if I do shoot something wide open or F4, then I can expect some USM needed? I have Fred Miranda's Intellisharpen plug in. Can I expect to apply this on my 2.8 to 4.0 shots? Thanks.

[timmyquest]

More often then not yes.

This is one of the things that makes a lens a good lens. What good is an f/2.8 lens if nothing is sharp at f/2.8.

[roanjohn]

Sharpness should vary at different F stops.

A good lens should be sharp wide open.......peaks a bit stopped down......and should lose sharpness if stopped down even more.

This is I think true with all lenses.

Even the best prime will be a bit sharper stopped down.


Ro1

[dixielandcandles]

Well, guess what? I just got the August '04 copy of Shutterbug mag and on page 62 is an article called "Wide Open". It states clearly that lenses will NOT be sharp when wide open. Oh well, so now, I'm wondering "What's the use to have a F2.8 lens when the image is soft at 2.8"??

[timmyquest]

Well, guess what? I just got the August '04 copy of Shutterbug mag and on page 62 is an article called "Wide Open". It states clearly that lenses will NOT be sharp when wide open. Oh well, so now, I'm wondering "What's the use to have a F2.8 lens when the image is soft at 2.8"??

Give me a couple mins, i'll show you why.

[roanjohn]

Well, guess what? I just got the August '04 copy of Shutterbug mag and on page 62 is an article called "Wide Open". It states clearly that lenses will NOT be sharp when wide open. Oh well, so now, I'm wondering "What's the use to have a F2.8 lens when the image is soft at 2.8"??

A mark of a great lens is its performance wide open (i.e. 85 f1.2 L, 50 f1.8 L etc....)

Some lenses are sharper than others.

When there's not enough light, a soft picture is more acceptable than one with motion artifacts............so use f2.8 as a last resort..........unless of course you have a lens that is already sharp from the get-go.

Ro1

[AzzKicker]

I soft image at f2.8 is better than no shot at ALL :)


for example most TC's will only work with lenses at 4.0 or faster. At least for AF to work.

[scottbergerphoto]

The optical quality of a lense is not uniform. The center portion of the lens is usually the best and it goes a little downhill from there. L glass is more expensive because Canon tries to keep excellent quality on them to the periphery. You can find tests for over a hundred lenses at www.photodo.com in the products section. You will see that the numbers improve as you start to stop down the lens.

Here is a pretty typical test of a Canon 50mm f/1.4:
http://www.pbase.com/scottbergerphoto/50mm_f_14__focus_test

Wide open it is a little soft. As you stop it down it sharpens up and remains sharp. Thses differences are sometimes not as noticeable on DSLR's due to the crop factor. The sensor is smaller then a true 35mm and is using a smaller area of the available lens in the center.
Regards,
Scott

[Cadwell]

Well, guess what? I just got the August '04 copy of Shutterbug mag and on page 62 is an article called "Wide Open". It states clearly that lenses will NOT be sharp when wide open. Oh well, so now, I'm wondering "What's the use to have a F2.8 lens when the image is soft at 2.8"??

Think about it... an f/2.8 lens may be a little soft at f/2.8 but will be quite sharp at f/4 and tack sharp at f/5.6. An f/4.0 lens will be a little soft at f/4.0, quite sharp at f/5.6 and tack sharp at f/8... now an f/5.6 lens.... can you see the progression?

Better quality lenses soften less wide open, but they all do it to some extent.

[Tom W]

Its very dependent on the lens. Yes, in almost all cases, a lens will be less sharp at the extreme wide aperture, and will suffer from diffraction softness towards the smallest aperture. Almost all lenses have their best qualities towards the middle apertures (between f/5.6 and f/11) with f/8 probably being the most common. But some lenses are better than others, and some have different characteristics than others.


There are some lenses that simply perform better wide open than others. For example, my 50/1.4 is a little soft at f/1.4, but improves dramatically by f/2.0. It is very sharp at f/2.8. On the other hand, my 85/1.8 is fairly sharp even wide open at f/1.8. It is about equal to the 50 at f/2 (almost wide open), and on down to smaller apertures, as best as I can tell.

There are some lenses that provide wide apertures that are pretty bad wide open. My Sigma 20 mm isn't great at f/1.8 (though admittedly, I haven't used it enough in the real world to say that with a great degree of certainty), but at f/2.8, its good, and at f/4, it has a very slight edge on my 17-40L in terms of sharpness (color is another story). So while it is capable of f/1.8, I'm most likely to use it as if it were an f/2.8 lens. Although, if I'm shooting in reasonable light, I'll probably turn to the 17-40 instead.

I've seen images from a Leica lens at f/2.0 that are very, very sharp, clean, constrasty, and colorful, similar to what a lot of good lenses do at f/4 or f/5.6.

[CyberDyneSystems]

The norm is certinly a bell curve.. Sharper at mid f/stops (f/8-f/16) and gradually decresing in either direction...

Lots of variation as to what aperture gives the best results though.

But.. the hallmark of the best "fast" lenses is there ability to be VERY sharp "wide open"

Shutterbug has certainly explained what is common,.. but there are exceptions to all rules.. and if you were to peruse the internet you'd read all kinds of agreeing ifo on which lenses do indeed buck the trend by rmaining VERY sharp wide open. (Heck I just posted such a list of these legendary lenses in this forum about a week ago)

Big primes are the obvious example.. these lenses are absolutely DESIGNED to shoot wide open nearly all the time.. and they are in fact the Sharpest lenses out there. People never stop them down because they need them to be sharper!