I'm confused.
If the most expensive lens out there are the fastest ones, then why do we need to stop them down in order to get the clearest, sharpest picture?
Dont we pay more for those low numbers??
I'm understanding, lately, that most lens are best around f8 .... give or take a couple of stops either way.
Somebody explain why we would pay nearly two thousand dollars for a low f stop lens when, if we get a better pic around f8, we couldnt be better off with a bottomed out f4?[:S]
I'm sure there are somebody in this forum will explain to you a lot better than I do, but before you read those posts, try to shoot with some fast lenses.(borrow or rent), and I think you will understand better.
Originally Posted by Bombsight
Ok, I'll bite. There's no need for apertures larger than f/5.6, because lenses lose sharpness at apertures wider than that. Also, because of the diffraction limits with dSLR sensors (something else for you to go learn about), images start to lose sharpness on most cameras somewhere from f/7-f/11, getting progressively worse with smaller apertures. So, Canon should really just stop making lenses with adjustable apertures, and instead make all their lenses with a fixed f/8 aperture. We'll still have shutter speed and ISO to adjust our exposures, so that should be fine.
Ok, back to reality...
On the most basic level, it's just physics. To design a lens with the same sharpness wide open as stopped down would require the resulting lens to be impossibly heavy for a given focal length. But, let's look at some of the expensive, fast prime lenses.
The 85mm f/1.2L is nearly $2K. That's a classic portrait lens - the goal of the fast aperture is a thin DOF. At typical portrait distances, you simply cannot isolate the subject from the background effectively with an f/4 lens. In fact, in portraiture, sharpness is not particularly desirable - there's a reason models' photos in magazine spreads are airbrushed.
The fast supertelehotos are very expensive. For example, the 400mm f/2.8 - at $7.2K that's six times what you'd pay for a 400mm f/5.6. i.e. $6K for two stops of light (with IS thrown in). These are the lenses you see on the sidelines at NFL games. Why? Because without those fast apertures, the photographers wouldn't get the shots. The fast aperture is needed for a high enough shutter speed. It's true that the ever-increasing high ISOs on todays dSLRs helps, but noise is worse than slightly diminished sharpness.
The point I'm making is that sharpness isn't everything. One of the problems with 'sharpness' is that people focus on that as a comparison measure, simply because it's easy to quantify. Especially for people just entering this realm, it gives them a nice set of numbers that they can compare. They can say, "Wow, I checked out the photozone.de test charts, and my 50mm f/1.8 that I paid $100 for is just about as sharp as that 50mm f/1.2L that costs $1500," ignoring all of the other factors that make the 50mm f/1.2L the lens that is it.
Well, the best shots may be around f8, but only for some circumstances. For example:
This is one of my photos, shot at f8. If I had used a still-sharp f/5.6 or f/4, I wouldn't have that nasty BG and would have been able to use lower ISO and faster shutter. Sound good?
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The other reason is that some types of photography require fast lenses. You can't shoot in candlelight with f/8 without using ISO 1,000,000.
And finally, many lenses are sharp wide open. Take my 300 f/4: I only stop down for DoF, not for sharpness.
my 2¢
brendan
Yes you pay for those wide apertures (low numbers like f/2.8, f/2, f/1.4, f/1.2). Now it is true that you get the clearest and sharpest pictures when you stop down, but keep in mind that when doing so you close down the aperture opening and that results in cutting downthe amount of light. For some situation you need everybit of light that your lens can gather to stop a particular action. Also keep in mind that expensive fast lenses are sharper wide open than cheap fast lenses. BTW sometimes you need to sacrificesharpness for speed.
Don't worry, you'll be fine []
First of all, ALL lenses are slightly/heavily sharper once stopped down. Not only big expensive ones.
Yes we do! But it's not only the low numbers but other factors play a big role as well. Colors, contrast, build quality. Sharpness is not the main factor in a lens...thank God []
Depends on what you think is best! Maybe most lenses are sharpest at this aperture, buy it doesn't mean that a portrait shot at f8 is at it's best []
You're right and wrong here. First of all, most people don't spend nearly 2k on a fast lens, but when we do, we get better pictures at lower apertures like f4 f5.6 etc.
You must also keep in mind, that you are talking about overall sharpness. For a lot of photographers center sharpness is more important. And those guys/girls don't need to stop down to get a better picture. The fast lenses you mention are most of the time very sharp in the center and only improving sharpness at the edges when stopped down.
Oh and I can promise you that a canon 17-55 f2.8 gives you a better picture at f8 than a canon 18-55 kit lens at f8. If you don't count sharpness as the main reason of a good picture.
Hope you're less confused about now. Otherwise good luck! [
Jan
I can come with a recent example.
For Christmas, my brother wanted me to take a few portraits of his family. He was specific about it. He wanted the shot in front of the tree, with a cross star effect on the lights( he uses white lights).
Because of this, I couldnt use a flash( would mute the lights), so available light only. I used a 24-70L, and had to use ISO 3200 to get an acceptable shutter speed. In this situation, sharpness isn´t critical, but the lenses aperture is. I couldn´t afford a 24L, so I had to compromise by raising the ISO( with a 24L I could shoot wide open at ISO 800). Needless to say, a fast prime like the 24L or 35L are on my shopping list this year.
Luckily, after a lot of work with Dfine, the photo came out very well.
Mostly an echo of what has been said, but put slightly differently:
Lens sharpness is better stopped down, but if your shutter speed is too
slow, your picture will be so blurry that it won't matter.
I can't speak for others, but I like fast lenses because
1) they allow fast shutter speeds or adequate shutter speeds in low light, and
2) they give a narrow DOF to isolate the subject and blur the background.
We don't always get a better picture. In ideal circumstances, we get a sharper picture. But circumstances are not always ideal, and sharpness is not always the only thing one cares about. With a high quality fast lens, you may need to be a pixel-peeper to see
the sharpness difference between f/2.8 and f/8, but you can see the
difference in background blur in a 4x6 print.
If you shoot only still subjects with a tripod or in good light and you don't need to blur the background, then by all means buy only slow lenses and stop them down to the diffraction limit or beyond.
Faster lenses (at least f/2.8) also allow the af to work better, but I doubt there are many shelling out big bucks for that reason alone.
Oh yes, because I need all the light I can get--and everything everyone else said. But also: RAZOR THIN DEPTH OF FIELD!! Which means you can do this:
Canon 24-70mmL taken at 2.8
And THEN, when they are SLEEPING (!) you can do this...
Canon 50mm 1.4 taken at 1.4
This comment almost brought a tear to my eye.
I almost always shoot my 35 1.4 wide open. The image is so smooth and dreamy. A lot of people will stop it down to at least 1.6 for sharpness, but I just can't do it. I feel like you are throwing out the character of the lens by stopping it down. Usually if you get familiar with a lens you can usually get a decently sharp shot wide open.
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