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Old Aug 28, 2007, 12:02 AM   #1
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I do not understand why fast lens are not super sharp at their minimum focal ?

If a 1.4 lens is only very sharp at 2.8, why do people call it a 1.4 ?

I would rather call it a 2.8... :?
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Old Aug 28, 2007, 4:30 AM   #2
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It is a 1.4 because that is where it starts and you can shoot at the wide aperture, however there are some reasons why every lens I've seen gets sharper as you stop down. Also if you are focusing correctly and aware of the narrow dof that is created at wide apertures then you should be getting usable results at f1.4.

Now to start I will say I'm not a lens technology expert however I have a little understanding so will try to give very basic reasons which I believe to be correct.

Spherical aberration is one of the main problems at wide apertures, and is caused by the various light frequencies being refracted at different angles as they go through the elements of the lens. By stopping down and narrowing the aperture light from less of the lens is captured so these differences are reduced. You can also get improved contrast when stopping down (again assisting sharpness) as you reduce the amount of light from the edges of the elements; at the edges you can get reflections from within the lens so getting the light away from the edge reduces the potential reflections.

I hope that gives a little insight to the subject, if not then you need an answer from a more clever person as that's pretty much all I have.

So basically you have a f1.4 lens however it is 'stronger' when stopped down, however if you are aware of and work within the limits you should get good results at f1.4.

Edit.... the problem with having been away for a while is not being up to speed with current topics, I've just read a few posts further down the thread and found http://forums.steves-digicams.com/fo...amp;forum_id=9which is covering this in quite a bit of detail.

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Old Aug 28, 2007, 7:19 AM   #3
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You're probably using this lens in low-light...

Try the same shot but with better lighting - As the contrast increases so is the appearance of sharpness. Just try to take some pictures outdoor wide open and you'll see a difference!
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Old Aug 28, 2007, 7:32 AM   #4
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Just curious as to what you would call a lens that wasn't super-sharp at any f-stop?


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Old Aug 28, 2007, 7:45 AM   #5
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Mark,

I had seen this string. I do understand that stopping down forces for a better light. I am looking more at a marketing explanation.

It looks to me that people are ready to pay more for a shorter focal, so I understand the will of makers to publicize the lowest focal possible.

It frustrates me though, as a beginner, to find out that the "true" range of the zoom, I mean where the pic is super sharp, is much narrower than what is advertised. If I purchase a 1.4 but can only shoot for decent sharpness at a minimum of 2, then that is no good. to me.

Yes indeed I am going for low light, but that is what a 1.4 for is purchased for isn't it ?



Anyway.... Thanks !

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Old Aug 28, 2007, 7:52 AM   #6
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philgib wrote:
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Yes indeed I am going for low light, but that is what a 1.4 for is purchased for isn't it ?
Yes you are right with this and I have a 50mm f1.8 which is not great at 1.8 however when I can't get away with 2.8 because it is too dark then getting a not fantastic shot is much better than getting no shot at all.

I would be really interested to see the sort of problem you are getting as even in these difficult situations I'm still happy to use the work for customers.

If you can post some photos that would really help us to help you in getting the shots you desire.
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Old Aug 28, 2007, 10:05 AM   #7
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philgib wrote:
Quote:
I had seen this string. I do understand that stopping down forces for a better light. I am looking more at a marketing explanation.

It looks to me that people are ready to pay more for a shorter focal, so I understand the will of makers to publicize the lowest focal possible.

It frustrates me though, as a beginner, to find out that the "true" range of the zoom, I mean where the pic is super sharp, is much narrower than what is advertised. If I purchase a 1.4 but can only shoot for decent sharpness at a minimum of 2, then that is no good. to me.
They market a lens based on what it's capable of, not on what it's especially good at.

That's what PhotoDo.com, SLRGear.com amd PhotoZone.de (among others) are for.
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Old Aug 28, 2007, 10:55 AM   #8
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A few things to add to Mark & Tcav's excellent comments:

1. Every lens out there performs BETTER when stopped down. It's pretty much a fact.

2. So the key is when buying a lens with wide aperture, does it perform GOOD ENOUGH at that aperture.

Point 2 is the tough one. Photography forums are littered with people that take way too many pictures of brick walls and signs and other measurebater tests looking at 100% crops to find the slightest flaw in a lens or camera. Anyone who is stating the 50mm 1.4 needs to be stopped down to 2.8 to be usable IMO falls into this category. Also, you have to realize - at the distances this lens was designed to work at (3-12 feet) and an aperture of 1.4 you only have a depth of field of about 7 inches (3.5" in front and 3.5" in back of your focus point). So if your focus is off just slightly your shot will suffer. That, to me, is a bigger issue with a lens like this - the practicality of shooting at 1.4. I can tell you shooting at 2.0 can be very challenging to get your focus correct. Shooting at 1.4 in anything other than a controlled environment (i.e. static subject in decent lighting) is going to be very difficult.
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Old Aug 28, 2007, 4:38 PM   #9
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Leica M lenses are apparently just as good (if not even better) wide open than stopped down.

Of course the Leica 50mm f1.4 costs £1850 and the Canon 50mm f1.4 costs £225. Zeiss are now making a very nice 50mm lens in Nikon mount that you can get a Canon adapter for, and the price is very reasonable too around £350 for Nikon mount and £375 for Canon. The Zeiss is manual focus only however.

And you do realize that these lenses don't zoom? It seems you are a bit confused. The focal length is 50mm always. The f-stop (1.4, 2, 2.8, etc) refers to the aperture.
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Old Aug 28, 2007, 5:10 PM   #10
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peripatetic wrote:
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Leica M lenses are apparently just as good (if not even better) wide open than stopped down.
It depends on what you mean by better.

Sharper with more contrast? Nope. They're better stopped down.

You can see MTF Charts here:

http://old.photodo.com/prod/lens/leicam.shtml

If you click on the technical data on Leica's own web site for the latest model of their 50mm f/1.4 Summilux-M, you'll see more MTF charts that show it's better stopped down:

http://www.leica-camera.us/photograp...enses/466.html

Direct Link:

http://www.leica-camera.us/assets/fi...e=file_158.pdf

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