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07-15-2013, 09:05 PM   #1
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Stopped down vs. same native aperture

Suppose you have two lenses. Lens #1 is a big 50mm, f/1.4. The front glass is big and clean! Lens #2 has the same focal length but is designed to natively be f/8. It may have a tiny front element.

Which lens has the potential to be sharper? Lens #1 stopped down to f/8 or lens #2 that is of the same focal ratio?

Would they literally produce the same image?

I understand that lenses can be well designed and built to meet a multitude of specs and performance ratings. Assume that there would be good engineers designing and building both lenses. Each team would do their best.

07-15-2013, 09:16 PM   #2
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Most lenses are sharpest when stopped down one or two stops. Wide open at f8 would, in theory, not be as sharp. Pretty soon you will start getting into diffraction issues, though.

I have no idea what a "native" f-stop is.
07-15-2013, 09:20 PM   #3
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In a "perfectly" designed lens the maximum resolution is wide open--and the more wide open the sharper. The theoretical resolution is inversely proportional to f/stop (due to diffraction). In fact wide open various les aberrations reduce resolution. Typically its close to a maximum 2-3 stops closed down. By f/11 on a reduced (1.5x) sensor dslr the resolution decreases, and by f/16 on a full frame. On a real lens--the f/8 lens or the f/1.4 lens may have greater resolution. It's a question of what the designer chose to maximize. And it is not a given the "best lens" is sharpest. It depends what the lens was designed to. Personally I don't consider sharpness (resolution plus contrast) to be the most important. BTW a very fast lens often maximizes resolution near wide open, and thus may not be the sharpest--but in fact the 50mm f1.2 Pentax is said to have excellent resolution at/near f/11. Lens design cannot be simplified to what is best.
07-15-2013, 09:35 PM   #4
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QuoteOriginally posted by 6BQ5 Quote
Suppose you have two lenses. Lens #1 is a big 50mm, f/1.4. The front glass is big and clean! Lens #2 has the same focal length but is designed to natively be f/8. It may have a tiny front element.

Which lens has the potential to be sharper? Lens #1 stopped down to f/8 or lens #2 that is of the same focal ratio?

Would they literally produce the same image?

I understand that lenses can be well designed and built to meet a multitude of specs and performance ratings. Assume that there would be good engineers designing and building both lenses. Each team would do their best.
I think I know what you are getting at. All other factors being equal, it would seem to me that a large piece of glass (f1.4) has to do more than a smaller (f4.5) piece. I would think it much easier to shape the smaller piece in an uncompromising way since it only has a limited range to cover. Obviously every lens is unique, and in theory there may be no difference at say f5.6. In practice, I have been impressed by smaller glass. Here is an example from a f4.5 lens on a small 1951 view finder triplet Reomar lens on film.




Last edited by arnold; 07-15-2013 at 09:53 PM. Reason: Added photo
07-16-2013, 06:06 AM   #5
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Um, a Pentax f1.2 stopped down to f2.0 will perform better than a Pentax f2.0 wide open at f2.0. But of course, faster lenses are usually also more expensive and harder to make, so the maker puts more effort into them. Faster lenses are often overall better than slower lenses, in terms of build quality, materials and coatings used, manufacturing precision, different lens design, more complex lens elements... its hard to find two lenses which are equally good in all specs, but only have a different sized max aperture. Maybe if you compare the FA and DA 35mm or FA and DA 50mm, but even these lenses arent equal in all respects.
In practice, you usualy want a lens stopped down one or two full stops for "optimal performance." You can also check lens review websites for the sweet spot on your lenses. Or if your camera has the MTF program line. Theoretical lenses don't exist in reality

Edit: Oh, but there are some good slow primes, like the SMC Pentax 35mm f3.5. Slow doesnt necessarily mean bad. I dont know how it stacks up to a FA 35mm stopped down to f3.5.

Last edited by Na Horuk; 07-16-2013 at 06:12 AM.
07-16-2013, 08:55 AM   #6
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QuoteOriginally posted by 6BQ5 Quote
Suppose you have two lenses. Lens #1 is a big 50mm, f/1.4. The front glass is big and clean! Lens #2 has the same focal length but is designed to natively be f/8. It may have a tiny front element.

Which lens has the potential to be sharper? Lens #1 stopped down to f/8 or lens #2 that is of the same focal ratio?
The question is too broad. Depends on the format being used and on the type of design of each lens. Some lenses are designed to perform the best wide open and other to be softer wide open than stopped down and so on.

May I suggest some reading:
Digital Camera Diffraction – Resolution, Color & Micro-Contrast

Also, take a look at the rest of the tutorials on that site. They are really good.
Learn Photography Concepts
07-16-2013, 09:23 AM   #7
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QuoteOriginally posted by 6BQ5 Quote
Suppose you have two lenses. Lens #1 is a big 50mm, f/1.4. The front glass is big and clean! Lens #2 has the same focal length but is designed to natively be f/8. It may have a tiny front element.

Which lens has the potential to be sharper? Lens #1 stopped down to f/8 or lens #2 that is of the same focal ratio?

Would they literally produce the same image?

I understand that lenses can be well designed and built to meet a multitude of specs and performance ratings. Assume that there would be good engineers designing and building both lenses. Each team would do their best.
Unfortunately, your question as asked makes no sense for all the reasons given above. E.g., if teams really did their best, all these 100,000$ lenses would be diffraction-limited wide open (what you called native aperture) and so, the F/1.4 lens would be way sharper. And anyway, F/8 is so small, that most lenses are diffraction-limited and therefore, your two lenses would perform equal.

So, let me rephrase your question to mean what sou probably had in mind:

Given two lenses of same focal, different apertures and similiar complexity of optical construction, which one would be sharper when stopped down to its resolution-maximizing aperture?

And the answer is ...
Ok, let me first explain something. All lenses have an aperture of maximum resolution. This optimum aperture is smaller towards the image edges and corners. E.g., a few top notch lenses have an aperture with maximum resolution which is F/2.8 in the center, F/4 at edges and F/5.6 in the corners. However, add a stop for most excellent lenses, esp. affordable ones. Obviously, your hypothetical F/8 lens has no chance to compete against such lenses.

Then, most lenses are designed to have acceptable performance wide open. And are sharpest stopped down 1 or 2 stops.

Therefore, I assume that a lens designed for between F/1.8 and F/2.4 has best chances to be the top performer at its aperture of optimum resolution.

So, your F/1.4 lens is much closer to this ideal, an F/8 lens will suck resolution wise.

Hope, this answers your curiosity.

Kind regards,
Falk
07-16-2013, 09:33 AM   #8
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QuoteOriginally posted by falconeye Quote
So, your F/1.4 lens is much closer to this ideal, an F/8 lens will suck resolution wise.
All you said, assumes we are ussing both lenses on APS-C sensor. I konw you know, just to clarify this also counts.

07-16-2013, 03:23 PM   #9
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Just a clarification/correction to my earlier post--the actual resolution of the lens is unrelated to sensor size.
When I said 1.5x vs full frame, and f/11 and f/16, I was thinking same/similar resolution in the print--which is why this rule of thumb is often given. As the cropped sensor requires additional 1.5 enlargement--for the same size print. Thus the resolution (if the prints are very large!) would be about 1.5 times larger (actually 1.414) at f/11 vs. f/16--assuming the lens is diffraction limited at f/11.
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