# Lense Diameter = Lense resolving power?



## kawasakiguy37 (May 27, 2010)

I had an idea today about lense resolution. Coming from a background in astronomy, the larger the front element usually meant it was a better telescope (more light gathering). With lenses we have an f-stop value, so this does not apply to light gathering. However, the large the diameter of the front element also produces a larger lense resolving power, or the potential resolution of the lense.

Is this true in photography as well? Do lenses with larger front elements  have a higher theoretical resolution?


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## robertwsimpson (May 27, 2010)

I think the larger elements are more of a forced issue because of the physical size of the aperture, which is dependent on the focal length.  That's why the long lenses with the smaller f-numbers have giant front elements... because they also have giant aperture mechanisms.


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## Dwig (May 27, 2010)

The light gathering capability of telescopes generally refers to the effect of diameter, actually area, of the objective on the number of photons collected from a point light source. With diffuse sources, those encountered in conventional photography and when telescopes are used to view/photography diffuse nebulae, it is not the absolute diameter of the objective, but the "focal ratio" (the classic telescope term) that is the controlling factor. Photographers generally refer to a lens' focal ratio as its f/stop, although the lens' spec's engraved on the barrel are traditionally in focal ratio notation (e.g. 1:1.4 instead of f/1.4).

It is true that Dawe's Limit does impact on photographic optics. As a rule, though, other issues intrude as a result of having to balance the optical performance over a rather wide field and to produce a flat image plane. Still, most camera lenses perform their best at near maximum aperture.


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## dxqcanada (May 27, 2010)

"The ability of a lens to resolve detail is usually determined by the  quality of the lens but is ultimately limited by diffraction."

Wikipedia


The larger diameter front elements are used for light gathering.

I have had some large diameter lenses that had poor resolution due to lens aberrations.
I have had some high resolution lenses with a not so large front lens element.


Telescopes with high resolution tend not to have lenses ... mirrors are used (which I believe you already know).


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## KenC (May 28, 2010)

What I remember is that the maximum aperture f number for a lens equals the focal length divided by the front element diameter.  So, for the same front element size on a longer focal length lens, the f number would be higher.  If you double the front element diameter on a given focal length lens, you get four times as much light (area is pi times d/2 squared), which is two stops, or half the f number, e.g., from 2.8 to 1.4.

I don't know if the element size can also affect resolution, but in any case, resolution is determined also by glass quality and diffraction effects due to the aperture blades.  Good glass quality is easier to achieve with smaller lens elements, so sometimes slower lenses are better for that reason.


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## iskoos (May 28, 2010)

I tent to agree with Robert. I am not saying this is definitely the only reason but I am sure the biggest factor in having larger front element diameter is relative aperture (f ratio).
I have a 50mm f/1.4 lens. The front element is larger than a typical kit lense but it is not so large. I have also seen 85mm f/1.2 lens and the front element is large!.. for the obvious reason.



> What I remember is that the maximum aperture f number for a lens equals the focal length divided by the front element diameter.


 
Relative aperture is focal length divided by the aperture diameter. NOT diameter of the front element. Yes, as the front element diameter gets larger, aperture diameter gets larger. But it is the aperture that controls how much light gets to image sensor/film not the front element. Thus we use aperture diameter in the relative aperture formula.


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## usayit (May 28, 2010)

All this talk about the size of the front element, the max aperture, and aperture diameter....   the OP asked specifically about "resolving" characteristics of photographic lenses.  

I can't comment much further as I am not an optics expert but I believe the ability of to gather light is different from the capability to resolve.  If I were to take a guess, the ability to gather light is related to the size of the glass BUT the ability to resolve is more a factor of optical design.

The main example I can think of in the photographic world would be the highly controversial Leica 50mm f/1 Noctilux.  In general, the consensus is that the 50mm Noctilux is not a replacement for other 50mm lenses hence why people have two 50s.   The Noctilux is the king of low-light... It is a HUGE glass that "gather's light".  Meanwhile, the much more compact 50mm lenses (Summilux, Summicron, Summarits) will "out resolve" the Noctilux fairly easily.


(Ideally, you want both... Big light gatherer that can resolve.  But that is a whole different story and complexity)


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## epp_b (May 28, 2010)

Sort of.

Larg front elements are both the product of a large aperture (which usually indicates a high-quality lens), but also of additional correction of aberrations.


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## Overread (May 28, 2010)

Hmm its got to be more than the front element size - my MPE 65mm has a front element that is beyond tiny and yet it has a fantastic resolution of fine details. I know that macro tends to bend a lot of common rules, but certainly that is one lens that does not need a large front element to show high resolution.


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## usayit (May 28, 2010)

Also, it is indirectly stating that all rangefinder lenses (typically very small elements with a filter sizes such as 39mm 46mm 49mm etc) are "less corrected" than the SLR counterparts which just does not make sense.


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## kawasakiguy37 (May 28, 2010)

Yea, I am not really inteested in "light gathering" as the F-stop number already measures this directly. I was merely thinking about resolution.....I shoot on a low res D2hs right now (only 4.1 megapixel), but eventually I want to go full frame and I dont want lenses that will be outperformed by a 18 or 21 megapixel sensor!


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## cfusionpm (May 28, 2010)

Here's a pretty technical article about lenses, apertures, and difraction: Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks

essentially, the fstop ratio (reletive diameter) can cause resolving issues the smaller it becomes (regardless of physical diameter).  

The higher the megapixel count, the lower aperture difraction kicks in.  Remember though, that this is on a per-pixel level.  Someone posted in another thread a great simple example.  Let's say you have a high res sensor and a four-pixel row reads BLACK, GRAY, WHITE, WHITE.  The grey area is something not perfectly resolved by the by the sensor and acts as a transition between the two.  But if you take a sensor half the size, the same space would only be two pixels: BLACK, WHITE.  So there's no graying, but there's also a loss half the detail.  In the end though, it's only really important if you are printing poster-sized images or cropping to show 75-100% original size.  When scaled down, the issue is almost non-existant.


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