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PostPosted: Wed Nov 11, 2009 3:37 pm 
I was wondering whether a sharper lens would suffer equal loss in resolving power as a less sharp lens resulting from noise at higher ISO or less (or perhaps more?). I CAN imagine it might lose relatively less than a less sharp lens but I have no idea whether this is indeed the case or not.

The less sharp lens would have less resolving power to begin with of course but how the losses in absolute and relative sense would be between two such lenses is something I'm curious about.

Does anyone here know anything about this?

Ben
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PostPosted: Wed Nov 11, 2009 4:56 pm 
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Hi Ben,

Good question. I don't know the answer from experiment so I always like to think of the boundary conditions to see what may be likely. Two scenarios spring to mind:
  1. Sensor noise (chroma plus luminance) results in image smearing over a much larger distance than either the good or the bad lens can resolve.

    In this case there wouldn't seem to be any point in using a sharper lens because the effect of sensor noise would swamp any extra fine detail it might deliver.
  2. Sensor noise is so small that the full theoretical resolution from all the pixels on the sensor is available.

    In this case the chances are that the poor lens might deliver less detail so using the good lens pays off
So your question is really about where the cross-over point is between these two scenarios. Intuitively one would suspect that the effects of smearing from noise and lens combine. Indeed, if one looked at this in terms of MTF graphs where 1 is perfect and 0 is utter rubbish the final MTF of lens plus noise would be obtained by multiplying the numbers together (so if noise MTF were 0.8 and the lens MTF were 0.9 the final MTF would be 0.72).

Again relying on intuition (always a dangerous game in physics) one would expect that for the two scenarios above:
  1. As sensor noise is absolutely horrible to start with let's assume an initial noise MTF of, say, 0.4. Then the final MTFs from an "OK" lens with an MTF of 0.8 and good lens with an MTF of 0.9 are just 0.32 and 0.36 respectively. In practical terms you wouldn't see any difference as both images would look horrible unless they were displayed as thumbnails.
  2. For virtually no sensor noise let's assume the MTF for noise is 0.95 - hardly a factor. Then for the same pair of lenses one would see MTFs of 0.76 and 0.855 respectively. Virtually all the extra performance of the good lens is delivered in the final image
Phew! That was arguably a lot of mumbo jumbo but, hey, it's written now so I may as well leave it in place to be shot down in flames as appropriate. :roll:

So where's that crossover point you wanted? I'd argue that it doesn't exist in practical terms. :shock:

The sensors on all DSLRs, and probably most compacts and superzooms though their lenses are fixed at the design stage, are capable of achieving low levels of sensor noise at the lowest ISO setting. So unless you never shoot in good lighting you can always get benefit from a better lens - this is scenario 2 above.

Of course if you always shoot in poor lighting with consequent high levels of sensor noise then scenario 1 applies and you could get by with a poorer lens. Except, of course, that a better lens may also have a brighter aperture allowing you to also reduce the ISO so you win because the brighter lens effectively improves the sensor noise MTF.

Maybe if one always had to shoot in bad lighting one could argue that investing in sharper (as opposed to brighter) glass wouldn't be worthwhile if smearing from sensor noise (expressed in pixels) is greater than, say, twice as large as the smearing (loss of resolution) from the lens. But that's total guesswork.

Bob.

P.S. For more on MTF curves check out this article.

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PostPosted: Wed Nov 11, 2009 6:01 pm 
Heh..interesting question and an equally interesting "dissertation" there Bob!

I see it a little differently myself - mainly because I consider "noise" and "resolution" power as two distinct non-overlapping phenomena.

I don't believe "noise" has any direct impact on "resolution of detail" in a picture...not in a strict sense anyway. High ISO noise occurs when the amplification of the signal go beyond what the camera can handle. This leads to a pattern of smears where clusters of pixels get attributed with the same values, presenting as "grain" to our eyes, but when looked at close up, they are clusters of pixels that are uniform in color and tonality.

Being an old audiophile, I tend to think of it the same way as a when you over-amplify a source-signal. The "noise" is when the audio is clipping..and instead of the fine gradations and details of a natural sound, you get a more or less constant tone.

This happens equally to very high resolution recordings and very low resolution recordings equally, when you over-amplify the signal.

Getting back to photography...if you take a clear noiseless image with a high-resolution lens and the same image with a lower-resolution lens and then apply a layer of "smear" on top of it, can you say that one lens has lost resolution more than the other? While I'd vote "no", it presents differently. A high-res image with blotches of smear will still appear more detailed in the the noise-free areas than a low-res image.
(assuming that the images are at same native resolution, since noise appears on a pixel-level, regardless of what the pixels show).

So visually I think I agree with Bob, but I think the "mechanics" of it are independent of each other.

Cheers :-)


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PostPosted: Wed Nov 11, 2009 7:22 pm 
I came to asking myself this question because in this topic a fellow forum member was considering a 'better' lens to help avoid noise issues. He mentioned the 12-60 which I know is a very good (sharp) lens.

Out of curiosity I revisited the results page of Gordon's review where a distinct difference in sharpness between the 12-60 and the (by no means bad) 14-45 lens can be seen and I wondered how these lenses would compare with increasing ISOs... AND I also wondered whether noise reduction in PP would be able to recover more detail from an image made with a lens with 'sharper noise' :D than from one that would have less 'sharp noise'.

As Bob mentioned there is of course a point where noise becomes so dominant that basically 'all is lost' and no lens can offer any improvement. But it's about that level at which noise becomes noticeable, where it starts to become an issue, that I wondered whether a better lens would be able to push that point just that little bit upward ISO-wise, perhaps in combination with PP NR as I mentioned.

Theory aside, the proof of the pudding....

So if someone would make a series of photos with two lenses with clearly different maximum sharpness in good light (100 ISO) and also at higher ISO AND also with additional PP NR I wonder what the results would be.

If anyone knows about such a series anywhere on the Interweb a link would be much appreciated. Alternatively someone here with sufficient drive and the necessary lenses could make such a photo series. (Why does the name Thomas suddenly spring to mind? ;) )

I'm curious to see what this thread will lead to.

Ben
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