Do we still need a TC?

Started Nov 11, 2013 | Discussions
Mark Ransom
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Re: Do we still need a TC?
In reply to awaldram, Nov 12, 2013

That was one thing I never considered - you might need the TC not for absolute reach, but for focusing and framing. As long as we have OVF this will be something to consider.

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Mark Ransom
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Re: Do we still need a TC?
In reply to paulkienitz, Nov 12, 2013

paulkienitz wrote:

Yeah, that is the question. The particular case that's most interesting is probably: what happens when you combine the K-3, the DA* 300, and the not-yet-available Pentax 1.4x AF RC?

Have you ever wondered why Pentax never released the 1.4x from the roadmap? Perhaps they realized that even if it's still viable today, it won't be with the camera they'll be making 2 years from now. Lenses need a longer lifetime than that to be worth the trouble.

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Mark Ransom
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Re: Basic error
In reply to James O'Neill, Nov 12, 2013

James O'Neill wrote:

Mark Ransom wrote:

A teleconverter works by magnifying the image that it gets from the primary lens. You could get the same effect by simply cropping, but the assumption is that the magnified image will be higher in quality than the cropped image.

No that's not the assumption. The TC can't possibly out put an image of any higher quality than went into it. So why do we have one ?

Sorry, I was unclear. I was talking about the final image quality, comparing the two results side-by-side. I would never claim that a TC could magically improve the lens.

If you record 1/2 the image with all the pixels (or all the film area) you will get better results than if you record all the image with all area and print half the result.

The net resolution is normally given as 1/net = 1/medium + 1/lens
for simplicity assume sensor and lens have equal res if the smallest the medium can resolve is 1/100th mm and the smallest the lens can resolve is also 1/100th mm the dot you can get when you put them together is 2/100mm. -i.e. 1/50th .

So lets say your lens can manage 1000 lines per image width and so can the sensor, net you get 500 lines per width. Crop to half the width you get 250 lines.

With a TC you're putting 500 of the 1000 lines onto 1000 lines of sensor res - net you 333 lines with the TC (less any loss from the TC itself)

I'm familiar with that formula for combining resolutions and it makes sense to me. There are a few assumptions that need to be questioned though. First, is it reasonable to assume that the resolution of the lens and sensor are approximately equal? That just happens to be the point where the formula works most dramatically; once one gets much lower than the other it tends to dominate. They may be equal, I just don't know how to tell. You alluded to another problem at the end, "less any loss from the TC itself". How much is the loss? I suspect it can be significant.

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Ian Stuart Forsyth
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Re: Basic error
In reply to MightyMike, Nov 12, 2013

MightyMike wrote:

James O'Neill wrote:

Mark Ransom wrote:

A teleconverter works by magnifying the image that it gets from the primary lens. You could get the same effect by simply cropping, but the assumption is that the magnified image will be higher in quality than the cropped image.

No that's not the assumption. The TC can't possibly out put an image of any higher quality than went into it. So why do we have one ?

If you record 1/2 the image with all the pixels (or all the film area) you will get better results than if you record all the image with all area and print half the result.

The net resolution is normally given as 1/net = 1/medium + 1/lens
for simplicity assume sensor and lens have equal res if the smallest the medium can resolve is 1/100th mm and the smallest the lens can resolve is also 1/100th mm the dot you can get when you put them together is 2/100mm. -i.e. 1/50th .

So lets say your lens can manage 1000 lines per image width and so can the sensor, net you get 500 lines per width. Crop to half the width you get 250 lines.

With a TC you're putting 500 of the 1000 lines onto 1000 lines of sensor res - net you 333 lines with the TC (less any loss from the TC itself)

If the parent lens out resolves the sensor by a factor of 2 then adding a 2x TC will still net you more detail than cropping. I'd suggest that many lenses still out resolve even high pixel count sensors by a good margin.

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I would have to agree with you Mike

Several people have been playing around with a Rasberry Pi camera board and installing them on 35mm and cropped lenses using 100mm and 200mm FL. You would be amazed with the detail that is being reached with these setups It would equate to a 225mp cropped sensor so I see no reason why you would not see an improvement in IQ with a 2X tc on a 60mp cropped camera body over just cropping

http://www.photoscala.de/Artikel/Von-Megapixeln-Viel-hilft-viel

http://translate.google.com/translate?hl=en&sl=de&tl=en&u=http%3A%2F%2Fwww.photoscala.de%2FArtikel%2FVon-Megapixeln-Viel-hilft-viel

This is a worth while read once you translate the link on diffraction

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Mark Ransom
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Re: relative resolution lens vs. sensor
In reply to Mark Ransom, Nov 12, 2013

Just coincidentally, I came across this which implies that we haven't reached diminishing returns in sensor resolution yet. Your assumption that the lens and sensor are approximately equal gets a good boost from this data, if anything the best lenses might still be out-resolving the sensor by some margin. If so then my question is answered.

http://www.dxomark.com/Reviews/Best-lenses-for-the-24M-Pix-Nikon-D7100-Part-I/Nikon-D7100-results-ultra-high-performance

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Ian Stuart Forsyth
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Re: relative resolution lens vs. sensor
In reply to Mark Ransom, Nov 12, 2013

Mark Ransom wrote:

Just coincidentally, I came across this which implies that we haven't reached diminishing returns in sensor resolution yet. Your assumption that the lens and sensor are approximately equal gets a good boost from this data, if anything the best lenses might still be out-resolving the sensor by some margin. If so then my question is answered.

http://www.dxomark.com/Reviews/Best-lenses-for-the-24M-Pix-Nikon-D7100-Part-I/Nikon-D7100-results-ultra-high-performance

this would interest you, using a 50mm lens on a would be 500mp FF sensor

http://www.dpreview.com/forums/post/51716149

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awaldram
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Re: relative resolution lens vs. sensor
In reply to Ian Stuart Forsyth, Nov 12, 2013

I seem to remember from my dim and distant past that 35Mp ff equates to the same resolution as I so 100 print film,

This would be about 24 Mp on apsC so today were approaching lens load that a 30 year old consumer print film would.

Given 30 years ago emulsions were available with much better resolution and those lens of the day still out resolved them, Today decent glass is not even sweating on today's sensors.

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Ian Stuart Forsyth
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Re: relative resolution lens vs. sensor
In reply to awaldram, Nov 12, 2013

awaldram wrote:

I seem to remember from my dim and distant past that 35Mp ff equates to the same resolution as I so 100 print film,

This would be about 24 Mp on apsC so today were approaching lens load that a 30 year old consumer print film would.

Given 30 years ago emulsions were available with much better resolution and those lens of the day still out resolved them, Today decent glass is not even sweating on today's sensors.

Before the K5 the largest concern with IQ when using a tc was how much light was lost but now we can use TC in light conditions that until recently was only a pipe dream. In pentax land its hard evaluate using TC due to the fact most are mismatched and most of the time used between different lens companies. With companies that build lenses and optimise the tc for use on those lenses you have some really good combos out there that deliver top of the line IQ.

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awaldram
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Re: Do we still need a TC?
In reply to Mark Ransom, Nov 12, 2013

Mark Ransom wrote:

That was one thing I never considered - you might need the TC not for absolute reach, but for focusing and framing. As long as we have OVF this will be something to consider.

To me it always the only reason I use a TC.

If the subject is to small for the focal length in use no amount of cropping will recover it.

Take a 50mm lens and go birding doesn't work, repeat with 300mm lens and it just about doable got to 600mm and it becomes a pleasure.

Now to get that 500-600 mm length (in rough order of IQ and ease of use)

Pentax 560mm £5500

Sigma 500 EX £3800

Sigma 300 f2.8 +2X or 1.4X £2500

Sigma 50-500 £1000

Pentax da* 300 +1.4 £1100

Now imagine you need 800-1000mm what do you do .?

The assumption you can stick a converter on a sows ear and somehow get a silk purse is one of the most pervasive wives tales on the web , In most cases cropping is considerably better.

Often sticking a converter on a cheap consumer zoom will introduce so many aberrations you will lose AF a clear indication the IQ has hit the floor .

But converters on glass that will support it (Mike did some tests on this and his table is a good source of reference) can produce very high quality results.

One other thought.

My old Sigma 70-200 EX does not have an anti reflective rear coating (DG) so in back lit situations can suffer contrast loss, Putting my sigma 1.4 DG converter on will resolve this.

So in this case the application of a converter increases the IQ of the native lens.

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Flashlight
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Re: Basic error
In reply to Ian Stuart Forsyth, Nov 12, 2013

Ian Stuart Forsyth wrote:

Several people have been playing around with a Rasberry Pi camera board and installing them on 35mm and cropped lenses using 100mm and 200mm FL. You would be amazed with the detail that is being reached with these setups It would equate to a 225mp cropped sensor so I see no reason why you would not see an improvement in IQ with a 2X tc on a 60mp cropped camera body over just cropping

I'm one of those people, although I think the PiCamera has a cropfactor of 10, so when filling a 35mm frame with that pixel density would give yo 10x10x5=500MP.

The first image is an overview of the scene shot @41mm (didn't have a 50mm image at hand). Look at the utter right to see where the crop (second image) has been taken:

I then mounted the camera sensor from the Raspberry Pi behind a Nikkor 50mm f/1.4D and shot an image @ f/4 (diffraction wil start @f/2.8 I think). Below you see a 100% crop of the center and next to it the size a likewise crop from a 36MP image should have been (select view original for full size):

It's obvious there's more detail than a blown up image from a 36MP sensor could give. There's some noise (already used some quick noise reduction) but it starts to look good. The above image was taken from RAW to circumvent the heavy built-in jpeg processing on the Raspberry Pi.

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Philip

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James O'Neill
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Re: Basic error
In reply to paulkienitz, Nov 12, 2013

paulkienitz wrote:

MightyMike wrote:

I'd suggest that many lenses still out resolve even high pixel count sensors by a good margin.

This is getting harder to believe. Today's crop-format sensors have a photosite spacing between 3.5 and 4 microns. I don't think there are a ton of lenses out there that can resolve finer than that. For one thing, the diffraction blur circle at f4 is already over 5 microns in theory, and f2.8 lenses are not usually sharper than f4 ones. In practice it seems possible to extract smaller detail than this, but it's an uphill fight.

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"A good photograph is knowing where to stand." -- Ansel

That's not how it works though. 6000 pixels * 4 microns = 24mm ... OK.

But do this thought experiment.

You scan a light beam one pixel with perfect edges which instantly from white to black, up and down a sensor with no bayer filter. The light is just bright enough to give maximum exposure level in the pixels it hits.
Your pattern of black and white stripes one pixel wide is reported by the sensor's pixels as 100%,0 100%, 0 ... Or is it? Suppose the alignment of the pattern was off by half a pixel, now the lines would go 50%,50%,50%,50% The average case is 75% , 25%,75%,25% an MTF of 50%.
The maximum number of line pairs we can record is half the number of pixels (people quote Nyquist here, a cycle is light/dark we need to have two samples per cycle, one to record light one to record dark)

Think of MTF as measuring "How much light strays into the 'dark' pixels". When the answer is "too much" you can't resolve light from dark. If the stripes are 2 pixels wide the average error is still 1/4 pixel but it's now 1/8 of a stripe so the light divides 12.5%/87.5% - MTF is 75%,  if the stripes a 4 pixels wide the error is 1/16th of a stripe so its 6.25/83.65%  MTF is 87.5% and so on.

Now imagine that the pattern that you are laying down isn't Black/white/black/white but black-grey-white-grey-black-grey-white. When you get close to the limit of lens resolution it doesn't go suddenly from perfect black/white to grey mush , instead the light spread at the transitions becomes such the grey edges get closer and closer together until the MTF of the pattern reaches the point where you say "there's still a lighter and darker, but the lines aren't properly resolved". If the lines are only just resolved then when  you add the effect of the sensor it drops below the minable MTF to call the result "resolved".  If one component massively out resolves the other when you use at close to the maximum res of the other it's MTF is nearly 100%.

So to get a end product that you call "resolved" the number of lines has to be inside the maxium resolution of both sensor and lens , so that when you add the edge blur of both together the result must be <= the maximum allowable edge blur to call that number of lines resolved. Since blur is proportional to  1/resolution we get this equation I learnt in the film days of 1/output = 1/lens + 1/medium

OK

Now put the bayer sensor back on. Now, however you line up your one pixel wide beam the sesnor records x% Red & Green1 (100-x)% Blue & Green 2, So all your pixels have the same red and blue values, and native red & blue pixels have the same green values and you can't resolve the lines any more.... so your lines must be at least 2 pixels wide (If you think of your 'white beam' as a red,green and blue beam, you have to record a red line, blue line, green line, and red , and blue pixels rows/columns are only 50% of the total)

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paulkienitz
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Re: Basic error
In reply to James O'Neill, Nov 12, 2013

I know how sampling works... and I don't see how any of that addressed my point or changes any of the numbers I mentioned.

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audiobomber
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Re: Do we still need a TC?
In reply to Mark Ransom, Nov 12, 2013

Mark Ransom wrote:

A teleconverter works by magnifying the image that it gets from the primary lens. You could get the same effect by simply cropping, but the assumption is that the magnified image will be higher in quality than the cropped image.

That's not what I believe at all. The main lens has a certain level of IQ. Adding a TC can only deteriorate the IQ by adding aberrations and losing sharpness. A good TC will only compromise the lens a little bit. Differences would only be visible in extreme conditions and at pixel level magnification.

The reason I need a TC with my DA*300 is that I frequently crop so hard that I run out of pixels (birding mostly). Having 24mp helps, but 24mp plus the TC is even better.

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John_A_G
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Also...
In reply to audiobomber, Nov 12, 2013

audiobomber wrote:


The reason I need a TC with my DA*300 is that I frequently crop so hard that I run out of pixels (birding mostly). Having 24mp helps, but 24mp plus the TC is even better.

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Dan

Lets not forget the AF portion of things.  When you have a larger subject presented to the AF array you have a better chance at accurate focus.  Especially given a focus point in a viewfinder is usually only an approximation of the actual point.

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lpammann
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Re: Also...
In reply to John_A_G, Nov 12, 2013

Both Dan and John give important reasons why a high-quality TC would be very useful for birding. Here is a 100% crop of a very distant Kestrel I shot Sunday using my K3+1.7AFA+DA*300 hand-held. I relied on the AF of the K3 and this TC to attain focus because he was very small in the viewfinder. Without the 1.7 I doubt I could have gotten him in focus, especially since it is so strongly backlit. With the TC there are 335 pixels from the top of his head to the bottom of the mouse's feet and there are 385 pixels from the kestrel's beak to the tip of his tail. i don't know what that corresponds to in the viewfinder, but 335 pixels is about 8.4% of the K3's vertical sensor dimension and 385 is about 6.4% of its horizontal dimension. Without the TC these dimensions would have been 4.9% and 3.8% respectively. I know the image is not great art, but I'm glad I had this TC on my lens. I would have preferred to have had a high-quality 1.4x AF TC on instead so that I could have switched instantly to shooting some Snow Geese who were flying by closer than this Kestrel without first having to manually focus most of the way.

LPA

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