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Messages - tim

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EOS Bodies - For Stills / Re: Too much dynamic range?
« on: November 21, 2012, 05:49:44 AM »
Here's a simplified example.  Suppose you have two sensors:

Sensor-A gives a signal between 0 and 1000mV, and has a noise of 10mV.
Sensor-B gives a signal between 0 and 1000mV, and has a noise of 1mV.

Sensor-A can just distinguish signals corresponding to 500mV and 510mV, but it cannot distinguish signals corresponding to 500mV and 501mV.  In other words Sensor-A can distinguish 100 levels.  And the ratio between the maximum and (average) minimum signals is 100, which is the same as saying it has a dynamic range of 100 (= 6.6 stops).

Sensor-B meanwhile can distinguish 1000 levels and has a dynamic range of 1000 (= 10 stops).

The noise determines both the dynamic range and the precision of the intermediate gradations.

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EOS Bodies - For Stills / Re: Too much dynamic range?
« on: November 21, 2012, 05:39:25 AM »
No really, it doesn't work like that.  The sensor noise determines both the dynamic range and the number of tones which can be distinguished.  They're inextricably linked.  If you want better accuracy of tones you need to reduce the noise, which automatically increases the dynamic range.

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EOS Bodies - For Stills / Re: Too much dynamic range?
« on: November 20, 2012, 07:59:08 PM »
So my point is: with lower DR camera we'll have lower tone delta

It doesn't work like that.  Suppose you have a sensor with a dynamic range of 10 stops.  That's (roughly) equivalent to saying that the noise is 1 part in 1000 of the maximum signal (2^10 = 1024), so you can only distinguish 1000 different shades.  Even if you put a 16-bit ADC on the sensor, you're still limited to 1000 shades.

Now I've glossed over a lot of things in the paragraph above.  For one thing noise is not constant over the tonal range: it gets larger at higher signal amplitudes, so in the example you'll actually be able to distinguish less than 1000 shades.

There's also a reason why you might use an ADC with a larger dynamic range than your sensor.  If you used a 10-bit ADC on a sensor capable of distinguishing 1000 shades, every shade in a scene would be assigned a definite value in the photograph.  Subtle gradients in the scene would be rendered as stepwise increments in the photograph, and under heavy processing this could become visible as a posterisation-type effect.  But with a higher bit-depth ADC the steps would be blurred out by the electronics noise so that they are not visible.  There's no extra information in the photo, but it looks nicer.

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EOS Bodies - For Video / Re: Shooting in B/W
« on: November 19, 2012, 05:50:40 PM »
Some people like shooting with the camera in B&W mode.  It's less flexible, but it helps them get into the B&W way of seeing things.  But everybody is different.  Try it and see what you think.

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Can you attach to the 600EX....by wire...other cheaper flashes....in case you wanted to for some studio shots.....

No, but you could attach the non-ETTL flashes to the camera's PC terminal.
It's also possible to use non-ETTL flashes with optical triggers, but in this case you lose automatic control of the 600EX output.

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EOS Bodies / Re: Canon EOS 5D Mark III Blinking Red AF Point Information
« on: November 15, 2012, 01:15:48 PM »
Kudos to Canon for trusting our intelligence enough to tell us the technical details of what they are doing.  Let's see if we can repay that respect.

I was pretty shocked by the poster above who asked for apologies and even money.  Can we please avoid badmouthing Canon for not delivering things they never promised?

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Street & City / Re: Post Hurricane Sandy black out photos.
« on: November 14, 2012, 05:20:19 PM »
etg9, I'm curious: what were people queueing for during a blackout?

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Lenses / Re: do image stabilisers decrease image quality?
« on: November 11, 2012, 06:41:08 PM »
1. Does IS mechanism reduce only the sharpness of a lens or causes any other type of IQ reduction?

The IS group is just a bunch of lenses, and so in principle it can suffer from all the same aberrations and imperfections as any other lens.  However my guess from looking at the design is that internal reflections are the most significant effect.  That would show up as flare and reduced contrast.

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2. Is the IS reduction of IQ enough that it is visible to the eye?

It's not something I worry about.  It's going to be a subtle effect, if visible at all.  To test for it you'd need a pair of lenses with the same optical formula except for the IS group, and I'm not aware of any such lenses.

Looking at Canon's 70-200 lenses, the IS versions are both better than their non-IS counterparts.  They are also quite a lot more expensive, and my supposition is that the price difference includes upgraded optics in addition to the IS.  So in practice, from the customer's point of view IS actually improves your image quality because you are buying a better lens overall.

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Canon EF Prime Lenses / Re: Canon EF 100mm f/2.8L Macro IS USM
« on: November 11, 2012, 06:02:04 PM »
Phixional, the lighting in your macro shots is very nice.  Are you using a reflector?

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Lenses / Re: do image stabilisers decrease image quality?
« on: November 11, 2012, 08:30:13 AM »
Physicist specialising in optics here.

Do image stabilisers decrease image quality?

Yes.  All other things being equal, they do, and for two reasons:
1. A certain optical formula requires a certain number of elements.  Adding extra elements above the minimum required causes problems, such as extra internal reflections and extra aberrations or distortions which then have to be corrected by altering the optical formula.  The overall formula is then no longer perfectly optimal.
2. The IS group puts extra constraints on the design of the rest of the lens.  In the case of telephotos there is usually plenty of space available for an IS group, but wide-angles are much more crowded.  Inserting IS forces the designer to reshuffle the main optical elements and make certain compromises in the design.

Here's the more interesting question:

How much do image stabilisers decrease image quality?

It depends.  It depends on the skill of the designer, and whether he/she is allowed to employ expensive materials, expensive lens coatings, and strict manufacturing tolerances.  There aren't too many examples of pairs of lenses which are identical except for addition/deletion of IS.  Canon's 70-200mm f4 lenses (figure below) are superficially identical, but actually have slightly different optical formulae.  In this case the (more expensive) IS version happens to be slightly better.



So if you're willing to pay for it, IS can -- in certain circumstances -- have little or no significant effect on image quality.  However, even with the best designers and most expensive manufacturing, there are physical limits.  Certain non-IS lenses will never have IS counterparts which are as good.

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Street & City / Re: Post Hurricane Sandy black out photos.
« on: November 10, 2012, 06:33:40 PM »
Fantastic pics.  It must have been quite an experience cycling around the city in the dark.  The second photo looked at first as if some daredevil climbing had been required to get the viewpoint!  Thanks to Google Earth I now know there's a rather magnificent walkway along Brooklyn Bridge, which I will definitely not miss on my next trip to NY.

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