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EspenB

Member Since 07 Jul 2011
Offline Last Active Jan 10 2017 03:43 PM
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Posts I've Made

In Topic: Quality of WA zoom lens for UW use

10 January 2017 - 03:29 AM

 

Yeah, I have also been puzzled by the DXO results measuring a lens to 3-4-5 MP on a 16 MP 4/3 sensor...

 

 

Here is my explanation:

 

You know the sampling theorem maybe:

 

f < 0,5 * fs

 

To sample audio up to 20 kHz you need a sampling frequency of more than 2 x 20 kHz = 40 kHz.

 

Its the same for images which can be described as sampled when its dvided up into pixels.

 

The sensor is the sampler and the sampling frequency is the pixel raster, i.e. 5184 x 3888 pixels (20 MP sensor).

 

Before the sampler/sensor there has been an optical low pass filter in basically all modern digital cameras up until recently.

 

Removing the optical low pass filter results in moiré and aliasing  because of the sampling  theorem limitations.

 

The bayer  filter which is usually used to get color from a  single sensor compunds the problem further. This gives you color moiré as well.

 

[Given the example above the low pass filter should  soften the image down to 2592 x 1944 pixels, effectively give you  a 5 MP image. :-) ]

 

[The nyquist freqency for the image sampler seems to be dictated by the number  of pixels at each mm on the sensor: 

http://www.edmundopt...em-performance/ ]

 

I don't know the details of the actuall level of optical low pass filters (anti alising filters) aka softening used in a system camera , but you can see an example here so the effect is quite dramatic:

 

http://thedigitalcam...ur-photographs/

 

On cameras which has removed the optical low pass filter you have two possibilities:

 

1. Reduce the offending moiré and aliasing manually in post processing.

 

2.  Use  an DSP which analyzes the image and removes/soften problem areas directly in camera. At least the new GH5 does this, probably the other M43 cameras from Panasonic as well.

 

Here are some more info on the AA-filter: http://www.outdoorph...go-no-low-pass/

 

"In high-quality digital maging systems,  optical low-pass filters (OLPF) are used to eliminate color Moire fringes.  An OLPF cuts off the lens MTF above the sampling frequency of the imager resulting an overall MTF curve that approximates a step function in spatial domain." http://www.optics-online.com/lpf.asp

 

 

The short summary here is that the lens+sensor resolution will never be the same as the pixel count on the sensor...

 

1.jpg


In Topic: Quality of WA zoom lens for UW use

09 January 2017 - 02:43 PM

Hi EspenB,

 

(But 3 pMpix with 20 Mpix sensor does NOT sound like excellent IQ to me).

 

Wolfgang

 

Yeah, I have also been puzzled by the DXO results measuring a lens to 3-4-5 MP on a 16 MP 4/3 sensor...

 

Can't say that I understand the real world implications. If taken figuratively all M4/3 optics are basically crap!

 

The best lens is the Olympus 75 mm f1.8

 

https://www.dxomark....-OM-D-E-M5__793

 

Sharpness is 13 MP.

 

This is followed by the more underwater friendly 12 mm f2.0 with 11 MPix sharpness.

 

I have the Oly 12 mm but have just tried it underwater once. Clearly I should use it more!


In Topic: Dome port size difference, zoom not compatible?

08 January 2017 - 07:34 AM

So espen, are u saying I'd get better image quality with the wwl and say, the kit lens, rather than an expensive dome and lens? And also, how would the macro quality be if I got a wet macro lens? Equal to let's say the 60mm Olympus lens (which is the one I had in mind)
 

 

I have personally no experince with the WWL-1. However, its designet to integrate directy with the water interface and can probably be thought as a modern answer to the old Nikonos optics.

 

The downfalls of the WWL:

As far as I can tell there is no possibility of manual focus. Operating it with a Panasonic 14-42 II lens i.e. the only gear available is for zoom. No port has an extra knob for focus operation.

Its heavy and expensive. But probably worth it considering the picture quality and corner sharpness. Others here have more direct experience.


In Topic: Quality of WA zoom lens for UW use

07 January 2017 - 11:57 AM

 

 

The DxOMark score for Sharpness is based on the Perceptual Megapixel (P-Mpix) concept that weights the Modulation Transfer Function (MTF) of the lens with the human visual acuity. Read more about Perceptual Megapixels.

 

https://www.dxomark....el-can-help-you
 

 

 

A number of factors can cause this loss in megapixels and resolution, including such lens defects as optical aberrations, light diffraction, or an ineffective anti-aliasing filter. The difference in number between a sensor’s megapixels and Perceptual MPix quantifies this loss.

 

You will find that no lens are perfect - it's quite the opposite.

 

Also the sharpness is not constand over the frame. Things allways gets worse at the edges. So a simplified averate rating doen't tell the whole picture.

 

Thus these sharpness figures are perhaps not the best figure to use for evaluating a system performance.

 

Generally 35 mm frame cameras and large optics are better. They are simply bigger and easier to make.

 

Still far from perfect. Find a few 36 - 50 MP FF cameras on DXO mark and check the "sharpness" factor with several lenses.

 

The camera might be 36 MP, DXO mark will say that lens XXX is only 10 to 15 MP "sharp". ;-)

 

Go figure.


In Topic: Are people leaving micro four thirds?

06 January 2017 - 02:14 AM

Hi Sully,

Just a quick bit of trivia. The Olympus underwater housings and ports are actually made by a company called AOI and branded Olympus.

 

 

"AOI is an Asian manufacturer that makes Olympus, Fuji, Sea & Sea underwater photography housings and accessories. Apart from making the Olympus branded products, they have come up with a value-priced port system that carries their own name. The AOI port system completes the Olympus micro four thirds housing system and makes them a serious contender against more advanced competitors."

 

Homepage: http://www.aoi.com.hk/page1.asp

 

AOI also seems to manufacture the RGBlue lights:

 

http://www.rgblue.jp/en/contact/