[Bertrand3000]

Hi,

I am well aware of the basis of exposure with aperture, speed and sensitivity. All are affecting exposure, but aperture also modifies depth of field and sharpness (diffraction and lens defect), and speed allow to freeze movement (or to make movement blur).

When it comes to ISO sensitivity, for what I have understood, the lower the better. Increasing ISO allows to close aperture (for depth of field increase) and/or to increase speed (to avoid movement blur), in the cost of noise.

But I have found an article (in French unfortunately), basically stating that increasing ISO would, according to the blog author, increase dynamic range and capture more colors : (L'exposition 3/3) Comprendre la sensibilité ISO en moins de 3 minutes | Tonton Photo

Is the blog author completely wrong, or is there something I missed about sensitivity?

And what is the nominaly sensitivity of the sensor, in the Pentax K-S2 (by nominal I mean the "ideal" ISO value, where the values are not increased or decreased by software)? 100?

[BrianR]

The two comparison photos are lacking in details - what were the shutter speeds and f-stop? Did the lighting change? White balance? It would take magical voodoo to go from an overexposed sky to a blue sky by doubling the iso and changing nothing else (likewise for the colour on the lemur & trees).

[sergysergy]

I agree with BrianR in that there has to be something else other than the higher ISO in those two pictures. If anything, I would expect the second one being overexposed everything else being equal.

[normhead]

As far as I know, the article is nonsense. But less Dynamic Ranges on the input device leads to more contrast and saturation on the output device, which could if you aren't adept at post processing lead to the appearance of more contrast and brighter colours.

The output device is the same whether you shoot high or low Dynamic Range, it all gets squeezed into the DR of the output device. So it doesn't matter how high the DR range is on the input device, if you have a 13 EV DR inout on your camera, and 10 EV out put on your viewing device the contrast will appear flatter than the real life appearance (less DR.) because you've diminished the DR to fit into smaller space. If you shoot a 7EV DR image and stretch it to a 10 EV colour space, you actually increase the DR of the captured image and things have more pop.. But if you want low DR/high contrast (from a high DR image) you can simply use levels to clip your 100 ISO image.

You always want to shoot 100 ISO where possible. 200-400 in a pinch. At 800 ISO you'll need to make sure your image isn't even slightly under-exposed to avoid noise in the shadows. At 1600 ISO there could be noise anywhere. The ISO of a digital camera doesn't really change. At 200 ISO you are clipping off the bottom stop of your dynamic range, so you've lost one stop. AT 400 2 stops. at 800 3 stops etc. because basicly all you are doing is moving your black point to the right.

But there are many days or lighting situations where you need about half the dynamic range available on the camera. SO even at 1600 ISO you have space on either side of your histogram, which tells you don't need the amount of dynamic range the camera can capture, for the image in front of you.

But main points:

Every stop of ISO decreases your dynamic range one stop.

Decreasing dynamic range on the camera, will increase appearance contrast, saturation and the brilliance of colour on the output device. But even then, shooting with higher ISO means a weaker signal, and a weaker signal means more noise.

Shooting at 100 ISO, you can create the same effect as shooting with a lower DR camera, by moving your black and white points on "levels."

It is always preferable to shoot 100 ISO and create a lower DR image in post if high contrast, saturation etc. is desired, that way you can change your mind. If you shoot high ISO you can't get back the DR you lost should you desire to do so.

The only reason for using a higher than 100 ISO is if you can't get the shutter speed you need at ISO 100 at the aperture you want. 100 ISO gives you the highest dynamic range, uses the most light to create your image and therefore gives you the lowest noise. Saturation, contrast and the dynamic range of your output can be dealt with in post processing more effectively than by decreasing the amount of light used to create your image, which is essentially what a higher ISO does.

( In real life, a standard print is not likely to be more than 7 EV. A TV or computer monitor will probably not be more than 9 EV.) A K-3 captures 13 EV and a K-1 captures 15 EV. That's why you have to bump saturation and contrast of a modern digital input to have the image look lifelike, so that the EV compression doesn't make everything look flat..)

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Last edited by normhead; 11-02-2019 at 08:03 AM.

[UncleVanya]

First, I haven't read the article... Apologies, but this is from my mobile and I wanted to add some context.

There are cameras like several Nikons and possibly the k-5, that offer a lower iso than the native iso of the sensor. In these cases I'm unless if the lower iso gives up anything vs the native iso (100 on the Sony sensors used in most apsc and full frame). In m43 there were some native iso 200 sensors that offered 100 as an extended range option as well.

In these cases does DR increase when you move up to the native iso?

[UncleVanya]

Ok I'm back after reading the article. I'm of the opinion, that some exposure differences and framing made the difference. The claim for the first two shots is that they are unprocessed but the camera always processes to some degree. There's a lot more greenery in the second shot which a Bayer sensor would get a lot of input from given the layout which has two times as many green receptors, this likely resulted in a lower relative exposure, which gave a more saturated look after processing (in camera) and retained more blue in the sky.

Controlled conditions and identical shots probably wouldn't lead to the same results.

[UncleVanya]

One more thing, there are positives to higher iso for very specific events. I shoot at very high iso when I want reduced resolution and a "grainy" feel.

[normhead]

One more thing, there are positives to higher iso for very specific events. I shoot at very high iso when I want reduced resolution and a "grainy" feel.

As much as I liked grain images on film, noise on digital is different. Grain on film had distinct patterns like snow flakes. Each one slightly different. Noise on digital has no such interesting characteristics.

[UncleVanya]

As much as I liked grain images on film, noise on digital is different. Grain on film had distinct patterns like snow flakes. Each one slightly different. Noise on digital has no such interesting characteristics.

I understand the point, but I disagree that the effect isn't interesting. Also to be fair grain can be emulated but that isn't what I'm after either.

This shot is an example of the result I'm after sometimes:


Generated from my Motorola o using tools.sportscard.trade

Iso 25k on a k-3

[sergysergy]

For me grain on digital only works with certain pictures. B&W is the best example.

[photoptimist]

The post is totally wrong about dynamic range and color. For most modern "ISO invariant" sensors, higher ISO always has worse dynamic range and worse color. For a few sensors that aren't ISO-invariant, higher ISO is no worse than lower ISO. Bt higher ISO is never better on these dimensions.

The point is that if the object is stationary, then the lowest ISO is always the best ISO for maximizing dynamic range and color. (For stationary objects, one typically sets ISO to the lowest value, aperture to the best value for DoF and the performance of the lens, and then lets shutter speed float with the light level.)

However, the broader point is correct: sometimes increasing the ISO is the key to a better picture, especially with non-stationary conditions.

Higher ISO has two major benefits:

1) One stop of added ISO cuts the motion blur in half. Sometimes freezing the subject is worth the added noise and loss of dynamic range.

2) One stop of added ISO increases the depth of field by about 40% and compared to a wide-open lens, has even bigger effects with corner sharpness. Sometimes getting more of the foreground and background in focus or getting better edge/corner sharpness is worth the added noise and loss of dynamic range.

The point is that sometimes you really want a specific shutter speed and specific aperture even if the it means a higher ISO. (That's where TAv mode is best.)

P.S. Any differences in the color/contrast of those lemur photos is due to changing lighting conditions and lens flare. The over-exposed white cloud in the ISO 200 image has fogged the frame. The ISO 400 image was taken with more blue sky and less bright cloud with less flare and more uniform lighting.

[aaacb]

Most processing isn't linear (just reduced dynamic range), so it's not as simple as low dynamic range leads to punchier images. Now of course you can post process, or set the camera's jpeg engine accordingly, instead of using high iso to reduce that dynamic range. This is what most people do Higher iso reduces dynamic range because the signal is amplified but the maximum (brightest) value is capped. As much as I don't care about these stats, dxomark has nice plots of dynamic range vs in-camera iso (go to the dynamic range tab Pentax K-3 - DxOMark) If the scene has less dynamic range than the dynamic range of the sensor/processing after amplification, then it's all good. And the amplification needs not be linear, one example is with the dual gain sensors that use a different model at higher ISOs.

[biz-engineer]

Positive effects of increasing ISO?

Higher ISO as opposed to under-exposure flushes more shadow detail in the RAW files. For example, ISO1600 is 4 stops above ISO100. Shooting at ISO100 underexposed 4 stops, data goes in the RAW file as is. When shooting at ISO1600 with 0 ev exposure compensation, the sensor data bits are shifted 4 positions to the left (MSB on left side notation) and some noise reduction is applied by image processor, then written in RAW file. That's why using ISO1600 offer more dynamic range then ISO100 underexposed by the same amount. That's why ISO100 image pulled shadow contain more details and more noise (minimal or no noise reduction applied to data prior to writting data into RAW format). Unless stacking underexposed exposures, the advantage goes to using a higher ISO setting.

[StiffLegged]

But I have found an article (in French unfortunately), basically stating that increasing ISO would, according to the blog author, increase dynamic range and capture more colors ... Is the blog author completely wrong, or is there something I missed about sensitivity?

He's wrong.

When shooting at ISO1600 with 0 ev exposure compensation, the sensor data bits are shifted 4 positions to the left (MSB on left side notation) and some noise reduction is applied by image processor, then written in RAW file. That's why using ISO1600 offer more dynamic range then ISO100 underexposed by the same amount.

Shifting the sensor data bits 4 positions left leaves the rightmost 4 bits empty, so added noise reduction is adding made-up (however it's calculated) data to these bits, not magically recovering shadow detail.

[jpipg]

He's wrong.




Shifting the sensor data bits 4 positions left leaves the rightmost 4 bits empty, so added noise reduction is adding made-up (however it's calculated) data to these bits, not magically recovering shadow detail.

The rightmost bits contain less information than the even the single least significant of the leftmost bits. In the case that you have a subject with narrow dynamic range, placing the values on the left side of the register gives far larger tonal resolution because those bits contain more data.

In the case when you have a scene with narrow dynamic range, you have a choice to put make those values high or low exposure. With digital, you can obtain more information by placing them in the high zones. This means that once you or the camera applies a curve to increase contrast, you can obtain a image with the desired appearance, more detail, with less noise and posterisation.

If you are in a situation where your aperture and shutter speed are fixed and you are shooting a low dynamic range scene then you will simply obtain a better image if you increase the iso. This is basically the idea behind ETTR