[tromboads]

You still with us OP? Clear as mud?

[clackers]

So can we all send our crop jobs / enlargements jobs to you because you figured out how to crop into an image without increasing noise?

You can't increase noise by cropping, because noise is a ratio, Class A.


If every twentieth pixel is noise, that is true inside the cropped area, and out.

What does not change is noise-per-pixel or dynamic range per pixel.

You're welcome.

As the number of pixels change, of course image noise and image dynamic range are affected.

When you double the amount of pixels, you also double the range of darkest (0) to brightest (all pixels at full capacity) image values.

Bizarre.


Where did you read this?

[clackers]

Also, the inverse square law regarding light fall-off, must play a factor in helping FF lenses on APS-C due to the smaller distance to the sensor, although I haven't worked out by how much..

Smaller distance to the sensor, Bossa?


In the words of another famous Australian, 'Please explain!'


The 'C' stands for cropped - the sensor is in the same position - where you'd also put film, it's just smaller.

[Class A]

You can't increase noise by cropping, because noise is a ratio, Class A.

You don't crop much, do you?

If you did, you would have noticed that heavy cropping can bring out image noise that otherwise remains hidden.

If every twentieth pixel is noise, that is true inside the cropped area, and out.

It doesn't work that way.

As a matter of fact, what we are interested in is signal-to-noise ratio (SNR). We can have a ton of noise but if we have a huge signal as well, the noise won't matter. Conversely, a small noise level can be very visible, if the signal level is low.

By cropping, you reduce the signal (after all, you crop out a lot of signal). That signal (and its noise) is no longer available to raise the signal level and help noisy pixels even out each other. As long as you have a lot of pixels contributing to a square mm of the final image, the individual pixel noise will cancel each other out (to some degree). The fewer pixels contribute, the more noisy that square mm in the final image will be.

If you do not believe your own eyes (when cropping) then perhaps the math in the article "Contrary to conventional wisdom, higher resolution actually compensates for noise" may convince you. See the formula on page 2 of the article how downsampling (using many pixels to calculate the values for fewer pixels) increases the SNR of the image (i.e., reduces its noise). The formula on the last page tells you exactly how much noise advantage you can expect from downsampling.

You are correct in stating that the same f-ratio leads to the same exposure, independently of the sensor size. That's correct, because exposure measures light intensity (e.g., lumen per square mm), which is independent of sensor size.

Let's say we are using the same pixels (same size and capacity) for an APS-C and an FF sensor. Then we are just using more pixels for the FF sensor in a larger area, right? If we now record the same scene with both sensors and achieve the same exposure (that means we are using the same f-ratio) then at pixel level we have the same noise (as exposure was the same). But when we now use all the many more FF pixels to downsample them to an image that has as many pixels as the one from the APS-C sensor, we increase the SNR. By a bit more than a stop (1.17 stops, in the case of Pentax APS-C).

To counteract this increase in SNR, we can stop down the FF lens by 1.17 stops, which by the way also establishes the same DOF as the APS-C lens produced. We then have an "equivalent" image with the same scene capture, same DOF, same noise levels. Even though the FF capture used a lower exposure, it doesn't matter, because we don't have to enlarge the FF image as much as the APS-C (which is equivalent to downsampling the FF image to APS-C dimensions).

Hopefully you can confirm that enlarging images increases noise. Every darkroom owner knows this.

Bizarre.

Where did you read this?

I did not read it anywhere. It makes sense intuitively. If you take four pixels that can be either on or off then you can capture five different overall image brightness levels. If you only have two pixels then you can capture only three different image brightness levels.

If our pixels always define the same brightness (i.e., our sensor sensels always have the same full-well capacity) then this increase in image brightness levels does not result in a finer resolution of brightness levels, but in an increase of dynamic range.

Remember that by projecting the same scene to a larger area (with the respective amount of more pixels), you reduce exposure, i.e., a single pixel now receives less light then before and we can use this gained headroom to capture more dynamic range in the scene.

Also, if you like another source: Ever wondered why DxOMark dynamic range measurements seem to be too good to be true, i.e., specifying more DR than the bit-range of the camera seems to support? The reason is that DxOMark state the DR in terms of a normalised 8MP sensor. If a camera has more than 8MP, it receives a respective increase in DR. The converse is also true, i.e., the DR for a 6MP camera would be stated lower than it is (for 6MP).

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Last edited by Class A; 07-14-2015 at 06:28 AM.

[osv]

When you push film you intentionally underexpose and then over develop. It's a basic technique that was taught in the first year of every photography class, ever.

my first film class was in 1977, with a 2 1/4 format camera, that had a waist-level viewfinder... it sucked

It essentially increases your ISO, and contrast, but also increases your grain size.

I do the same thing in digital, by exposing to the left which is in essence an underexposure

his stated example was shooting iso400 when he should have used iso3200... that's not exposing to the left, it's gross underexposure!

his real failure was claiming that "In film photography you can't even bump the ISO. The "equivalent" method is called push processing"

Read more at: https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/299360-equi...#ixzz3fsxkPJAJ

i repeatedly proved that you bump film iso by using a different speed of film, not with push processing. ......obviously there are limits to how far you can go with film asa.

---------- Post added 07-14-15 at 10:09 AM ----------

Good list, but Norm did account for FOV in the next post:
"Nikon D800 at ƒ4 and 50mm at 10 feet away has a DoF 8' 8.9 inches to 11'8" approx. 3 feet.
A nikon D7000 at 33mm @ ƒ2.8.. and 10 feet away has a DoF of 8' 7.8 to 11' 10" approximately 3 feet."

he only went to the dof table because i rode his *** about it :-) he's left out portions of the equivalence formula in every post that he's made to this thread, including that one.

he's repeatedly posted that equivalence proponents don't consider dof, which we both know is not true

[boriscleto]

Smaller distance to the sensor, Bossa?


In the words of another famous Australian, 'Please explain!'


The 'C' stands for cropped - the sensor is in the same position - where you'd also put film, it's just smaller.

The 'C' stands for Classic. There were 3 APS formats.APS-H (High Definition, 30.2x16.7 mm, 16:9), APS-C (Classic, 25.1x16.7mm, 3:2) APS-P (Panoramic, 30.2x9.5mm, 3:1)

The C and P formats are made by cropping, so it could have been 'Cropped', but it wasn't...


But that doesn't have anything to do with focal flange distance, that is a mirror vs mirrorless argument

Nikon made the APS Pronea SLRs, they kept the same focal flange distance. You can use the Pronea lenses on a Nikon APS-C DSLR with a slight modification...

[starbase218]

<cut>

^^ what he said

[normhead]

You guys do realize, that that DxO stuff is purely theoretical? Because they keep saying stuff that's just not true. How many times do they say "assume that". Now go through the whole article and see how many times they say that, then work out the odds of them actually being accurate, because most of it is not true. TO me it's amazing that you guys can take all this theoretical micro-analysis, which may be true in a perfect theoretical sense, but can't be applied in the real world because they've made too many incorrect assumptions. The big scientific phrase missing from the DxO pages. The missing phrase is "if and only if."

Now assume that the same exposure times and identical ISO settings are used with a low-resolution camera and with a high-resolution camera having four times as many pixels. Since each high-resolution pixel is intrinsically less sensitive, a higher gain (either analog or digital) is applied to the signal, yielding more noise.

The above statement is true if and only if each high-resolution pixel is intrinsically less sensitive. What are the odds of that? See this is DxO. It's not science, it's pseudo science. The fact is, when comparing different systems and, the only thing that matters is "what is the point at which the original signal has been amplified and by how much? That should be critical to determine how much noise the camera will produce. DxO doesn't address it. Because before 800 ISO or whenever it is serious amplification cuts in there is no difference in amplification gain, just longer and shorter exposures. And looking at APS-c and FF , yes you need more light, but you have to use twice as much light to normalize DOF, so much of the time you're in exactly the same boat. If you use half the size pixel but twice as much light to normalize DOF, the light hitting the smaller well is the same, the noise is the same.

And further more, to normalize DoF you'd have to shoot a stop smaller aperture, and that is half the light. SO DxO has done in their example exactly what every FF pragandist has done since the beginning of time. Forgoten to normalize for DoF. Pretend that nothing in the camera system exists but the sensor and that nothing else matters.

What we have here is amateurs getting lost in the minute details of a micro analysis, and missing out on the big picture. That statement I quoted from DxO is absolutely correct, and absolutely irrelevant when trying to understand the difference between two camera systems. Why ? Because there is no analysis of the factors that companies can use to reduce noise, discussion of the sensitivity of various sensors, including the differences between a traditional sensors and a BSI sensors or organic sensors and items of that type. It looks at pixel size in a vacuum.

It's just really sad when people come out and quote it as if it's some kind of absolute fact. That's made possible by omitting that important mathematical construct, "if and only if".

It's a valid explanation of a very small part of photographic hardware, but to have people quoting it as some kind of hard science that actually describes real world systems, that's pretty bizarre. Rather than science, it's more like mathematical theorem, not scientific research into the real world.

It describes one teeny tiny little part of big and very complex systems. And doesn't go into sensor sensitivity, which you would think would be part of the same article, if it were to have any value at all. Unfortunately companies noise reduction algorithms which are also going to have a tremendous affect on this bit of minutia are proprietary and probably beyond the scope of little companies like DxO. They do a good job of measuring the results of the complete system in low light, but have as much clue as to how they got that way as you or I do. Hence when two cameras use the same sensor yet one is rated 7 points higher, they don't know any more than we do. They just tell us it what it is.

[CFWhitman]

My take on this is that "equivalency" is never perfect. There are too many variables involved, and thus not really any such thing as equivalency. I tend to agree with Lowell. It's about learning to use your equipment to get what you want, and learning when you need to add another piece of equipment because what you have won't quite give you what you want.

This is not to say that comparisons can't be useful (notice how I said "comparisons" rather than "equivalency" ). If the original poster is still paying any attention, then I would explain it a bit differently:

So I am to understand that when I'm using my tamron 17-50mm 2.8 on my k5 it is equal to a 25-70mm lens now does this affect the aperture also or is it still considered an f2.8 ?

When you use the Tamron 17-50mm 2.8 on your K-5 your angle of view will be similar to 25 - 75 mm on a 35mm sized sensor.

Your aperture as far as light gathering ability goes will (theoretically, because some lenses have truer aperture ratings than others) be similar at f2.8 as any other lens that is f2.8. That is, the same ISO to f-stop ratio will get the proper exposure.

Your aperture as far as depth of field goes will be different because depth of field varies with focal length as well as aperture and you are shooting at a different focal length. Wider aperture gives less depth of field, and longer focal length gives less depth of field. So when you shoot at a shorter focal length as with APS-C, then you would need a wider aperture to get the same depth of field.

ISO/noise ratio is governed by pixel size. There are two types of noise involved with digital cameras: shot noise and read noise. Shot noise is almost completely dependent on area. Read noise is dependent on the number of pixels (more pixels mean more read noise). When signal is low (high ISO settings) then the fact that you get more read noise per pixel is the deciding factor and larger pixels give you less noise overall. When signal is high (low ISO settings) then the greater signal more than compensates for increased read noise and you actually get less noise with more pixels (because you have plenty of light to go around). Of course, you'll generally notice noise more when there is less signal, at high ISO settings, so for practical purposes you get less noise with larger pixels.

[Class A]

Your aperture as far as light gathering ability goes will (theoretically, because some lenses have truer aperture ratings than others) be similar at f2.8 as any other lens that is f2.8. That is, the same ISO to f-stop ratio will get the proper exposure.

That's wrong (as are further statements you make about pixel size and noise that I won't address).

Many people have this misconception of "aperture for light gathering" vs "aperture for DOF". There is only one aperture with a certain diameter and the latter determines both "light gathering capability" and "DOF".

The fact that there is less DOF is caused by the additional light gathered. The fact that less light is gathered causes more DOF.

It is true that exposure only depends on the f-ratio, but to equate that with "light gathering ability" is 100% misleading. The Pentax-Q 01 lens is stated to have a widest aperture of f/1.9. Now everyone knows is that this doesn't mean you get the shallow DOF that you can get with an FA 43/1.9 on an FF camera. What a lot fewer people know is that while you technically get the same exposure with f/1.9 independently of the format size, the noise levels that you get with the Pentax-Q 01 lens at f/1.9 are the same as with an FF frame lens at f/10.64. Therefore, the focal length of the Pentax-Q 01 lens is FF-equivalent to 47.6mm and its "light gathering" ability is FF-equivalent to "f/10.64".

Using the example of the OP, if you convert the focal length range of the 17-50/2.8 into its FF-equivalent range, you also need to convert its f-stop to its FF-equivalent. The 17-50/2.8 on APS-C hence allows you to take the same pictures as a 26-75/4 on FF.

[starbase218]

Here's another thought. Equivalence may not be an exact science, and your mileage may certainly vary, because not all cameras are equal. But I think most of us probably get a new body every few years. So even if a certain FF setup deviates from what you expect based on equivalence (esp. with regard to noise), it may still be of value to you as an indicator of the capabilities of a fullframe system in general. After all, you may replace your fullframe body in a few years, and who knows how that sensor will perform?

[dtmateojr]

You do not understand f-stop at all. It's very basic physics. F-stop controls the amount of light hitting the sensels. Sensor size is immaterial.

---------- Post added 07-15-15 at 09:30 ----------

For the sake of everyone, allow me to explain the very basic concept of f-stop. This is the sort of explanation that even kids should be able to understand. After this, I will stop responding to this thread. Buckle up...

Assume that you are shooting a uniform light source and comparing m43 and FF sensors. To get the same angle of view, we need to use 25mm and 50mm lenses respectively. Assume for now that the lens openings are the same for both.

The effect of increasing focal length is magnification. Longer focal lengths project larger images on the focal plane. If a subject is projected as a 1x1 square at 25mm it will be projected as a 2x2 square at 50mm. The same amount of light coming from the source is therefore spread more thinly over a wider area by the longer lens. This effectively reduces the amount of light PER UNIT AREA hitting the focal plane (sensor). Every sensel behind the longer lens will receive four times LESS the amount of light compared to the smaller sensor. This results in underexposure of the larger sensor. It's like spreading a spoonful of peanut butter on different sizes of loaves. The smaller loaf will have more peanut butter per bite.

To compensate for the unfair spread of incoming light, we need to
1) increase the opening of the longer lens, or,
2) reduce the opening of the shorter lens

The ratio between the focal length and lens opening is your f-stop. That's why for the same f-stop, each sensel receives exactly the same amount of light. Sensor size is immaterial. This is the physics of photography in its entirety.

[lguckert79]

Yeah I'm still here I just watched a video from Tony Northup and he had a good way of explaining the aperture when on a crop sensor so I'm good and I thank everyone's expanation's

[kh1234567890]

The fact that there is less DOF is caused by the additional light gathered. The fact that less light is gathered causes more DOF.

[tromboads]

I must remove my subscription from this thread but it's just so funny!