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Two interesting observations about "equivalent focal length"
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Jan 18, 2021 11:22:32   #
CHG_CANON Loc: the Windy City
 
Rongnongno wrote:
Actually the big deal is that pixels have no size when 'dots' are limited by the device used. When each pixel is represented by a dot you get a size.

Monitors do not display 'pixels' but dots. Depending on resolution you get more dots on screen than on another. Remember the DPI rating of monitors????

Implication? If you print a full image at 72DPI you will have a larger print compared to one printed at 300DPI. The resolution cost is 'corrected' by the viewing distance.


Dell defines my monitor as 1920x1080 pixels. How many dots is that?

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Jan 18, 2021 11:30:02   #
Rongnongno Loc: FL
 
CHG_CANON wrote:
Dell defines my monitor as 1920x1080 pixels. How many dots is that?

Ever heard of pixel representation?
Dell can use any terminology they like, the resolution is in DPI.

You rightly indicated the idea that a dot can and is usually the result of many pixels merged together.

If you have an image 19200*10800 displayed as a whole on your monitor 100 pixels are represented by 1 dot.

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Jan 18, 2021 11:33:18   #
CHG_CANON Loc: the Windy City
 
Rongnongno wrote:
Ever heard of pixel representation?
Dell can use any terminology they like, the resolution is in DPI.

You rightly indicated the idea that a dot can and is usually the result of many pixels merged together.

If you have an image 19200*10800 displayed on your monitor 100 pixels are represented by 1 dot.


Really, in January 2021, you're going to say my pixel-based image on a pixel-based monitor is displayed in dots?

BTW: where do I go into my digital cameras and change the DPI value? Give me the menu and model where I can make this change, please.

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Jan 18, 2021 11:34:45   #
Rongnongno Loc: FL
 
CHG_CANON wrote:
Really, in January 2021, you're going to say my pixel-based image on a pixel-based monitor is displayed in dots?

BTW: where do I go into my digital cameras and change the DPI value? Give me the menu and model where I can make this change.


The terms Dots Per Inch (DPI) and Pixels Per Inch (PPI) are commonly used interchangeably to describe the resolution of an image.

Take a picture of your monitor, blow it up and see the dots. Good luck seeing the pixels.

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Jan 18, 2021 11:43:56   #
CHG_CANON Loc: the Windy City
 
The hot new phone is the Samsung Galaxy S21. The technical specs are 6.2-inch Flat FHD+ Dynamic AMOLED 2x, and 2400x1080 pixels

Q1 How many dots is the Samsung Galaxy S21?

I went to BestBuy.com and found the LG - 43" Class UN7000 Series LED 4K UHD Smart webOS TV is advertised as 4X the pixels of Full HD

Q2 How many dots are in this LG 4K TV?

Even the BestBuy Q&A states: Resolution refers to the number of pixels, horizontally and vertically, used to display the video. The higher the resolution, the finer the detail will be.

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Jan 18, 2021 11:47:26   #
a6k Loc: Detroit & Sanibel
 
CHG_CANON wrote:
Lot of words ... but what is the ratio?

How many dots are in a pixel? If you like it the other way: how many pixels are in a dot?


I depends on what you mean by a dot in this case. If you are using a single color then it is CYMK vs RGB and I don't know the right answer because for dot-matrix printers, the dots are a single color and they mush together. If you use other printing methods it works differently. I am not anything close to being a print expert.

I know, however, that it's 1:1 the way CaptureOne, etc. compute print size.
https://www.howtogeek.com/292081/how-big-of-a-photo-can-i-print-from-my-phone-or-camera

Do I get a gold star?

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Jan 18, 2021 11:48:09   #
a6k Loc: Detroit & Sanibel
 
CHG_CANON wrote:
Dell defines my monitor as 1920x1080 pixels. How many dots is that?


It is not dots. It is pixels. Dots are on a print.

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Jan 18, 2021 11:50:53   #
a6k Loc: Detroit & Sanibel
 
BebuLamar wrote:
You contradicted yourself. You said DPI isn't the same as PPI (which is correct) but because a dot isn't a pixel then 3000pixels wide image printed at 300dpi won't be 10" wide. Just like Paul asked you how many dots in a pixel or how many pixel in a dot.


I don't believe this is as correct as you may think.
https://www.howtogeek.com/292081/how-big-of-a-photo-can-i-print-from-my-phone-or-camera

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Jan 18, 2021 11:51:58   #
CHG_CANON Loc: the Windy City
 
a6k wrote:
I depends on what you mean by a dot in this case. If you are using a single color then it is CYMK vs RGB and I don't know the right answer because for dot-matrix printers, the dots are a single color and they mush together. If you use other printing methods it works differently. I am not anything close to being a print expert.

I know, however, that it's 1:1 the way CaptureOne, etc. compute print size.
https://www.howtogeek.com/292081/how-big-of-a-photo-can-i-print-from-my-phone-or-camera

Do I get a gold star?
I depends on what you mean by a dot in this case. ... (show quote)


So, if you text-search your link above that has the title "How Big of a Photo Can I Print from My Phone or Camera?", you will find no references to 'dpi' nor 'dot' within the text of the post.

So, is that link to define the ratio of dots to pixels (or pixels to dots?) or was the link to support some other point?

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Jan 18, 2021 11:52:37   #
Longshadow Loc: Audubon, PA, United States
 
a6k wrote:
I depends on what you mean by a dot in this case. If you are using a single color then it is CYMK vs RGB and I don't know the right answer because for dot-matrix printers, the dots are a single color and they mush together. If you use other printing methods it works differently. I am not anything close to being a print expert.

I know, however, that it's 1:1 the way CaptureOne, etc. compute print size.
https://www.howtogeek.com/292081/how-big-of-a-photo-can-i-print-from-my-phone-or-camera

Do I get a gold star?
I depends on what you mean by a dot in this case. ... (show quote)


Pixels are not a single color?

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Jan 18, 2021 11:56:46   #
a6k Loc: Detroit & Sanibel
 
Strodav wrote:
So help me understand what you are getting at. The idea of EFL is if you have a camera with a different sensor size than the standard 35mm sensor and you want to get the same field of view as a 35mm camera, what focal length lens do you need for your crop sensor camera. The formula EFL = actual lens focal length / sensor crop factor is well known. I do not see DPI in the formula, nor do I see pixel density. The only ambiguity I know of is if you use equal diagonal field of view vs equal horizontal field of view. My understanding is that the diagonal field of view is more common.

With that, please explain why the formula needs to be modified to include pixel density. I am especially confused by the 300ppi variable, which is only there as a place holder for a meaningless image size.
So help me understand what you are getting at. Th... (show quote)


Thanks for asking a good question. My inquiry into this subject was about the usual "crop factor" and EFL. I am only using print size as a way to show that the final result will be different than expected if the crop factor is the only number used. My crop frame camera has a factor of 1.5. So I should get the 400 mm lens to look exactly like a 600 mm lens on a FF camera. But since the FF camera could have such different pixel density, my largest practical prints, even with the same lens, will vary in size a lot. The reason is simply that the EFL is insufficient for the purpose of comparing results. It's fine for angle of view, btw.

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Jan 18, 2021 12:08:20   #
User ID
 
lamiaceae wrote:
I have worked with cameras in formats from APS-C, 35mm, 6x6 cm, 6x7 cm, and 4x5" to 8x10"! And I don't see the point to all that math voodoo, I just learned to work with each format with various lenses. Unless you are doing and publishing scientific photos who needs to know precisely the size of something in an image. It is art and composition and aesthetics are what is important. And did anyone ever answer how many pixels per dot or dots per pixel. For digital photography the only metric that matters is pixels per inch. Who ever said a 24MP APS-C sensor is the same as a 24MP FF sensor?
I have worked with cameras in formats from APS-C, ... (show quote)
Well, your APSC in that case has 6000 sensels per inch while the FF has 4000. Thus they both have 4000x6000 arrays.

So, working under favorable conditions, their images will be essentially identical if using appropriate lenses. Therefore identical same size prints at the same DOTS per inch will result.

Pixels are digital and dots are analog. There is no fixed correlation between them. You can crop down to a 20x30 pixel detail and print it 20x30 inches at 300 dots/inch. Acoarst, such a print visually renders only 1 pixel/inch even though it’s a 300dpi print. 300dpi and 1ppi in the same print.

Numbers above were selected to make the math easier to grasp. You could substitute any numbers you want and send it to your printer. Results will be somewhere between Chuck Close and Piet Mondrian. It’s all good.

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Jan 18, 2021 12:12:44   #
CHG_CANON Loc: the Windy City
 
Sony makes digital cameras along with TVs. They must know the importance of dots, right?

From BestBuy, they're also advertising the Sony 55" Class X750H Series LED 4K UHD Smart Android TV. In the specifications, the screen is 3840 x 2160 pixels. But, this TV is 12-inches bigger than the LG model.

Q1 Do these tech specs mean the Sony has more dots than the LG?

Q2 How can both LG or Sony fail to define the 'dots' in their pixel-based products?

Q3 Where on a Sony a7II do I find and change the DPI value?

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Jan 18, 2021 12:26:49   #
User ID
 
CHG_CANON wrote:
Sony makes digital cameras along with TVs. They must know the importance of dots, right?

From BestBuy, they're also advertising the Sony 55" Class X750H Series LED 4K UHD Smart Android TV. In the specifications, the screen is 3840 x 2160 pixels. But, this TV is 12-inches bigger than the LG model.

Q1 Do these tech specs mean the Sony has more dots than the LG?

Q2 How can both LG or Sony fail to define the 'dots' in their pixel-based products?

Q3 Where on a Sony a7II do I find and change the DPI value?
Sony makes digital cameras along with TVs. They mu... (show quote)

RTFM x3. You won’t find any answers cuz, well, you know why, but it might infotain you for a while, which is what really matters anywho ;-)

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Jan 18, 2021 12:33:47   #
bleirer
 
a6k wrote:
My first point is that since the usual print DPI is a constant (300), the pixel density of the image has to be part of the true measurement of "equivalent focal length" (EFL). I have attached a snapshot of a spreadsheet that I use for sensor and lens comparisons. In it you can see that the image size difference between the two cameras is 40.3%.
51.2/36.5=1.4027.

The nominal difference should be 750 vs 600 according to Sony and that is only 25%.
750/600=1.250.

Then I shot one exposure from each camera at exactly the same distance of the same object. I used Preview to display each image at "actual size" which is a 1:1 "crop". I measured the sizes of the two images on the screen with a metric ruler and the difference was 43%. Since the number of pixels on my monitor is constant, it's equivalent to a print for this purpose.

Using this method, reality closely supports theory. The true EFL must include pixel density in the calculation.

I don't have a convenient formula for "true EFL" but this provides a way to compare individual camera-lens combinations more accurately.
My first point is that since the usual print DPI i... (show quote)


What you say is interesting, but I'm not sure you are getting at equivalent focal length but maybe some other idea like pixel density vs. resolution. Just for the sake of understanding, if i went into photoshop and resampled a photo to different pixel dimensions, you seem be saying by doing that i would be changing the equivalent focal length?

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