MEGAPIXEL MATH

[Daggilarr]

If I take a shot with a 24mp camera, then crop it by 50% is the result 12mp?

If I merge three shots taken with a 24mp camera, in Lightroom merge, as in HDR. What is the MP of the resulting image?

Are these really dumb questions?

Thanks in advance


Francis

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[Dennis]

If I take a shot with a 24mp camera, then crop it by 50% is the result 12mp?

If you mean 50% of the height (and 50% of the width) then it's 6MP.

If I merge three shots taken with a 24mp camera, in Lightroom merge, as in HDR. What is the MP of the resulting image?

Should be roughly 24MP (I'm not sure if it might crop a little if images aren't perfectly aligned). HDR merge overlays images, unlike panorama stitching, which increases image size.

- Dennis
--
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[dwalby]

If I take a shot with a 24mp camera, then crop it by 50% is the result 12mp?

If I merge three shots taken with a 24mp camera, in Lightroom merge, as in HDR. What is the MP of the resulting image?

Are these really dumb questions?

typically the crop/zoom percentages are expressed in linear terms, so if you display the image at 50% zoom, or crop to 50%, you typically have 1/4 the pixels of the original image (0.50*0.50=0.25).

I don't use Lightroom, but if what you're describing is blending the brightness of three identical, or nearly identical images into a single image, it would have the same number of pixels as the original image.

 

[Dan Marchant]

If I take a shot with a 24mp camera, then crop it by 50% is the result 12mp?

It depends what you mean by 50%. it is simply a count of the number of pixels in the image. Length x height. If you had a (fictional) image that was 5,600 pixels long and 4,300 high then it is about 24,000,000 pixels (24 megapixels). If you crop the image to 4500 x 2,150 you have removed half the pixels and it is a 12 megapixel image.

If, as mentioned above you cropped 50% from the length and 50% from the height then you actually have a quarter of the pixels so 6 megapixels.

If I merge three shots taken with a 24mp camera, in Lightroom merge, as in HDR. What is the MP of the resulting image?

24 mp well not exactly but.....
Merging to HDR does not generally make a larger image (pixel size). It just uses some image data from each image to create a new version at the same pixel size. So the file size (megabytes) may be larger due to the extra data but image size in pixels remains the same (megapixels).

If you merge to panorama on the other hand the images are stitched together to make one large image, so the megapixel size would increase. By how much would depend because panorama merges often result in parts of the image being lost because they don't line up/get cropped off.

Are these really dumb questions?

Not dumb... just not useful. Megapixel is really just a marketing term used to sell cameras. An easy single number that customers can read and think "oh this must be better than the 20 megapixel one". It tells us very little about an image that is useful. It is just a count of the number of pixels in an image. That image could be 2 pixels high and 12,000,000 pixels long or 5,600 pixels long and 4,300 high.... in either case it is 24 megapixels.

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[Daggilarr]

What I don’t get is, why then are the pictures from a 12mp camera the same size as pictures from a 24mp camera? I had thought this was about pixel density.

 

[Chris R-UK]

What I don’t get is, why then are the pictures from a 12mp camera the same size as pictures from a 24mp camera? I had thought this was about pixel density.

I think that you have misread Dan's post. Nowhere does he say what you are suggesting.

If you are measuring the "size" of the "pictures" in pixels, then they are clearly not the same. If you print at, say, 20"x16" then the "pictures" will be the same size/dimensions but the 24MP camera will have higher resolution for the print and the quality will therefore be somewhat better.

 

[Bobthearch]

What I don’t get is, why then are the pictures from a 12mp camera the same size as pictures from a 24mp camera? I had thought this was about pixel density.

They should not be.

Pictures from a 12mp camera are generally 4,000 x 3,000 pixels. 4x3=12

Pictures from a 24mp camera are generally 6,000 x 4,000 pixels. 6x4=24

Now if you crop the height and width of the 24mp camera by 50%, you get:

3,000 x 2,000 pixels, or 6mp. 3x2=6

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[Daggilarr]

I so appreciate you all for taking the time to help me understand this stuff. One of the reasons I ask this is because I want to have a simple kit and not carry around loads of lenses. I now have a Sony A6k with an 18135 lens, and for most things this is fine. However, now and again I find I need a little more reach, so was exploring how much I can consider cropping without too much loss of IQ.

Or, indeed, what kind of kit would be more suited to my needs.

 

[Chris R-UK]

I so appreciate you all for taking the time to help me understand this stuff. One of the reasons I ask this is because I want to have a simple kit and not carry around loads of lenses. I now have a Sony A6k with an 18135 lens, and for most things this is fine. However, now and again I find I need a little more reach, so was exploring how much I can consider cropping without too much loss of IQ.

Have you tried cropping your images from the A6000 to see what IQ you get? If you are just shooting to post on the web then you can get away with cropping a lot more than you can if you want to make large prints.

It really depends on what you are doing with your images and what IQ you need.

 

[Dennis]

I so appreciate you all for taking the time to help me understand this stuff. One of the reasons I ask this is because I want to have a simple kit and not carry around loads of lenses. I now have a Sony A6k with an 18135 lens, and for most things this is fine. However, now and again I find I need a little more reach, so was exploring how much I can consider cropping without too much loss of IQ.

That's more complicated than your original question. Not all megapixels are created equal. Put a Zeiss Otus lens on a 24MP full frame body, a kit zoom on a 24MP APS-C body and grab a 24MP (or as close as you can find) pint & shoot.

Which of those is going to produce the biggest sharp prints ?

DXOMark attempts to resolve some of this with their "perceptual megapixels" rating, but common consensus is that the methodology is too secret and the results too inconsistent to take seriously. The basic idea is that, for any given lens & camera combination, you take a photo at a particular focal length and f-stop, then you see how far you can downsize it before you start to lose actual detail (i.e. if you start with a really fuzzy image, you can probably downsize it from 24MP to 3MP, then resize it back up again, and make a print with just as much detail).

What all of this means is that whether you start with 16MP or 24MP or something else isn't what's important - more important is just how sharp that 18-135mm lens is at the long end. You can crop it to 6MP and you can find websites that will tell you that 6MP is good enough for a print of a certain size, but if you're cropping from a zoom at its longest focal length, it may not be a detailed 6MP.

The other potential issue with cropping is the increase in noise. If you take a 24MP image and crop it to 6MP, at 100%, the noise is identical, but if you print both to the same size (or display both online at the same size), the crop looks noisier. At low ISOs, this isn't an issue - just something to consider if you get into situations where you want to crop by a lot *and* you're shooting at high ISO. (Basically, whatever print size you're looking to make from your cropped image, picture the image quality being equal to what you'd get making a print twice as big from the uncropped image).

So you really want to know how good a cropped image from the 18-135mm is ... and you need to know how good an image you actually need for what you do.

Since you have the camera and the lens, you're in the best position to see for yourself.

- Dennis
--
Gallery at http://kingofthebeasts.smugmug.com

 

[NO x]

Your low-resolution monitor might be tricking you into thinking the images are the same size because it resizes them to fit on the display.

 

[Daggilarr]

Your low-resolution monitor might be tricking you into thinking the images are the same size because it resizes them to fit on the display.

-

My monitor is fairly good. It is a 27” iMac, albeit from 2010. I do wonder what is the best, most honest (if that makes sense) software to view on is. I use Lightroom to edit.
Francis

Francis de Aguilar

Explore Francis de Aguilar’s 654 photos on Flickr!

www.flickr.com

 

[D Cox]

I so appreciate you all for taking the time to help me understand this stuff. One of the reasons I ask this is because I want to have a simple kit and not carry around loads of lenses. I now have a Sony A6k with an 18135 lens, and for most things this is fine. However, now and again I find I need a little more reach, so was exploring how much I can consider cropping without too much loss of IQ.

That depends on the subject -- some subjects are more critical for image quality than others. It's also greatly affected by accuracy of focus and any trace of camera or subject movement.

If you expect to crop, use a faster shutter speed.

Or, indeed, what kind of kit would be more suited to my needs.

Depends on whether you are shooting little birds or big birds or aircraft.

Sigma make some good long zoom lenses.

 

[Ellis Vener]

There’s numbers (dimensions measured in pixels) and there is technique, and then there are tools.

assiming you are using the same camera and lens, are processing using the same software, and viewing on the same monitor/display, then we can ignore tools as a factor and are left with technique.

Are you using a tripod?

What are your processing settings?

If you are printing, what size are you printing?

 

[tbcass]

If I take a shot with a 24mp camera, then crop it by 50% is the result 12mp?

No it's 6mp. If the 24mp image is 6000x4000 that equals 24mp. Reduce the image by 50% and you get 3000x2000= 6mp.

If I merge three shots taken with a 24mp camera, in Lightroom merge, as in HDR. What is the MP of the resulting image?

24mp. The 3 images increase the dynamic range but not the resolution.

Are these really dumb questions?

No they are not but some of the answers above are making it seem a lot more complicated than it really is. Simple questions deserve simple straight forward answers.

--
Tom

 

[Daggilarr]

Okay, still not sure what this is all about.

Say I take a shot and view it as a 10x8. It has been shot on a 24mp sensor.

I decide that it needs cropping so a take 25% off the base and 25% off the side.

I then view the cropped image at 10x8

Is the cropped image 12mp? Or, to put it another way, is the cropped image the equivalent of an image shot on a 12mp sensor?

 

[Tony Beach]

Okay, still not sure what this is all about.

Say I take a shot and view it as a 10x8. It has been shot on a 24mp sensor.

I decide that it needs cropping so a take 25% off the base and 25% off the side.

I then view the cropped image at 10x8

Is the cropped image 12mp? Or, to put it another way, is the cropped image the equivalent of an image shot on a 12mp sensor?

It would be the equivalent of shooting on a smaller format sensor that is 11.25 MP. To put that in perspective, if you were to have an 8x10 print made from that file it would have 375 pixels per linear inch, which is more than enough.

 

[Ellis Vener]

I

Okay, still not sure what this is all about.

Say I take a shot and view it as a 10x8. It has been shot on a 24mp sensor.

I decide that it needs cropping so a take 25% off the base and 25% off the side.

I then view the cropped image at 10x8

Is the cropped image 12mp? Or, to put it another way, is the cropped image the equivalent of an image shot on a 12mp sensor?

If the camera has a sensor with the same height x width ratio 1:1.25 sensor.

since you have Lightroom there is a very straightforward method of determining the resolution of the cropped image in pixels. Crop the image and measure the height and width of cropped image in pixels. Then multiply the number of pixels that will give you the resolution in megapixels of the cropped image.

For example if you start with a sensor with a 24mp sensor (4000x6000 pixels) and the cropped photo measures 4000x5000 pixels, the resolution is 20mp. If the cropped photo measures 2000x2500 pixels, the resolution of the cropped photo is 5mp.

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[Gerry Winterbourne]

Okay, still not sure what this is all about.

Say I take a shot and view it as a 10x8. It has been shot on a 24mp sensor.

The first thing is that the sensor has an aspect ratio of 3:2 but 10 x 8 is 5:4 so some of the 24MP have been lost just by the photo size. Instead of 24MP it is now 20MP (5,000 x 4,000).

I decide that it needs cropping so a take 25% off the base and 25% off the side.

You've reduced the pixel dimensions to 4,000 x 3,200 so the new image is 12.8MP.

I then view the cropped image at 10x8

Is the cropped image 12mp? Or, to put it another way, is the cropped image the equivalent of an image shot on a 12mp sensor?

See above - it's 12.8MP. But that's not the end of it. The resolution of a picture depends on the lens as well as the sensor. Let's assume the lens resolves 200 lines/mm (typical of several Sony A-mount lenses). Cropping by 25% reduces that to 1600.

Using the 12MP sensor without cropping uses the full resolution of the lens so even if the MP counts match the cropped image won't be as sharp.

 

[Dennis]

Okay, still not sure what this is all about.

Say I take a shot and view it as a 10x8. It has been shot on a 24mp sensor.

So the native resolution is 4000x6000 and you're viewing it cropped (because the aspect ratio is wrong).

I decide that it needs cropping so a take 25% off the base and 25% off the side.

So instead of 4000x6000, the resolution is 3000 x 4500.

That's 13.5 MP.

I then view the cropped image at 10x8

Is the cropped image 12mp?

It's made from a crop from the 3000x4500 image - the aspect ratio of the image you cropped on your computer is 2:3 while the print is 4:5 so you're viewing an image made from 3000 x 3750 or 11.25MP.

Or, to put it another way, is the cropped image the equivalent of an image shot on a 12mp sensor?

You cropped your 24MP image to a 13.5MP image. By cropping off 25% off the base and 25% off the side, you reduced it to .75*.75 or .5625 of the original.

Take a 4x6" print. Cut off 25% along the bottom. Now it's a 3x6" print. Cut off 25% from the side. Now it's a 3x4.5" print. Your 24 square inch print has been reduced to a 13.5 square inch print.

By the way, reducing it to 3/4 of the original gives you the same field of view as a lens with a focal length that's 4/3 the original. So it's like simulating a 400mm lens with a 300mm lens. You lose a lot when cropping by much.

- Dennis