Jump to content

Recommended Posts

This has been brought up several times so I thought I'd try and show the maths behind it and how it can be changed to recalculate new sizes, should Alamy change the minimum size requirement. I'll start with the basic formula  for full size images then move on to show how to work out for cropped images.

 

Basic Formula

 

(File width (pixels) x File height (pixels) x 3 (for the three channels, red, green and blue) )

________________________________________________________________________> = 17MB uncompressed

                                                   1024 x 1024

 

 

The 1024 x 1024 comes from the fact that each pixel of information is stored in a byte of data and there are 1024 bytes in a kilobyte (KB) and 1024 KB in a MB

 

So for my Nikon D200 an uncropped image saved as an 8bit TIFF would be:

 

3872 x 2592 x 3

____________ = 28.71MB in size, well over the current minimum of 17MB

  1024 x 1024

 

 

The slightly trickier part comes in if you need to crop the image. For images with the same aspect ratio we use the properties of similar triangles:

 

similar.jpg

Ax / AD    =  Ay / CD  where Ax and Ay are the cropped width and cropped height sizes, respectively. Substituting what we know for my D200:

 

Ax / 3872  = Ay / 2592 and transposing:

 

Ax =  (3872/2592)Ay  (equation 1)

 

Where does this get us? If we substitute this for the width in the basic formula at the start of the post we can find the minimum height for the cropped image:

 

3872 x Ay x Ay x 3

______________ = 17 which becomes:

2592 x 1024 x 1024

 

 

11616 Ay2

__________    = 17, then cross multiplying this becomes:

2717908992

 

 

Ay2 = 3977656 so the minimum height for a cropped image from my D200 would be:

 

Ay = 1994.4 or 1995 pixels

 

Substitute this back into equation 1 above to get the minimum width:

 

Ax = (3872/2592)x1995 = 2980 pixels

 

If you keep the same aspect ratio for your crop then you only need one of the dimensions if you crop is Photoshop as there is a preset for this. Should Alamy change the 17MB minimum file size just substitute the new size in the basic formula at the start of this post.

 

If you do not keep the same aspect ratio then you will need to put what data you do have into the basic formula and transpose for the unknown dimension.

 

Please remember that when you convert your images to JPEG the file sizes will be much smaller than this.

 

I hope this helps those struggling to work out file sizes.

Edited by Carl Skepper

Share this post


Link to post
Share on other sites

Very interesting and thank you for the hard work. Too much for me though I just check in Alamy sizecheck software available free.

 

Allan

Share this post


Link to post
Share on other sites

This is what I'd call the image size. To my mind the "file size" is the room the file takes up on the disc, which is generally different and rather smaller for jpgs. It is the confusion between "file size" and "image size" that is part of the problem people have with understanding Alamy's requirements. IMO

  • Upvote 1

Share this post


Link to post
Share on other sites

Forgive my skepticism but I think that those who struggle to understand file size requirements are going to blow brain fuses looking at your equations. I think the main problem over time has been the confusion between file size and image size (as JohnB has posted while I type). Image size is called pixel size by Adobe and in the general literature.

Share this post


Link to post
Share on other sites

Very interesting and thank you for the hard work. Too much for me though I just check in Alamy sizecheck software available free.

 

Allan

 

You're welcome. Hehe, the easiest solution is always the best ;)

 

This is what I'd call the image size. To my mind the "file size" is the room the file takes up on the disc, which is generally different and rather smaller for jpgs. It is the confusion between "file size" and "image size" that is part of the problem people have with understanding Alamy's requirements. IMO

 

Yep but image size is directly related to file size. By using the D200 as an example I was hoping to show how to calculate the minimum image size uncompressed. I will edit to make that clearer

 

Forgive my skepticism but I think that those who struggle to understand file size requirements are going to blow brain fuses looking at your equations. I think the main problem over time has been the confusion between file size and image size (as JohnB has posted while I type). Image size is called pixel size by Adobe and in the general literature.

 

Don't be afraid of the equations, they are made more scary because of the big numbers. I was also concerned a little about even whether to post this and whether I would be teaching my grandmother to suck eggs. However if you need to crop an image it is better if you know what the limits are and you won't know that without doing the maths. Just substiture your own cameras file sizes where I have put mine and you will get the minimum cropped size.

 

........  :blink:

 

Just shoot in RAW >>>> post process in Tiff   >>>>>>  happy and Tiff size > 17Mb  >>>>>>>  convert to Jpeg and upload  -_-

Not exactly rocket science, is it?  :rolleyes:

 

Cheers,

Philippe

 

This would be the trial and error approach. By knowing the minimum file sizes before you start it saves any uncertainty before you crop.

Share this post


Link to post
Share on other sites

 

Forgive my skepticism but I think that those who struggle to understand file size requirements are going to blow brain fuses looking at your equations. I think the main problem over time has been the confusion between file size and image size (as JohnB has posted while I type). Image size is called pixel size by Adobe and in the general literature.

 

Don't be afraid of the equations, they are made more scary because of the big numbers. I was also concerned a little about even whether to post this and whether I would be teaching my grandmother to suck eggs. However if you need to crop an image it is better if you know what the limits are and you won't know that without doing the maths. Just substiture your own cameras file sizes where I have put mine and you 

I'm not afraid of equations. I did maths to degree level back in the 70s and I've been into digital imaging since the 90s. But I am aware that a lot of people are afraid of maths and I think that those who find it difficult to understand the Alamy guidance will probably find your equations very daunting. And besides, as already pointed out, it's the confusion between pixel size and file size on disk that is the most common problem. 

Edited by MDM

Share this post


Link to post
Share on other sites

 

 

........  :blink:

 

Just shoot in RAW >>>> post process in Tiff   >>>>>>  happy and Tiff size > 17Mb  >>>>>>>  convert to Jpeg and upload  -_-

Not exactly rocket science, is it?  :rolleyes:

 

Cheers,

Philippe

 

This would be the trial and error approach. By knowing the minimum file sizes before you start it saves any uncertainty before you crop.

 

 

Depends on your software. I do  my final converting of RAW to jpg in Photoshop. Not only does it tell you the size of the image in the bottom left corner, if resizing it tells you the new size as you are resizing. So can't go wrong. And if you always check the bottom left corner before saving, you are good to go.

 

Jill

  • Upvote 1

Share this post


Link to post
Share on other sites

 

Don't be afraid of the equations, they are made more scary because of the big numbers. I was also concerned a little about even whether to post this and whether I would be teaching my grandmother to suck eggs. However if you need to crop an image it is better if you know what the limits are and you won't know that without doing the maths. Just substiture your own cameras file sizes where I have put mine and you will get the minimum cropped size.

 

I'm not, they're just not necessary, and I do know what the limits are without doing the maths because PS and LR do it for me.

It's possible to over-complicate this.

Share this post


Link to post
Share on other sites

Hehe I understand where you are all coming from but it is obvious some people have difficulty with this, for whatever reason. Also some may not use Photoshop. Several useful methods have been discussed here so hopefully there is enough information for those that do want it. ;)

Share this post


Link to post
Share on other sites
Rule of thumb:

 

3:2 image - longest side at least 3000 pixels

 

1:1 image (square) - each side at least 2500 pixels

 

That works for me for over 99% of the time. If you really want to get closer to the minimum size, then it's helpful to understand the maths, be comfortable with converting bits to bytes, and know how the data from each pixel is represented.

 

In Windows you can see the pixel dimensions under Properties. I'm sure that the Mac has something similar. 

Share this post


Link to post
Share on other sites

The Hypotenuse is a useful dimension when describing a lens. "Normal" lenses are the hypotenuse  of the film/sensor area; wide are less than that and telephoto are greater.

 

Maybe that works out to slightly wider than what we think of as normal. For example, for 35mm film, it works out to 43.27mm, whereas we might say it was 50mm.

 

For 6x6 (actually 56x56mm), it's 80mm, like the normal lens on a Hasselblad. A portrait telephoto is roughly 2x the hypotenuse.

Share this post


Link to post
Share on other sites

How big is a 17Mb image file on a 4K screen (3840 x 2160 pixels)?

On an 8K screen (7680 x 4320 pixels)?

 

 

300px-8K_UHD%2C_4K_SHD%2C_FHD_and_SD.svg

 

Maybe consider those pixel sizes as a minimum for now; for the future.

 

wim

Share this post


Link to post
Share on other sites

I just let Photoshop figure things out, but there are lots of calculators like this one online.

Share this post


Link to post
Share on other sites

What I would really like to know is why the Alamy moderators don't pin one of these threads so that it doesn't come up every month or so...

  • Upvote 1

Share this post


Link to post
Share on other sites

What I would really like to know is why the Alamy moderators don't pin one of these threads so that it doesn't come up every month or so...

 

+1 :)

Share this post


Link to post
Share on other sites

Hi. Thanks for all this information. I would like to know how it applies to 4x5 negatives scanned. Ive scanned a bunch (using a good scanner, Epson V700) at what seems high resolution but they come out 4,3Mo. Assuming Mo is the same as MB (? ) then its not nearly enough for Alamy, which says it should be at least 17MB. Is this correct? Advice? 

Forgive my digital ignorance.

 

thanks

Margo

Share this post


Link to post
Share on other sites

Hi. Thanks for all this information. I would like to know how it applies to 4x5 negatives scanned. Ive scanned a bunch (using a good scanner, Epson V700) at what seems high resolution but they come out 4,3Mo. Assuming Mo is the same as MB (? ) then its not nearly enough for Alamy, which says it should be at least 17MB. Is this correct? Advice? 

Forgive my digital ignorance.

 

thanks

Margo

 

Hi Margo,

 Apologies for the delay in replying but I haven't had chance to visit the forum in the last few days. In answer to your question I'm not sure what 4,3Mo is so can't answer that and I also don't scan negatives. However if you follow Alamy's advice (quoted below) about scanning them they should be fine:

 

 

Film formats
  • 35mm originals

    To achieve a 70MB file from a full frame 35mm transparency, requires a scan resolution of approximately 4000ppi.

  • 6 x 6cm and 5 x 4" originals

    The larger the film area, the lower the scanning resolution needs to be to achieve a 70MB file. Medium format transparencies require a scan resolution of around 2000ppi, whilst a 5 x 4" original will need to be scanned at around 1000ppi

 

Here's the link to the relevant page: http://www.alamy.com/contributor/help/scan-images.asp

Edited by Carl Skepper

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.