Pixels Versus File Size

[markg]

Ok, so i m on my 2nd digital camera. And i m ok at it but really a layman.

Heres what i dont understand. My 5MP Pentax produces files about 3.5MB in size. A friends Olympus 7.1MP produces files of less than 2 MB. Why is that.

I have compared two pictures using ACDSee software and they confirm that the photos are indeed 5MP and 7.1MP respectivly.

I just dont get it at all..............she can take almost twice as many photos as me on her memory stick.

Heres another one. I took a photo for my passport of my head and shoulders, 3.5MB in size and then cropped it to the correct size for printing. It came down to 500kB............but the quality is the same. E.g. i had a tiny bead of sweat on my forehead and its clearly visible on both the photos. The big one and the small file sized one. I just dont get it

[guardian]

I'm no expert, but I think the size of a jpeg file varies according to the algorithm that a camera's software uses to compress it from RAW. Probably the Olympus is compressing the file more and this is not good as many useful data are thrown away during compression.

As for the quality of the picture you cropped, the print quality would be the same for upto a certain size, after which you will a drop in the quality of the cropped file. There is a maximum size limit that a file can be printed upto, after which artefacts begin to become visible.

Hope this helps.

Guardian

[Tywais]

Most digital cameras have several modes they can operate in. Often you can select the resolution that you want to use, such as 3264x2468 (my 8M pixel max resolution) or you can select a lower range (eg. 1632x1234) given lower resolution but smaller file size thus more pictures per card.

Then there are the various formats that the camera can save in. Raw camera (native) format is the full data uncompressed and unmodified by the camera processors. This usually is the largest file type (my case 12MB) and is when you want no modifications by the camera of the image. The next type, not all cameras support, is .tiff. This is also going to be a large file because it is saved as non-lossy, that is no loss in quality if they use compressed or uncompressed tiff, the later can be as large as the Raw format.

Most cameras will have .jpg format and this is a lossy compression format in that it compresses the information using algorithms that allows it to remove some information (color) with small effect in quality. Jpg can be saved in several compression levels, the higher the compression level the more loss in picture quality but smaller file size. Mine has something like fine, extra fine, ultra fine - each being less compression higher quality and larger file size.

So, depending on the settings of your neighbors camera (perhaps they want more pictures on a card but not concerned about some quality loss) and your camera.

Sorry about the length but wanted to cover all bases.

[astral]

If you have the option it is best to shoot in the RAW then nothing is lost,

and the editting adjustment options are much more comprehensive.

[Chang_paarp]

Ok, so i m on my 2nd digital camera. And i m ok at it but really a layman.

Heres what i dont understand. My 5MP Pentax produces files about 3.5MB in size. A friends Olympus 7.1MP produces files of less than 2 MB. Why is that.

I have compared two pictures using ACDSee software and they confirm that the photos are indeed 5MP and 7.1MP respectivly.

I just dont get it at all..............she can take almost twice as many photos as me on her memory stick.

As with most things boy play with bigger is better. The larger file size is related to less compression. You could change the settings on your camera to get more pictures on the card but given the price of memory and the loss of quality why?

I see lots of people buying high mega pixel cameras and using the lower size and quality options to fit more photos on the cards. Then when they want me to crop and print them or enlarge the images they say the camera is cr@p because the result is no good.

Heres another one. I took a photo for my passport of my head and shoulders, 3.5MB in size and then cropped it to the correct size for printing. It came down to 500kB............but the quality is the same. E.g. i had a tiny bead of sweat on my forehead and its clearly visible on both the photos. The big one and the small file sized one. I just dont get it

A passport photo is typically 35mmx45mm. When printed on a photo lab print it needs less than 700x900 pixels. If you have taken a full frame image with your 5MP camera, and then cropped down to the correct proportions, you will have more than enough data to make passport photo.

You can then re-size it for the passport photo. It may look very clear on the screen (72dpi) and equally good at passport photo size, but don't try to take it too big. Again compression can come into the file size.

The lab I use can give reasonable quality results down to about 100ppi, I have printed acceptable passport sized photos from 125x185pixel images. (They were not going to be used on passports.) I do not recommend going that small but it can be done. Again bigger is better here as the passport folk want the best possible photo to start with so they can do their biometric measurements.

[markg]

Thanks for your replies though i m still trying to get my head around it.

7MP for 1.5Meg and 5MP for 3 Meg

It just doesnt add up.........

I guess i ll get it one day.

Maybe you just cant teach an old dog new tricks after all !

[Tywais]

Thanks for your replies though i m still trying to get my head around it.

7MP for 1.5Meg and 5MP for 3 Meg

It just doesnt add up.........

I guess i ll get it one day.

Maybe you just cant teach an old dog new tricks after all !

If you tell me your camera make and model maybe I can show you how to change the settings to get a smaller file size. But again the smaller the file size the less resolution you have so depends on what you are trying to do. For web pages and showing on your computer, the lowest resolution will be fine, for prints the higher resolutions are better.

[fxm88]

7MP for 1.5Meg and 5MP for 3 Meg

It just doesnt add up.........

Post the pics and someone here will tell you exactly why the one is so much bigger than the other.

[markg]

Thanks for your replies though i m still trying to get my head around it.

7MP for 1.5Meg and 5MP for 3 Meg

It just doesnt add up.........

I guess i ll get it one day.

Maybe you just cant teach an old dog new tricks after all !

If you tell me your camera make and model maybe I can show you how to change the settings to get a smaller file size. But again the smaller the file size the less resolution you have so depends on what you are trying to do. For web pages and showing on your computer, the lowest resolution will be fine, for prints the higher resolutions are better.

Hello. Its a Pentax Optio S55.

It's ok, i know how to turn the settings down to make the file size smaller.

Thats not really a huge issue for me as i have a large mem stick now.

I was just trying to learn something about the relationship between pixels and file size. From my laymans point of view, 5 million pixels should produce the same file size. Its 5 million pixels at the end of the day. I understand what one poster said about the lossy and no lossy compresion formats. Although presumably most "store bought" Joe Schmoe cameras just save pics as JPegs which is the same compression irrespective of camera make?

I was also surprised, and still dont fully understand, that using ACDsee i can reduce the file size of a picture, but it still remains at 5 mega pixels.........that puzzles me. And also the quality "seems" to be the same............for example, i could still zoom in on my PC after i had reduced a picture and see fine detail. (The tiny bead of sweat on my head for example)

Well anyway, i m not going to lose sleep over it. Its just that i have a rigid and logical thinking mind and this upsets my equilibrium !!

Mark

[andyfletch]

Have you tried taking ( as near as is possible) the same shot with both camera's, with both settings as close to the same as you can get? Then you can at least get a direct comparison of the two............just a thought

[ZZZ]

jpeg is just the format of the file. Another aspect is what is usually called Quality, most cameras have settings such as Fine, Normal and Economy. These different settings will compress the pictures with different loss. Fine will create a picture with less loss than Economy and the picture will be more detailed and the file will be larger.

The loss will not be very evident if you take a picture of say a black and white sign but when taking a picture of a face the loss will be very noticeable.

Using jpeg a picture taken of an object with few colors and shades will be much smaller than a picture taken of an object with many colors and shades. This is simply because large areas of a picture where the color is the same can be easilly compressed. Take a picture of a white wall and then a picture of a complex object, with the same camera setting and you will see that the picture of the white wall is smaller than the picture of the complex object.

"My 5MP Pentax produces files about 3.5MB in size. A friends Olympus 7.1MP produces files of less than 2 MB. Why is that."

This is because of the way the different Quality settings work in the different cameras and depends on the object you take a photo of.

[markg]

zzz kinda making a bit more sense now.

What i seem to be finding is that all this huge amount of pixel's is mainly a load of rubbish for the snapper who is never going to print more than 6 x 4 prints.

I have found, by reading some other posts, that basically 3.5 mega pixels is about the ideal size for printing 6 x 4 photos with no loss around the edges. That size therefore cuts down the file size and lets me take a whole load more photos per memory card.

The average size of camera now in shops is probably 8 mega pixels - where will it all end. Will we see 20 mega pixels being the average in 12 months time? Probabaly. And frm what i can see, for the joe shmoe average snapper, its useless.

Another poster suggested taking the same shots with 2 different cameras.

I did something similar. I took one shot of a face. Then "saved as" and cut it down from 3.5MB to about 700 kB. I then compared the 2 on my compute screen. There didnt appear to be any difference. I was clicking the "forward and back buttons" and there was no difference.

I didnt print them out.

Man - its confusing this photography!

[astral]

Comparing images on the screen it not really a test.

The screen only has 72dpi resolution.

Printing A3 size would be a better idea.

[autonomous_unit]

Here is a clumsy analogy to help you think about the size... people keep talking about quality and that is sort of like how rich and deep of a paint layer can you lay down. Bytes in the file are like some small volume of paint. When you take your big, high quality umpteen megapixel picture, you've used a lot of paint. When you crop out a small part in the middle, that part still has the same quality but it only contains a small amount of the total paint proportional to the area you kept.

Where this analogy falls down is in the way digital pictures are scale-free. A high quality picture takes the same number of bytes whether you "paint" it on a thumbtack or on the side of a barn. But, if you think of the number of pixels across like a length, then it makes sense. When you cropped out the part with your face, you cut both the height and width of the picture, and the area decreased as the multiple of these two changes. For example, if I had a 3 foot by 5 foot wall and I cut out a 1.5 foot by 2 foot piece in the middle, the area has gone from 15 square feet to only 3 square feet.