JPEG's varying files sizes

Here's something I've been meaning to ask for a while. What determines

differences in JPEG file sizes in a digital camera when no settings

have changed?

 

After shooting different scenes in jpeg format, and without changing

any settings (like resolution or image quality setting) other than

Time and Aperture values, files that I download from the camera have

varying compression ratios.

 

So what is lost, if anything, in the above example ? And what is the

explanation for that variation ?

 

PS: This question has nothing to do with quality settings in Photoshop

when saving a file in jpeg. It's also not a jpeg vs tiff format question.

How compressable the image data is. That is, the complexity of patterns in the image

determine how much can be safely left out. JPG is a 'lossy' compression format, where some

data will get thrown away to enable smaller file sizes.

<br><br>

<a href="http://en.wikipedia.org/wiki/Lossy_compression">This</a> article explains it in

more detail.

Two aspects:

 

-the first stage of compression is a variable-rate compression, which discards information below a certain level (not below a certain size).

 

-the second stage is a lossless bit-level compression, which is also variable-rate (almost by definition).

The tonal value of each different shot has a different total when you are saving in jpg.

Try taking a shot of a scene and then cover the lens with your hand and take a second exposure, then compare the file sizes ....frankly I do not know exactly what is going on but I see the effect in every download of shots. So I don't think anything has been lost but rather a different amount of information had to be recorded to represent the image ... sorry to be rather vague :-)

 

One day I must try that test when saving in TIFF and RAW to see what is happening in those recording methods.

 

What happens in PS on saving a jpg is similar to you adjusting the compression in the camera Basic, Fine etc .... and resolution change is similar to re-sizeing.

To a large extent the subject determines the file size. Try taking a photograph of a well-lit complicated scene with lots of colors - like a downtown city-scape. Now take a photograph of something with just one or two colors like a brown paper bag with some writing on it. The city-scape jpeg will likely be much larger because well, there's more "stuff" there - more colors, more tones, more things to represent. Here's a neat trick: Take 5 or 6 shots of the same thing. Now look at the jpeg sizes and find the largest one. Usually it'll be your sharpest shot of the bunch. Good luck!

I've now found out that camera manufacturer also have a role in determining JPEG compression ratio.

 

I borrowed a Kodak digicam last week and played with it for a while. Average file size of some 50 pictures of very different subjects was exactly 25% of camera resolution. That's way too much compression.

 

The average file size for a Canon SD500 came out at 40% of resolution.

So it varies. My hunch is that the higher the compression ratio for jpeg, the higher the handicap for printing larger 8x10's.

I beg to differ on the statement that 25% is far too much compression.

 

In actual fact when you are working at 25% there is very little difference from working with TIFF and what is discarded makes very little difference in the final comparison between working at 25% or 100%.[TIFF or RAW]

 

By 25% I assume that equals what Nikon call FINE or 1:4 compression in the manual. Similar to Pansonic's "seven dots".

"By 25% I assume that equals what Nikon call FINE or 1:4 compression in the manual. Similar to Pansonic's "seven dots"."

 

Nope. Just comparing file size on disk with camera resolution.

Jack ... your reply is so cryptic I miss your point.

 

25% is a quarter or put another way is 1:4 or 4:1