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11-17-2016, 09:19 AM   #1
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How can I predict RAW file sizes?

My one-and-only dSLR is a Pentax K-x which a 12.4 MP sensor. I am shopping for a replacement body, and I'm trying to anticipate how much the larger RAW files from more recent cameras are going to be.

Is there any way to calculate the approximate size of RAW files (DNG), based on the sensor's megapixels, or from pixel dimensions of the images produced -- or do the file sizes vary so much based on model, lens selection, scene complexity, ISO, etc. that it's not possible to predict with any accuracy?

I shoot in DNG, only, and it looks like my recent files vary from 9MB to 18MB, with the majority being around 10-14 MB.

My list of potential replacements includes the following:

Pentax K-S1 or K-S2, 20.12 MP
Pentax K-5IIs, 16.28 MP
Pentax K-70, 24.2 MP
Pentax K-3, 24.35 MP

The K-70 and K-3 are "14 bit" vs "12 bit" for the others. I don't really know what that means, but I assume it affects the DNG file sizes?

Can anyone tell me what size of DNG files I can expect to get from these cameras?

thanks

11-17-2016, 09:39 AM   #2
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As a rule of thumb, the Mb are roughly the same as the Mp - but it varies a little from image to image depending on the amount of detail etc - pixel-shifted images in the K-70 will be about maybe 3-4 times the size of normal ones - very rough, but fairly accurate - and in line with your observations with your current camera
11-17-2016, 09:40 AM   #3
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My rule of thumb for DNG size is 'Number of MPx + 6' Mo. The K-3 files are around 30Mo, the K-1 files are about 42Mo. If I remember well my K-5 files were a bit above 20Mo... Hope this helps !
11-17-2016, 10:13 AM   #4
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QuoteOriginally posted by runswithsizzers Quote
Pentax K-5IIs, 16.28 MP
The DNG files from my k5iis for this year average 22.7MB in size. They range from 12.1MB to 33.8MB. The more detail the larger the file, but gorme's "MP+6" approximation holds up on average for me

11-17-2016, 03:26 PM   #5
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It's important to note that - if the OP isn't already aware of that - DNG produced by Pentax cameras are losslessly compressed.
This means that, while no information is discarded, the amount of saved space depends on the quantity and quality of the information contained in the picture.

A black frame snapped with the lens cap on, or a bokeh-only picture, for instance, will yield a low file size.
OTOH a crisp, colorful f/8 panorama with lots of fine detail (tree branches etc.) will produce much bigger files.
11-17-2016, 04:19 PM   #6
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QuoteOriginally posted by gorme Quote
My rule of thumb for DNG size is 'Number of MPx + 6' Mo. The K-3 files are around 30Mo, the K-1 files are about 42Mo. If I remember well my K-5 files were a bit above 20Mo... Hope this helps !
Gorme - great - that does help!

In my case, the MP+6 rule seems to be a maximum. For my K-x, 12MP+6 = 18MB. A quick finder search turned up 1,730 DNG files. Of those only two were 18 MB, and only 23 were 16MB or greater. Based on your experience, would you (or anyone else) say the MP+6 rule tends to be more of an average, or more of an upper limit? (BrianR has already reported the DNG files from his k5iis *average* 22.7MB in size - thanks for that info)

I'm a little scared of doubling or tripling my file sizes because I don't know if it will complicate my work flow. I can work around increased storage needs, but if I buy a new camera, I don't want to find out I also suddenly need a new computer to run the bigger files in Lightroom/Photoshop. For my needs 16MP is probably enough resolution, and the K-5IIs would be a step up from my K-x. But if I'm going to upgrade, I'd rather get a more modern camera than the K-5IIs, which is only a couple of years newer than my K-x.

LensBeginner - interesting observation about the effects of exposure and sharpness on file size. My immediate thought was to check the dates on my DNG files to see if there was a bump when I switched to primes (vs. the kit zoom) due to an increase in sharpness. Not really. But I did observe that the vast majority of my smallest files are grossly underexposed - or a small moon in a big black sky - that sort of thing.
11-17-2016, 04:34 PM   #7
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QuoteOriginally posted by runswithsizzers Quote
*snip*

LensBeginner - interesting observation about the effects of exposure and sharpness on file size. My immediate thought was to check the dates on my DNG files to see if there was a bump when I switched to primes (vs. the kit zoom) due to an increase in sharpness. Not really. But I did observe that the vast majority of my smallest files are grossly underexposed - or a small moon in a big black sky - that sort of thing.
Not underexposure.

Imagine you have to describe an image, which description will be shorter, "all pitch black" or "tree branches on the foreground, with red and yellow leaves, on the middle ground a house, red tiled roof, background hills and fields etc."?

Uniform images (hence the black frame and the bokeh example) have large areas with little variation, hence little information, which can be compressed more effectively than a pattern with every pixel markedly different from the next one.


Last edited by LensBeginner; 11-17-2016 at 04:42 PM.
11-18-2016, 04:24 AM   #8
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I would say that for the 14 bits files, the MP+6 rule tend to be the average. It would be interesting to know how the Pentax cameras are computing the number of images available. When you select your format, they say how many you can store, but I don't know what their methodology is.
Theoretically speaking, the DNG file contains the image + the info about the image (1 small document made by the camera manufacturer and 1 standardized document with the basic resolution, time, lens used, modes of the camera, distance to the subject, flash.... ).
From a theoretical point of view the maximum file size for a DNG in 14 bits is supposed to be "MP*14bits/8 + size of info'MB. For the K-1 files, this would give some huge 63+MB files (50% more than the actual size I have). But the DNG format uses lossless compression to reduce the size of the file. Which means that if you have some large areas of the same values you are going to save up some space.

So the conclusion is that I don't have a theoretical answer to the average size (maybe there are some people knowing more about the DNG format and the lossless compression they use around), but there is one for the upper limit
11-18-2016, 05:25 AM   #9
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For my K-3 I'm in the low thirties megabytes say 32-33 Mb. Remember the DNG file contains a low resolution Jpeg in addition to the RAW data. The Jpeg's impact on file size will be related to image complexity and will also be influenced by some of the Jpeg processing options you set in the camera.

---------- Post added 18th Nov 2016 at 10:09 PM ----------

I did prepare for the transition from a K-x to the K-3 by doing an upgrade of my computing environment primarily because I had max'd out on RAM per slot for the generation of RAM I was using. RAM is the most critical element. Not enough and your pc spends way too much time performing page swaps. I had no path forward for RAM upgrades so bit the bullet for a full system refresh.

Last edited by southlander; 11-18-2016 at 05:31 AM.
11-18-2016, 07:02 AM   #10
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QuoteOriginally posted by runswithsizzers Quote
I'm a little scared of doubling or tripling my file sizes because I don't know if it will complicate my work flow. I can work around increased storage needs, but if I buy a new camera, I don't want to find out I also suddenly need a new computer to run the bigger files in Lightroom/Photoshop.
What kind of hardware do you have now? You can download sample dngs from a few sources (reviews on Pentax Forums, or at Imaging Resources), to at least see the kind of chugging Photoshop may give you. 3 years ago, when I moved from 6mp to 16mp, I noticed a considerable slowdown on my old hardware, which was already borderline at the time- e4400 dual core + 2GB ram. Upgrading to a quad processor and more importantly 4 GB of ram (still low but the parts were free), made it tolerable for me. I'm sure a k3 would have forced a complete upgrade right away (with as much ram as I could afford), but it really depends on what kind of editing you do and how patient you are.

QuoteOriginally posted by gorme Quote
It would be interesting to know how the Pentax cameras are computing the number of images available. When you select your format, they say how many you can store, but I don't know what their methodology is.
It looks like the k5iis constantly recalculates assuming the pictures yet to be taken will all be just shy of 33MB, so a very conservative estimate (I should mention I'm using 1 MB to mean 1024*1024 bytes). This is very noticeable during a shoot of the night sky when the "available photos" counter can tick one off for every 2 or even 3 photos taken. As noted above lots of black works well with the lossless compression they used, and many of my smaller sized DNGs are of the night sky.

The camera manuals give a table of "Approximate Image Storage Capacity by Size". The k5ii manual states 58 RAW for a 2 GB card, which matches the cameras conservative calculations.
11-18-2016, 11:23 AM   #11
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Thanks BrianR for your feedback on this. I must admit that I don't check the number of images left very often when shooting thanks to the dual camera slots.
I agree, you should get some raw files from the reviews (dpreview and pentaxforum mainly) and see how they work for you. Adding a bit of RAM may also help your computer. I think that the 24MPx are OK for any modern i5 or i7 computer.
11-19-2016, 02:06 PM   #12
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Thanks to all!

My 5 year old iMac has an i5 processor, and 8 gb ram which is upgradeable to 16, I believe. Considering having the HD replaced with SSD. Someday. One thing I cannot upgrade is the GPU. Lightroom can use hardware graphics with some cards, but not mine.

I believe I have some 50gb DNG files created by my HDR software. I will test those out tonight.

I'm really frustrated that I may have missed the last of the new K-3s, now that I've finally decided that's the one I want. I'd rather have the flash than the GPS.

Oh well, a used K-3 is a better fit for my bank account.
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