What is the effect of Long Exposure on file size

Question

Is it a rule that long exposure will produce a larger size file compared to a shorter exposure? It seems logical (you are writing more data to the memory card when you expose more time).

What about shooting long exposure at night and at day? I believe that shooting in a day light will produce a larger file than shooting at night for the same exposure time (I've no idea why I believe so)

Asked by Akram Mellice

Answer

No, it doesn't work that way. The image file isn't built up as the exposure goes on, but rather is made from a full read of the sensor when the exposure is complete. So, you're not writing more data to the memory card when you expose for a longer time.

Each photosite — one "pixel" on the sensor — is a counter that goes up as it's hit by more photons. It's actually an analog device, but when read at the end of exposure, a single digital value is produced (usually 12 or 14 bits). This value is simply the total amount of light that site received. If the particular pixel is all dark, it'll be 000000000000, and if it's all light, it'll be 111111111111. There's no record of how long it took that all-full sensor to get to that state — it could be very, very bright with a wide aperture, so you could get that value in ¹⁄₁₀₀₀th of a second. Or, it could be so dark out that it takes 30 minutes to get the same result.

In the end, though, it's the same single value. And all together, there's no more or fewer values no matter how long the exposure is.

There is another factor, though. Some files will compress better than others. Patterns compress well, and of course large identical areas compress best of all. Arbitrary detail compresses poorly, and random data worst of all. Since noise is by definition random, very noisy images produce the largest files. There isn't a direct correlation to exposure length here, but longer exposures may have more noise as the sensor heats up. So, that may be a practical consideration, but it isn't because of the accumulation of data per se.

Answered by mattdm

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Can a Pentax K-5 record more than 4 GB of video with an SDXC card?

Question

When using an SDHC card, the Pentax K-5 can only record up to 4 GB of video in a single clip. This is said to be because SDHC cards use the FAT32 file system, which has a 4 GB maximum file size. At the highest video size and quality settings, this permits only about 5 minutes of recording in a single clip.

There are suggestions in various forums that the use of an SDXC card (64 GB+) with the exFAT file system will allow recording larger files, up to the 25 minute hard cutoff imposed by the camera. However, those suggestions often seem to be based on logic ("it ought to work that way") rather than testing.

Does anyone know, through actual experience or reference to reliable sources, whether the K-5 can record more than 4 GB when an SDXC card is used?

Answer

No it cannot. It will only write a 4 GB file using the Motion-JPEG codec.

There are a number of cameras which use the AVCHD format and they can often record longer clips because the simply split the video stream into separate files. The K-5 is not one of them.

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How to avoid jpeg compression artefacts when saving photos in web resolution?

Question

When I save a web-resolution image (for example, 1920×1024px), flat areas tints often have damaged shades of color. I'm saving in Photoshop CS4 with quality 2 in standard mode, because I need the files to be less than 300k. How can I make my JPEG images this small without the compression artefacts?

An example: http://www.screencast.com/users/capsize/folders/Jing/media/f858e229-1f05-431c-98ce-5334918aa1f3

Answer

If you save a JPEG image with an extremely low quality level, you WILL get compression artifacts. Its just a simple fact of JPEG lossy compression. If you wish to avoid compression artifacts, use a higher quality setting than 2. You won't need to save at maximum quality, as most images can be saved with a fairly low quality setting without noticeable loss in detail or visible artifacts.

One thing to note is that smooth gradients and JPEG do not mix well. JPEG compresses small blocks of the image, and when it comes to smooth gradients, it tends to make all the pixels in a block the same or very similar colors. This creates distinct banding (posterization) that can be quite visible. The only way around this is to use a higher quality setting, however that will obviously increase your file size.

An excellent article that might be of help can be found here:

• An Analysis of Lightroom JPEG Export Quality Settings

The above is for Lightroom, however the fundamental concept still applies to Photoshop. The important point from the article above is that the lower range of quality settings for JPEG tend to have a minor effect on image size, and it is only when you get to the upper few brackets that image size starts to climb by any significant degree. I would suspect that you could change your quality setting from 2 to 5 and only incur a small hit to your image size.

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What is a reasonable file format for film scanning?

Question

I'm looking to use my CanoScan 8600F to digitally archive the photos I've shot on film.

The issue I've stumbled on is deciding on a file format. Ideally I'd like something that is lossless but with some reasonable compression.

So far I've tried (over the last few years):

TIFF - File sizes seem to be pretty big

JPEG2000 - Photoshop elements 4 supported it, but doesn't seem to have a lot of support else where

DNG - Not sure if this is an option for scanning

I'm also looking for some guidance on resolutions to scan at... this was asked in another question "What to consider when scanning 35mm film (and scanning in general)" but wasn't directly answered, other than to say that scanners were only optically capable of 1500dpi.

With some exceptions I don't expect to make prints other than maybe 4x6s.

I currently use Adobe Lightroom with my DSLR and my new laptop included Photoshop Elements 9, but I haven't used PE9 yet.

Answer

I would say TIFF is probably the best format. JPEG 2000, like JPEG, is still a lossy compressed format when you really try to save space (the lossless version can compress a bit, but not nearly as much as the lossy form, and some forms of the "lossless" wavelet compression still can't fully reproduce the exact original image.)

When scanning in an original or master image, it's really best to maintain as much image detail and color depth as you can, and TIFF is an ideal format for this. It is guaranteed to be lossless, supports a wide variety of color depths including high color depths, has very broad support across many applications on multiple platforms, and even supports layers and other advanced objects that can be created with applications like Photoshop. TIFF also supports the storage of metadata, like JPEG.

I am not sure if DNG is an option for directly scanning film, and even if it was, I am not sure what the benefit of using DNG over TIFF for a film scan is. DNG has more merit in the digital RAW workflow, as it supports storing native camera RAW data and metadata, which really wouldn't be of much use for a film scan (which is always going to result in RGB pixels anyway.)

Regarding resolution, I guess it might depend on the nature of the film. If you are scanning very grainy film, you might not need to scan at an extremely high resolution, however scanning at too low of a resolution will likely even interfere with grain detail (which does have an aesthetic appeal to many, and that aesthetic might be diminished at too low of a resolution... and I would consider 1500dpi to be fairly low these days.) I'm a digital photographer, however I have researched film quite a bit as I have a strong interest in large format. To my understanding, a low-speed, fine-grain film like Velvia 50 is easily capable of over 3000dpi, which is double the dpi mentioned in the article you have linked. I have done some 35mm negative scans of old film just for kicks, and scanning up to 4800dpi (the maximum optical native of my scanner) produces an astonishing amount of fine detail. I would say scan at the highest resolution you can so long as you don't see negative returns, and I wouldn't be surprised if 4800dpi or even as much as 6000dpi was necessary to extract all the detail from your film.

If the initial file size worries you, you can always downscale your master file a bit from a high resolution scan, which should help improve sharpness a bit while also saving a little disk space. High-ISO black and white film will generally require less DPI, however keep in mind that film grain is not ubiquitous in size and distribution, and the full quality and shape of a single grain may require many pixels to render fully.

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How do I constrain filesize of a bunch of photos?

Question

I have a bunch of photos of jewelry that I need to upload to a website. Unfortunately there is a 2MB file size limit. Most of the photos are JPEGs over 3MB.

I want to keep the aspect ratio, meta data, and highest quality possible at under 2MB. Adobe Bridge will allow me to set an image size limit when I export, but not the file size.

Does anyone know of a way to render the images with a max filesize limit?

Answer

I don't believe Bridge has this functionality.

Photoshop and Lightroom (with a mogrify plugin) have this ability to set a file size based export.

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