Ask Tim Grey

Today’s Question: The same image exported twice from Lightroom with same pixel dimensions but one at 72 ppi and one at 300 ppi are the same file size. I would have thought that the 300 ppi file would have been larger. Why isn’t it?

Tim’s Answer: The issue of resolution continues to be one of the more common sources of confusion in photography, in part because information about an image is often presented in a way that can be a little confusing.

When it comes to the pixel per inch (ppi) resolution for an image, I think it is best to think about this as simply being a metadata value. It has absolutely no bearing whatsoever on the “real” information in your photographic image.

The overall size of an image is determined by how many pixels are in that image. Adding confusion, that total volume of pixel information is often referred to as resolution as well. In other words, sometimes the term “resolution” refers to the total volume of information (pixel dimensions, megapixels, etc.), and sometimes it refers to the density of information (how many pixels per inch, for example).

Forget about printing for the moment, and think about the image size as it appears at a 100% scale in Photoshop (for example) as well as the size of the file saved on hour hard drive. The number of pixels is the primary factor here.

This makes sense when you consider how the appearance of an image in Photoshop at a 100% zoom setting changes based on how many pixels are in the image. If we have a square image that is 10 pixels on each side, that image will look very small in Photoshop even at a 100% zoom setting. A square image that is 10,000 pixels on each side will look very different, with the image being so big that we can only see a small portion of the image when viewed at a 100% zoom setting.

If the pixel dimensions remain the same, with only the pixel-per-inch resolution changing, the file size will not change. The ppi resolution only affects (for the most part) how the image is printed. In other words, when you send that 10,000 pixel-per-side image to the printer, how do you want the pixels spread out on the page? If you spread them out really far (perhaps only 72 pixels per inch) you’ll be able to make a very big print, but the quality won’t be very good. If you keep the pixels pretty close together (perhaps 360 ppi) you’ll have a smaller print, but that print will have great image quality.

In both examples above, the number of pixels didn’t change, so the file size would be the same (all other things being equal). All that changed is a simple metadata value that provided information on how the pixels should be distributed on the page when printed.

There are, of course, other factors that impact file size. These include (among other things) the bit depth of the image, compression applied to the image, and whether layers are included with the image. However, all other things being equal, changing the ppi resolution for an image will have absolutely no impact on file size. The number of pixels in an image is the key factor in overall file size, as well as for the potential output size for the photo.

It is worth noting, by the way, that if you had specified the output size in inches (for example) instead of pixels, this would have made a difference. For example, to create a square image that is one inch on each side at 300 pixels per inch, the resulting pixel dimensions will be 300×300 pixels. At ten inches on a side at 300 pixels per inch, the resulting pixel dimensions would be 3,000×3,000. And the ten inch image at 72 pixels per inch would be 720×720 pixels. So describing image dimensions in inches at a given pixel per inch resolution may result in different pixel dimensions. But if pixel dimensions are fixed, the file size is fixed (all other things being equal, of course).

Today’s Question: In an earlier Q&A [from January 23rd] you refer to underexposing by raising the ISO.

There is some confusion here somewhere. If one keeps all other parameters constant and raises the ISO isn’t this equivalent to using a faster film and therefore one would, relative to the earlier ISO, be over exposing? What am I missing?

Tim’s Answer: I would be more than happy to clarify.

When I was referring to the notion of raising the ISO, resulting in an under-exposed image, I wasn’t trying to suggest that raising the ISO setting actually caused the image to be darkened. Rather, I was referring to the impact of ISO on overall exposure and image quality.

I think it will be helpful to talk about specific exposure settings in order to help clarify. So, let’s assume a “sunny 16” exposure with an aperture of f/16, a shutter speed of 1/125th of a second, and an ISO setting of 100.

If I raise the ISO setting by two stops (to 400) and adjust other settings to compensate, I might end up with an aperture still set to f/16 but a shutter speed of 1/500th of a second. So, you could reasonably suggest that the faster shutter speed (the shorter exposure duration) would cause the image to be darkened, but that the higher ISO setting caused the image to be brightened to the same degree, resulting in an exposure that is exactly the same as would be achieved with the prior settings.

The key thing that I think photographers need to understand is how each setting affects the final image, and that is why I refer to the “underexposure” issue when you raise the ISO setting. More on that in just a moment.

The aperture primarily affects, of course, the depth of field in the scene. The shutter speed has primary control over the degree to which motion is frozen (or not) in the photo. And the ISO setting determines (in many respects) the amount of noise in the photo.

When you raise the ISO setting you are making a change that will have a brightening effect on the photo, all other things being equal. But you aren’t doing so by “magically” increasing the sensitivity of the image sensor.

So, to my point about raising the ISO resulting in a reduced exposure, let’s take a look at the exposure settings referenced above. At an ISO setting of 100 I referenced an aperture of f/16 and a shutter speed of 1/125th of a second. Raising the ISO to 400 resulted in a change to a shutter speed of 1/500th of a second at f/16.

But going from a shutter speed of 1/125th of a second to a shutter speed of 1/500th of a second represents two stops of exposure reduction. We’ve caused two stops less light to actually reach our image sensor. The image sensor can’t magically collect more light, or be more sensitive to the light. The result is that we’re actually taking a photo that is two-stops under-exposed, and the camera is then applying amplification to the signal information that was recorded to create the effect of a brighter exposure. In the process, noise will result.

To be fair, today’s digital cameras do a remarkable job of applying amplification through higher ISO settings without creating excessive noise. And there are a variety of ways you can mitigate the noise after the fact. But if you think of a higher ISO setting as representing an underexposed image that needs to later be brightened considerably, I think (and hope) it will provide a useful way for you to evaluate the ISO setting relative to other exposure settings. In other words, I hope this information helps encourage you to avoid raising the ISO setting on your camera unless it is necessary for your other exposure goals, in order to minimize the amount of noise in the final image.

Today’s Question: Our club has a good quality digital projector but some people are not always happy with how their images look when projected. Would it make a substantial difference if we took the extra step of calibrating it or will we always see a difference between the projected image and the display on a calibrated monitor?

Tim’s Answer: Calibrating and profiling your digital projector will indeed have a tremendous impact on the accuracy and consistency of the display of projected images.

Put simply, if all club members calibrate the monitor display they use for reviewing and optimizing their photos, and you calibrate the projector being used to display the images at your club meetings, you can expect a very good match between what the photographer created on their own computer and what is being displayed by your projector.

The key is to make sure everyone is calibrating to the same target values. For example, you could specify that everyone should calibrate to a color target of 6500 Kelvin and luminance of 100 candelas per square meter (cd/m2). If everyone uses the same values, you will achieve a high degree of consistency across multiple displays.

To actually calibrate the digital projector, you’ll need a monitor calibration package that supports digital projectors as well as standard displays. One such package that works very well is the X-Rite ColorMunki Display package, which you can find here:

http://timgrey.me/colormunkidisplay

Today’s Question: When one is confronted with extreme low-light conditions, could you discuss the pros and cons of shooting at high ISO with in-camera high ISO noise reduction engaged versus purposely underexposing a photograph at lower ISO and subsequently correcting for the underexposure and reducing the noise during post-processing?

Tim’s Answer: When you raise the ISO setting in your digital camera, you are effectively under-exposing the photo, possibly to an extreme degree. Therefore, it is worth considering (as suggested in today’s question) how to minimize the risk of noise associated with that under-exposed photo.

The basic choice here is how to compensate for an under-exposed photo. Your two options are to either increase the ISO setting in the camera, or to leave the ISO setting at a low value (with a photo that is therefore underexposed) and save all adjustments for your image optimization workflow after the capture.

Put simply, you will generally get better results (often much better results) by raising the ISO setting as compared to simply under-exposing. To be sure, it is best to use the lowest ISO setting possible for the conditions in order to minimize the amount of noise in a given photo, because raising the ISO setting translates into amplification of the signal being gathered by the image sensor. That amplification translates to increased noise in the image.

However, while a high ISO setting increases the amount of noise in a photo, severely underexposing the image will produce (in most cases) far worse results. This was actually the subject of an article called “ISO Illustrated” that I published in the December 2013 issue of Pixology magazine.

I most certainly recommend keeping the ISO setting as low as possible to minimize noise. However, that doesn’t mean using a shutter speed that is too slow, or underexposing the image. When the situation requires a higher ISO setting to achieve a proper exposure, by all means raise the ISO setting. You may need to mitigate the noise in post-processing, and even with noise reduction the photo may not exhibit optimal quality. But the quality will still be better than if you had simply kept the ISO setting at a low value and under-exposed the photo.

Today’s Question: Is there any way I can batch process multiple image sets through HDR Pro in Photoshop, or am I restricted to one set at a time?

Tim’s Answer: In theory there are a few possible ways you might batch process your high dynamic range (HDR) captures using Photoshop or other HDR-processing software. However, I don’t recommend this approach, for two main reasons.

First, HDR processing can be rather labor-intensive. Especially when merging the data to a 32-bit per channel source HDR image (which then gets tone-mapped to produce the final result, or saved to be tone-mapped with other software), there is a tremendous amount of data being processed. In other words, it would be relatively easy to overwhelm even a very powerful computer system.

Second, when it comes to HDR processing there is tremendous variability in terms of the specific settings used for each set of photos. That is especially true when it comes to the tone-mapping phase of processing, where you’re applying adjustments and creative effects to the HDR image. However, it can also be an issue for the initial captures, especially as it relates to alignment, chromatic aberration adjustments, and ghosting removal.

Photoshop does not offer a batch-processing feature as part of the HDR Pro tool. I have seen some scripts that enable batch processing for HDR in Photoshop, and it is possible to batch-process images with Photomatix software as well (among other solutions, I’m sure).

But the bottom line in my mind is that you’re not missing anything, and that it is worth it (in my opinion) to “manually” process each set of photos for an HDR capture. I’ll add, by the way, that while HDR Pro in Photoshop does a good job of creating the initial HDR image, I consider other tools to be better at the tone-mapping portion of the process. For example, you can perform the tone-mapping work in Lightroom for a 32-bit per channel HDR image, or you might look at third-party products such as HDR Efex Pro (part of the Nik Collection from Google) for processing your RAW captures.

Today’s Question: I have a question about publishing to Facebook [from Lightroom]. When setting up all the parameters, I see once you have chosen an album. But you can’t change it? I may want to publish different pictures to different albums on Facebook. Can that be done via a single Facebook profile, or must I set up a separate profile for each album?

Tim’s Answer: It is true that once you have established a Publish Service for Facebook within Lightroom, you aren’t able to change the album to which that Publish Service is actually publishing. While this may seem odd, and it certainly can be a bit of an inconvenience, this limitation is based on the notion that the Collection that is used as the basis of publishing to Facebook is essentially synchronized with that Facebook album. Changing the target album could create a degree of confusion (or unintended consequences) with respect to the connection between Lightroom and Facebook.

You can, however, configure multiple Publish Services within Lightroom so that you have the option to publish to different albums on Facebook. You can create a new Publish Service by clicking the “plus” icon to the right of the Publish Services header on the left panel in the Library module and then choosing “Go to Publishing Manager” to bring up a dialog where you can establish the settings. Even easier, if you already have an existing Publish Service for Facebook, you can right-click on the header that says “Facebook” and choose the “Create Another Publish Service via ‘Facebook’” from the popup menu.

I think the thing to keep in mind here is that the Publish Services option to share photos on Facebook uses a synchronization approach. In other words, you aren’t simply posting a photo to Facebook from Lightroom, and then having Lightroom “forget” about the publication of the image. Rather, the photos you share are retained in a special Collection, and you can then review the photos you’ve published on Facebook using that collection. You can even interact with the comments for the photos on Facebook from directly within Lightroom.

Taken in this context, I think it is a little more clear why you can’t simply change the target album for the Publish Service for publishing to Facebook. For this reason, my general approach is to create a single album specifically for sharing from Lightroom, not intended to serve as an organizational tool within Facebook, but rather aimed at simply providing an easy way to share individual photos using Facebook.

Today’s Question: What is your opinion on using lens correction in Lightroom? Do you think it squishes the pixels around enough to have an effect on an images? I use it many times and have not seen any degradation in sharpness when using it.

Tim’s Answer: Overall I would say the various Lens Corrections adjustments in Lightroom’s Develop module (and the exact same adjustments in Adobe Camera Raw) produce excellent results that can greatly improve the overall appearance of a photo.

When it comes to the various perspective correction options, there’s no question that a close examination will show some degradation in overall image quality. You’ll generally see, for example, a slight reduction in sharpness in the areas that have been reshaped most significantly.

However, in my view, the very minor degradation in image quality is more than made up for by the benefits provided by the Lens Corrections adjustments. The chromatic aberration removal offers a tremendous improvement for images that need this work, the Upright controls can quickly straighten out an image that exhibits barrel distortion or perspective issues, and the Manual set of controls allow you to fine-tune the image to perfection.

In short, the various Lens Corrections controls can be invaluable when it comes to improving the overall look of a photo, and the problems they solve can represent significant issues for a photo. I never hesitate to make use of these various adjustment options for images that need the improvement.

Today’s Question: I use Lightroom and Photoshop and just bought a new Nikon camera. I can now shoot Raw format pictures. Since there are many Raw formats from the various Camera manufacturers and they are “non-standard” would you recommend converting to DNG format on import to Lightroom? What are the Pros and Cons of standardizing on DNG?

Tim’s Answer: The Adobe Digital Negative (DNG) file format was created as a publicly documented file format that can effectively replace the variety of proprietary RAW capture formats that have been created by camera manufacturers. In fact, a variety of digital cameras allow you to use the Adobe DNG format as an in-camera capture format to replace what would otherwise be a proprietary RAW capture format.

I don’t personally convert my RAW captures to DNG, for a few reasons.

First of all, despite the fact that there really aren’t any “real” reasons I should feel this way, I don’t feel comfortable deleting my original RAW captures after converting to the DNG file format. That means I would actually be increasing the amount of storage space used by my photos, since I would be keeping two copies (at least) of each image.

I also make use of a synchronization solution for backing up my photos (which I covered in an article in the September 2014 issue of my Pixology digital magazine). With this approach, the DNG file format represents a disadvantage when it comes to backing up my images. Any changes to metadata for a DNG file means the file itself is updated, and so the entire DNG file must be backed up again. With my approach of retaining the original RAW captures and saving metadata out to XMP “sidecar” files, the backup only needs to copy the small XMP file when I make changes to metadata for a photo.

In addition, I prefer to retain the “private” metadata that many digital cameras write into RAW captures. Granted, that data can generally only be accessed by using the RAW processing software from the camera manufacturer, but I like the idea of preserving any of that information just in case it becomes useful at a later date. It is possible to embed the full RAW capture in the DNG file to work around this issue, but that would also result in a rather large file compared to my original RAW captures.

There are, of course, some advantages to the DNG format. To begin with, as noted above, the DNG format is publicly documented, so that if the proprietary RAW capture format you would otherwise use is one day no longer supported by any software, you could still feel confident that the DNG format could be processed, even if new software had to be created based on the public file specification. This isn’t something I feel is of any significant concern, but some photographers feel differently.

Some photographers also prefer to have their metadata updates written into the single DNG file, rather than having those updates saved in a separate XMP sidecar file. As noted above, because of my approach to backing up my photos, the XMP approach is actually a benefit for me.
The DNG file format also provides a workaround for using software that doesn’t support the latest RAW capture formats. For example, perhaps you are using Adobe Camera Raw to process your RAW captures, but have an older version that doesn’t support a new capture format. You could use Adobe’s free DNG Converter software to convert those RAW captures to the DNG format, so they can then be processed with your older software.

Finally, the DNG file format produces files that are generally smaller than the original RAW capture, without actually losing any information, thanks to lossless compression. In general you can expect a DNG file to be about 20% smaller than the original RAW capture, though your specific results may vary.

Ultimately it is up to each photographer to choose whether a conversion to the DNG file format makes sense to them. I prefer to make use of the proprietary RAW capture format employed by the cameras I photograph with. But if your camera supports the DNG format, you could certainly use DNG as a capture format. And a variety of software tools allow you to convert to the DNG file format. The key is to consider the various advantages and disadvantages based on your specific workflow needs.

Today’s Question: While I was watching one of your video lessons that covered sharpening in Photoshop, it struck me that you were using a value of less than one pixel to define the sharpening effect. How can a sharpening filter work on partial pixels?! Shouldn’t the value for the size (Radius) only be in whole numbers?

Tim’s Answer: I think the way I would describe this (understandable) source of confusion is that the number of pixels for the Radius setting is really a shorthand of sorts that goes beyond a simple number of pixels. In other words, you aren’t really having an effect that goes down to a level smaller than a single pixel, as that’s not possible with pixel-based editing. Instead, you are having an effect that is tapered across multiple pixels.

So, when you set the Radius to a value of a half pixel, you aren’t really having an effect on half of a pixel at a contrast edge, but rather are using a value of 0.5 as part of the (somewhat complex) formula that determines how much contrast is enhanced along existing contrast edges within the photo.

Keep in mind that the size of the sharpening effect (often referred to with a “Radius” value) interacts with the strength of the sharpening effect (often referred to with an “Amount” value). In addition, with many sharpening filters there are additional values you can adjust or factors that are used to determine exactly how the contrast enhancement of sharpening is applied to various areas throughout the image.

So, the Radius setting really relates to the size of the transition area for the contrast being enhanced. In some cases, with a very strong sharpening effect, the formula will cause a change with values that suggest granularity of one-tenth of a pixel. That’s not because the pixels are being divided into smaller units (they aren’t), but rather because the formula is blending the effect into the image. So in some ways you can think of the Radius setting as defining the degree of transition for the effect.

To get a sense of this, you can create a simple graphic in Photoshop. For example, create an image that is half black and half white, with a hard transition from black to white. Then apply a very slight blur to the image, so that there is a transition from black to white that spans perhaps ten pixels in the image. Then choose Filter > Sharpen > Unsharp Mask from the menu to bring up the dialog for the Unsharp Mask dialog (I recommend this filter for the test because it is a simpler sharpening filter than Smart Sharpen). Set the Amount to 500% (the maximum value) and the Threshold to 0 (the minimum value), and evaluate the effect in the image as you adjust the value for Radius.

If you conduct this little experiment, you’ll see that even a change of 0.1 or 0.2 pixels for Radius will have an impact on the sharpening effect in the photo, with higher values causing the gradation in the image to become smaller than they appear with a lower value for Radius. Again, this is just part of the complexity of the formula in play for this (and other) sharpening filters.

Today’s Question: I have a Lightroom question. I’m trying to either sort or find files that have no keywords in either a folder or collection. I know the symbol in the bottom right of the thumbnail but there’s no way to sort with that. Any suggestions?

Tim’s Answer: There is actually a very easy way to filter images to show only those that don’t have any keywords assigned to them. The icon you refer to is a “badge” indicating that keywords are assigned to a given image. While there isn’t exactly a filter based on the absence of that badge, there is a filter for “No Keywords”.

You can navigate to a specific folder containing the images you want to filter, of course. Then you want to view the Library Filter Bar, which can be enabled by choosing View > Show Filter Bar from the menu if it isn’t already visible. You can also use the backslash key (\) to toggle the display of the Library Filter Bar.

Next, to clear any existing filters, click the “None” option. Then click the “Metadata” option to display a set of columns that allow you to filter your images based on a variety of metadata values. Make sure all of the columns are set to the “All” option initially, so the images are not being filtered. Then click the header for the right-most column, and choose “Keyword” from the popup.

This will present a list of the keywords that have been assigned to any of the images that are currently available (in other words, the images in the folder you’re currently browsing). You will also find a “None” option on the list. Click that option, and the images will be filtered to include only those without any keywords assigned.

You could, of course, combine a variety of different filters in addition to the lack of keywords, in order to further refine the range of photos being displayed. But as you can see, it is quite simple to filter a set of photos in Lightroom to include only those without keywords assigned to them.