Aperture is not what it used to be

In the olden days, as they say – that’s way back last century, photographers were taught to stop down the lens for best sharpness and depth of field. With 35mm cameras that was modified a little – apertures no longer went all the way to f/64. LJK_3724-640Smallest apertures were only f/16 on some lenses as seen in my photo here of a fifty year old lens.

Still, stopping down remained a good rule. Lenses sported aperture scales with the f-numbers, distance scales that were pretty accurate, and depth-of-field marks, as seen here, that made the relationship between aperture and depth of field pretty clear. In the photo here, the lens is set to the hyper-focal distance for f/16, about 8 feet. You can tell that the scale shows that objects from 4 feet to infinity would be sharp.

That brings us to “sharp”. The “sharpness” of objects in a photograph are always dependent on a number of different things. The resolution capability of the recording medium being just one factor. How a photo is reproduced and viewed is another. What looks tack sharp in a small print might be unacceptably fuzzy in a wall-sized enlargement.

LJK_3720 (3872x2592)Film for a long time was a limiting item, especially as cameras moved to smaller formats. One big problem lurking in the background always was “diffraction”. Yes, camera lenses are diffraction limited optics at the smaller aperture sizes. The aperture is that “hole” through which the light reaches the recording medium, nowadays, the sensor.

The distance from that aperture to where the light bundle converges to a point for an infinitely far object on the other side, is called the “focal length”. The focal length divided by the diameter of the aperture is called the “f-number”. The light actually never “converges to a point”. This is where the laws of physics wag a big finger. Diffraction happens, it is the cantankerous, ornery, way of light bending slightly around corners instead of proceeding in a straight line. The smaller the light bundle, the more light won’t converge to a point. We will let the physicists worry about “Airy disks”, the point here is that light is smeared out and small detail becomes fuzzy, the larger the f-number, the worse it gets.

As digital cameras get ever smaller, this problem becomes more bothersome. The individual sensor elements, “pixels” to all of us, are already smaller that what the optics can resolve. As the aperture is set to a smaller hole, the “spread” of the light increases. So the minimum aperture for some camera lenses is limited. I have seen f/11 as the smallest aperture on some cameras. Smartphones, which have absolutely tiny sensors, go to the extreme: No stopping down of the lens. They work at full aperture all the time.

I ran an interesting experiment. file-sizes-NEFI mounted my camera firmly on a tripod a good distance from my subject. This subject was a barren tree some two hundred feet from the camera. I was careful to exclude any foreground. I turned off auto-focus and vibration reduction. Then I took a series of photos at apertures from the widest, f/5.6, to the smallest, my camera goes to f/36. The first indication of how image quality is affected can be seen from the file sizes of the images. The seven images were taken at f/5.6, f/8, f11, f/16, f/22, f/32, and f/36. I let the camera choose the shutter speed.

file-sizes-JPGNotice that file size gets larger, reaches a maximum for f/11, then declines. When the raw images were translated to JPG format (at 100% quality), the differences became even more noticeable. Photo file formats reduce the file size by eliminating repeating values. The simplest way to explain this is to say that when a pixel has the same value as the previous one, instead of recording that value, a “ditto” is recorded. So if three pixels have the same value, the file says effectively “three times xxx”. As a consequence photos with less detail produce smaller file sizes since there are fewer different values. This is a gross oversimplification, of course.

Can you tell the difference in the pictures? You sure can!

comp

Here are small cutouts from the images. These cutouts are from the JPG images since this is how most photos get distributed and viewed.

Here, in a larger view the extremes, the f/5.6 image, the f/11, and the f/36:

comp3

I was far enough from the tree so that the small twigs were narrower in the image than the pixels of the camera sensor. The first couple of photos show improvement in the image as the lens aberrations decrease as the lens is stopped down. Then, after f/11 (third image in the top row, center image of the larger views) diffraction and some other effects take over.

The moral of this story is this: For best image quality, do not go to small apertures, large f-numbers.

For my camera the deterioration in this test starts at about f/11. This is a DX format Nikon D-60. For full-size sensor cameras, the sensors, and the pixel elements are larger thus the diffraction effects will be less. For smaller cameras, especially the point-and-shoot pocket cameras, image quality will decline starting at even larger apertures.

To give you an idea of the size of the areas shown above, here is the full image with the section indicated.

LJK_3707-Fm640

Camera manufacturers either help out, or cover up, depending on your point of view. You don’t see aperture scales on lenses any more. Distance scales are only on professional lenses. Cameras are programmed to do the best they can, and this means working at the largest aperture possible, not the smallest one.

.:.

© 2011 Ludwig Keck

Making a photo greeting card – image editing and manipulating

Over on the other side of Café Ludwig you can see my Happy Holidays greeting post. Here I would like to tell about the process and tools I used to transform a photo into an abstract image. This also gives me the opportunity to introduce some newcomers to the concept of layers in photo editing. Layers is a powerful feature found in image editing programs. These programs can be intimidating because there is so much to learn to master them. For this project I did not use a photo editor at all, I used Microsoft Office Word 2010. This “text editor” has some great “Picture tools” that are simple and easy to use.

I started with a nice enough photo, but wanted to make the picture to be more like a painting. The “Artistic effects” tool called “Cutout” reduces the continuous tones into just a few shades. After trying several settings I liked the effect best with six shades. The resulting images was nice enough for me to use on greeting cards. For this project I wanted to do a little more. Here are close-ups of the candle in the photo. The first (left) image is the original photo, the second (middle) picture is the results from the “Cutouts” effect transformation. More about the third image a bit farther down.

candle-3v

I wanted to add a glow and light “spikes” around the candle flame. The glow should go behind the flame. When using terms like behind or above we come to “layers”. In Word it is quite easy to place one image over another one. That’s all there is to layers, one image over another. In “Picture tools” there are also tools for setting “transparency”. The first one I used was picking one color to be transparent. The third image (on right) shows the result when I picked the black color to be transparent. In this case the “white paper” is seen wherever the image had been black. stack

Now I could put this image over another and see the layer underneath wherever the picture had been black. Let me describe what I did in a little more detail. The picture on the right here will explain the steps and the layers.

I used the drawing tools to make a black rectangle. This would serve as replacement for the black that I had made transparent in the photo.

I made a smaller black rectangle and placed a blue radial gradient on it. You can see it at the right. I put this rectangle, or “layer” over the large black one in place so the center of the blue glow would wind up behind the candle flame. I then positioned the image layer over both of these.

The glow behind the candle was now in place.

I used drawing tools again to make a small four-pointed star. In Word you can freely rotate an image, so I set the angle to an orientation that looked good to me. The star was also filled with a gradient of yellow color. It is pretty hard to see in the “stack” illustration here. Placing that over the flame completely hid the details behind the star. This is where the “transparency” setting comes in. This setting is for the whole image, not for a single color as I used before. A transparency of 100% means that the image is like a sheet of glass with nothing on it. Set to 0% means that the image blocks everything behind it. At intermediate settings the image is more or less faded and allows details from a lower layer to shine through.

There was a little problem once I had the transparency just right for the spikes. The center of the flame was now covered with the star which showed detail. So I made a small oval shape, the size of the flame part that I wanted to be completely white. I placed this shape over the flame. That was a bit hard to do with a white oval – it was hard to see. So I made the oval red. You can see it just barely in the stack illustration. Once it was in the correct place I reset the color to white.

This completed the picture part of my project. I had made the black rectangle a bit smaller than the photo so the two candy decorations would hang out of the “frame”.

Word, of course, is great for text and text effects, so the “Happy Holidays!” message was easily added. Here is a view of the finished project.

HappyHolidays

I enjoyed preparing this image to wish you the best for this holiday season and for the year to come. Maybe my explanations also gave you some insight and ideas for your own enjoyment.

 

 

JPG Options in Windows Live Photo Gallery

Windows Live Photo Gallery can display most image types, but can fix, edit or enhance, just some file types. Most cameras store images in a format called JPG and many also in a “raw” format. JPG is so widespread and common, most users don’t deal with anything else. Photo Gallery uses JPG by default when you make a copy, or resize a picture.

The JPG format is a “lossy” compression method. It creates files that are smaller than the “raw” images produced by a camera. This makes storage and sharing easier. The term “lossy” implies, of course, that something is lost. The more the image is compressed, the smaller the file size, but also the lower the quality of the image as fine detail is lost. JPG files can be created in a range of quality, usually expressed as a percentage. Photo Gallery is set to 98% as it comes. This can be adjusted.

To modify the JPG quality setting of Photo Gallery, proceed as follows:

  • Click the “File” button, upper left.
  • Click Options.
  • In the Options dialog click Edit.

The JPG quality slider can be set to 100% (max right) and as low as 80% (max left).

Here is a photo which I saved in a couple of sizes and several quality settings. The image here is at quality 80%.

For this photo here are the file sizes. The file names should make clear the size and compression quality. The original file as it came out of the camera is the camera’s “raw” format (the extension for this Nikon camera is NEF). Note the huge difference between the 80% and 100% JPG files, more than five times! For the smaller sized image the difference is not as large, but still substantial.

How different is the quality? Here are enlarged sections of the smaller image. Click on the image to see the comparison more easily.

You can see differences, however, the largest differences are in areas where there is little detail, as in the sky areas of this picture. If you are going to display pictures on a web site or in a blog like here, the lower quality is perfectly acceptable. The benefit is that the much smaller file sizes will cause the pictures to be displayed much faster. Of course, using up less space on your computer or the online server is a significant benefit.