What Does F-Ratio Mean?

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This is part of the subject which was discussed in lesson 3, lenses, light and magnification of the Photo 101 course that I am following. It looks at brightness and f-ratio’s, magnification and field of view.

I found this lesson a harder one than the other two, although it is explained very well. I had to read through it a couple of times before I really got it. And this was partly because I had missed a key sentence, so it just shows it pays to reread an article!

F-ratio

My favourite part of the lesson though was the explanation of f-ratio, what it is and how it works. This is one of those photography terms that I hear people talking about and read about, but was not really sure what it was or what it meant.

F-ratio describes the relationship between the diameter of the lens and the focal length. F-ratio = focal length divided by diameter of the lens.

The f-ratio of a lens is always written on it somewhere. I always wondered what all those numbers on a lens meant!

If I put this information together with what I learned in the first two lessons then it is building a picture. Of how the design of a camera uses what we know about light and how it behaves to create an image. And we can then choose the right lens or settings for the situation, depending on how much light there is available to create the best image possible.

The different parts of the puzzle are starting to come together…

Homework

The homework this week was quite hard as most of the options were suited for DSLR’s, but that was fine because there was an option for point and shoots which was to use the macro setting on the camera.

My camera is a cheap point and shoot and I have been having problems getting the macro setting to work successfully. I keep it at the minimum distance that the camera suggest of 0.6m. And I found using a tripod and the self timer gives the best results. But I think I have done alright with the photos below (they are better than previous ones), although there is a bit of digital noise and they are not completely macro, but the best the camera will allow.

Mandy

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How to Make a Camera Obscura

Making a Camera Obscura is one of the homework choices for lesson 1 of the Photography 101 course on the Digital Photography School Blog.

What is a Camera Obscura?

Camera Obscura or ‘dark chamber’ is one of the earliest ways of producing an image, the idea is very old and first documented a 1,000 years ago.

Basically it is a lightproof box or room with a small hole in one end, light travels through the hole and forms an image on the opposite side or wall to the hole. Because of the way that light travels and behaves the image that is produced is up side down.

Due to the small hole only letting a bit of light through the image is faint but has very good perspective and detail. If you made a larger hole the image would be stronger, but the more rays of light that pass through the hole the more they overlap, and consequently the image is not as sharp. The small hole directs the light better.

This gives us a basic understanding of how the aperture works on a camera, and how the size of the aperture and the amount of light that passes through it affects the image that is created.

So what did I do?

I decided to use my spare room to make my camera obscura, it only has one window and is very handily box shaped. I completely covered the window in a combination of foil and masking tape, making sure there was no light getting through, or as much as I could anyway. Then I made a small hole in the middle of the foil. I also made a screen for the image and propped it up in front of the window. My wallpaper is textured and I didn’t want to be too optimistic about how well the image would make it to the wall!

It didn’t work the first time…

These things never do. It worked in the fact that there was an image on the screen, but it was a reflection of the foil on my window. It was a very sunny day so the foil image was quite strong, because the foil was doing what it does best and reflecting that well (and getting quite hot) I think it was starting to cook the window.

So I had to redesign slightly and cover the back of the foil with paper to stop it reflecting. That did the trick though and sure enough an image of the houses across the road appeared on the screen, but up side down. And not as strong as the foil image, but I could see the detail of the clouds and the chimneys of the houses.

Was it worth it?

Oh yes, it was absolutely great when the image appeared on the screen. I really felt like I had achieved something, and I could say I had 100% created that image from basically foil, paper and masking tape. I was made up with it, and it was a lot of fun.

Unfortunately as much as I tried to take a photo of what I had done, my camera (a point and shoot) was not up to the job of shooting in such low light. So I haven’t got any photos of it, but I have shared some pictures of what the camera obscura looked like so you can get a feel for how it worked.

Mandy

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