f-Stop Tables

This page was last updated: January 9, 2003

I recently purchased a digital camera and studio flash to take portraits. I was confused by the flash meter and how it can display fractional 1/10 and 1/3 f-stops. My confusion was further confounded by the fact that my digital camera was only allowing me to set f-stops at weird settings such as f-7.6, f-6.7, and f-5.3. I wanted to know more about the calculation of f-stop and how the number was arrived. I did some quick search on the Internet, and was only able to find limited references on this subject.

I want to share what I have learned so far in the following sections:

1. A brief explanation of f-stop.
2. A few links to other sites on this topic.
3. f-stop table in +1/10 steps.
4. f-stop table in +1/3 steps.
5. Instructions on how to use my Studio Flash Light Ratio Calculator Excel spreadsheet.
6. Link to download the spreadsheet with the tables and Flash Ratio Calculator.

Please e-mail me if you find any incorrect information .

1. f-Stop

f-stop is derived by dividing the focal length of a lens by the diameter of the lens opening. There are a series of f-stops defined as full stops. Each f-stop would allow either half as much or twice as much light compare to the next f-stop. With a lens of a fixed focal length, the bigger the diameter opening of the lens will result in a smaller f-stop value. A f-stop of 11 would mean a much smaller opening in the lens than a f-stop of 1.4.

Sometimes it can get confusing when f-stop is used to describe what one sets on a lens or what is measured on a flash meter or what is the f-stop for a studio flash. However, once you understand what a f-stop means, hopefully the usage will also be less confusing.

Comments from Bob Shell (January 8, 2003):

An f-stop is the ratio between the focal length of the lens and the *apparent* size of the lens opening as viewed through the front. It must take into account the magnification factor of all lens elements in front of the diaphragm, because it is the size of the opening that the light "sees" as it passes through the lens, not the actual physical diameter of the diaphragm opening.

It is this fact that allows companies to make constant aperture zoom lenses which maintain a constant f-stop when the focal length changes, because such lenses are designed so that the magnification factor (diopter value) of all elements in front of the diaphragm changes as focal length is changed to hold the aperture value constant.

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2. Links

• CK's Digital Camera Page- This is a good site by Dr. Ching-Kuang Shene. It has an extensive overview on Exposure.
• http://www.davis.k12.ut.us/chs/photo/fstops.html- a basic one page summary of f-stops.
• Page on Film speed, EV values, and f-stops
• Link to Depth of Field Calculator- http://dfleming.ameranet.com/dofjs.html
• Link to Bob Shell's talk on "The Myth of 18% Gray, Lighting Ratios, and Other Photographic Mythology"

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3. f-Stop +1/10 Table

Definitions:

1. F-stop = SQRT(2) ^ Stop Number
2. Stop number can be incremented by decimal point.
3. Half a stop is 0.5 Stop number more, 1/3 stop is .33333 stop more, etc.
4. The calculated stop number for 1/10th is below and the 1/3 table is in the next section.
5. SQRT means the "square root of", and ^ means "to the power of"
6. Please note that f-5.7 and f-11.3 are calculated values- 5.6 and 11.0 are commonly used.

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4. f-stop +1/3 Table

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5. How to use the Studio Flash Ratio Calculator Spreadsheet

Most people will probably find this totally useless. Anyway, it is free for you to download and use.

1. When using studio flash, sometimes it is important to know the light ratio between the main light and the fill light.
2. With some of the new non-SLR digital cameras, a lot of the f-stops are not set at the normal values. For example my Nikon Coolpix has f-7.6 at 21.4 mm.
3. Start by selecting the f-stop you want to use on your camera. I will use a normal full stop value of f-8.0 for this example.
4. Enter the f-stop at the "Enter f-stop" cell in blue, then hit return to calculate the other f-stops.
5. Decide on the light ratio you want between the bright part of your studio photo and the darker part.
6. Using a final f-stop of 8.0, if you want to see a 4:1 ratio, you would look up the "Calculated f-stop" section and find f-4.0.
7. You start by adjusting your fill light so that your flash meter would show f-4.0.
8. With the flash meter pointing back at the camera and make sure the flash meter sees both the fill and main light, you should adjust your main light to obtain a final composite studio flash at f-8.0.
9. What this means is that the bright part of your photo, which sees both the fill and the main light, would be 4 times as bright as the dark part, which only sees the fill light.
10. This calculation comes in handy when your camera limits you to fractional f-stops.
11. You can print out the f-stops you normally use for your studio work and use it as a reference.

Sample Calculation using a final f-stop value of 8.0. You need to download the spreadsheet to acutally use the calculator:

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6. Download the Excel Spreadsheet

You may need to right click to save the spreadsheet: f-Stop Table and Studio Flash Ration Calculator Excel Spredsheet

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