Whats the differnece between f-stop and aperture?

[Gordo]

I recently started an Intro to photography course. The first lesson included a discussion on Depth of Field. I decided to take my Digital Rebel out for a series of shots at different f stops. I was looking at the photos and decided to print the f stop and shutter speed on each photo. I opened the EXIF information in Adobe Photoshop Elements and was surprised to see a value for the f-stop and one for the aperture value that were different. I thought that the f-stop and aperture were one in the same, and I would have expected them to be the same value.

If you look at the properties of the file in windows it itemizes an f-number and lens aperture separate, but the values are the same.

Can someone explain the difference between an f-stop and aperture, and why Elements is showing a different value?

Thanks Gord

[Crocuta]

They're not exactly the same, even though they're addressing the same issue. The aperture is the diameter of the opening in your lens. The f-number is the ratio of the aperture to the (effective) focal length of the lens. So an f-stop of f/16 is an aperture whose diameter is 1/16 of the focal length of that lens.

As an example, if you had a setting of f/16 on a 300mm lens, the aperture would be roughly 18.75mm. We always take note of the f-number because that's a consistent measure of the light reaching the film (or CCD), but different lenses will need different apertures to achieve the same f-number.

I don't know for sure what numbers the Rebel is recording, but my best guess is that the aperture is being recorded in an absolute scale (in mm) and the f-stop is being recorded in its ratio form in the way we're all used to seeing it.

You should be able to check and see if I'm right. Take the two numbers you mentioned from the EXIF as well as the focal length for that image. Multiply the aperture value by the f-stop value. If it equals the focal length then that's what's happening. There may be some error since the f-numbers are all rounded off, but it should be close.

[Suhit Gupta]

Both Aperture and F-stop basically determine how much light is entering the camera lens. Bigger aperture = more light, while bigger F-stop = lesser light entering the lens. So they are reciprocals of each other.

A good definition of F-stop, off of PBS.org is "The scale used to measure the size of the opening of the iris (the opening that lets light in) on a lens. F-stops can be confusing, because the larger the number, the smaller the opening (iris), less light is let in, on the other hand the smaller the number, the larger the opening, more light is let in. Common f-stops are 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, and 22."

Suhit

[Gordo]

I now have a better understanding of the difference, and it makes sense.

However, when I tried the formula it didn’t work: f-stop * aperture = focal length, 22 * 8.9 = 196 when the actual focal length was 18mm. At the other end it was 3.5 * 3.6 = 12.6 when actual focal length was 18mm

[Crocuta]

I now have a better understanding of the difference, and it makes sense.

However, when I tried the formula it didn’t work: f-stop * aperture = focal length, 22 * 8.9 = 196 when the actual focal length was 18mm. At the other end it was 3.5 * 3.6 = 12.6 when actual focal length was 18mm

Oh well, so much for my guess! My definitions of aperture and f-stop are correct, but Canon must be recording something else in those EXIF fields or possibly they're expressing at least one of them in another scale. Do let us know if you ever find out. I'm curious now.

[Gordo]

I am just starting an Intro to Photography course and my next class is Saturday morning. The topic this Saturday includes f-stop, so will ask my instructor.

Thanks for the info.

[Gordo]

Crocuta, you formula is correct. Specifically: “You can calculate an f-number, if you are keen or don’t have much of a life, by dividing the lens focal length by the diameter of the aperture.” This is a quote from some of the material provided this week.

Based on your formula I took all of the values from the EXIF data for all of the photos that took. All had the exact same focal length, and yet not one the combinations of “aperture * f-stop” gave the correct answer nor where they a consistent number. This leads me to believe that the Aperture number being recorded in the EXIF data by the Rebel is incorrect. The lens I was using was the one that was sold with the camera as part of the kit. (EFS 18-55mm

Fortunately, I don’t believe it really matters to my photography.

[Crocuta]

Crocuta, you formula is correct. Specifically: “You can calculate an f-number, if you are keen or don’t have much of a life..."

Hey! I resemble that remark... :lol:

Fortunately, I don’t believe it really matters to my photography.

Too true. Everyone needs to know how the f-stop and shutter speed interact to allow a certain amount of light to the film or CCD, and how changing them affects depth of field and the ability to stop action (respectively). But no one that I know ever really calculates the actual aperture in mm. If it weren't for the mystery of these numbers, I wouldn't have either. While I'm sure you could pursue it further if you wanted, I think your time would be better spent by taking pictures.

[Suhit Gupta]

Oh well, so much for my guess! My definitions of aperture and f-stop are correct, but Canon must be recording something else in those EXIF fields or possibly they're expressing at least one of them in another scale. Do let us know if you ever find out. I'm curious now.
Yeah, I back your definitions. It is weird that the numbers don't add up. But then I guess I can see the discrepancy; after all, note that when we give focal length on most digital SLR lenses, they usually have to be multiplied by some factor that the camera usually adds. This must be one of those cases, time to investigate further.

Suhit

[socrates63]

For a good book on understanding f-stops, shutter speed, and the interplay between the two for proper exposure, I recommend Bryan Peterson's "Understanding Exposure." I believe a new edition was recently published. I bought mine almost ten years ago.

It's easy to read and has plenty of sample photos for illustration. Photography newbies will definitely benefit.