f-stop different from aperture? ?

[bigbelly]

Hi,

I tested a canon 28-105 usmII 3.5-4.5 with my eos 10d today.
pics looked quite ok. the only odd thing happened when i read the exif data in photoshop (file-info).
I noticed that F-stop had always different values than Aperture Value. in this example :

http://www.egocat.com/canon/exif.htm


you can see that f-stop has a value of 8.0 but aperture Value has a value of 6.0
i might be completely mistaken but shouldn't those 2 values be the same? if not, i'd be glad to read why they are different.

I actually was thinking of buying the lens so i would like to be reassured that the difference in those values are normal :)

thanks

t

[RGorrill]

Hi,
What program are you using to get the EXIF data? Those two values should not be different and I also note that the Maximum aperture value is strange. I use Breezebrowser and this is the EXIF data that I get:

File: 104_0472.CRW
File size: 6,325KB
Image Serial Number: 104-0472
Camera Model: Canon EOS 10D
Camera serial number: 032XXXXXXX
Firmware: Firmware Version 1.0.0
Owner: Robert B. Gorrill
Date/Time: 2003:05:19 10:38:21
Shutter speed: 1/250 sec
Aperture: 9.5
Exposure mode: Program
Flash: Off
Metering mode: Evaluative
Drive mode: Single frame shooting
ISO: 100
Lens: 28.0 to 105.0 mm
Focal length: 28.0mm
Subject distance: 655 m
AF mode: One-shot AF
Image size: 3072 x 2048
Image quality: Raw
White balance: Auto
Color space: sRGB
Saturation: High
Sharpness: Normal
Contrast: Normal
Tone: Normal
Custom Functions:
CFn 13: Assist button function: Select home position

And this is the data from the File Viewer Utility:

File: 104_0472.CRW
File size: 6,325KB
Image Serial Number: 104-0472
Camera Model: Canon EOS 10D
Camera serial number: 032XXXXXXX
Firmware: Firmware Version 1.0.0
Owner: Robert B. Gorrill
Date/Time: 2003:05:19 10:38:21
Shutter speed: 1/250 sec
Aperture: 9.5
Exposure mode: Program
Flash: Off
Metering mode: Evaluative
Drive mode: Single frame shooting
ISO: 100
Lens: 28.0 to 105.0 mm
Focal length: 28.0mm
Subject distance: 655 m
AF mode: One-shot AF
Image size: 3072 x 2048
Image quality: Raw
White balance: Auto
Color space: sRGB
Saturation: High
Sharpness: Normal
Contrast: Normal
Tone: Normal
Custom Functions:
CFn 13: Assist button function: Select home position

Bob

[bigbelly]

i used photoshop 7 when i got that exif data..
i niticed that with breezebroweser or file viewer utility there is no F-Stop value at all un the exif .. only the aperture..

i am a bit confused now

the lens i tried is a second hand.. so i wonder if it might b broken

[RichardtheSane]

Trust the breezebrowser info. I noticed the exif you posted suggests a max aparture LOWER than the reported aparture. Buggy?
That is the software, not the lens.

[SoCal69]

I noticed the same discrepancy in my Photoshop EXIF info. There is a listing for aperature and one for f-stop. Sometimes the values match, sometimes they don't. I always just ignored f-stop and went with the aperature value. My EXIF also contains a listing for shutter speed and another for exposure time. so far I have not noticed a discrepancy in the exposure/shutter numbers, but I haven't really been looking either.

[J.A.F. Doorhof]

Same here f-stop is different from Aperture.
I use ACDSee 5.0 for the EXIF data.

Greetings,
Frank

[Longwatcher]

bigbelly wrote:


you can see that f-stop has a value of 8.0 but aperture Value has a value of 6.0



I think I have this figured out.

f-stop will be a set number of which 8.0 is one.
the aperture value of 6.0 indicates a fraction of an f-stop so I suspect the software is telling you that the aperture was set at 6.0, but the equivilant f-stop was 8.0.

full f-stops are
f/3.5, f/5.6, f/8, f/11, f/16 and f/22

So if F-stop always falls on one of these numbers, my theory whould be correct, if it doesn't then back to the drawing board. You might wonder why it did not go to 5.6, my guess would be that it either goes to next one up or 6.0 is actually closer to 8 in actual value.

Just my guess and research,

[SoCal69]

Your theory doesn't seem to hold up. I have one example where the Aperature in the EXIF reads 5.5 and the f-stop reads 6.7.

[slejhamer]

Get ready for some math!

The square root of 2 raised to the power of the aperture value gives you the f-stop value.

The square root of 2 raised to the 6th (aperture) is 8 (f-stop), and the square root of 2 raised to the 5.5th is f/6.7.

Both numbers are correct as displayed. All f-stops are derived from powers of the square root of 2. For each aperture stop (which I will call a "click" to avoid confusing the terminology,) you raise it to the sq. rt. of 2 to get the f-stop. Aperture click 1 = f/1.4, click 2 = f/2, click 3 = f/2.8, click 4 = f/4, etc.

Go with the F-stop; this is the number you dialed in.

[Webster]

Aperture is the actual size of the opening. In the formula f/x=a, f is the focal length of the lens, x is the f-stop, a is the aperture. That's why you see f-stop shown as f/5.6.

If you have an f-stop of 8 and an aperture of 6, I'm guesssing the focal length was 48mm.

[bigbelly]

hi,

actually focal length was 105mm

[daveh]

bigbelly wrote:
hi,

actually focal length was 105mm



Here's my guess ;)

6 is the aperture setting that was sent to the lens. However, because this is a variable max aperture lens, setting the same physical aperture size results in an effective exposure of f6 when shot wide (max f3.5) and f8 when shot narrow (max f4.5).

This guess should be easy enough to test - see if 6 = 6 when shot wide. (If I had a variable max ap lens I would have tested it before posting.)

[bigbelly]

i shot the picture with aperture priority and i set it to f8.

[daveh]

That was my assumption. The exposure value you set is the real effective exposure that you get.

For a simpler example consider what happens when you want to shoot at f4.5. If you're wide, then the lens needs to stop down to f4.5. If you're tele, 4.5 is wide-open. Same setting on the camera, but the lens needs to do something different.

[slejhamer]

I guess the math was too tough for you?



:p


Try it at any setting. The math is correct. F/stop = square root of 2 raised to the power of the aperture. Nothing to do with exposure or rounding to the next highest full stop.

Why anyone would care to have this info is beyond me, but the question has been answered.

[daveh]

Not sure if that was meant for me but the math was trivial. However, with such limited input, I can come up with lots of explanations to relate two numbers :)

[Osmium]

Classic case of using a slejhamer to crack a nut... ;)

Thanks for the info!

[slejhamer]

Yep, the math is trivial. That's why I don't understand why Photoshop's EXIF bothers with it. Must be there for the geeks (yes, me.)

Sorry 'bout the limited input. The guts of it are pretty simple: to double the area of a circle, the radius (or diameter, or either, if I recall correctly) must be multiplied by the square root of two. Thus, each click of the aperture ring ( 1 - 2 - 3 - 4 , etc.) multiplied by 1.414 (the square root of two) gives you the f/stop for that setting.

Hope that helps.

[daveh]

No, I get that, I just meant that when someone says he has "6" and "8" without any more explanation of the fields, I can come up with multiple transforms from one to the other.

[SoCal69]

So what you are saying is that the aperature is simply telling us how many times we have halved the area of the aperature from a hypothetical fully open aperature (one that hypothetically has an f-stop of 1.0 eg: focal length 70mm/aperature 70mm=f-stop 1.0). Aperature of 6 simply means we have halved the area of the aperature 6 times, resulting in an f-stop of 8.0 (70mm focal length/8.75mm aperature = f-stop 8.0). So the result is not lens dependent... in other words, the aperature number and corresponding f-stop should be the same no matter which lens you use.

[slejhamer]

Correct, SoCal. [In fact, I should have started at "click" zero (before the first halving) to get f/1.0] The reason this is independent of the lens is that we are talking about the number of clicks (or number of halvings, to keep with your terminology,) whereas in the "Focal Length divided by Aperture" equation we use the actual physical measurement of the aperture diameter.

However, mathematically speaking, you can substitute "the square root of two raised to the power of the aperture setting" for the F/stop variable in the equation "Focal length divided by F/stop" to calculate the physical measurement of the aperture, or vice versa.

Whew! I hope to never have to think about this again!

:)