So, is f(?) constant or not?

Donnie

Donnie

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so what I've had rattling round the back of my mind on and off is..............

if f2.8 / f4 / f 11 is what it says it is on my canon / nikon crop sensor then is that the same on my full frame of the same make???
 
The F number is a specification of the lens itself and has nothing to do with the body you attach it to. It is a ratio of the focal length to the objective apperture diameter.

Your question is like asking if you car still has the same horsepower if you are wearing jeans or a suit.:thinking:
 
The exposure is the same if you set f4 and then fit the lens to APS-C, FF or anything else.
 
When you see f/4 or f/8 or whatever it's the focal length divided by the diameter of the aperture - and nothing else.

This applies to all optical devices - you'll see it quoted for telescopes where there's no sensor involved at all! So your aperture f number applies ONLY to the lens - nothing to do with what it's attached to.
 
What they all said about what the F no. actually means (the size of the aperture as a ratio of the focal length of the lens).

However, a larger imaging device requires a higher f no. to maintain the same DoF. That mostly gets flipped by photographers who often aim for a shallow DoF, which is easier to get with larger image area (sensor or film).
 
Oddly, the f number can change on a nominally f/2.8 lens (or presumably other "constant" f stops), most commonly on primes. Macro lenses tend to lose a stop or 2 as they focus closer. Some systems report the actual f stop while others pretend it doesn't happen and report the set aperture.
 
Nod said:
Oddly, the f number can change on a nominally f/2.8 lens (or presumably other "constant" f stops), most commonly on primes. Macro lenses tend to lose a stop or 2 as they focus closer. Some systems report the actual f stop while others pretend it doesn't happen and report the set aperture.
Click to expand...

Both the f/number and focal length are only true at infinity focus.

Macros are interesting, because when focused down to 1:1 (subject life-size on sensor) the effective aperture will have got darker by exactly two stops. Canon doesn't report this change (though of course TTL metering automatically compensates) while Nikon does. They are both kinda right and wrong, though it makes no difference in practise.

There is a further twist, because a Nikon 105mm f/2.8 macro will show f/4.8 (from memory) at 1:1 when it should show f/5.6 for a two stops theoretical drop. The fact that it shows less is down to focus-breathing, suggesting the focal length is more like 80mm-ish at 1:1.
 
Thanks for that Richard, I was hoping you'd be along to tell us more. I knew what happened (but not exactly what!) but not how and why. Just had a play with my Nikkor 105 and yes, at 1:1 it reports f/4.8, only speeding up to f/2.8 at about 2m. No idea what is actual focal length is at 1:1 (or how to measure it accurately - as long as it's 1:1, it doesn't matter to me anyway!)
 
Nod said:
Thanks for that Richard, I was hoping you'd be along to tell us more. I knew what happened (but not exactly what!) but not how and why. Just had a play with my Nikkor 105 and yes, at 1:1 it reports f/4.8, only speeding up to f/2.8 at about 2m. No idea what is actual focal length is at 1:1 (or how to measure it accurately - as long as it's 1:1, it doesn't matter to me anyway!)
Click to expand...

Thanks Nod :) My estimate of 80mm-ish above was a guess. Applying some maths means that if the aperture ratio is correct at f/4.8, then the focal length of the Nikon 105mm macro actually drops to 90mm at 1:1.

There is a practical aspect to this focus-breathing business with macros, because as the focal length reduces it impacts on the minimum working distance (front of lens to subject). Canon's two 100mm macros are a case in point: the older 100mm f/2.8 has a MWD at 1:1 of 15cm, the newer 100mm L is 13cm, and the Nikon 105mm macro sits in-between at 14cm. More interesting than relevant really :D
 
OK, cheers all, some interesting stuff there, it was just a query that's been rattling round in my head.

For some daft reason I thought that a set aperture on a full frame camera would expose more than one on a crop sensor purely through the size of the sensor :cuckoo:
 
Donnie said:
OK, cheers all, some interesting stuff there, it was just a query that's been rattling round in my head.

For some daft reason I thought that a set aperture on a full frame camera would expose more than one on a crop sensor purely through the size of the sensor :cuckoo:
Click to expand...

It does, but in a different way. In terms of exposure level per square mm, if you see what I mean, full-frame and crop are the same at a given f/number, but since the FF sensor is more than twice the area, total photon capture is more than double.

In practical terms, this is what drives FF's better high ISO performance, and also it also allows lenses to perform better, because FF demands less resolution for a given print size. These are the two fundamental advantages of larger sensors. It also changes the depth of field characteristics for a given angle of view and f/number, FF delivering less, that can be considered either an advantage of disadvantage.
 
In the days when large format with bellows cameras was the norm, you always measured the extension to calculate the exposure factor.
Up to the Weston 3 exposure meter they had log scales, which you could use as a slide rule to work it out for you. very simple when you know how. You did not work out a revised aperture you calculated a revise exposure time.
I still keep a weston 3. Though a pocket slide rule is easier... much easier than a calculator as the entire exposure series is set at once.
 
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