Aperture and lens speed

[table1349]

One of the things that seems to be overlooked in this whole discussion is the fact that most lenses are not exactly what they are advertised. There are very few true 50mm lenses out there or any other focal length for that matter. They are close, a bit under, a bit over but generally not exactly what they are marked. This isn't some scheme on the part of lens makers but a matter of practicality. There are standardized lens sizes and the manufactures mark those lenses to the closest standard size. Do you really want to decide between a 48.8mm lens, a 49.2mm lens or a 51.3mm lens all listed as f1.4. Do you seriously even want to think about it or care?
I own both the 70-200 f2.8 USM II and a 200 mm f2.8. The 200mm f2.8 is slightly faster than the 70-200mm f2.8 at 200mm. No surprise and no bother either. It's the nature of lens making.

 

[greybeard34 7]

One of the things that seems to be overlooked in this whole discussion is the fact that most lenses are not exactly what they are advertised. There are very few true 50mm lenses out there or any other focal length for that matter. They are close, a bit under, a bit over but generally not exactly what they are marked. This isn't some scheme on the part of lens makers but a matter of practicality. There are standardized lens sizes and the manufactures mark those lenses to the closest standard size. Do you really want to decide between a 48.8mm lens, a 49.2mm lens or a 51.3mm lens all listed as f1.4. Do you seriously even want to think about it or care?
I own both the 70-200 f2.8 USM II and a 200 mm f2.8. The 200mm f2.8 is slightly faster than the 70-200mm f2.8 at 200mm. No surprise and no bother either. It's the nature of lens making.

Only slightly, given the number of elements in the zoom compared to the fixed, that's quite an accomplishment.

 

[sk66]

And then there's something about effective aperture in macro lenses that I'll figure out if I ever decide to get one.

Actually, "effective aperture" applies to more lenses than the "physical aperture" does. It's just like "focal length" is not typically a physical measurement, it's an "optical measurement." The focal length of a lens can easily exceed its physical dimensions.

But like I said before, none of this is really worth worrying about IMO. In 30+ yrs I've never had it affect anything that I have noticed.... 2.8 is 2.8...

 

[WayneF]

And then there's something about effective aperture in macro lenses that I'll figure out if I ever decide to get one.

Actually, "effective aperture" applies to more lenses than the "physical aperture" does. It's just like "focal length" is not typically a physical measurement, it's an "optical measurement." The focal length of a lens can easily exceed its physical dimensions.

But like I said before, none of this is really worth worrying about IMO. In 30+ yrs I've never had it affect anything that I have noticed.... 2.8 is 2.8...

Right, lens transmission certainly was an issue in the 1930s, but with improved modern lens coatings today, anyone worrying with this now ought to be out taking pictures instead.

Macro lenses are a different issue. Lenses focus closer by extending the front elements farther out (excepting internal focus lenses). This focus extension makes the lens longer, and increases their focal length. The marked focal length is valid only for infinity focus. But the optical physics say to achieve macro 1:1 magnification, the lens focal length is extended to be 2x longer (than infinity).

That extension changes fstop number = focal length / effective aperture diameter.

The aperture diameter is (about) the same, but 2x focal length is 2x aperture number, which is two stops less if at 1:1 magnification. (loss is not the right word, f/stop is simply computed with different numbers)

Internal focus lenses can do other things, like also shifting relative position of the aperture itself. So many Nikon macro lenses show slightly less than two stops at 1:1 (but still, very nearly two stops, because that is physics). Macro lenses recalibrate to show the correct final fstop value. The Nikon macro lenses (the internal focusing ones) probably shows f/4.8 at 1:1 instead of the theoretical f/5.6 (two stop loss from f/2.8). That is about 1/3 stop less than the full two stops.

And you may notice your Nikon VR 105mm will show maximum aperture of say f/3 if focused at 4 feet. You have to get near to 9 or 10 feet to see the full f/2.8 maximum. This is just normal physics, due to the lens extension.

 

[sk66]

Macro lenses are a different issue.

Yes, but it still doesn't "matter."
99.99% of the time, a lens does what it does, the meter adjusts, and the exposure is "correct." All lenses of similar design behave the same and will be doing the same thing... it doesn't really matter if you know what/how.

The only way I can see this stuff affecting someone is if they were using handheld metering and a lens that lies when adjusted (i.e. Canon macro's in macro mode)... or when using a manual bellows and handheld metering.
I suppose it might be surprising to some for the recorded exif aperture to be different than what was set... And maybe that's why Canon macros don't report it...

 

[rambler]

Okay try this: Say you have a big prime lens, 600mm. When set to an aperture of 5.6, let's say that the diameter of the opening is 1". And, you have a small prime lens, 35mm. When set to an ap. of 5.6 the diameter of the opening is 1/2". It does not matter that the 5.6 apertures are different diameters. Why? Because of the ratios.

Say you have a gallon jug. Fill it half with sand and half with sugar. You have a half gallon of sand and a half gallon of sugar. The ratio is 1:1

Take a two gallon jug and fill it half with sand, half with sugar. You now have more sand (one gallon) and more sugar (one gallon) than you had with the one gallon jug, but the ratio is the still same, 1:1

PS Of course, I was really referring to physical effective aperture size and when you come to a fork in the road, take it.

 

[AlanKlein82 21]

F stops are standards used by the industry. If the amount of light was different between lenses for the same stop, your meter would be useless. F/2.8 is f2.8 and F/8.0 is f/8.0 regardless the lens. More important is to learn that large apertures decrease depth of field DOF and small apertures increase DOF. Beyond that, spend time making your pictures more interesting.

 

[table1349]

F stops are standards used by the industry. If the amount of light was different between lenses for the same stop, your meter would be useless. F/2.8 is f2.8 and F/8.0 is f/8.0 regardless the lens. More important is to learn that large apertures decrease depth of field DOF and small apertures increase DOF. Beyond that, spend time making your pictures more interesting.

Alan, not so with the meter being useless. The meter gives you a different set of readings for the same correct exposure. My 70-200 f2.8 vs my 200 f2.8 above is a perfect example. With the 200mm at f2.8 I get a slightly faster shutter speed or I can lower my ISO a bit as compared to the 70-200 at 200mm at f2.8 to achieve the same exposure.
If what you are saying was the case than any other lens with something different than what the camera is programed for would also fool the meter.

 

[Vince.1551]

It's pointless arguing. Go do a practical test!!!

 

[AlanKlein82 21]

F stops are standards used by the industry. If the amount of light was different between lenses for the same stop, your meter would be useless. F/2.8 is f2.8 and F/8.0 is f/8.0 regardless the lens. More important is to learn that large apertures decrease depth of field DOF and small apertures increase DOF. Beyond that, spend time making your pictures more interesting.

Alan, not so with the meter being useless. The meter gives you a different set of readings for the same correct exposure. My 70-200 f2.8 vs my 200 f2.8 above is a perfect example. With the 200mm at f2.8 I get a slightly faster shutter speed or I can lower my ISO a bit as compared to the 70-200 at 200mm at f2.8 to achieve the same exposure.
If what you are saying was the case than any other lens with something different than what the camera is programed for would also fool the meter.

Then it appears that the settings on the lenses are not correct. Otherwise it defeats the whole point of having standard readings. For example, I shoot a film camera that has no exposure meter. I use a separate hand held meter. I set the lens shutter and aperture according to the meter regardless of the lens I use. Of course there could be differences between the lenses settings that I just haven't noticed in the final results. Bit it would be a real pain to have to change the settings based on which lens I use and not rely directly on my meter.

Curious, how different are the speeds between the two lenses for that f/2.8 setting you're getting at 200mm? What object is being metered; are the views exactly the same on both lenses? Has the light changed even in the slightest during the time you changed lenses?

 

[WayneF]

I own both the 70-200 f2.8 USM II and a 200 mm f2.8. The 200mm f2.8 is slightly faster than the 70-200mm f2.8 at 200mm. No surprise and no bother either. It's the nature of lens making.

B&H specs say the 70-200 mm lens has 23 elements in 19 groups. Says the 200mm f/2.8 has 9 elements in 7 groups.

Each air-glass surface loses about 4% transmission (due to reflectance). 19 groups, 38 surfaces, 0.96^38 is 20% transmittance (if uncoated).

But modern coatings reduce this reflectance loss to almost nothing. It really is not a modern problem (at least it is solved now), however 19 groups is still much more than 7 groups, and some slight difference can be detected. Something for the techies to argue about, but no one else will notice it (today).

 

[sk66]

Then it appears that the settings on the lenses are not correct. Otherwise it defeats the whole point of having standard readings. For example, I shoot a film camera that has no exposure meter. I use a separate hand held meter. I set the lens shutter and aperture according to the meter regardless of the lens I use. Of course there could be differences between the lenses settings that I just haven't noticed in the final results. Bit it would be a real pain to have to change the settings based on which lens I use and not rely directly on my meter.

That's the whole point, "everything" is just an approximation... even handheld incident meter readings in constant light can vary slightly (direction it's pointed, dome/disk, etc)... it just may not vary enough to matter or display. Most handheld meter's have an EC adjustment capability for this reason, or they can be calibrated to a particular lens/camera. But I've never seen/heard of it being necessary.
I've never known anyone to take a handheld incident reading (in a new setup) and not fine tune their exposure after the initial test shot... heck, that's why polaroids were so popular in the studio with film. They allowed fine tuning the exposure/lighting before committing to burning up several rolls of film.

The built in camera meter doesn't really care about the settings it just needs to know how much light it's getting now, and how much it's going to change when the exposure is taken. So as long as the "changes" are accurate (i.e. f/2.8 doesn't have to be 2.8, but f/4 better be one stop less) the final exposure will be "accurate." Just like with film you might set an ISO for a roll of film different from it's rating.
The funny thing is, a DSLR can be more "accurate" than you can be... it can use SS's/ISO's that you can't set manually. But it really doesn't matter that much, how accurate does it need to be really? What is a "correct exposure?"

 

[table1349]

F stops are standards used by the industry. If the amount of light was different between lenses for the same stop, your meter would be useless. F/2.8 is f2.8 and F/8.0 is f/8.0 regardless the lens. More important is to learn that large apertures decrease depth of field DOF and small apertures increase DOF. Beyond that, spend time making your pictures more interesting.

Alan, not so with the meter being useless. The meter gives you a different set of readings for the same correct exposure. My 70-200 f2.8 vs my 200 f2.8 above is a perfect example. With the 200mm at f2.8 I get a slightly faster shutter speed or I can lower my ISO a bit as compared to the 70-200 at 200mm at f2.8 to achieve the same exposure.
If what you are saying was the case than any other lens with something different than what the camera is programed for would also fool the meter.

Then it appears that the settings on the lenses are not correct. Otherwise it defeats the whole point of having standard readings. For example, I shoot a film camera that has no exposure meter. I use a separate hand held meter. I set the lens shutter and aperture according to the meter regardless of the lens I use. Of course there could be differences between the lenses settings that I just haven't noticed in the final results. Bit it would be a real pain to have to change the settings based on which lens I use and not rely directly on my meter.

Curious, how different are the speeds between the two lenses for that f/2.8 setting you're getting at 200mm? What object is being metered; are the views exactly the same on both lenses? Has the light changed even in the slightest during the time you changed lenses?

Alan, exact f stops are a product of physics derived from a mathematical formula. This an excellent read on the mathematics of f.stops. Please Make A Note: The Mathematics of f/stop Aperture Numbers

As I mentioned in my first post, the focal length listed on lenses is not an exact science. There are standard, accepted and used focal lengths 12mm, 24mm, 35mm, 50mm. 70mm, 100mm, 135mm etc. Virtually all lenses are slightly off from those focal lengths. Lens manufactures however list there lenses as the closest standard focal length. A lens with a true and accurate focal length of 48.5mm is not going to be marked and marketed as a 48.5mm but as a 50mm lens. Have you ever looked to purchase a 71 to 198.6 mm lens? No, because it is marketed as a 70 to 200 mm lens. Thus the slight but irrelevant difference in true f stops. Just as a manufacture of that 48.5 mm lens marks it as a 50mm lens that same lens having an actual f stop of 2.71 is still listed as an f2.8. The slight variances would only become relevant if lens f stops were in 1/10th increments just as my hand held meter can be set to 1/10 f stops instead of the standard 1/2 or 1/3 stops. Even then the relevance to photography for the most part would be slight.

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